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IEEE Magnetism In Nanotechnology & Electronics Conference 2007

Organized by the IEEE Magnetics Society – Chapter of Northern Virginia and Washington DC 25 – 26 June 2007, National Institute of Standards and Technology, Maryland, USA

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This event is sponsored by

Chapter of Northern Virginia and Washington DC

IEEE Magnetics Society

Conference Proceedings Editor: Philip W.T. Pong

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Contents IEEE IMAGINE Conference 2007 3

Conference Committee – Program Committee and Organizing Committee 4

Conference Program 5

Conference Abstracts for Session: Monday, June 25, 2007, 09:15 AM – 10:45 AM

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Conference Abstracts for Session: Monday, June 25, 2007, 11:15 AM – 12:15 PM

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Conference Abstracts for Session: Monday, June 25, 2007, 01:15 PM – 03:15 PM

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Conference Abstracts for Session: Monday, June 25, 2007, 03:45 PM – 05:15 PM

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Conference Abstracts for Session: Tuesday, June 26, 2007, 09:00 AM – 10:30 AM

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Conference Abstracts for Session: Tuesday, June 26, 2007, 11:00 AM – 12:00 PM

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Conference Abstracts for Poster Session 37

Tutorial Materials for Quantum Statistics of Nanomagnets

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Tutorial Materials for A Vector Preisach Model

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Tutorial Materials for Computational Magnetism

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Appendix – NIST Map & Emergency Phone Number 95

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IEEE IMAGINE Conference 2007 –

Ieee MAGnetism In Nanotechnology & Electronics Conference 25 – 26 June 2007 Lecture Room A, Building 101, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland, MD 20899

Introduction:

The IEEE Magnetics Society Chapter of the Northern Virginia/Washington DC is organizing the IMAGINE Conference 2007 to provide an occasion for the magnetism researchers in this region and nearby to meet, interact, and exchange ideas. In addition, this is a good opportunity for postdoc/junior researchers and graduate students to present their research and gain exposure in the magnetism community. Student to be our contributors Just like the APS March meeting, a student is guaranteed an opportunity to present an oral or poster presentation. Postdoc/junior researchers to be Rising Stars Postdoc/junior researchers who show potential in the field are invited to make presentation as Rising Star Lecturers. They will also have opportunities as session chairs in the conference. This year, five Rising Star Lecturers are selected:

Tingyong Chen, Johns Hopkins University

Seok-Hwan Chung, National Institute of Standards and Technology/University of Maryland

Prasanta Dutta, West Virginia University

Christian Heiliger, National Institute of Standards and Technology

Amshumali Mungalimane, Virginia Tech

Congratulations! IEEE Magnetics Society Distinguished Lecturers In addition, the 2007 IEEE Magnetics Society Distinguished Lecturers are invited to lecture during this event as the keynote speakers. This event is FREE for IEEE members.

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IMAGINE

Conference committee

President of the Northern Virginia / Washington DC chapter: Can Korman, George Washington

University

Conference Chairman and Secretary of the chapter: Philip W. T. Pong, National Institute of

Standards and Technology

Executive Secretary: Se Young O, National Institute of Standards and Technology

Program Coordinator: Brian Maranville, National Institute of Standards and Technology

Student Assistant: Reza Rock, University of Delaware

Program committee

Lawrence Bennett George Washington University DC

Edward Della Torre George Washington University DC

Robert McMichael National Institute of Standards and Technology Maryland

Organizing committee

Sharat Batra Seagate Research Pennsylvania

Alan Edelstein Army Research Laboratory Maryland

Romel Gomez University of Maryland College Park Maryland

Charles Graham University of Pennsylvania Pennsylvania

David Lederman Western Virginia University West Virginia

Samuel Lofland Rowan University New Jersey

Michael McHenry Carniegie Mellon University Pennsylvania

Chaoying Ni University of Delaware Delaware

Daniel Reich Johns Hopkins University Maryland

Zareh Soghomonian Electromagnetic Systems, General Atomics DC

Gordon Yee Virginia Tech Virginia

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IEEE Magnetism in Nanotechnology & Electronics Conference 2007

Conference Program

Monday, 25 June, 2007

08:45 AM Registration

Session Chair: Philip Pong, NIST

09:00 AM Philip Pong Opening remarks, orientation, safety

09:15 AM

Robert McMichael Tutorial - Computational Magnetism

09:45 AM

Michael Donahue Plenary Talk - Overview and Outlook of the OOMMF Micromagnetic

Modeling Package

10:15 AM Antonio Zambano High-throughput Investigations of Exchange Coupling Interaction in Soft/Hard Magnetic Bilayer Systems for Development of Nanocomposite Magnets

10:30 AM Paul Ohodnicki Induced Anisotropy and Core Losses in Transverse Field Annealed Co-Rich Amorphous and Nanocomposite Alloys

10:45 AM

Break & Posters

Session Chair: Ed Della Torre, George Washington University

11:15 AM Alex Shapiro Reversal Asymmetry in CoPt Thin Films

11:30 AM K. Gilmore First-Principles Calculation of Precession Damping in Itinerant Ferromagnets

11:45 AM ✩Tingyong Chen Enhanced Curie Temperature and Spin Polarization in Mn4FeGe3 Compound

12:00 PM ✩C. Heiliger Influence of Amorphous Electrodes on Tunnelling Magnetoresistance Effect in Fe/MgO/Fe

12:15 PM

LUNCH & Posters

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Session Chair: Robert McMichael, NIST

01:15 PM


Distinguished Lecturer

- Matthias Bode

Imaging Magnetic Surfaces with Atomic Resolution

02:15 PM Miyeon Cheon Exchange Bias and Uncompensated Magnetization in FexNi1-xF2/Co and FexZn1-xF2/Co Bilayers

02:30 PM Makoto Murakami Exchange Bias Study between Ferromagnetic Metals and Multiferroic Materials

02:45 PM

Alan Edelstein Plenary Talk - Advances and Barriers in Magnetometry

03:15 PM

Break & posters

Session Chair: ✩Prasanta Dutta, West Virginia University

03:45 PM J.R.

Hattrick-Simpers Combinatorial Exploration of Magnetostriction in FeGaX Ternaries

04:00 PM Patrick Downey Mechanical Properties of Iron-Gallium Nanowires with Applied Magnetic Field

04:15 PM ✩Amshumali

M.K. Room Temperature and Near Room Temparature Coordination Polymer Magnets

04:30 PM Mark Harvey Mono- and Di- substituted TCNE Analogs that Form Room Temperature and Near-Room Temperature Molecule Based Magnets

04:45 PM

Ed Della Torre Tutorial - A Vector Preisach Model

05:15 PM Adjourn

✩ - Rising Star Lecturer

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Tuesday, 26 June, 2007

Session Chair: Lawrence Bennett, George Washington University

09:00 AM

Lawrence H. Bennett Tutorial - Quantum Statistics of Nanomagnets

09:30 AM


Distinguished Lecturer

- Sarah Majetic

Magnetic Nanoparticles: Self-Assembly and Nanoscale Behavior

10:30 AM

Break & Posters

Session Chair: Brian Maranville, NIST

11:00 AM ✩Seok-Hwan

Chung Sensing Bio-conjugated Magnetic Nanoparticles in Fluids

11:15 AM ✩Prasanta Dutta Superparamagnetism and Blocking in Thiol-capped Gold Nanoparticles

11:30 AM Vivek Singh Magnetic Properties of 4nm Ni Nanoparticles Dispersed in SiO2 Matrix

11:45 AM Sean McCooey Microwave Measurements of the Complex Permeability and Permittivity of Fine Magnetic Particles at Frequencies from 4 GHz to18 GHz

12:00 PM Close

✩ - Rising Star Lecturer

Posters:

1. Kelsey Miller FeCo- Co-Ferrite Core Shell Structures for Heat Sources in Thermoablative Cancer Therapy

2. Sridhar

Patibandla Spin Transport Studies in Nanowires

3. Robert Booth Modeling of Microwave Absorption of Magnetic Microwires

(throughout the whole conference)

4. Robert

McMichael Variation of Thin Film Edge Magnetic Properties with Patterning Process Conditions in Permalloy Stripes

5. Se Young O X-ray Diffraction Study of the MgO Growth for Magnetic Tunnel Junctions

6. CNST Introducing the NIST Center for Nanoscale Science and Technology

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Conference Abstracts for

Monday, June 25, 2007

09:15 AM – 10:45 AM

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Computational Magnetism

Robert D. McMichael

Magnetic Materials Group, NIST, Gaithersburg, MD 20899 USA Micromagnetics addresses the magnetism of magnetic domain configurations on mesoscopic length scales, forming a connection between microscopic materials properties and the performance of magnetic devices and materials. In this tutorial, I will review the assumptions of micromagnetics, starting with the basics of magnetization motion in isolated magnetic moments, and adding the local energy and interactions terms that lead to the interesting domain patterns in magnetic materials. Combinations of these tems yeild the characteristic length scales of the magnetization, and consequently, the typical sizes of magnetic features such as vortices, cross-ties and domain walls. For computational micromagnetics, the smooth, continuous character of the classical magnetization vector field is replaced by a discrete set of local magnetic moments. The discretization scheme is typically either a regular rectangular grid, which enables efficient computation of dipole-dipole interactions, or a finite element – boundary element discretization that facilitates modeling of shapes with curved or sloped surfaces. Additional choices between different computational algorithms are made depending on the type of output that is desired. For calculations of the magnetization dynamics, typically on nanosecond time scales, one needs to solve the differential equations that govern the magnetization motion. For calculations of equilibrium magnetization configurations, the precessional motion of the magnetization is often not important, so energy minimization schemes can be employed. A universal challenge in computational physics is ensuring the quality of the output. This is a challenge for modelers and for consumers of modeling information alike. Several common computational pitfalls and their characteristics will be discussed. I will conclude with example calculations of the precessional normal modes, i.e. quantized spinwaves, in a few magnetic nanostructures.

