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Studies of Thin-Film Magnetism Using Nuclear Resonant Scattering - - PowerPoint PPT Presentation

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Workshop on Nuclear Resonant Scattering and Data Analysis November 16 19, 2017, Advanced Photon Source Studies of Thin-Film Magnetism Using Nuclear Resonant Scattering Ralf Rhlsberger Deutsches Elektronen-Synchrotron DESY, Hamburg

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Ralf Röhlsberger

Deutsches Elektronen-Synchrotron DESY, Hamburg

Workshop on Nuclear Resonant Scattering and Data Analysis November 16 – 19, 2017, Advanced Photon Source

Studies of Thin-Film Magnetism Using Nuclear Resonant Scattering

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Outline

  • 1. Hyperfine Interactions:

Temporal beat patterns as fingerprints of magnetic properties

  • 1. Magnetic structure of thin films, multilayers and

nanostripes

a. Spin structure of exchange-spring magnets b. Magnetic order in Fe/Fe-oxide heterostructures c. Spin-structured multilayers for precision spintronics

  • 1. Magnetic dynamics in thin films and nanostructures

a. Spin precession at ferromagnetic resonance

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3

w

2

w

1

w

Temporal beats

Nuclear Resonant Forward Scattering of Synchrotron Radiation

Pulsed broadband excitation

  • f hyperfine-split nuclear

levels

The 14.4 keV nuclear resonance of 57Fe t0 = 141 ns, G0 = 4.7 neV The beat pattern is a fingerprint of the magnetic structure of the sample:

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 4

Magnetic Hyperfine Interaction

Dm = +1

From:

  • Phys. Rev. B 67, 245412 (2003)

Energy dependence

Dm = -1 Dm = 0

Directional dependence à Dipole emission characteristics, described by vector spherical harmonics

Magnetization direction

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 5

Magnetic Hyperfine Interaction

Directional dependence

From:

  • Phys. Rev. B 67, 245412 (2003)
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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 6

Electric Hyperfine Interaction

Directional dependence

From:

  • R. Röhlsberger, Springer Tracts in

Modern Physics, Vol. 208 (2005)

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 7

Diffraction as Method of Structure Determination

Diffraction pattern

Illumination by a spatially extended beam Array of slits A(x) Position x Momentum transfer q Intensity |A(q)|2

Beat pattern

Illumination by an energetically extended beam Array of resonances A(E) Energy E Time t Intensity |A(t)|2

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | ANL/APS, 16 – 19 November 2017 | Page 8

Optical Properties in the X-ray Regime

Scattering amplitude (f-Z)/r0

Fe K-edge

n = 1− 2πρ k0

2

f

Index of refraction

δ ≈10−5 −10−6 for λ = 0.1nm

n =1−δ

Critical angle of total reflection

ϕc = 2δ

Si Pd

Example: Fe

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | ANL/APS, 16 – 19 November 2017 | Page 9

Scattering amplitude f/r0

57Fe: nuclear resonance

G0 0.5 µeV

X-ray Optical Properties of Matter (Example: 57Fe)

Fe K-edge

E0 = 14.4 keV G0 = 4.7 neV t0 = 141 ns g e

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Structure Determination in SpaceTime (2)

Real Space: Thin-film interfaces Energy Space: Nuclear resonances

Combination of thin-film-interference with nuclear resonant scattering: CONUSS module KGIN

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 11

Temporal beats and magnetization direction

The temporal beat pattern sensitively depends on the

  • rientation of the

Magnetization M relative to the Photon wavevector k0 à Use isotopic probe layers to investigate the depth dependence

  • f magnetic properties
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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 12

Permanent Magnets: Evolution of the Energy Product

Exchange hardening in nanostructured two-phase systems: Hard phase with high coercivity Soft phase with high magnetization

Magnet volumes at constant magnetic energy

  • R. Skomski and J. Coey:

PRB 48, 15812 (1993)

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | ANL/APS, 16 – 19 November 2017 | Page 13

Ferromagnet/Antiferromagnet

Magnetic hysteresis Magnetic hysteresis

Exchange bias Hard magnet/Soft magnet Exchange spring

minor loop major loop

What happens at the interface ?

