epsrc-jsps uk-japan core-to-core workshop on heusler …ah566/core1/abstracts.pdfepsrc-jsps uk-japan...
TRANSCRIPT
EPSRC-JSPS UK-Japan Core-to-Core
Workshop on Heusler Alloys
16 March 2016
Departments of Electronics and Physics
University of York
EPSRC-‐‑JSPS Core-‐‑to-‐‑Core Workshop on Heusler Alloys
Wednesday, 16 March 2016 Departments of Electronics/Physics, University of York
12:00 Lunch G/N011
12:50 Opening remarks G/N120 Prof. Kevin O’Grady (Dept. of Phys., Univ. of York)
13:00 Session I: Magnetic and transport properties of Hesuler alloys G/N120 Chair: Gonzalo Vallejo-‐‑Fernandez
13:00 Prof. Masafumi Shirai (RIEC, Tohoku Univ.) Electronic structure and magnetic properties at interfaces between Co-‐‑based Heusler alloys and MgO barrier
13:30 Miss Chris Yu (Dept. of Phys., Univ. of York) Exchange bias induced at a Co2FeAl0.5Si0.5/Cr interface 13:45 Mr John Sinclair (Dept. of Phys., Univ. of York) Electrical characterisation of Néel transitions 14:00 Dr Gavin Bell (Dept. of Phys., Univ. of Warwick) Polarised neutron reflectivity of half-‐‑metallic epitaxial films 14:30 Mr William Frost (Dept. of Elec., Univ. of York) Development of a CPP-‐‑GMR Heusler Alloy Junction 14:45 Mr Haokaifeng (Jason) Wu (Dept. of Phys., Univ. of York)
Magnetic and structural properties of Mn2VSi alloy grown at elevating temperatures
15:00 Coffee/Tea break G/N011
15:30 Session II: Structural properties of Heusler alloys G/N120 Chair: Atsufumi Hirohata
15:30 Dr Masahito Tsujikawa (RIEC, Tohoku Univ.) Perpendicular magnetic anisotropy and its electric field modification in the MgO/Co-‐‑based Heusler alloy interface
16:00 Mr Teodor Huminiuc (Dept. of Phys., Univ. of York) Novel antiferromagnetic materials for spintronic devices 16:15 Dr Barat Kuerbanjiang (Dept. of Phys., Univ. of York)
The role of chemical structure on the magnetic and electronic properties of Co2FeAl0.5Si0.5/Si(111) interface
16:45 Zlatko Nedelkoski (Dept. of Phys., Univ. of York) The role of interfaces on the spin-‐‑polarization
17:00 Closing remarks G/N120 Prof. Masafumi Shirai (RIEC, Tohoku Univ.)
19:00 Dinner Deans Court
Session I: Magnetic and transport properties of properties of Heusler alloys 13:00~15:00 G/N 120
Chair: Gonzalo Vallejo-‐‑Fernandez
Electronic structure and magnetic properties at interfaces between Co-‐‑based Heusler alloys and MgO barrier
Masafumi Shirai
Research Institute of Electrical Communications, Tohoku University
Huge tunneling magnetoresistance (TMR) ratio over 2,000% was observed at low temperatures for magnetic tunnel junctions (MTJ) composed of Co-‐‑based Heusler alloys and MgO barrier. However, the TMR ratio of the MTJ remarkably decreases with increasing temperature. It was proposed that the spin-‐‑flip and/or inelastic tunneling processes via interfacial resonant states (IRS) in the minority-‐‑spin gap are responsible for the reduction of the TMR ratio in the MTJ with half-‐‑metallic electrodes [1]. Indeed, we found that the IRS appeared at the Co2MnSi/MgO junction are mainly composed of Co or Mn 3dε orbitals having the Δ2 or Δ5 symmetry [2]. Since the Δ1 electrons predominantly transmit the MgO barrier, the IRS hardly contribute the tunneling conductance in the MTJ with thicker MgO barrier. On the contrary, in the MTJ with thinner MgO barrier, the IRS could play a crucial role in reducing the TMR ratio. We proposed that the harmful IRS can be eliminated by inserting CrAl-‐‑layer into the Co2MnSi/MgO interface [3].
