Magnetic heterostructures as seen by x-rays and neutrons

Hartmut ZabelExperimentalphysik/FestkörperphysikRuhr-Universität Bochum, Germany

MMSPN, 7-9 December, 2001, Budapest, Hungary

Thin films for magnetism, electronics, spintronics, superconductivity, optics,

corrosion protection...

Magnetic filmsGMR heterostructures

S

S

Proximity effects and tunneling

Lateral structures

F - pinned

Exchange bias

• Dimensional scaling of Ho/Y

• Exchange bias of Co/CoO

• Enhancement of interface sensitivity

Topics:

ξx

ξz

ξy

Scaling behavior of the critical temperature with thickness:

ξx

ξz=0 ξy

Connection between correlation lengthand suszeptibility:

C

Cv

TTT −== − εεξξ ,0,

1)(

22±+

±=ξ

χχqq

0 10 20 30 40 50 60 70 80 90 100

1020304050

60708090

100

T/T c

Thickness t

( ) ( )( )

λ−

⋅=

∞−∞

0ttb

TtTT

C

CC

λ=1/νλ=1/νλ=1/νλ=1/ν

||J

⊥J JTk cB 41=

How does coupling affect the critical temperature?

Scaling behavior of complex spin structures

Q

ΛΛΛΛSDW

SDW magnetism in Cr Spin helix in Ho

How does the Ho spin structure and the Néel temperature depend on interlayer coupling?

Ho

Single film

Ho

Y

Ho

Y

Ho

Coupledsuperlattice

Ho

YHx

Ho

YHx

Ho

Breaking the coupling with

hydrogen

V. Leiner, D. Labergerie, H. Zabel

Bulk Holmium Phase Diagram

W.C. Koehler et al. PR 151 (1966) 414G.P. Felcher et al. PRB 13 (1976) 3034M.J. Pechan and C. Stassis, JAP 55 (1984) 1900

30° lock + cone inc. helix para

0 20 131.5 T[K]

9.68 µB

-τ +τ

G [001]

I +τ

Magnetic peaks from a spin helix are expected at: �

±= GK

4 6 8 10 1214 1618 20

0,00

0,05

0,10

0,15

0,20

30Å

4 6 8 10 1214 1618 200,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

2θ [deg]

Intensity [a.

46Å

4 6 8 10 121416 1820

0,2

0,4

0,6

0,8

1,0

1,2

1,4

100 Å

6 8 10 12-0,020,000,020,040,060,080,100,120,14

1000 Å

(00.τ) as a function of the Ho thickness tHO

Magnetic signal from single Ho films

tHo

Y

Y

Ho

Order parameter for a single 4.6 nm thick Ho-film

RKKY-type Interactions in Ho/Y Superlattices

RKKY-type interlayer coupling mediated by Y conduction electrons

RKKY coupling between localised Ho 4f - moments

5s

5s

IEC

4f

4f

Ho

Ho

tYY

Magnetic τ-peak for a single thin Ho film:Y

Ho

Y

I

Q

Magnetic τ-peak for a coupled Ho/Y superlattice:

Ho τ-peak⊗ =

I I I

Q

Modulation by the chemical periodicity

Magnetic Signal from 30 x [77Å Ho/52Å Y]

V. Leiner, D. Labergerie, M. Ay, H. Zabel, 2001

Temperature dependence of the Ho/Y(00.0+τ) peak after hydrogen loading of the Y-spacer

Simulation of τ-peak for uncoupled and weakly coupledHo-blocks

Scaling of the Néel – Temperature

Tunability of the exchange coupling in Fe/V superlattices with hydrogen

V. Leiner, K. Westerholt, B. Hjörvarsson, H. Zabel, to be published

Ker

r an

gle

Fe /V

-2000 -1000 0 1000 2000

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003

(3) (13)

p(H2) = 0 mbar

beforeH-loading

afterH-loading< 10 mbar

-0,003

-0,002

-0,001

-2000 -1000 0 1000 2000

0,000

0,001

0,002

0,003

8 mbar

Ker

r an

gle

Exchange Coupling in Fe/Cr Superlattices

Λ

Q||

Q⊥

Λ

Q||

Q⊥

R. Siebrecht, A. Schreyer, H. Zabel, 2000

( )+ ( )+10 Oe 215 Oe

0,0 0,5 1,0 1,5 2,0 2,5 3,00

20

40

60

80

100

120

ferroantiferro

(Fe2V13)100 m agnetic phase diagram

T C,N

(K)