Figure1. Top: normal modes of a 350 x 160 nm ellipse of Permalloy, 5 nm thick in zero applied field. Modes are excited by field pulses having different symmetry properties. Bottom: Spatially averaged power spectra of the dynamic magnetization.

Tutorial

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Overview and Outlook of the OOMMF Micromagnetic Modeling Package

M.J.Donahue

National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Micromagnetics is the study of the magnetic behavior of materials at the nanometer length scale; this scale is large enough to allow a continuous approximation to the magnetization, but small enough to reveal fine details of magnetization transitions. As illustrated in Figs. 1 and 2, interpretation of experimental results is often aided by the numerical solution of micromagnetic models [1], and moreover these models are often able to describe and predict behavior outside the reach of experiment. The simulation results presented in Fig. 2 were computed using the Object-Oriented Micromagnetic Modeling Framework (OOMMF) [2], which is a widely-used public domain computer program developed at NIST. This talk will provide an overview of the OOMMF package, which is a complete, fully three dimensional, micromagnetic simulation system supporting both GUI and batch mode interfaces. This presentation will also include an introduction to some new features in current development, such as spin-momentum transfer, staircase-edge artifact correction (Fig. 3) [3], improved self-magnetostatic interaction computation, and multi-threaded support for multi-core/multi-processor machines. References [1] M.H. Park, Y.K. Hong, B.C. Choi, M.J. Donahue, H. Han, S.H. Gee, Physical Review B 73 (2006) 094424 [2] M.J. Donahue, D.G. Porter, “OOMMF User's Guide, Version 1.0,” NISTIR 6376, National Institute of Standards and Technology, Gaithersburg MD, USA 1999 [3] M.J. Donahue, R.D. McMichael, “Micromagnetics on curved geometries using rectangular cells: error correction and analysis,” to appear in IEEE Trans. Magn.

Fig.2 – OOMMF simulation of the sample in Fig. 1. Inside each domain wall are two counter-rotating vortices with three distinct high-divergence subregions (indicated by color) that generate the stray fields imaged in Fig. 1.

Fig.3 – Edge mode frequency simulations for a thin square rotated with respect to the computational grid. For θ ≠ 0, staircase-like edges result in serious computational artifacts (upper curve), but these errors can be largely corrected (lower curve).

Fig.1 – MFM image of a 65 nm thick, 2 µm NiFe ring, in a remanent “onion” state. The stray field sampled by the MFM probe shows three high-intensity regions above each head-to-head domain wall.

Plenary Talk

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High-throughput Investigations of Exchange Coupling Interaction in Soft/Hard Magnetic Bilayer Systems for Development of Nanocomposite Magnets

A. Zambano1, H. Oguchi1, M. Yu1, and I. Takeuchi1

S. Lofland2 D. Josell3 and L. A. Bendersky3

J. P. Liu4

1Department of Materials Science & Engineering and Center for Superconductivity Research,

University of Maryland, College Park, MD, 20742. 2Department of Physics and Astronomy, Rowan University, Glassboro, NJ, 08028.

3National Institute of Standards and Technology, Gaithersburg, MD, 20899. 4Department of Physics, University of Texas at Arlington, Arlington, TX, 76019.

Exchange coupled hard/soft magnet nanocomposites are being pursued for creating permanent magnets with substantially enhanced maximum energy products (BH)max. The identification of the parameters that govern the exchange interaction is crucial for the improvement of nanocomposite magnets. As a simple 1-dimensional model of such magnets, bilayer thin films are frequently used to probe and identify the parameters that govern the exchange interaction. But despite extensive effort, the factors that rule the exchange coupling interaction in such bilayer systems have not been unambiguously established. For this purpose, we use the high-throughput approach1 to study thin soft-magnetic/hard-magnetic bilayer films designed to delineate subtle variations of the exchange coupling interaction on magnetic constants. On single chips, multiple samples are grown by e-beam evaporation varying composition and layer thicknesses. The magnetic hysteresis loop for each sample is rapidly measured using magneto-optical Kerr effect (MOKE) measurements. The analysis of numerous samples at the same time allows us to have unique information which would be impossible to obtain from separately made single samples. We will show examples of studies of libraries of CoPt/(Fe, Co or Ni) and SmCo/(Fe, Co or Ni), where we experimentally characterized the exchange coupling interaction by measuring the coupling length (λ) and the nucleation magnetic field (HN) as function of soft layer thickness (ts). We will discuss the general advantage of using this technique to study numerous systems at the same time and obtain the role played by various parameters on exchange coupling interaction. This work is supported by ONR MURI N00014-05-1-0497

References

[1] I. Takeuchi, O.O. Famodu, J.C. Read, M.A. Aronova, K.-S. Chang, C. Craciunescu, S.E. Lofland,

M. Wuttig, F.C. Wellstood, L. Knauss and A. Orozco, Nat. Mater. 2, 180 (2003).

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Induced Anisotropy and Core Losses in Transverse Field Annealed Co-Rich Amorphous and Nanocomposite Alloys

P. R. Ohodnicki1, M. E. McHenry1, and D. E. Laughlin1, M. A. Willard2

1Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

2U.S. Naval Research Laboratory, Materials Physics Branch, Code 6340, Washington, D.C. 20375

Field processing of amorphous and nanocomposite soft magnetic alloys can result in significant improvements in core losses. Recent work on transverse field annealed Co-rich Co,Fe-based nanocomposite alloys has demonstrated low core losses and large field induced anisotropies (Ku~1800J/m3) of great technical interest for use in high frequency applications [1]. In addition, the HiTPerm-type Co-rich alloys exhibit an interesting tendency for preferential nucleation of bcc phase even for Co:Fe ratios within the single phase fcc region of the binary Fe-Co phase diagram [2,3]. Several Co-rich amorphous alloy compositions have been synthesized through single-roller wheel melt spinning. Toroidal tape wound cores have been annealed in a transverse field of 2T for 1 hour at temperatures above and below the primary crystallization temperature. Using AC permeametry, dynamic losses and the anisotropy field Hk have been measured for frequencies ranging from 1-500 kHz for both the field annealed amorphous ribbons and field crystallized ribbons. For a Co:Fe ratio of 0.85:0.15, a qualitative difference in the variation of the Ku with 1hr annealing temperature is observed for a (CoFe)NiZrBCu alloy as compared to a (CoFe)NbSiB alloy. The peak values of Ku are quite similar for the two alloys, however the peak value is observed for the amorphous ZrB-based alloy while the value of Ku increases with increasing annealing temperature above the primary crystallization temperature for the NbSiB-based alloy. In addition, the high frequency losses (f=300kHz) increase significantly for the ZrB-based alloy after crystallization while the high frequency losses (f=500kHz) for the amorphous and crystallized NbSiB-based alloy are quite similar. The observed difference in the trend of Ku with increasing annealing temperature suggests a possible difference in the dominant mechanism responsible for the field induced anisotropy of these two alloys. [1] Y. Yoshizawa, S. Fujii, D. H. Ping, M. Ohnuma, and K. Hono, Scr. Mater. 48, 863 (2003). [2] M. A. Willard, T. M. Heil, and R. Goswami, Metall. Mater. Trans. A (in press). [3] P. R. Ohodnicki, S. Y. Park, H. K. McWilliams, K. Ramos, D. E. Laughlin, and M. E. McHenry, J. Appl. Phys. (in press).

