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TCD April 2007 1
Chapter 10
Experimental Methods
10.1Materials preparation
10.2 Magnetic fields
10.3 Atomic-scale magnetism
10.4 Domain-scale measurements
10.5 Bulk magnetization measurement
10.6 Excitations
10.7 Numerical methods
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10.1 Materials Preparation
10.1.1 Bulk material
Metals: Melt in an arc furnaces or a rf induction furnace.
Heat treat in a resistance furnace (controlled temperature or atmosphere.
Amorphous metals are produced by rapid solidification - melt spinning
Insulators: Mill components e.g. CoO + Fe2O3 ! CoFe2O4 . Grind and fire nx
Mix ions in solutions. Precipitate gel as a precusror.
Crystals:
seed
temperature
Bridgeman method
seed
Czochralski method
Arc melter Glovebox X-ray Diffractometer SQUID magnetometer
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10.1.2 Thin films
Physical vapour deposition
source
Substrate 400 - 1000 C
Evaporation: Thermal
e-beam e.g. 10 kV, 1A
Mean-free path " = 6/P "in mm, P in Pa.
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cap
film
substrate
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Pulsed-laser deposition (PLD)
ns pulses of UV light
! 1 J cm2 on the target, ! 10 Hz.
directed plume cos11#
Growth rate ! 1 nm s-1
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Molecular-beam epitaxy (MBE)
Carried out in UHV 10-7 - 10-9 Pa Needed to avoid conamination of a slowly-growing film by residual gas.
Time for a monolayer
$t = (12MkBT/M)1/2/Pa2
e..g Oxygen a ! 0.2 nm, P= 10-5 Pa, $t ! 6 0s
Growth rate < 0.2 nm s-1
• Franck-van der Merwe
• Volmer-Weber
• Strannsky-Krastanov
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10.1.3 Small particles
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Sputtering
Use Ar gas, Ar+ ions are acceleratedtowards the cathode (target). A glow-discharge is formed.
Target-substrate distance ! 100 mm
To enhance the ionization of Ar, a magnetic field is applied with a ‘magnetron’’
Growth rate ! 10 nm s-1
DC sputtering for metals. P ! 0.05 - 1 Pa
RF sputtering for insulators. 13.56 MHz P ! 0.02 Pa
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Base pressure < 3 x 10-8 Torr
2 Target Facing Target guns (MgO)
Base pressure < 3 x 10-7 Torr
6 Series-III S Guns (DC& RF)
!!Chamber AChamber A!! Chamber B Chamber B
MgO
6 targets
2x3-clusters2 tfts
Multiple-target sputtering tool
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Seed Layer
AFFM
Tunnel Barrier
Bottom Contacts
Top Contacts
Tunnel Barrier
AAFM
Middle Free layer
DMTJ stack
Ta5
Ru50
Ta5
NiFe5
IrMn10
CoFe2
Ru0.85
CoFeB5
MgO2.5
CoFeB3.5
MgO2.5
CoFeB5
Ru0.85
CoFe2
IrMn10
Ta 5
Cu10 50
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Chemical methods
Electrodeposition.
Good for thick films of alloys of metals which are not too electronegative. e.g. Ni78Fe22
1 microamp mm-2 deposits a monolayer in 5s.
1 milliamp mm-2 deposits 40 nm s-2.
Chemical vapour deposition (CVD). Use organometallic precursors, decompose byheated substrates or laser light.
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Superconducting magnet 10 T
Electromagnet 1 T
Helmholtz coils 0.01 T
10.2 Magnetic fields
10.2.1 Generation Steady fields
Bitter magnet 33 T
Polyhelix
Hybrid magnet 42 T record
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54 T 75 ms pulsed magnet, Nijmegen
100 T pulsed magnet laboratory, Dresden
50 MJ capacitor bank
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10.2.2 Field Measurement
a) Search coil b) Rotating coil
b) Hall generator d) 77Rb vapour magnetometer
E = -Nd%/dt
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10.2.3 Shielding
Static: Soft magnetic shields. Permalloy µ > 10,000
Superconducting shields, exclude flux penetration
High-frequency:
Faraday cage.
