Nuclear Magnetic Resonance in ferromagnetic
multilayers and nanocomposites: Investigations of
their structural and magnetic properties
Christian MENY, Pierre PANISSOD
Institut de Physique et Chimie des Matériaux de StrasbourgUMR 7504 ULP-ECPM-CNRS, BP 43, 23 rue du Loess, 67034 Strasbourg Cedex 2, France
QMMRC - IPCMS Winter School 2009, Feb. 09-13, Seoul
Outline of the talk
• Introduction: NMR in ferromagnets: an analysis tool among others
• Basis of NMR and particularities of NMR in ferromagnets
• Structural information by NMR• Local symmetry
• Local chemical environment
• Magnetic information by NMR• Hyperfine field profile
• Field and temperature dependent measurements
• Local magnetic susceptibility: 3D NMR in Ferromagnets• Restoring field
• Magnetization reversal inhomogeneity
• Magnetic anisotropy inhomogeneity
• Conclusion
NMR in ferromagnets: an analyses tool among others
• Free surface
– Electron Diffraction : RHEED, LEED: Growth mode, 2D Surface structure, Orientations
– Auger Spectroscopy : AES: Growth mode, Surface diffusion & segregation– Imaging : STM, AFM: Direct topological view
• Volume
– Xray Diffraction : XRD : Xtal Structure, Super-period, Texture, Interface roughness
– Transmission Electron Microscopy : TEM : Stacking, Grain structure, Superlattice coherence, Misfit dislocations, Interface roughness, chemical analyses when combined with EELS/EDXS
– Extended Xray Absorption Fine Strusture : EXAFS Chemical & Topological Short Range Order
– Rutherford Back Scattering : RBS : Concentration profiles – Hyperfine techniques: Nuclear Magnetic Resonance, Mössbauer
Spectroscopy : Chemical & Topological Short Range Order
Bulk Structure
Xtal StructureXtal OrientationTexture, Stacking
MosaicityGrain Size
Impurities atdefects
Interface nanostructure
Extended defects,Grain boundaries
Point Defects,Impurities
XRD EXAFS NMRTEM
Interface Structure
Long RangeRoughness
InterfaceCompound
AlloyedInterfaceDislocations
XRD
SurfaceAFMSTMAES
LEEDRHEED
TEM
NMR
Outline of the talk
• Introduction: NMR in ferromagnets: an analysis tool among others
• Basis of NMR and particularities of NMR in ferromagnets
• Structural information by NMR• Local symmetry
• Local chemical environment
• Magnetic information by NMR• Hyperfine field profile
• Field and temperature dependent measurements
• Local magnetic susceptibility: 3D NMR in Ferromagnets• Restoring field
• Magnetization reversal inhomogeneity
• Magnetic anisotropy inhomogeneity
• Conclusion
• NMR signal can be obtained for all nuclei with non zero nuclear spin
• Quantum mechanics: Resonant Absorption of Photon
Basis of Nuclear Magnetic Resonance
Spin 1/2H0 = 0
hωLPhoton
H1(ω)+1/2
−1/2
H0 ≠ 0 Resonance: ω = ωL
H0: Static magnetic field: Zeeman Effect H1: Radiofrequency field : Resonant Absorption
Observable Elements & Sensitivity
To increase sensitivity:Low Temperature : NMR signal increases as 1/T (Curie law)High Field : NMR signal increases as ωωωω2 =γγγγ2H2
In practice: Organic materials: H, F, P (17O, 14N, 13C)Metalloids: Ga, B, As, In (Si, Se, Te)Normal metals: Li, Na, Al (Be, K) Transition Metals: V, Mn, Co, Cu, Nb, La, Re (Y, Pt, Ta)Rare Earths: Pr, Eu, Tb, Ho (Nd, Gd)
H HeLi Be B C N O F NeNa Mg Al Si P S Cl ArK Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br KrRb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I XeCs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
MediumGood Bad No
NMR in ferromagnets: Particularities
•In ferromagnets the magnetic field H0 is the field inside the material on the nuclei sites (For example 20 Tesla for Cobalt nuclei in bulk Co).
•It is called the Hyperfine Field (HF)•The HF strength depends on the local symmetry and local chemicalenvironment of the probed nuclei
•An NMR spectrum in a ferromagnet is measured by frequency sweeps• it is a number of atoms versus a resonance frequency.