∆q /q (%)

2201

22

L)Lk()Lkcos(J)L(J

F

FRKKY ∝=

L

J

Crtical temperature as a function of interlayer exchange coupling for Fe 2 ML/VHx 12 ML

The ADAM Reflectometer at ILL

http://www.ill.fr/YellowBook/ADAM/

Magnetic resonance x-ray scattering of Ho at theLIII - absorption edge (8074 eV), sp-channel

Troika beam line, ESRF

C. Sutter, E. Weschke, R. Meier, C. Schüßler-Langeheine, G. Grübel, D. Abernathy

31nm Ho16.4nm Ho

Resonant enhancement of the magnetic superstructure satellite at the Ho MV edge

Reflectivity of a 109-ML-thick Ho film (a) well below and (b) in the vicinity of the Ho MV edge

0.05 0.10 0.15 0.20 0.25 0.30

(000+τ) T = 40 K

(a)

(b)

τ

109 ML Ho/W(110)

Refle

cted

X-R

ay In

tens

ity

(arb

. uni

ts)

Scattering Vector (c*)

hν = 1338 eV

hν = 900 eV

at resonance

below resonance

1.5 2.0 2.5 3.0 3.5 4.0 4.5

x raysneutrons

46Å HoT=70 KIn

tens

ity (a

rb. u

nits)

q(nm-1)

2 3 4q(nm-1)

2 3 4q(nm-1)

E. Weschke, C. Schüßler-Langeheine, A. Yu. Grigoriev, H. Ott, G. Kaindl, V. Leiner and H. Zabel, to be published

Resonant enhancement of the magnetic superstructure satellite at the Ho MV edge, BESSY

(00.0 +τ) peak of a single 4.6 nm thick Ho(00.1) film

Critical scattering from a 4.6 nm thick Ho film

XRMD results, BESSY

Short range order diffuse scattering

E. Weschke, C. Schüßler-Langeheine, A. Yu. Grigoriev, H. Ott, G. Kaindl, V. Leiner and H. Zabel, to be published

Order parameter and inverse correlation length

106 108 110 112 114 1160

2

4

6

8

10

TN = 110 K

Integrated Intensity (arb. units)

Temperature (K)H

WH

M (1

0-3 Å

-1)

Exchange bias

T<TN

F-layer

AF-layer Magnetic field

Mag

netiz

atio

n T>TN

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

Ker

rRot

atio

n [a

. u.]

2nm CoOCo

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

1. magnetization reversal after field cooling

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

2. reversal

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

Trained hysteresis

MOKE measurements of a CoO/Co bilayer

Magnetic field (Oe)

(00.1)

F. Radu, M. Etzkorn, T. Schmitte, R. Siebrecht, V. Leiner, K. Westerholt, H. Zabel

Polarized neutron reflectivity Y

XM

Q

θφ φ

nµ�

1. Non-spin flip(NSF) scattering: the Y-component

Y

XM

Q

θφ φ

nµ�

Ym,, M(θpRR 2)sin2 ∝=− −−++

2. Spin-Flip (SF) Scattering:

the X-component

Y

XM

Q

θφ φ

nµ�

xm,, M(θpRR ∝== −+−+ )cos

HE

M

HM=0

+,+ -,-+,-

Hex

Bc HH ±

H

Neutron Scattering:

+,+ -,-

+,- Hex

H

Magnetometry:

H

Rotation

H

Wall motion

CoOCo

2nm

PNR results from a CoO/Co bilayer

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

Ker

rRot

atio

n [a

. u.]