Figure 1: Room temperature dynamic B-H Loops (f=3kHz) for Co-rich Amorphous and Nanocrystalline Toroidal Cores. a) No field (NF) and transverse field annealed (TMF) nanocrystalline cores annealed at 530C for 1hr (Blue=(Co0.85Fe0.15)83.6Ni4.4Zr7B4Cu1, Red =(Co0.85Fe0.15)78.4Nb3.6Si9B9). b) and c) TMF amorphous and/or nanocrystalline (CoFe)NbSiB and (CoFeNi)ZrBCu cores, respectively, annealed at varying temperatures for 1 hour.

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-5

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-60 -40 -20 0 20 40 60

(CoFe)NbSiB f=3kHz

B (k

G)

H (Oe)

530C TMF

350C TMF

480C TMF

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5

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(CoFe)NiZrBCu f=3kHz

B (k

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350C TMF

450C TMF

a) b) c)

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11:15 AM – 12:15 PM

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Reversal Asymmetry in CoPt Thin Films.

R.D. Shull, V.I. Nikitenko, and AJ. Shapiro

Magnetic Materials Group, National Institute of Standards and Technology, Gaithersburg, MD 20899

How quickly and easily a material reverses its magnetization direction upon application of a reversed magnetic field is dictated by the mechanism for reversal. The mechanism also determines how that material can be used in applications.

In a ferromagnet (e.g., the common magnet), the easiest process for reversal occurs through the nucleation of a local region or domain with reversed magnetization that subsequently grows throughout the material, consuming neighboring domains. Normally, such nucleation does not care in which direction the field is applied. Recently, directionally dependent nucleation was observed by scientists in MSEL in ultrathin (0.6 nm) Co and in [Co (0.6 nm)/Pt (3 nm)]n (n = 1, 2, 4) multiplayer films with magnetization out of the plane. This anomalous effect was discovered using the magneto-optic indicator film (MOIF) technique invented in MSEL. The new magnetic domains nucleate at different positions for applied fields pointing up than for those pointing down. These hitherto unknown asymmetrical magnetic domain nucleation centers result in different reversal fields for opposite field directions. Other anomalous effects, such as differing rates of reversal, were also found. MOKE images of the evolution of the [Co/Pt]4 domain structure with m0H = +23.4 mT (a),(b), and after a magnetic field switch to m0H = - 23.4 mT (c),(d) are presented below.

These results are of great consequence for magnetic device designers: magnetic devices and recording media based on such materials will have to be adjusted for field directionality. Implications are particularly timely since Co/Pt multilayer films with perpendicular anisotropy are planned as the next generation highest density recording media. . In addition to the industrial implications, fundamental theories of magnetization reversal will also need to be changed.

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First-principles Calculation of Precession Damping in Itinerant Ferromagnets

K. Gilmore1,2, M.D. Stiles1, and Y.U. Idzerda2

1 Electron Physics Group, National Institute of Standards and Technology, Gaithersburg, MD 20899

2 Department of Physics, Montana State University, Bozeman, MT 59717

The performance of magnetic devices depends increasingly on the rate at which the magnetization dynamics are damped. While these dynamics can successfully be modeled with the Landau-Lifshitz-Gilbert equation, predictive use of this phenomenological expression requires prior knowledge of the damping rate. This has limited the success of recent efforts to develop new materials with specific or lowered damping rates for various applications. Further, material constants, such as the damping rate, are expected to change as dimensions are reduced to the nanoscale. A thorough understanding of the relaxation process and an ability to predict the damping rate of new materials or systems with reduced geometries would provide a clear benefit to the development of new devices. Toward this goal we have performed first-principles calculations of the damping rate of bulk Fe, Co, and Ni, the simplest metallic systems. The minimum calculated (previously measured) damping rates for these metals are: a_Fe = 0.0013 (0.0021), a_Co = 0.0011 (0.0026), and a_Ni = 0.017 (0.024). While the calculated values are smaller than the measured values, results of the calculation would be increased by properly including the orbital contribution to the magnetization. Our calculations demonstrate that we have identified the dominant mechanism for damping in these systems.

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Enhanced Curie Temperature and Spin Polarization in Mn4FeGe3

Compound

T. Y. Chen and C. L. Chien Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218

C. Petrovic

Department of Physics, Brookhaven National Laboratory, Upton, NY 11973

With a good lattice match to semiconductors such as Ge, the intermetallic ferromagnetic compound,

Mn5Ge3, is a promising spin injector with a high spin-injection efficiency for the next generation

electronics, the spintronics. The Curie temperature (TC) of Mn5Ge3, though much higher than that of

magnetic semiconductors, is still below room temperature. The spin polarization of an epitaxial

Mn5Ge3 film has been shown to be about 42%. In this work we investigate magnetic properties of a

metallic Mn4FeGe3 compound and determine its spin polarization using point-contact Andreev

Reflection. We show that by replacing one Mn atom with Fe, the TC of Mn4FeGe3 can be enhanced to

320 K while maintaining its high conductivity. Most importantly, its spin polarization is over 60%,

significantly larger than that of Mn5Ge3.

Fig.1 – Magnetization and resistivity of Mn4FeGe3 as a function of temperature in various magnetic fields.

Fig.2 – Representative PCAR spectra of Mn4FeGe3/Nb pointcontacts. Open circles are experimental data and solid lines are fittings to theoretical models with red line for the BTK model and blue line for the diffusive model.

Fig.3 – Spin polarization as a function of Z factor. The solid line is a polynomial fitting to guide the eye.

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Influence of Amorphous Electrodes on Tunnelling Magnetoresistance

Effect in Fe/MgO/Fe

C. Heiliger1,2,3, M. Gradhand3, P. Zahn3, I. Mertig3

1Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-8412

2Maryland NanoCenter, University of Maryland, College Park, MD, 20742 3Department of Physics, Martin Luther University,D 06099 Halle, Germany

The extensive research on tunnelling magnetoresistance effect (TMR) has recently focused on Fe/MgO/Fe due to the possibility of growing high quality crystalline tunnel junctions [1,2]. Such crystalline junctions reveal TMR ratios up to several hundred percent. These high ratios were predicted theoretically [3,4] and can be explained by the electronic structure [5]. In typical theoretical investigations, semi-infinite leads of Fe are assumed. In a real tunnel junction, however, the thickness of the Fe electrodes is only a few monolayers. In addition, Djayaprawira et al. [6] found experimentally that tunnel junctions with amorphous FeCoB leads but a crystalline MgO barrier had a high TMR ratio over 200%. Here we theoretically investigate the role of the structural order of the ferromagnetic leads. We present ab initio calculations to clarify the role of amorphous iron in direct contact to the tunnelling barrier. In this case the high TMR ratio is drastically decreased to less than 50%. Following this result the question will be addressed: How many crystalline ferromagnetic monolayers are necessary to obtain a high TMR ratio?

References [1] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nature Materials 3 (2004) 868 [2] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yang

Nature Materials 3 (2004) 862 [3] J. Mathon, A. Umerski, Phys. Rev. B 63 (2001) 220403 [4] W. Butler, X.-G. Zhang, T. Schulthess, J. MacLaren, Phys. Rev. B 63 (2001) 054416 [5] C. Heiliger, P. Zahn, I. Mertig, Materials Today 9 (2006) 46 [6] D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe,

S. Yuasa, Y. Suzuki, K. Ando, Applied Physics Letters 86 (2005) 092502

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IMAGINE



01:15 PM – 03:15 PM

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Magnetics Society Distinguished Lecturer for 2007 IMAGING MAGNETIC SURFACES WITH ATOMIC RESOLUTION

Dr. Matthias Bode University of Hamburg Fueled by the ever increasing data density in magnetic storage technology and the need for a better understanding of the physical properties of magnetic nanostructures, there exists a strong demand for high resolution, magnetically sensitive microscopy techniques. The technique with the highest available resolution is spin-polarized scanning tunneling microscopy (SP-STM) which combines the atomic resolution capability of conventional STMs with spin sensitivity by making use of the tunneling magnetoresistance effect between a magnetic tip and a magnetic sample surface. Beyond the investigation of ferromagnetic surfaces, thin films, and epitaxial nanostructures with unforeseen precision, it also allows the achievement of a long-standing dream: the real space imaging of atomic spins in antiferromagnetic surfaces. The lecture addresses a wide variety of phenomena in surface magnetism which in most cases could not be imaged directly before the advent of SP-STM. After starting with a brief introduction of the basics of the contrast mechanism, recent major achievements will be presented, like the direct observation of the atomic spin structure of domain walls in antiferromagnets and the visualization of thermally driven switching events in superparamagnetic particles consisting of a few hundreds atoms only. To conclude the lecture, recently observed complex spin structures containing 15 or more atoms will be presented.