•Among the “traditional” ferromagnets (Fe, Co and Ni), •Co has the highest sensitivity (104 time larger than Fe for example)
Spin 1/2H0 = 0
hωLPhoton
H1(ω)+1/2
−1/2
H0 =HFResonance: ω = ωL
Outline of the talk
• Introduction: NMR in ferromagnets: an analysis tool among others
• Basis of NMR and particularities of NMR in ferromagnets
• Structural information by NMR• Local symmetry
• Local chemical environment
• Magnetic information by NMR• Hyperfine field profile
• Field and temperature dependent measurements
• Local magnetic susceptibility: 3D NMR in Ferromagnets• Restoring field
• Magnetization reversal inhomogeneity
• Magnetic anisotropy inhomogeneity
• Conclusion
NMR Frequency and Local symmetry: Bulk CoSpin Echo Intensity
210 215 220 225 230Frequency (MHz)
Stackingfaults
hcpCobalt
fccCobalt
FCC Dominant
210 215 220 225 230Frequency (MHz)
Stackingfaults
hcpCobaltfcc
Cobalt
HCP Dominant
x5
Pure samples: Frequency is the fingerprint of the atom site local symmetry
Example : Bulk Structure of Co Thin FilmsQuantitative analysis
0
20
40
60
80
100
0 1000 2000 3000 4000 5000
fcc fractionhcp fraction
3% 30%Stacking faults:
Volume Fracion (%)
Layer Thickness (Å)Electrodeposited on very flat Au film
Initial growth mostly hcp (
Outline of the talk
• Introduction: NMR in ferromagnets: an analysis tool among others
• Basis of NMR and particularities of NMR in ferromagnets
• Structural information by NMR• Local symmetry
• Local chemical environment
• Magnetic information by NMR• Hyperfine field profile
• Field and temperature dependent measurements
• Local magnetic susceptibility: 3D NMR in Ferromagnets• Restoring field
• Magnetization reversal inhomogeneity
• Magnetic anisotropy inhomogeneity
• Conclusion
NMR frequency and Chemical environment
200 220 240 26050 100 150 200 250
Spin Echo Intensity
100 150 200 250
Co0.90Cu0.10
Frequency (MHz)
0 Cu nn
1 Cu nn
2 Cu nn
2 Ru nn
1 Ru nn
0 Ru nn
Frequency (MHz)
Co0.90Ru0.10
3 Ru nn
Frequency (MHz)
Co0.90Fe0.10
1 Fe nn
2 Fe nn
0 Fe nn
3 Fe nn
•Alien atoms as nearest neighbor of the probed atom shifts the resonance frequency.The value and sign of the shift depends on the nature of the neighboring atom. (Cu: -16 MHz;
Ru:-25 MHz; Fe: +10 MHz)
NMR frequency and Chemical environment : quantitative analysis
Decomposition of NMR spectrum with Gaussian lines.
NMR intensity increase as 1 NN : grows as C,2 NN : grows as C2,3 NN : grows as C3,C: Concentration in alien atoms
Random distribution of the alien atoms among the nearest neighbors (NN) of Co.
The line intensity follows a binomial law
40 80 120 160 200 240Frequency (MHz)
Spin Echo Intensity
0 Crnn2 Crnn
4 Crnn
1 Crnn
3 Crnn
Co0.86Cr0.14
Co0.96Cr0.040 Crnn
1 Crnn
2 Crnn
CoCr: Trend towards segregation at low concentration
CoCu: Strong trend towards segregation(immiscible elements)
CoFe: Close to solid solution
CoRu: Perfect solid solution
Relative line intensities (0, 1, 2, 3 NN)& binomial probability law(Random site occupancy)
NMR frequency and Chemical environment : quantitative analysis
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.00 5 10 15
Impurity Content (%)
CoRu
CoFe
CoCu
CoCr
Configuration Probability
Problem - Bulk Not Understood
CoPt alloys:– 2 sets of satellites
• narrow upwards
• broad downwards
– Related to position of the nearest neighbor among the 12 NN ?
– Long range interactions between Pt impurities ?