Focusing on the spin-‐‑flip tunneling processes caused by magnetic excitations in the ferromagnetic electrodes, we evaluated tunneling conductance of Co2MnSi/MgO/ Co2MnSi MTJ by tilting the direction of magnetic moments of each atomic layer. As a result, we conclude that the thermal fluctuation of Co moments in the interfacial region is responsible for the reduction of the TMR ratio with increasing temperature since the exchange stiffness of the interfacial or sub-‐‑interfacial Co-‐‑layer is much weaker than that of bulk Co2MnSi [4]. For achieving higher TMR ratio at room temperature, we need to find the Heusler alloys possessing higher magnetic anisotropy and/or stronger exchange stiffness at the interface of the MTJ.
This work was partly supported by Grant-‐‑in-‐‑Aid for Scientific Research form JSPS/MEXT and by JSPS Core-‐‑to-‐‑Core Program, A. Advanced Research Networks “New-‐‑Concept Spintronics Devices”.
References [1] P. Mavropoulos, M. Lezaic, and S. Blügel, Phys. Rev. B 72, 174428 (2005). [2] Y. Miura, et al., J. Phys.: Condens. Matter 19, 365228 (2007). [3] Y. Miura, et al., Phys. Rev. B 78, 064416 (2008). [4] Y. Miura, K. Abe, and M. Shirai, Phys. Rev. B, 83, 214411 (2011).
13:00
Exchange bias induced at a Co2FeAl0.5Si0.5/Cr interface Nga Tung Chris Yu,1 Andrew Vick,1 Nobuhito Inami,2 Kanta Ono 2 and Atsufumi Hirohata 1
1 Departments of Physics and Electronics, University of York 2 Institute of Materials Structure Science, High Energy Acceleration Organization (KEK)
Among HMFs, Heusler alloys hold greatest potential due to their theoretically predicted half-‐‑metallicity, structural matching with substrate materials and high Curie temperatures (Tc) at room temperature [1]. Co2FeAl0.5Si0.5 (CFAS) is an attractive half metallic material because of its high Tc (~1000K) and finite band gap in one spin channel [2]. For spintronics device application large remanent magnetisation is one of the requirements and it can be achieved by exchanged bias.
In order to engineer the strength of an exchange bias to introduce anisotropy in a cubic Heusler alloy layer, crystalline strain has been induced at a ferromagnet/antiferromagnet interface by lattice mismatch in addition to the conventional interfacial exchange coupling. [CFAS/Cr]3 structures have been prepared by ultrahigh vacuum molecular beam epitaxy. The magnetic and structural properties have been characterised to investigate the exchange interactions at the CFAS/Cr interfaces. Due to the lattice mismatch between Cr and CFAS layers to be 1.4%, the maximum offset of 18 Oe in a magnetisation curve has been measured for the case of a 2 nm thick CFAS/0.9 nm Cr interface at 193K. And no annealing was performed. The half-‐‑metallic property of CFAS has observed to remain unchanged, which agrees with the theoretical prediction by Culbert et al. [3] Such a strain-‐‑induced exchange bias may offer insight of the interfacial interactions.
Reference [1] I Galanakis et al, J. Phys. D: Appl. Phys. 39, 765 (2006). [2] N. Tezuka et al, Appl. Phys. Lett., vol 94, 162504 (2009). [3] C. A. Culbert et al, J. Appl. Phys. 103, 07D707 (2008).
13:30
Electrical characterisation of Néel transitions John Sinclair, Teodor Huminiuc, Haokaifeng Wu, Gonzalo Vallejo-‐‑Fernandez
and Atsufumi Hirohata
Departments of Physics and Electronics, University of York
In the recent years there has been a large increase in demand for Iridium due to its applications in magnetic memory and storage. This is due to its ability to form a sheet antiferromagnet with a blocking temperature above 300K when combined with Manganese. Unfortunately as a consequence of this increase in demand for Iridium, the price has soared making it necessary to explore other options for creation of antiferromagnetic layers in spintronic devices.