Matthias Bode received the diploma in physics from the Free University of Berlin, Germany, in 1993, and the Ph.D. degree in physics from the University of Hamburg, Germany, in 1996. Based on his works on spin-polarized scanning tunneling microscopy he received the habilitation in experimental physics from the University of Hamburg in 2003. Since 1996 he is a Research Staff Member at the Institute of Applied Physics at the University of Hamburg. In the past 10 years Dr. Bode developed spin-polarized scanning tunneling microscopy, a magnetic imaging technique

with a resolution down to the atomic limit. His research explores correlations between structural, electronic, and magnetic properties of epitaxial nanostructures with a special interest in frustrated antiferromagnetic surfaces, superparamagnetism, and new magnetic phenomena. Dr. Bode has published more than 80 peer-reviewed papers, three review articles, and three book chapters. In 2003 he was awarded the Philip-Morris Award for research.

(Information transcribed from www.ieeemagnetics.org)

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Exchange bias and uncompensated magnetization in FexNi1-xF2/Co and

FexZn1-xF2/Co bilayers

Miyeon Cheon and David Lederman

Department of Physics, West Virginia University,

Morgantown, WV 26506-6315 We have studied the exchange bias using antiferromagnetic FexZn1-xF2 and FexNi1-xF2 thin films grown via molecular beam epitaxy. FexZn1-xF2 is an ideal dilute antiferromagnet (AF) that behaves as a random field Ising model system and FexNi1-xF2 is an antiferromagnetic with a random anisotropy. A large uncompensated magnetization was observed in the hysteresis loops of samples grown with a thin Co film overlayer whose sign was correlated with the sign of the exchange bias field (shift of the center of the loop, HE). In the FexNi1-xF2/Co bilayer system we were able to reverse the sign of the uncompensated magnetization by applying higher fields at low temperatures due to the weak AF anisotropy, thus enabling us to change the sign of HE at low temperatures (5 K). The coercivity of the uncompensated magnetization was very large at low temperatures (~15 kOe, depending on x) but decreased rapidly as the temperature was increased, until it became comparable to the coercivity of the ferromagnetic overlayer. At this temperature HE disappeared, resulting in a low blocking temperature compared to the AF Néel temperature TN. Curiously, the uncompensated magnetization had an exchange bias of its own and a temperature dependence that decreased with increasing temperature near TN with a critical exponent of ~0.4, close to the bulk ordering value of 0.37. This indicates that the uncompensated magnetization is located within the bulk of the AF, and not just at the interface. An Fe0.36Zn0.64F2/Co sample also showed a large uncompensated magnetization, but it was only possible to reverse it at a temperature close to TN using the available field range (70 kOe), illustrating the importance of the AF anisotropy. This work was supported by the National Science Foundation (grant 0400578).

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Exchange Bias Study between Ferromagnetic Metals and Multiferroic

Materials

Makoto Murakami1, J. Hattrick-Simpers1, S. Fujino1, S.-H. Lim1, L. G. Salamanca-Riba1, S. E. Lofland2, S.-W. Cheong3, J. Higgins4, M. Wuttig1 and Ichiro Takeuchi1,4

1Department of Materials Science and Engineering, University of Maryland,

College Park, MD 20742 2 Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028

3Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854-8019 USA 4Center for superconductivity Research, University of Maryland,

College Park, MD 20742

We are studying exchange bias at ferromagnet layer/multiferroic interfaces to understand the nature of magnetism in multiferroic materials, and to explore tuneable multiferroic device applications (Fig.1). Co 5 nm layers have been deposited by high –vacuum sputtering on surfaces of epitaxial BiFeO3 and TbMnO3 thin films as well as on LuMnO3 single crystals. To study out-of-plane exchange bias, [Co/Pt] multilayers have been deposited by e-beam evaporation on surfaces of epitaxial Cr2O3 . Epitaxial BiFeO3, TbMnO3 and Cr2O3 films were prepared by pulsed laser deposition on SrTiO3, YSZ and c-sapphire substrates, respectively. c-axis oriented LuMnO3 crystals were grown by the Bi2O3 based flux method. Magnetic properties of the Co/multiferroic bilayers are measured using SQUID, VSM, MOKE and XMCD. In BiFeO3, we find that the bilayers exhibit exchange bias even at room temperature as shown in Fig.2. In the TbMnO3 system, increase of coercive field and exchange bias was clearly observed below the Néel temperature. In LuMnO3, we observe positive exchange bias as well as switching of the sign of the exchange bias depending on the cooling procedure. This behavior may be related to the frustration in Mn spins. In the Cr2O3/[Co/Pt] system, the shape of hysteresis curve is controlled by the cooling procedure of the bilayers. Difference in the exchange bias behavior between different multiferroic materials will be discussed. The effect of electric field on exchange bias is currently under investigation. Supported by ONR N000140110761, ONR N000140410085, NSF DMR 0094265, DMR 0231291, MRSEC DMR-00-0520471, and the W. M. Keck Foundation.

Fig.2 – Magnetic hysteresis curves of BiFeO3/Co bilayer films on (100), (110), and (111) SrTiO3

substrates.

Fig.1 – Schematic of spin valve on multiferroic device.

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Advances and Barriers in Magnetometry

Alan Edelstein

U.S. Army Research Laboratory, Adelphi MD 20783 The considerable recent progress in magnetometry will be reviewed. Lower cost total field magnetic sensors with good sensitivity may result from chip scale atomic magnetometers. New developments in vector magnetometers include magnetic tunnel junctions with MgO barriers that exhibit magnetoresistance values of 470% at room temperature, magnetoelectric sensors that generated a voltage without any input voltage, and a device, the MEMS flux concentrator, that will mitigate the effect of 1/f noise. A few of the barriers in using magnetometers and in MEMS fabrication will be discussed.

Plenary Talk

23

CONFER



03:45 PM – 05:15 PM

24

Combinatorial Exploration of Magnetostriction in FeGaX Ternaries

J.R. Hattrick-Simpers1, D. Hunter1, C. Long1, K.S. Jan1, J. Cullen1, S.E. Lofland2, M. Wuttig1, I.

Takeuchi1

1Department of Materials Science and Engineering, University of Maryland, College Park, MD

20742 2Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08012

The unusually large magnetostriction and complex dependence of magnetostriction observed in Fe1-xGax (0 ≤ x ≤ 30) has attracted a large amount of interest in recent years [1,2]. As Ga is added the magneostriction exhibits two peaks, one at 20 atm% Ga the other 30 atm% Ga, with values of 270 ppm, an increase of a factor of 9 from that of pure Fe [1]. These two peaks have been previously attributed to separate effects; the first peak is associated with the establishment of DO3 order, the second with a maximum in magnetoelastic coupling. The addition of ternary elements to Fe-Ga offers the possibility of further enhancing magnetostriction, increasing workability, and probing the physics underlying the multiple peaks in magnetostriction. This work focuses on the synthesis and characterization of FeGaX (X=Pd, Cu, Al) ternary composition spread samples. A high sensitivity, high-throughput measurement technique for measuring the magnetostriction of cantilever bimorph spread samples was developed and employed to monitor the trend of magnetostriction across the ternary phase diagram. The addition of Pd, a low solid-solubility element in Fe, was found to rapidly decrease the overall magnetostriction of Fe-Ga and also to stabilize the DO3 phase to lower Ga contents. The addition of Al, a high solid-solubility element in Fe, was found not to alter the compositional dependence of magnetostriction. Additions of Al up to ~10 atm% were found to slightly increase the magnetostriction observed in Fe70Ga30. References [1] G. Petculescu, K.B. Hathaway, T.A. Lograsso, M. Wun-Fogle, A.E. Clark Journal of App. Phys. 97 (2005) 10M315-1 [2] M. Wuttig, L. Dai, J. Cullen App. Phys. Letts. 80 No. 7 (2002) 1135

25

Mechanical Properties of Iron-Gallium Nanowires with Applied

Magnetic Field P. R. Downey and A. B. Flatau

Aerospace Engineering, University of Maryland, 3181 Martin Hall, College Park, MD 20742

This study details the characterization of the mechanical properties of magnetostrictive iron-gallium nanowires when subjected to an applied magnetic bias field. Researchers are interested in this alloy because it exhibits moderate magnetostriction with good mechanical behavior such as high tensile strength and machinability [1], creating new possibilities for sensor and actuator applications. In particular, the motivation for this research is to use the magneto-mechanical coupling inherent in these electro-chemically deposited nanowires [2] to mimic the transduction of biological cilia for acoustic, flow, or tactile sensing. The experiments are conducted with a custom manipulator stage designed for use within a scanning electron microscope (SEM). Individual nanowires are extracted from an array and statically loaded in tension between two cantilevers of calibrated stiffness (as shown in fig. 1), where the applied stress and strain can be calculated from measurements made by micrograph analysis. The resultant graphs (fig. 2) provide insight into the Young’s modulus and ultimate tensile strength of the nanowires. This data is also measured dynamically by using a piezoelectric actuator to excite the fundamental resonance of cantilevered nanowires and calculating the required stiffness. The effect of magnetic field on these values is determined by performing the tests in the presence of a dc field to observe any delta-E effect similar to that displayed by the bulk material.