– Second neighbor shell ?150 175 200 225 250Frequency (MHz)
Spin Echo Intensity
CoPt0.10
CoPt0.06
CoPt0.02
Examples : Magnetic multilayers
• Frequency fingerprint ofChemical/Xtallographic Environment
• Frequency Ranges correspond to Different Sample Parts
60 100 140 1800.0
0.2
0.4
0.6
0.8
Co-X admixture(other mixed planes)
Co-X admixture(1st mixed plane)
x10
Bulk grain boundaries
Perfect 111 interface
Spin Echo Intensity (arbitrary)
Frequency (MHz)210 220 230 240
0
2
4
6
8
Stacking faults(& hcp Co Μ//c)
hcp CobaltM⊥c
fcc Cobalt
Interface spectral range Bulk spectral range
Co/Cu multilayer
.../Cu/Co/Cu/... Interface SpectraNearly perfect interface (111). Single satellite line corresponding to Co atoms with 3 Cu neighbors. Traces of Co with 2 Cu neighbors.
Excellent multilayered structure. The 3 Cu neighbors line is 30% of the interface spectrum.
Sharp interfaces but numerous monoatomic step defects
Mixed interfaces with some flatter areas (3 Cu neighbors line resolved). Steep concentration profile.
Mixed interfaces. Satellite lines of Co with 1 and 2 Cu neighbors show the presence of several planes containing a weak Cu concentration (3 to 8 %).
Frequency (MHz)50 100 150 200 250
Spin
Echo Intensity
THO
IBM
IEF
PHI
Modeling of Interface SpectraMonoatomic Step Defects
Simulated Spectra
d=2, l=2d=2, l=4
d=2, l=2d=4, l=2
12 1110 9 8 7 6 5 4 3 2 1 0Number of Alien Nearest Neighbors
Interface Model
Parameters of the modeld : Average distance between stepsl : Average length of straight parts
Application : small roughness
Cross section view
l
Top view of mixed plane
d
Spin Echo Intensity
120 140 160 180 200 220 240Frequency (MHz)
Ru/Cu 8Å/Co24Å/Ag 2.4Å/Cu20Å /Ag 2.4Å /Co24Å
Average distance between steps: 2.8 at.d.Average straight length: 1.6 at.d.
Cross section view
l
Top view of mixed plane
d
Modeling of Interface SpectraMonoatomic Step Defects
Modeling of Interface SpectraDiffuse Interface
Cross section view
3 Mixed Planes6 Mixed Planes
1 Mixed Plane2 Mixed Planes3 Mixed Planes
12 1110 9 8 7 6 5 4 3 2 1 0Number of Alien Nearest Neighbors
Simulated Spectra Interface Model
Parameters of the modelConcentrations in each plane
(Concentration profile)Example : Linear profile
Application: Large admixture >2ML
C1C2C3C4C5C6C7
.../Cr/Co/Cr/... Largely Mixed Interfaces
Last fullCo plane
1st mixedplane
2nd mixedplane
Spin Echo Intensity
Frequency (MHz)50 100 150 200 250
[Co16Å/Cr8Å]Cr con
centration
at.%
Hyprfine Field (% of bulk)
100
75
50
25
0
100
75
50
25
0-1 0 1 2 3 4 5
Atomic Layer Index
Co Hyperfine fieldCr concentration
Outline of the talk
• Introduction: NMR in ferromagnets: an analysis tool among others
• Basis of NMR and particularities of NMR in ferromagnets
• Structural information by NMR• Local symmetry
• Local chemical environment
• Magnetic information by NMR• Hyperfine field profile
• Field and temperature dependent measurements
• Local magnetic susceptibility: 3D NMR in Ferromagnets• Restoring field
• Magnetization reversal inhomogeneity
• Magnetic anisotropy inhomogeneity
• Conclusion
Magnetic information: Hyperfine Field Profile
Interface Model :Concentration profile
Side output :Average HF in each atomic plane
Average HF an estimate of Co magnetic moment at interfaces
0 1 2 3 4 50
20
40
60
80
100
Ru Concentration(%)
Co Hyperfine Field(% of bulk)
Interface Profiles
Interface Plane Index (1: last pure Co)
NM R T h eo ryC R u H F /H F b u lk µ /µ b u lk C R u
0 1 .0 0 1 .0 0 00 0 .9 9 1 .0 3 02 .5 0 .9 4 0 .9 7 01 7 0 .7 5 0 .8 9 2 55 0 0 .4 0 0 .6 3 5 08 2 0 .0 4 0 .1 3 7 5
Frequency (MHz)50 100 150 200 250
Co /Ru
Bulk Co planes
3rd mixed plane50%Ru
2nd mixed plane17%Ru
1st mixed plane2.5% Ru
Spin Echo Intensity
32ÅMBE Grown
X Multilayers
Magnetic Properties by NMRWith additional external static magnetic field
210 215 220 225 230
Spin Echo Intensity
Frequency (MHz)
External field0 Oe
500 Oe1000 Oe1500 Oe
Tann = 1000°CTexp = 4.2 K
Lower frequency lineLarge Co clusters, multidomain, fcc :• No demagnetizing field (217 MHz)• Presence of Bloch walls (no shift in low field)
Upper frequency lineSmall Co clusters, single domain, fcc :• Not hcp, no HF anisotropy (no broadening with Hext)• Demagnetizing field -6 kOe (223 MHz)• No Bloch wall (shift -γ Hext from the smallest Hext)
FCC Co ?