One of the most promising groups of materials for this application are Heusler alloys, with a number predicted to exist in an antiferromagnetic state [1]. To achieve this it will be necessary to characterise the magnetic ordering temperatures of the alloys of interest, namely Ni2MnAl and Fe2VAl. Sheet resistance measurements are achieved using 4 point measurement with Van der Pauw geometry in an Oxford Instruments cryostat. After measuring twice with current applied in perpendicular directions an area independent resistivity measurement can be calculated. To ensure that this measurement technique is viable initial testing has been carried out on single crystal Chromium, shown in figure 1.
This technique will allow for measurement the Néel temperature for the two alloys mentioned above and also exploration of the dependence of the Néel temperature on annealing time and crystallinity. These properties are controlled through the use of a vacuum annealing furnace and deposition parameters on the HiTUS sputter deposition system used to produce the films.
240 250 260 270 280 290 300 310 320 330
9.0
9.5
10.0
10.5
11.0
Resistance (µΩ)
Temperature (K)
TN = 311K
Figure 1 -‐‑ Electrical resistivity of single crystal Chromium with Temperature.
Reference [1] D. J. Singh and I. I. Mazin, Phys. Rev. B 57, 14352 (1998).
13:45
Polarised neutron reflectivity of half-‐‑metallic epitaxial films
Gavin Bell
Departments of Physics, University of Warwick
Polarised neutron reflectivity (PNR) is a powerful probe of magnetic multilayers, able to measure depth-‐‑resolved magnetic moments among other properties. The technique will be introduced along with the complementary X-‐‑ray reflectivity (XRR) method and simultaneous data fitting techniques. Results from several spintronic multilayer structures will be discussed.
14:00
Development of a CPP-‐‑GMR Heusler Alloy Junction William Frost and Atsufumi Hirohata
Department of Electronics, University of York
We report improvements made to the perpendicular magnetic anisotropy of the Heusler alloy Co2FeSi in a GMR stack. Perpendicular magnetic anisotropy (PMA) in Heusler alloys is most commonly induced using an MgO layer where hybridised orbitals induce the anisotropy.1 For a practical GMR device this is not possible as a Silicon substrate is used and the barrier layer has to be conductive. In this work Vanadium has been used as a seed layer to produce a Co2FeSi (110) textured film with a rocking curve FWHM of 5.4o. Lattice mismatch between the (110) lattice directions induces a strain in the Co2FeSi layer shown by a decrease in the lattice constant by 0.5% as compared to the bulk value.2 This strain increases the PMA in the Heusler alloy and has been shown to be interfacial in nature by varying the thickness of the Co2FeSi layer.
A top interfacial layer of Vanadium was then added to induce strain at both interfaces of the Co2FeSi layer in order to increase the PMA, as would be present in a GMR device. However the magnetic anisotropy remained strongly in plane. The thickness of the Co2FeSi layer has been reduced showing an increase in the perpendicular anisotropy with decreasing film thickness. At a Co2FeSi layer thickness of 4 nm a much higher level of perpendicular anisotropy is achieved when coupled with a second interfacial layer of Vanadium.
Alternative seed layers have also been used in an attempt to decrease the thickness of the seed layer. The addition of an Fe buffer between the V and CFS radically changes the crystal structure of the CFS layer. This has been investigated by varying the thickness of the V, Fe and CFS layers.
Acknowledgement: This work was partly funded by Seagate Technology, Derry.
[1] Z.Wen et al., Appl. Phys. Lett. 98, 242507 (2011). [2] W. Frost and A. Hirohata, IEEE Trans. Mag., (in press).
Fig.1: Perpendicular M-‐‑H loops showing the effect of the second Vanadium interface.
Fig. 2: M-‐‑H loops of a 4 nm Co2FeSi layer showing an improved PMA.
