nuclear magnetic resonance in ferromagnetic multilayersand...

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










• 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










• 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










• 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










• 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










• 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)


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


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

nuclear magnetic resonance in ferromagnetic multilayersand...

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