210 220 230 240
Temperature4.2 K77 K
300 K
Spin Echo Intensity
Frequency (MHz)
Tann = 800°CHext = 0 Oe
When Co Clusters are superparamagnetic:No NMR signal
Lower frequency lineLarge Co clusters ferromagnetic :• Constant intensity
•Upper frequency lineSmall Co clusters superparamagnetic• Loss of intensity with increasing temperature• Large distribution of blocking temperatures
Large distribution of sizes :Coexistence of small single domain clusters and of large multidomain clusters
Magnetic Properties by NMRNMR measurements versus temperature
Outline of the talk
• Introduction: NMR in ferromagnets: an analysis tool among others
• Basis of NMR and particularities of NMR in ferromagnets
• Structural information by NMR• Local symmetry
• Local chemical environment
• Magnetic information by NMR• Hyperfine field profile
• Field and temperature dependent measurements
• Local magnetic susceptibility: 3D NMR in Ferromagnets• Restoring field
• Magnetization reversal inhomogeneity
• Magnetic anisotropy inhomogeneity
• Conclusion
NMR - Macroscopic ViewpointLaboratory frame Rotating frame (ω)
Mn(ωL)
H0
x
y
z
H1(ω)
Pulsed H1 Pulse Duration: τ Turn Angle: ω1τ
Particular Turn Angles π/2 : ⇒Mn⊥H0 π : Mn Reversal
Measured NMR signal is proportional to the magnitude of the nuclear
magnetization in the XY plane: Maximum when turn angle ω1τ=γΗ1τ= π/2
Mn(ωe)
H0 - ω/γHe
H1
Z
X
Y
ω ω ω ω ≠≠≠≠ ωωωωLω ω ω ω ≠≠≠≠ ωωωωL
Off - Resonance
Y
Mn(ω1)
H1X
Z
ω = ωω = ωω = ωω = ωLω = ωω = ωω = ωω = ωL
At Resonance
H0 - ω/γ=0
Log(H1)
Hoptimum
Spin Echo Intensity
1/3 1 3
Ext
erna
lS
tati
c F
ield
H0
10 t
o 10
0 kO
e
RF Field H110 to 100 Oe
Non Magnetic Sample
Bopt = π/2γτ
1
Maximum of NMR signal depends only on the properties of the probed Nucleus and on the experimental set up:
τ is a fixed experimental parameterγ : the gyromagnetic ratio of the nucleus
NMR intensity (Spin echo intensity) vs RF field strength
Restoring Field related to :
• Anisotropy field (single domain behavior)
• Coercive field (domain nucleation or domain wall stiffness)
• Exchange bias, Coupling field (coupling energy between layers/grains)
(ferro)H2HF/H optres πωτη ==
HF: Hyperfine Field10 to 500 kG
Hr : Restoring Field
Excitation B1 = η η η η H1:B1/H1 = HF/Hr = η
For the maximum NMR intensity:
Hr=HF*Hopt(ferro)/Bopt(non ferro)
Hr=HF*Hopt(ferro)2γτ/π
Magnetic Sample
RF Field H10.01 to 1 Oe
µe
RF Field B1=ηH110 to 100 G
Hr
HF
1/η ⇒ Hr
Log(H1)
Soft
Log(H1)Spin Echo Intensity
1/η ⇒ Hr
Hard
η
η
Enhancement factor & Restoring Field
fcc
Co
210 215 220 225 2300
200
400
600
800
1000
Restoring Field (Oe)
Frequency (MHz)
hcp
Co M⊥c
Stac
king
Faul
ts
3D NMR spectra in ferromagnets
How to visualize inhomogeneities
NMR Intensity vs• Frequency : StructuralNMR• RF Field : Magnetic stiffness
Line of
Crest
Scaled RF Field (Oe)
"Single Xtal" hcp Co
210215
220225
230
40100
200400
fcc Co
StackingFaults
hcp CoM⊥c
Frequenc
y (MHz)
3D Curves
The larger the strength of the H1field we need to apply, the stiffer the part of the sample under investigation
From softestGrain boundaries (small grains)Interface planesStacking faults (zones of)Bulk hcp (M⊥c axis)Bulk fccBulk hcp (M//c axis)
To hardest
Structure & Magnetic Stiffness
/NiFe/FeMn22/CuCo75Spin Valve (IBM)
Gra
inbo