-30 -20 -10 0 10 20 30
-1.0
-0.5
0.0
0.5
1.0 M/MS
H (Oe)
In Plane Perp.
14:30
-500 -250 0 250 500
-1.0
-0.5
0.0
0.5
1.0 M/MS
H (Oe)
V/Co2FeSi/V V/Co2FeSi
14:45 Magnetic and structural properties of Mn2VSi alloy grown at elevating temperatures Haokaifeng Wu, Teodor Huminiuc, John Sinclair, Atsufumi Hirohata, Gonzalo Vallejo Fernandez
and Kevin O’Grady
Departments of Physics and Electronics, University of York
IrMn has currently been used in a magnetic read-‐‑head element in a hard disk drive (HDD) due to its high thermal stability of the antiferromagnetism corrosion resistance. Because of the increasing demand for the HDD and the scarcity of Ir, this study aims to replace IrMn with an antiferromagnetic Heusler alloy. 100nm thick Mn2VSi films were deposited on Silicon substrates with a 18nm silver seed layer and a 3nm aluminium capping layer using a HiTUS system. The substrates were heated at above 250℃ during the deposition. The films were then characterised by vibrating sample magnetometry (VSM) and X-‐‑ray diffraction (XRD).
Fig. 1. XRD θ-‐‑2θ scans for the Mn2VSi films grown at different temperatures.
We have investigated the crystallization and surface roughness co-‐‑relation of the Mn2VSi film. Growth at higher temperature provides highly crystallized films which show a sharp peak in XRD intensities at 2θ ≅ 45° indicating the (220) Heusler alloy peak. In particular, the growth at 488℃ provides the strongest Heusler peak however unfortunately high temperature growth also increases surface roughness.
In order to verify that the Mn2VSi thin film is antiferromagnetic, a 5nm ferromagnetic CoFe layer was deposited on the top of the Mn2VSi layer. When materials with ferromagnetic (F) ordering and antiferromagnetic (AF) ordering are in close contact and are field cooled through the Neel temperature (TN) of AF, exchange bias is induced in the F layer. We used the setting temperature of 65℃ for 30 minutes to make the AF layer becomes paramagnetic and the domains are aligned along the direction of the field when an external field 5000Oe is applied for sample grown at 250℃. The exchange bias about 10 Oe was observed as shown in Fig.2. As the growth temperature was increased no exchange bias was observed. This may indicate the intermixingbetween the Mn2VSi and CoFe layers occur at higher temperature.
Fig.2. Hysteresis loop for Mn2VSi/CoFe film grown at 250℃ measured at 100K and room temperature.
-45
-35
-25
-15
-5
5
15
25
35
45
-1500 -1000 -500 0 500 1000 1500
measured at room temperaturemeasured at 100k
Magnetization (emu/cm3)
Applied field (Oe)
\
Session II: Structural properties of Heusler alloys 15:30~17:00 G/N 120
Chair: Atsufumi Hirohata
Perpendicular magnetic anisotropy and its electric field modification in the MgO/Co-‐‑based Heusler alloy interface
Masahito Tsujikawa and Masafumi Shirai
Research Institute of Electrical Communications, Tohoku University
The half-‐‑metallic Heusler alloys are the promising candidates for the application of spintronic devices due to the excellent physical properties such as a high spin-‐‑polarization, low damping constant. For the application of magnetic tunnel junction (MTJ), large perpendicular magnetic anisotropy (PMA) is also necessary to overcome the thermal fluctuation. Recently, the large PMA is observed for the interface between MgO and full-‐‑Heusler Co2FeAl. In this study, we estimate the magnetic anisotropy energy at the MgO/Co2XAl (X = V, Cr, Mn, Fe) interface by using first principles calculation, and discuss the physical origin of interfacial PMA on the half-‐‑metallic alloys. Furthermore, we also discuss the potential of the electrical control of interfacial PMA.