[1] R. Kellogg, A. Russell, T. Lograsso, A. Flatau, A. Clark, and M. Wun-Fogle, Acta Materialia 52 (2004) 5043.

[2] P. McGary, L. Tan, J. Zou, B. Stadler, P. Downey, and A. Flatau, J. Appl. Phys. 99 (2006) 08B310.

Fig.1 – Micrograph of nanowire attached to opposing AFM tips in the manipulator device prior to tensile testing.

Fig.2 – Tensile stress vs. strain results for a 14.1 µm long, 135 nm diameter nanowire without applied field. Measured modulus of 59.4 +/- 6.3 GPa, tensile strength of 1202 MPa.

26

Room Temperature and Near Room Temperature Coordination

Polymer Magnets Amshumali M.K, Mark D. Harvey, Joseph M. Zadrozny, Alexis Wells, Kim Joyce and Gordon T

Yee*

Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212

From the perspective of possible commercialization, one important goal in the field of molecule-based magnets is the synthesis of air stable materials with critical temperatures (Tc) above room temperature. The emergence of soft ferrimagnetism in V[TCNE]2, TCNE = tetracyanoethylene, has sparked interest in the development of new radical anion bridged magnets that are ordered above room temperature.1 Since no TCNE-based magnet is both ordered at room temperature and air stable, it is necessary to find a tunable TCNE analogue which hopefully could be paired with an oxygen stable cation to give a room temperature magnet. Working toward this goal, recently we have synthesized a number of fluoro- chloro- Bromo and trifluoromethyl substituted phenyl tricyanoethylene compounds (Fig.1), for the replacement of TCNE and used them to form their analogous vanadium-based magnets.2,3 Depending on the substitution’s position on the phenyl ring, the majority of them are near room temperature ferrimagnets, and few compounds have magnetic ordering temperature above the room temperature. Trends in Tc with substitution can only be partially explained by correlation with electronic properties of the building blocks as determined by DFT calculations and electrochemical measurements.

CN

CN

NC CN

CN

NC

F3C

CN

CN

NC

Br

CN

CN

NC

F

CF3

CN

CN

NC

Cl

Cl

Fig.1: Examples of TCNE analogues. References: 1) J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, J.S. Miller, Science 252 (1991) 1415. 2) J.M. Zadrozny. J.Under Graduate Materials Research .2 (2006) 42. 3) M.D. Harvey, J.T. Pace, G.T.Yee. Polyhedron (2006), doi:10.1016/j.poly.2006.09.097.

27

Mono- and Di- substituted TCNE Analogs that Form Room Temperature and Near-Room Temperature Molecule Based Magnets

Mark D. Harvey, M. K. Amshumali, Joseph A. Zadrozny, Alexis Wells, Kim Joyce and Gordon T.

Yee*

Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 24061

The emergence of molecule-based magnets in the 80’s[1] has led to the discovery of the soft magnetic coordination polymer V[TCNE]2 (TCNE = tetracyanoethylene) with an ordering temperature (Tc) above 350 K.[2] This amorphous material contains a TCNE radical anion that allows for effective magnetic communication throughout the polymer. Despite its high Tc, extreme air sensitivity has plagued its potential usefulness. In addition, a surprisingly small number of related magnets have been developed to probe the nature of high Tc in this family of compounds.[3] In this work, various mono- and di- substituted phenyl rings replace a nitrile group in TCNE to create new acceptors that serve as “tunable” probes of the magnetic properties of the corresponding vanadium(II) coordination polymers (Figure 1). Magnetic measurements have thus far provided a picture of the effect that substitution has around the phenyl ring: substituents located in the 2, 3, 5 and 6 positions yield higher Tcs when compared to the unsubstituted analog, PTCE (Figure 2, X = H). Substitution in the 4 position suppresses the ordering temperature of the polymer magnet. Assuming the changes in steric bulk of the substitutions relative to a hydrogen atom are minimal, electronic effects would be the logical cause for changes in ordering temperatures. The electronic properties of the radical anion acceptors are discussed in terms of the cyclic-voltammetry data and DFT calculations. One goal of this project is to be able to tune molecule-based magnets that could provide insight into the creation of stable, high Tc materials. References: [1] J. S. Miller, J. C. Calabrese, H. Rommelmann, S. R. Chittipeddi, J. H. Zhang, W. M. Reiff, A. J. Epstein, J. Am. Chem. Soc. 109 (1987) 769. [2] J. M. Manriquez, G. T. Yee, R. S. McLean, A. J. Epstein, J. S. Miller, Science 252 (1991) 1415. [3] M. L. Taliaferro, M. S. Thorum, J. S. Miller, Angew. Chem. Int. Ed. 45 (2006) 5326. (and references therein)

NC CN

CN

Xn

X = H, F, Cl, Br, CF3, Me, OEt

NC CN

CN

2

3

4 5

6

Figure 1. PTCE (phenyltricyanoethylene) backbone with labled positions where substitutions have been made.

Figure 2. A list of various TCNE analogs that have been developed by our group.

28

Advanced Preisach Modeling

E. D. Torre

Institute for Magnetics Research, George Washington University, Washington, DC 20052

Preisach modeling has been successfully used for many years to characterize the magnetic properties of devices. These models were essentially one dimension models. Lately, they have been expanded in scope to include accommodation, thermal aftereffect, and most recently the have three dimensional capability.

Dr. Della Torre, author of Magnetic hysteresis, and developer of many of these extensions, will give a tutorial lecture on the basic model and how it has been extended recently.

[1] E. Della Torre, Magnetic Hysteresis, Piscataway, NJ: IEEE Press, 1999. [2] E. Della Torre , E. Pinzaglia , E. Cardelli, "Vector modeling: Part I, Generalized hysteresis model," Physica B, 372, Feb. 2006, pp. 111-114. [3] E. Della Torre, E. Pinzaglia, E. Cardelli, "Vector modeling: Part II, [4] Ellipsoidal vector hysteresis model - numerical application to a 2-d case," Physica B 372, Feb. 2006, pp. 115-119. [5] L. Yanik, A. Yarimbiyik, and E. Della Torre, "Comparison of the differential equation accommodation model with experiment," J. Appl. Phys. 99, 08D706 (2006).

Tutorial

29


Tuesday, June 26, 2007

09:00 AM – 10:30 AM

30

Quantum Statistics of Nanomagnets

Lawrence H. Bennett

George Washington University, Ashburn, VA 20147 At a temperature T and chemical potential ς, the average number of quantum particles in a single particle state i with energy Ei is:

<Ni> = 1 / {e(Ei-ζ)kT +/- 1 } `.

The particles are fermions when the + sign is used and bosons when the - sign is used. Most physicists understand the properties of fermions with its application to electronic band structure in semiconductors, but they have a more difficult time with bosons. In this tutorial, I will explore various questions that appear to cause difficulty, including the phenomenum of Bose-Einstein condensation (BEC), the meaning of chemical potential, and the differences (or similarities) between BEC in real particles such as atoms, and quasiparticles, such as photons or magnons. I will stress the special situation of nanomagnets.

Tutorial

31

Magnetics Society Distinguished Lecturer for 2007 MAGNETIC NANOPARTICLES: SELF-ASSEMBLY AND NANOSCALE BEHAVIOR

Prof. Sara A. Majetich Carnegie Mellon University

The magnetic behavior of a monodomain nanoparticle was first described by Stoner and Wohlfarth nearly sixty years ago, yet this simple system is frequently invoked in discussions of high-density magnetic recording media, magnetic refrigeration materials, and a host of biomagnetic applications. Here we will examine two cross-cutting themes of current research on magnetic nanoparticles: self-assembly and nanoscale magnetic behavior. Different types of superstructure can be self-assembled from the same type of particles. In organic solvents, two-dimensional arrays with long-range order can be formed using Langmuir layer techniques. These monolayers are also used as nanomasks for crystallographically oriented thin films, which provide an alternative approach to preparing nanoparticle arrays for data storage media. Faceted three-dimensional single “grain” nanoparticle crystals are formed by colloidal crystallization methods. Magnetic field gradients can also be used to guide self-assembly. For example, gold-coated iron oxide particles can be used to image self assembly dynamics in aqueous media, in response to patterned magnetic elements, using plasmon scattering and dark-field optical microscopy to track single particles. The ability to make magnetic nanostructures creates a need for new tools that enable us to visualize their magnetization patterns. Small-angle neutron scattering provides average magnetic correlation lengths within three-dimensional assemblies, where correlations of hundreds on nanometers may be present at low temperature. Electron holography shows real-space magnetization patterns of magnetic monolayers, where vortices and transverse domain walls are present as low energy excitations. Scanning probe techniques have the potential for single-particle-per-bit magnetic information storage.