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50

-1500 -1000 -500 0 500 1000 1500

10

20

30

40

50

60

70

80

90

Spin

Flip

Inte

nsiti

es [c

ts/se

c]

Magnetic Field [Oe]-1000 -900 -800 -700 -600 -500

10

20

30

40

50

60

70

80

90

Spin

Flip

Inte

nsiti

es [c

ts/se

c]

Magnetic Field [Oe]

(+,+)

(-,-)

(+,-)

EVA-Neutron Reflectometer at the ILL with a 3He polarisation analyser:

B. Nickel A. Rühm, W. Donner, J. Major, H. Dosch, A. Schreyer, H. Zabel, and H.Humblot, Rev. Sci. Instr. 72, 163 (2001)

Diffuse scattering from Co/CoO bilayer

d ≈1- 10 nm0.1nm

0,0 0,1 0,2 0,3 0,4 0,5 0,61E-81E-71E-61E-51E-41E-30,010,1

1

X-r

ay re

flect

ivity

Scattering vector

2π/d

Enhancement of interface sensitivity

PNR of 2 nm Fe/ 150 nm Nb on sapphireADAM neutron reflectometer, ILL

K. Theis-Bröhl, R. Siebrecht, H. Zabel, 2001

Substrate

Nb - filmFe

Substrate

Nb - filmFe

0,00 0,05 0,10 0,15 0,201E-5

1E-4

1E-3

0,01

0,1

1

FeNb MBE1214: low temperature T =1.5 K

uunddndun

Ino

rmal

ized

to m

onito

r

Q [Å1]

bcohMoV MoV MoVV V

Λ

bcohMoV MoV MoVV(D) V(D)

bcohMoV MoV MoVV(D) V(D)

Contrast matching for enhancing interface sensitivity

V. Leiner, B. Hjörvarsson, J. Birch, H. Zabel, submitted

Vanishing of 1. order peak by D- loading of V- layers:

After deuterium loading

Neutron wave guide (resonator) with polarized neutrons

Cd

Cd

Fe

FeTi

PSD

Reflector

V.K. Ignatovich, F. Radu, V.L. Aksenov, Yu.V.Nikitenko, Yu. M. Gledenov, P.V. Sedyshev

Frank Laboratory, Dubna

-1000 0 1000 2000 3000 4000-6,00E-008

-4,00E-008

-2,00E-008

0,00E+000

2,00E-008

4,00E-008

6,00E-008

8,00E-008

1,00E-007

1,20E-007

1,40E-007

1,60E-007

1,80E-007

2,00E-007

bcoh+pmag

bcoh bcoh-pmag

scat

terin

g le

ngth

th ickness

Magnetic film SubstrateSurface

Florin Radu

0,000 0,005 0,010 0,015 0,020 0,025 0,030

0,0

0,2

0,4

0,6

0,8

1,0

R R++ R+- R-- R-+

Ref

lect

ivity

Scattering vector Q

Resonances below the critical edge of total reflection

Magnetizationvector 45°

0,000 0,005 0,010 0,015 0,020 0,025 0,030

0,0

0,2

0,4

0,6

0,8

1,0

R Rpp Rpm Rmm Rmp

Ref

lect

ivity

Scattering vector

Magnetizationvector 0°

Resonances below the critical edge of total reflection

Summary� Neutrons reveal the spin structure and exchange

coupling in magnetic superlattices� Neutrons can distinguish between wall motion

and rotation during magnetization reversal� Resonant magnetic x-ray scattering provides

element selective information on magnetichysteresis and spin structures

� Interface sensitivity can be enhanced by resonant techniques and/or clever design of the sample

Thanks to: F. RaduT. SchmitteM. EtzkornK. Theis –BröhlK. WesterholtV. LeinerR. Siebrecht

B. Hjörvarsson, UppsalaJ. Birch, LinköpingE. Weschke, FU Berlin

Funding through DFG-SFB 491, BMBF