15:30
Novel antiferromagnetic materials for spintronic devices Teodor Huminiuc,1 Jan. Balluff,2 Markus Meinert,2 Günter Reiss,2 Kevin O’Grady 1
and Atsufumi Hirohata 1 1 Departments of Physics and Electronics, University of York
2 Department of Physics, Bielefeld University
Antiferromagnetic (AF) materials are used as pinning layers for GMR and TMR devices. The standard alloy used for exchange bias is IrMn but Ir is a scarce element hence an alternative AF is required.
Ni2MnAl and Ru2MnGe are Heusler alloys with potential to form an AF phase. The structural and magnetic properties of these alloys have been studied. It is predicted that the partially disordered B2 phase of the Heusler alloys will develop an AF structure [1]. MnN also shows good AF ordering and thermal stability with high values of Hex at room temperature. Epitaxial and polycrystalline thin films of these materials were deposited using magnetron co-‐‑sputtering and an IBD system. The thickness of the films was varied from 10 to (100 ± 2) nm. The samples were annealed or grown on heated substrates at temperatures in the range 250 to 700oC.
Prior to crystallisation the samples exhibited paramagnetic behaviour. The degree of B2 ordering increases with both annealing time and temperature. A shifted loop (Hex=45Oe) is observed in epitaxial grown samples at low temperatures (100K) for all the alloys listed as seen in Figure 1. They have been used as pinning layers for the ferromagnetic Fe, Co2MnSi, Co2FeSi and Co. An increase in coercivity is observed after field cooling the polycrystalline bilayers. The substrate/AF/F interfaces have been studied using cross sectional TEM as seen in Figure 2. Seed layers, of Cr/Ag and doping with Co/Al have been investigated to promote crystallisation of the AF and an increase in the Néel temperature.
[1] Acet et al, J. Appl. Phys, 92, 3867 (2002).
16:00
Figure 1: Hysteresis loop of field cooled epitaxially grown Ru2MnGe/Fe bilayer on MgO.
Figure 2: Cross sectional TEM image of Ru2MnGe deposited on a Si/SiO2 substrate.
Si
SiO2
Ru2MnGe
8 nm
2 n m2 n m Fe 2 nm
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-60
-40
-20
0
20
40
60 m(µemu)
H(kOe)
T=100KH cool=20kOe
Hex
The role of chemical structure on the magnetic and electronic properties of Co2FeAl0.5Si0.5/Si(111) interface Barat Kuerbanjiang and Vlado Lazarov
Department of Physics, University of York
We study high quality Co2FeAl0.5Si0.5/Si(111) heterointerface for spintronic applications. Electron energy loss spectroscopy shows that in a narrow interface region there is a mutual inter-‐‑diffusion dominated by Si and Co. Aberration-‐‑corrected scanning transmission electron microscopy reveals that the film has B2 ordering. The film lattice structure is unaltered even at the interface due to the substitutional nature of the intermixing. First-‐‑principles calculations performed using structural models based on the microscopy results, show that the increased Si incorporation in the film leads to a gradual decrease of the magnetic moment as well as significant spin-‐‑polarization reduction. These effects can have significant detrimental role on the spin injection from the Co2FeAl0.5Si0.5 film into the Si substrate, besides the structural integrity of this junction.
16:15
The role of interfaces on the spin-‐‑polarization
Zlatko Nedelkoski and Vlado Lazarov
Department of Physics, University of York
By using first-‐‑principles calculations we show that the atomically abrupt Co2FeSi(111)/ Si(111) interface have reversed spin polarisation at the interface. This unfavourable spin-‐‑electronic configuration can be completely avoided by introducing a monolayer of CoSi2 at the interface. We show that such layer is energetically favourable to exist at the interface. This was further confirmed by direct observations of CoSi2 nano-‐‑islands at the interface, by employing atomic resolution scanning transmission electron microscopy. The monolayer of CoSi2, in addition, shifts the Fermi level from the valence band edge (abrupt interface case) towards the conduction band edge of Si. The significantly improved half-‐‑metallicity and smaller energy difference between the conduction band edges of the film and Si make this interface highly desirable for device applications.
16:45