Sara Majetich received the A.B. degree in chemistry at Princeton University and the M.S. degree in physical chemistry at Columbia University. Her Ph.D. was in solid state physics from the University of Georgia. She did postdoctoral work at Cornell University. Since 1990 she has been a faculty member, and now full professor, in the Physics Department at Carnegie Mellon University. Her awards include the Ashkin Award for excellence in teaching, the Carnegie Mellon University Undergraduate Advising Award, and a National Young Investigator Award from the National Science Foundation. She has three patents and over 100 publications. Her research

interests focus on magnetic nanoparticles and nanocomposites and their applications.

(Information transcribed from www.ieeemagnetics.org)

32

IMAGINE


Tuesday, June 26, 2007

11:00 AM – 12:00 PM

33

Sensing Bio-conjugated Magnetic Nanoparticles in Fluids

Seok-Hwan Chung

Center for Nanoscale Science and Technology National Institute of Standards and Technology, Gaithersburg, MD 20899

The recent development of bio-conjugated magnetic nanoparticles offers various opportunities for their applications in the biomedical field. In particular, widespread research in the use of magnetic nanoparticles for biosensing has been stimulated by recent progress in magnetic field sensors. Here we demonstrate a substrate-free biosensing approach based on the Brownian rotation of ferromagnetic nanoparticles suspended in fluids. The signal transduction is through the measurement of the magnetic ac susceptibility as a function of frequency. The susceptibility changes due to the modification of the hydrodynamic radius of bio-conjugated magnetic nanoparticles upon binding to target bio-molecules, as shown in Fig. 1. In addition to the conventional magneto-electrical technique, we demonstrate a magneto-optic approach to measure the ac susceptibility of magnetic nanoparticles suspended in fluids. Our magneto-optic measurement is able to detect nanoparticle densities at least three orders smaller than commercial electrical systems. This increased sensitivity allows the magneto-optic technique to be potentially useful for measuring properties of the magnetic nanoparticle’s local environment, such as temperature, viscoelasticity or molecular binding. The advantage of this substrate-free approach includes its relative simplicity and integrity compared to the methods that use substrate-based stray-field detectors.

References [1] S. H. Chung et al., Appl. Phys. Lett. 85 (2004) 2971 [2] S. H. Chung et al., J. Appl. Phys. 97 (2005) 10R101 [3] S. H. Chung et al., J. Magnetics 11 (2006) 189

Fig.1 – Imaginary part of the acsusceptibility of an avidin-coated magnetite nanoparticle before (solid circles), after (open circles) binding to S-protein, and after (open squares) binding to biotinylated T7 bacteriophage [1].

34

Superparamagnetism and Blocking in Thiol-capped Gold

Nanoparticles P. Dutta1, M. Anand2, S. Pal1, C. B. Roberts2 and M. S. Seehra1

1Department of Physics, West Virginia University, Morgantown, WV 26505

2Department of Chemical Engineering, Auburn University, Auburn, AL 36849

The observation of ferromagnetism in thiol-capped gold nanoparticles (Au-NP) has been the focus of considerable attention in recent years [1-3]. Since the naked Au NP are not ferromagnetic, the ferromagnetism has been suggested to results from holes produced by charge transfer from Au to S atoms in dodecanethiol (inset of Fig. 1). However, in none of these studies [1-3], observation of either a blocking temperature TB or Curie temperature Tc has been reported. In this work, we report the observation of TB ⋍ 50 K (Fig. 1) in 5 nm dodecanethiol (DT)-capped Au-NP (Fig. 2 and Fig. 3) but only diamagnetism in 12 nm DT-capped Au-NP. For T < TB = 50 K, the strong temperature dependence of coercivity Hc, saturation magnetization Ms and exchange-bias He in the field-cooled sample confirm the blocked state resembling ferromagnetism with Hc ⋍ 250 Oe, He ≃ - 40 Oe, and Ms ⋍ 10-2 emu/g at 5 K. The observed electron magnetic resonance line shows expected shift, broadening and reduced intensity below TB. A magnetic moment µ ⋍ 0.006 µB per Au atom attached to DT is determined using a model which yields Ms varying as 1/D, with its source being holes in the 5d band of Au produced by charge transfer from surface of Au to S atoms in DT (Inset of Fig. 1). This model shows that the size-induced magnetism in Au-NP has its origin in the charge transfer to the capping agent, thus providing a new route to create magnetic materials in otherwise diamagnetic NP.

[1] P. Crespo et al, Phys. Rev. Lett. 93 (2004) 087204; ibid, 97 (2006) 177203. [2] A. Hernando, P. Crespo, M. A. Garcia, Phys. Rev. Lett. 96 (2006) 057206. [3] Y. Yamamoto et al, Phys. Rev. Lett. 9 (2004)116801.

Fig. 1 – Magnetic susceptibility χ (M/H) vs. T at H = 500 Oe for the ZFC and FC modes. The inset is the schematic illustration of Au-SH bond to dodecanethiol.

Fig.2–Transmission electron microscopy picture of chemically synthesized DT-capped gold nanoparticles.

Fig.3 – Particle size distribution histogram of the nanoparticles. The solid line is fit to log-normal distribution with D = 4.8(1.4) nm.

0 1 2 3 4 5 6 7 8 9 100

10

20

30

Freq

uenc

y (%

)

Diameter (nm)

0 100 200 300

2.0

2.4

2.8

3.2

3.6 FC

ZFC

50 K

H= 500 Oe

χ (1

0-6 e

mu/

gAu

Oe)

T(K)

•Au

o

CH3

SH(CH2)11

35

Magnetic Properties of 4nm Ni Nanoparticles Dispersed in SiO2

Matrix

Vivek Singh1, M. S. Seehra1, and J. Bonevich2

1 Physics Department, West Virginia University, Morgantown, WV 26506

2 National Institute of Standards and Technology, Gaithersburg, MD 20899

We report detailed investigations of the magnetic properties of Ni nanoparticles embedded in Ni/SiO2

(15/85) nanocomposite prepared by sol-gel precursor method[1]. Transmission electron microscopy (TEM) showed uniform dispersion of Ni nanoparticles (NP) with mean size D≈ 3.8nm (σ=0.7) and amorphous nature of the SiO2 matrix(fig.1&2). X-ray diffraction studies showed Ni-NP with D≈3nm without any contamination from NiO. Measurements of the dc magnetization (M) vs temperature (T) for the ZFC and FC modes in H = 50 Oe yielded a blocking temperature TB ≈30K. For T > TB, M/MS scales approximately as H/T expected for superparamagnetism with average magnetic moment/particle, µP ≈ 3500µB. Measurements of ac susceptibilities χ΄ and χ΄΄ at f = 0.1, 1, 99.9, 499 and 997Hz showed increase of TB with increase in f. The data for χ΄΄ is used to determine Φ=∆Tm /Tm ∆log10 f with Φ=0.11(0.1) where ∆Tm is the shift in the peak temperature Tm of χ΄΄ with frequency f [2]. This value of Φ=0.11 signifies negligible interparticle interaction [2]. The coercivity HC ≈ 200 Oe is constant from 2 to 6K and then decreases on approach to TB. A similar anomaly below 6K is observed in temperature dependence of the remnance Mr. Finally, the observed decrease of TB with applied field H (fig.3) is found to fit the equation TB(H)=33(1 – H/3200)m with m=2 for H < 1000 Oe and m=3/2 for 1000 < H < 3200 Oe, in line with theoretical predictions[3]. References [1] E. R. Elite, et-al, J. Nanotechnol. 2, 89 (2002) [2] J. L. Dormann, L Bessais, D. Fiorani, Solid State Phys. 21, 2015 (1988) [3] R. H. Victoria, Phys. Rev. Lett. 63, 457 (1989); D. Walton, ibid 65, 1170 (1990)

Fig.3 – Decrease in blocking temperature with applied H with the solid lines as fits.

Fig.1 – TEM of nickel nanoparticles embedded in silica matrix.

Fig.2 – Particles size distribution histogram of nickel nanoparticles with the solid line representing the fit to log-normal distribution.