unda
ries
Inte
rface
100 120 140 160 180 200
10
20
30
40
50
60
70
Frequency (MHz)
Restoring Field (Oe)
Spectrum
fcc
Co hc
p C
o
220
Frequency= environment-High frequency, bulk Co : Co atoms surrounded by other Co atoms
-The frequency gives the local symmetry (fcc or hc Co)
-Low frequency, interface: Co atoms by Co and Cu atoms
Radiofrequency strength vsfrequency :-shows how magnetically stiff each environment is
Magnetization reversal initiated where the sample is the less stiff
Coupling Oscillations
• Interface planes partly decoupled from inner planes • softer than bulk (usual)• more sensitive to AF coupling (expected)
• AF coupling only partly transferred to inner planes
10 15 20 25 30 350
200
400
600
800
1000
1200
1400
Bulk Hr
Interface Hr
MR
Restoring Field (Oe)
Ru Thickness (Å)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Magn
etoResistan
ceRatio (%
)
/RuCo32Å X Multilayers
Magnetization Process Homogeneity
Discriminates between • Inhomogeneous
magnetization process
And• Coherent rotation of anet remanent magnetization
e.g. Biquadratic Coupling
Co/Cu Multilayers
Co15ÅCu9ÅAF + F coupling
Co15ÅCu15ÅFerromagnetic coupling
Co10ÅCu10ÅBiquadratic coupling ?
180200
220
1000 300 100 30180
200220
1000 300 100 30Scaled RF Field (Oe)
180200
220
1000 300 100 30 Frequency
(MHz)
M/Ms
-8k -4k 0 4k 8k
-1.0
-0.5
0.0
0.5
1.0
External field (Oe)-8k -4k 0 4k 8k
-1.0
-0.5
0.0
0.5
1.0M/Ms
External field (Oe)
Inhomogeneities of Co Dispersion, Inhomogeneities of Magnetic Anisotropy: Co clusters in Cu
100 1000 10000 1000000
50
100
150
200
Hres, 2ν = 200 MHz
Spin Echo Intensity
Hres, 1
100 1000 10000 1000000
100
200
300
400
500
600
Scaled RF Field strength (Oe)
Hres, 2
Hres, 1
ν = 215 MHz
Spin Echo Intensity
50 100 150 2000
100
200
300
400
experimental spectrum Stif magnetic Co clusterssoft magnetic Co alloy
Spin EchoIntensity
Frequency (MHz)
Spectrum decomposition into two phaseswith different magnetic anisotropyStif : Pure fcc Co clusters + interface
Soft : CoCu alloy grains
Evidence for the existence of two phaseswith different magnetic anisotropies
The cluster size of the two populations is similar but because of their chemical composition their anisotropy is different
Magnetic measurements: 2 blocking temperatures; 2 different cluster sizes?
ConclusionsNMR allows to get specific information about of the structural and
magnetic properties in different parts of the sample
Bulk Co
• Structure of the bulk of the layers and their respective magnetic stiffness
Small grains lead to soft layers because of an increased number grain boundaries
At interfaces• Morphology of the Interfaces
• Magnetization profile. • Magnetic stiffness depends on interface disorder and defects
Between layers• Coupling strength• Discriminates between coherent and incoherent magnetization process.
In granular systems
• Distribution of sizes, magnetic anisotropy, superparamagnetism.
Ref:C. MENY, P. PANISSOD Nuclear Magnetic Resonance in Ferromagnetic Multilayers and Nanocomposites: Investigations of Their Structural and Magnetic Properties
In Modern Magnetic Resonance, G. Webb, Ed., Springer, Heidelberg, 2007. and references therein
Kam sah hamnida
Thank you
Merci
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