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10.3 Atomic-scale magnetism
10.3.1 Diffraction
Elastic scattering; 2d sin# = n" & = ghkl
Ineastic scattering; & = ghkl + q
K’ - K = &; '’ - ' = (
Differential scattering cross section
)diff = d2)(&,(,T)/d&d(
K = 2*/" ghkl = 2*/dhkl
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Intensities of Bragg reflection are proportional to the square of the structure factor
Fhkl = +I fi exp (i&.ri) = +I fi exp (hxi + kli + mzi)
The sum is over the i atoms in the unit cell at (xi, yi, zi)
X-ray tube
Synchrotron
Cu Kedge 8.98 keV
1016 photons s-1
5 GeV ,mc2 ! 104 mc2
"c = 0.00714/B,2m
1017 photons s-1 in 0.1%bandwidth
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SmCo5. Sintered magnet c || & Powder
Magnetic scattering of X-rays is 106 times weaker than X-ray scattering.The effect can reach 1% near an absorption edge.A good method for Sm, Gd. (huge neutron scattering cross section)
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Magnetic neutron scattering
P = 1.91reS fS or if both spin and orbital moments are present 1.91re(S fS + (1/2) L fL )
fL,S = [J(J+1) ± S(S+!) - L(L+1)]/[2(J+1)]
Unpolarized neutrons: |Fhkl|2 = |+I bi exp (i&.ri)|2 + |+I pi µi exp (i&.ri)|2
polarized neutrons |Fhkl|2 = |+I (bi + ".p) exp (i&.ri)|2
Magnetic interaction vector µ = m - &(&.m)/&2
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The triple-axis spectrometer allows scan so be made at constant E or constant q.
The dispersion relation for any excitation can be mapped out.
q
E
Antiferromagnetic magnon
2*/a
Inelastic neutron scattering
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The triple-axis spectrometer
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10.3.2 Spectroscopy
Absorption and photoemissio nprocesses for a single photon
X-ray photoemission spectroscopy (XPS)
X-ray absorption spectroscopy (XAS);
X-ray absorption fine structure (EXAFS). Measure structure near the X-ray absorpftion edge - element specificlocal structure
X-ray magnetic circular dichroism (XMCD). Measure difference of absorption for left and right circular polarisedlight. Deduce <S> and <L> for the element
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Mossbauer spectroscopy of FeFe
111(1+cos2#)3,4 (±1/2 !-/+1/2 )
4024 sin2#2,4 (±1/2 !±1/2 )
3333(1+cos2#)1,6 (±3/2 !±1/2 )
k -, k || ,powderlines
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Mossbauer spectra of BaFe12O19 There are five different sites. The ferrimagnetic sublattices areseparated by an applied field.
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10.3.3 Electronic structure
Spin-polarized densities of states for SmCo5
Dispersion relations for spin-polarized electrons E(k)are acomplete description of theelectronic structure of a solid.
UV Photoemission spectroscopygives some information on thedispersion rlations and the densityof states N(E)
Computation Is now the mainsource of informations, especiallydensity functional theory (DFT)calculations in the local spin densityapproximation (LSDA)
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10.4 Domain-scale measurements
10.4.1 Stray field methodsy
Stray-field methods for observing domain structure, a) Bitter method (magnetic colloid)b) Magnetic force microscopy and c) scanning electron microscopy
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10.4.1 Radiation methods
Faraday effect and Kerr effect
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00
5
10
15
20
25
30
35
0.6nm
MOKE spectra Pt(2nm)/Co(t)/Pt(2nm)
RA
S u
nits
photon energy
0.5nm
0.8nm
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
Hysteresis Loops Pt(2nm)/Co(t)/Pt(2nm)
RA
S u
nits
Current Electromagnet [A]
0.5nm3.6eV
0.6nm3.6eV
0.6nm(4eV)
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Faraday effect spectrum of BaFe12O19
Kerr image of a polisher surface of Nd2Fe14B
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Imaging schemes in transmission electron microscopy. Fresnel and Foucoult
Samples are Nd2Fe14B
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A TEM image of a thin foil of melt-spun Nd2Fe14B showing domain walls oinned at grain boundaries.
Left Fresnel image, right Foucallt image. Top is a lattice image showing the planes in the structure.
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10.5 Magnetization
10.5.1 Open circuit
Force methods; a) Faraday balance, b)Torque magnetometer c)Alternating gradient force magnetometer
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Flux methods: a) Extraction b) Vibrating sample magnetometer (VSM) c) Superconducting quantum interference device (SQUID)
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Measurement of anisotropy field, Ha = 2K1/Ms H is the internal field.
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10.5.2 Closed circuit
Schematic illustration of a hysteresigraph for measuring B (or M) as a function of the internal field H in acylindrical sample. On the right are the compensated coils needed to measure M and the potential coil usedto measure H.
The compensated coil has n1A1 = n2A2. The emf is then proportional to (N1 - N2) Amµ0M
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Use of a potential coil to measure potential difference between two points of a magnetic circuit
The long coil has cross section a and n turns m-1. .% = µ0na(/x - /y).
Measure .% with an integrating voltmeter. E = -Nd%/dt.
The potential coil
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10.6 Excitations
10.6.1 Thermal analysis The methods of thermal analysis involve heating a small sample ata uniform rate (e.g. 10 K min-1), and recording some parameter.
Differential thermal analysis; differential scanning calorimetry thermogravimetry thermopiezic analysis
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10.6.2 Spin waves
Inelastic neutron scattering.
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10.7 Numerical methods
A grid used to obtain numerical solutions of differential equations using the finite difference method in two dimensions
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A triangular mesh used fro two-dimensional finite element calculations