1 2 3 4 5 6 70

2

4

6

8

10

12

14

1 2 3 4 5 6 70

2

4

6

8

10

12

14

CO

UN

TS

PARTICLE SIZE (nm)0.0 0.5 1.0 1.5 2.0 2.5 3.0

05

10152025303540

T B(K

)

H (kOe)

TB(H)=33(1 - H/3200)m

m = 2

m = 3/2

36

Microwave Measurements of the Complex Permeability and Permittivity of Fine Magnetic Particles at Frequencies from 4 GHz to18 GHz

Sean McCooey, Mahmut Obol, Nawaf N. Al-Moayed, Kim N. Nguyen, Usman A.Khan and

Mohammed N. Afsar

Department of Electrical Engineering, Tufts University, Medford, MA 02155 The total U.S. market for ceramic powders in 2006 was roughly 1.26 billion pounds with a worth of $2.2 billion. This is projected to increase to 1.57 billion pounds worth $3.4 billion by 2011. The global market for all nano materials in 2005 was 9 billion tons worth $13.1 billion. Similarly, projections are 10.3 billions tons worth $20.5 billion by 2010. Therefore, measuring nano powders is useful in adding a dimension for the quality control science and various physical applications. Also, the rapid development of nano magnetic materials for applications in IC technology and biomedical sciences demands the accurate magnetic and dielectric characterization of these materials based on their complex permeability and permittivity. Although various measurement techniques are available to define such nano materials, a waveguide technique was selected to measure these materials based on their response to microwaves in a broadband frequency region. Using the Agilent 8510C VNA and a standard TRL calibration set, the complex permeability and permittivity of the powders in table.1 were measured from 3.95 GHz to 18.0 GHz. Where the average diameter of nano particles was less than 40 nanometers, were purchased from Sigma-Aldirich Inc. Results indicate that the relative permeability and permittivity of the nano powders are slightly different than the values for the corresponding solid compounds. The data shows that Fe3O4 behaves like a semiconductor material while NiCr2O4 exhibits dielectric properties similar to those of air. The NiCr2O4 may be an excellent compound for material coating applications. Some nano powders have the same permeability as air while their permittivity is slightly different then air because of varying densities and the possibility of the effects of the different cations in the compounds. It should be also noted that for the materials with low permittivity and permeability similar to that of air, the reflection uncertainty, Γ~d should be taken into account when determining the complex parameters; µµµ ′′−′= j and εεε ′′−′= j . Also measured was the planar hexaferrite powder Ba2Co2Fe12O22 from Trans-Tech, in which samples had an average particle diameter of five micrometers and density of 2.095 g/cm3. Results show that the relative permeability is about 1.5 up to 7 GHz. Its relative permittivity, affected by larger particle size and a higher density than the nano powders, is around 3.0. The unique insulating properties of this material may be useful for inductor applications across a broad GHz frequency range since there have not been any magnetic materials reported to have this capability. The waveguide technique was also employed for the measurements of single walled carbon nanotubes (SWCNT) and multi walled carbon nanotubes (MWCNT). As expected, these carbon nanotube materials demonstrated very conductive behavior indicated by the imaginary component of the measured permittivity. Furthermore, these carbon nanotubes demonstrated an induced diamagnetic property in the broad GHz frequency region. Confirmation of these diamagnetic properties is being sought through other measurement techniques. Such that the SQUID was deployed to have the moments of carbon MWCNT; emuM 61020 −×−≈ by ZFC was recorded which is in comparable order to S21 magnitude in power.

Table 1 Average Values for Nano Powders from 3.95 – 18.0 GHz

Nano Powder BaFe12O19 SrFe12O19 CuFe2O4 Fe3O4 CuFe2O4Zn NiCr2O4 Fe2NiO3Zn

ρ (g/cm3) 0.74 0.86 0.97 1.50 1.24 0.39 1.01

rε 1.51-j0.07 1.55-j0.07 1.61-j0.12 5.34-j1.14 1.56-j0.07 1.06-j0.05 1.25-j0.07

rµ 1.13-j0.15 1.10-j0.11 1.01-j0.03 0.96-j0.23 1.02-j0.03 1.08-j0.09 1.07-j0.015

37


Poster Session

38

FeCo- Co-Ferrite Core Shell Structures for Heat Sources in

Thermoablative Cancer Therapy Kelsey Miller1, A.H. Habib2, M. Bockstaller1, and M.E. McHenry1,2

Dept. of Mat. Sci. and Engr.1 and Physics2, Carnegie Mellon Univ., Pittsburgh PA

P.M. Chaudhary3 University of Pittsburgh Cancer Institute, Hillman Cancer Center, Pittsburgh, PA

Heating of ferrofluids with AC magnetic fields holds promise for thermoablative cancer therapies [1]. Dissipation results from Neél and Brownian relaxation of particles in a fluid. Rosensweig [2] considered the nanoparticle size dependence of power dissipation in magnetic oxide based fluids. We have calculated that FeCo nanoparticles have significantly higher heating rates, exceeding those of magnetite and maghemite (at non-invasive frequencies, ~300 kHz, AC field amplitudes of 50 mT and nanoparticle volume fractions φ = 0.1). Fig. 1 shows comparative heating rates as a function of particle size for FeCo. Important observations are: (1) the material magnetization sets the scale for heating rate and (2) magnetic anisotropy (entering the expression for Neél relaxation) determines the particle diameter, D, where a maximum heating rate occurs. These alloys are therefore predicted to have superior heating rates at sizes appropriate for colloidal suspension. FeCo nanoparticles have other advantages in forming stable, protective cobalt-ferrite shells [3] which aid in their functionalization and biocompatibility. We have produced FeCo/CoFe2O4 nanoparticles by plasma torch synthesis followed by immersion in various concentrations of sodium oleate surfactant to form stable ferrofluids. Transmission electron microscopy (TEM) studies of these ferrofluids reveal a narrow particle size distribution with some agglomerates and a mean particle size of 20 nm (Fig. 2). Experimental heating has been performed in an AC magnetic field at the non-invasive frequency of ~220 kHz. Fig. 3 shows temperatures exceeding 45°C achieved in our ferrofluid at times less than 50 seconds.

[1] K. Okawa, et al., J. Appl. Phys. 99, 08H102, 2006. [2] R. E. Rosensweig, J. Magn. Magn. Mater. 252, 370 2002. [3] Z. Turgut, CMU Ph.d Thesis, 2000.

Fig. 1 – Comparative heating rates as a function of particle size for acicular equiatomic FeCo nanoparticles with shape anisotropy

Fig. 2 – Heating rates of FeCo/CoFe2O4

ferrofluids at 220 kHz

Fig. 3 – TEM image of ferrofluid showing CoFe2O4

shell, both spherical and faceted particles, as well as particle size distribution

39

Spin Transport Studies in Nanowires

S.Patibandla1, G.C.Tepper2, S.Bandyopadhyay1

1 Department of Electrical and Computer Engineering 2 Department of Mechanical Engineering

Virginia Commonwealth University, Richmond, VA 23284

Spin transport studies in semiconductor nanostructures has attracted great deal of research interest due to its promising role in implementing spintronic devices which operate at a low power and enhanced data processing speed. Also, since spin coherence time in semiconductors is much longer than charge coherence time [1, 2], spin is considered to be the ideal candidate for encoding qubits in quantum logic gates. The popular structure used to study spin relaxation in any paramagnetic material is the “spin valve”. The spin relaxation studies were performed using Ni/Ge/Co nanowire spin valves in which the spacer Ge nanowire is 50 nm in diameter and ~200nm in length as shown in the TEM image of Fig 1. The magnetoresistance measurements were performed using Physical Properties Measurement System. Fig 2 shows one typical measurement performed at 1.9K. In spite of the large background AMR effect (1% in the field range 0 to 0.6 Tesla) [3], we can clearly see tell-tale resistance peaks whose leading and trailing edges occur at fields approximately corresponding to the coercive fields of the Co and Ni nanomagnets [4,5]. It is possible to estimate the spin relaxation length in the Ge spacer from the measured resistance change ∆R associated with the spin valve peak and we follow the model of ref. [3, 6] modified for the classical spin valve geometry for the calculations. The experimental techniques employed to fabricate and characterize these nanowire spin valves will be presented and the spin relaxation mechanisms will be discussed with the necessary details.

Fig1. TEM micrograph of released Fig2. Magnetoresistance nanowires (scale bar is 20nm) data for Ni/Ge/Co nanowire spin valve

Reference: [1] J.Kikkawa and D. Awschalom, Physical Review Letters, Vol. 80, 19, 4313-4316, 1998. [2] P.Mohanty, J.Jariwalla, R.Webb, Physical Review Letters, Vol.78, 17, 3366-3369, 1997. [3] T. Ohgai, L. Gravier, X. Hoffer, M. Lindeberg, K. Hjort, R. Spohr and J-Ph. Ansermet, J. Phys. D: Appl. Phys., 36, 3109 (2003). [4] M. Zheng, L. Menon, H. Zeng, Y. Liu, S. Bandyopadhyay, R. D. Kirby and D. J. Sellmyer, Phys. Rev. B, 62, 12282 (2000). [5] H. Zeng, M. Zheng, R. Skomski, D. J. Sellmyer, Y. Liu, L. Menon and S. Bandyopadhyay, J. Appl. Phys., 87, 4718 (2000). [6] F. J. Jedema, A. T. Filip, and B. J. van Wees, Nature (London), 410, 345 (2001).

40

Modeling of Microwave Absorption of Magnetic Microwires

Robert Booth, Sam Lofland

Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028 Recently [1], it was discovered that wires with electrodeposited magnetic metals can have large field-dependent changes in microwave absorption. This interesting property has potential applications such as magnetic field sensors or tunable absorbers. We model the experimental observations using the dynamic permeability tensor from the Landau-Lifschitz equation. We solve vector wave equation f using the quasistatic approximation for infinitely long axially symmetric cylinders for azimuthal anisotropy and the rf field along the field axis. There are marked field and frequency dependencies. The simulations agree reasonably well considering the approximations used in describing the magnetization process. References [1] H. García-Miquel, S. M. Bhagat, S. E. Lofland, G. V. Kurlyandskaya, and A. V. Svalov, J. Appl. Phys. 94, 1968 (2003).

-300 -200 -100 0 100 200 300 400 500 600

AP9,65 GHzHDC|| hrf _|_P

ower

Abs

orbe

d (a

rb.u

.)

H (A/m)

Fig.2 – Calculated power absorption as afunction of relative magnetization.

Fig.1 – Field dependence of microwave absorption of a microwire with electrodeposited Co.

0.2 0.4 0.6 0.8 1

2´10-19

4´10-19

6´10-19

8´10-19

1´10-18

Pow

er a

bsor

bed

(arb

. u.)

M/Ms

41

Variation of Thin Film Edge Magnetic Properties with Patterning

Process Conditions in Permalloy Stripes

Robert D. McMichael1, Brian B. Maranville1 and David W. Abraham2

1Magnetic Materials Group, NIST, Gaithersburg MD 20886

2IBM Thomas J. Wastson Research Center, Yorktown Heights, New York 10698 We present the first quantitative measurements of how magnetic thin film edge properties depend on patterning conditions. The magnetic properties of thin film edges were determined by measuring the ferromagnetic resonance of "trapped-spinwave modes" or "edge modes" in transversely magnetized stripes [1,2]. Because these edge modes are localized by the inhomogeneous fields near the edges, they are sensitive to the edge conditions. Analysis of the edge mode frequency as a function of applied field yields two edge properties: 1) an edge saturation field, which is an effective in-plane edge anisotropy, and 2) an effective out-of-plane edge anisotropy field [3,4]. We demonstrate these edge characterization techniques using 250nm-wide stripes in 20nm-thick Permalloy, and we track changes in the edge properties as a function of ion mill etch depth. With increasing etching depth, the side wall angle changes from 47 deg to 80 deg and the edge saturation field nearly doubles. The correlation between side wall angle and magnetic edge properties is largely confirmed by micromagnetic modeling of the magnetization dynamics in stripes with different edge geometries. Other micromagnetic modeling shows that surface anisotropy and magnetization dilution near the edge also have significant effects on edge properties. [1] J. Jorzick, S. O. Demokritov, B. Hillebrands, M. Bailleul, C. Fermon, K. Y. Guslienko, A. N. Slavin, D. V. Berkov, and N. L. Gorn, Phys. Rev. Lett. 88, 047204 (2002). [2] J. P. Park, P. Eames, D. M. Engebretson, J. Berezovsky, and P. A. Crowell, Phys. Rev. Lett. 89, 277201 (2002). [3] B. B. Maranville, R. D. McMichael, C. L. Dennis, C. A. Ross, and J. Y. Cheng, IEEE Trans. Mag. 42, 2951 (2006). [4] B. B. Maranville, R. D. McMichael, S. A. Kim, W. L. Johnson, C. A. Ross, and J. Y. Cheng, J. Appl. Phys. 99, 08C703 (2006).

Figure 1. The side wall angle of the edges depends on the etch depth.

Figure 2. The magnetic properties of the edge depend on the side wall angle.

42

X-ray Diffraction Study of the MgO Growth for Magnetic Tunnel Junctions

Se Young O1,2, Chan-Gyu Lee2, W.Egelhoff1, W.T. Pong1

1Magnetic Materials Group, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

2School of Nano & Advanced Materials Engineering, Changwon National University, #9 Sarim-dong, Changwon, Gyeongnam 641-773, Republic of Korea

MgO based magnetic tunnel junctions (MTJs) show a long magnetoresistance(MR) effects and are currently the most promising ones for the application in magnetoresistive devices such as magnetic random access memory (MRAM), magnetic read heads and magnetic sensors [1,2]. A large tunneling magnetoresistance (TMR) is critical to the success of this application. There are reports showing MTJs made with MgO(200) as the oxide barriers exhibiting TMR as high as 400% to 500% [3,4]. In this work, MgO oxide layers were deposited under different sputtering conditions and underlayer structures and characterized by X-ray diffractometry (XRD) theta-two theta measurement in order to study and optimize the MgO (200) crystal structure. Our results show that high sputtering power and short sputtering distance is important to the formation of MgO(200) structure. The titanium deposition before sputtering helps to keep the chamber clean and allow the CoFe underlayer and MgO crystallize better. The XRD pattern of a sample is shown in Figure 1.

30 40 50 60 700

2000

4000

6000

8000Si

(400

)

Si (2

00)

MgO

(200

)

MgO

(111

)

Inte

nsity

(cou

nts)

2 Theta (deg) Figure 1. XRD patterns of 10Ta/200NiFeCuMo/15CoFeB/300MgO (unit in Angstrom)

References [1] S.S.P.Parkin, C. Kaise, A.Panchula, P.M.Rice, B.Hughes, M.Samant, and S.-H.Yang, Nature

Mater., 3 (2004) 862 [2] S.Yuasa, T.Nagahama, A.Fukushima, Y.Shzuki, and K.Ando, Nature Mater., 2 (2004) 868 [3] S.Ikeda, J.Hayakawa, Y.M.Lee, R.Sasaki, T.Meguro, F.Matsukura and H.Ohno, Japanese Journal

of Applied Physics., 44 (2005) L1442 [4] S.Yuasa, Appl.Phys.Lett., 89 (2006) 042505

43

Introducing the NIST Center for Nanoscale Science and Technology NIST’s new Center for Nanoscale Science and Technology (CNST) consists of a Research Program and the Nanofab, a shared-use facility providing economical access to state-of-the-art nanofabrication and nano-measurement tools. The CNST Nanofab is an advanced nanofabrication facility available to both NIST and external users. The Nanofab houses a multi-million dollar suite of state-of-the-art nanofabrication and nanomeasurement equipment. The tools within the Nanofab are designed to accommodate a wide variety of materials and substrate sizes from small pieces to conventional size wafers. The tools and operating procedures have been selected to provide hands-on users with easy-run operations with minimal time. Alternatively, the tools can be operated by one of CNST’s process engineers. Some of these state-of-the-art tools are: • Focused ion beam/SEM • Nano-imprint lithography system • Electron beam lithography system • Furnaces and CVD tools for nanoscale film deposition with high purity and uniformity • Dry etch tools to transfer photolithographic nanoscale imaging into thin film materials • Metal deposition tools for creating electrical contacts and magnetic devices • Wet chemistry tools to clean and etch materials in a controlled, safe environment. • Inspection tools such as FESEM, ellipsometry, profilometry, and for electrical testing. The Research Program has the mission of advancing measurement science for future electronics, nanofabrication, and energy conversion and storage. The research areas include nanomagnetics, atomic scale characterization and fabrication, nanoscale measurement and fabrication using laser-controlled atoms, and modeling nanostructures in mesoscopic environments. New research areas in Nanophotonics and Nanoplasmonics aim at the interaction of light with subwavelength, nanoscale structures, in support of chip-based optical information processing. Some of these facilities offer access to prototype measurement or fabrication tools that operate beyond the state-of-the-art of commercially available alternatives. Opportunities exist for collaboration with scientists from industrial research laboratories, universities, and other government laboratories, as well as from within NIST.

44

Tutorial materials for

Quantum Statistics of Nanomagnets

60


A Vector Preisach Model

78

CONFER


Computational Magnetism

95

Appendix A – NIST Map & Emergency Phone Number

In case of emergency, call 301-975-2222

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