Exchange-Coupled Bilayers

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 14

The Spin Structure of Magnetic Bilayers

Fe on FePt Soft – magnetic Fe Hard – magnetic FePt with uniaxial anisotropy

  • Exchange coupling at the

interface: Parallel alignment of Fe and FePt moments

  • With increasing distance from

the interface: Coupling becomes weaker

  • External field H induces spiral

magnetization

  • Return to parallel alignment for

H = 0

Fe FePt

Exchange-Spring magnets

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Imaging the Internal Spin Structure of Exchange-Spring Magnets

Time spectra of nuclear resonant scattering 20 mm 0.7 nm 57Fe 3 nm Ag 10 nm Fe 30 nm FePt

  • R. Röhlsberger et al., PRL 89, 237201 (2002)

= 160 mT

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 16

Domain wall compression with increasing external field Fe on FePt

160 mT 240 mT 500 mT

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 17

Depth profiling of magnetic properties Minimize the magnetic free energy

  • E. Fullerton et al. PRB 58, 12193 (1998)

Exchange Anisotropy Dipolar energy

Divide the layer system into N sublayers of thickness d Simulation of Exchange-Spring Layer Systems

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 18

Depth Dependence of the Oxide Phase

RR, H. Thomas, K. Schlage, T. Klein, J. Magn. Magn. Mater. 282, 329 (2004)

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 19

New magnetic order in Fe/Fe-oxide heterostructures

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 20

20

57Fe 56Fe

M1

H=70mT

M1

56Fe-oxide

M1 M2 M1 M2

k0 H=70mT H=70mT k0 k0 Dq = 60o

  • 70 mT field

Dq = 85o

  • in remanence

(1) New magnetic order in Fe/Fe-oxide heterostructures

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 21

N times Fe(2nm) native FeOx(1nm)

Oxide / insulating spacer layer:

à efficient eddy current damping1

1G.S.D Beach and A.E. Berkowitz, PRL 91, 267201 (2003)

Eddy current problem at high frequencies : Possibilities with new materials?

Fe/Fe-oxide Heterostructures:

Materials with high magnetisation and low electrical conductivity

(1) New magnetic order in Fe/Fe-oxide heterostructures

Magnetic thin films and nanostructures for high-density magnetic recording à Read heads are subject to high-frequency magnetic fields

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 22

Fe/Fe-Oxide Multilayer: Magnetic Superstructure

k0 B

56Fe 56Fe-oxide

15x

57Fe

  • Pure magnetic origin

k0 B B

Electronic reflectivity Nuclear reflectivity Superstructure peak

B=70 mT 10.6 Å 23 Å

Iron layers probed by 57Fe

  • Canted moments of Fe-layers
  • Th. Diederich et al., PRB 76, 54401 (2007)
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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 23

B !

B (mT)

  • 1200
  • 20

20 1200

Coupling strength and magnetization reversal

Fe/Fe-oxide Multilayers

What is the origin of the magnetic coupling ? à Investigate the nature of the buried native oxide !

  • Th. Diederich, S. Couet, and RR, Phys. Rev. B 76, 54401 (2007)
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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 24

à Coupling is mediated by an antiferromagnetically

  • rdered oxide (at room temperature) !!

Ideal systems, 2 possibilities: FM coupling AFM coupling Real system, roughness: Spin frustration at the interfaces Non collinear coupling

Non collinear coupling in the system Fe/Fe-oxide/Fe

The strong coupling between the Fe layers excludes an RKKY type of interlayer coupling

QUESTION: How can the native oxide be antiferromagnetically ordered at room temperature?

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 25

Oxidation of 8 Å Fe at 5x10-5 mbar

Evolution of the native oxide during growth

Reference spectra

Mixture of phases at saturation An in-situ XAS study

1 Langmuir (L) = 10-6Torr s

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 26

Fitting XANES : Linear combination of reference spectra allows to extract the relative concentration of the different species Some metallic iron exists even at saturation

Chemistry of the free native oxide

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 27

Deposition of iron on its native oxide

Chemistry of the buried native oxide

Complete reduction to an FeO like buried oxide!! Fe deposition Oxidation

  • S. Couet et al.,
  • Phys. Rev. Lett. 101, 056101 (2008)
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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 28

The Fe/Fe-Oxide Story: Summary

  • Native oxide exhibits a mixture of

chemical states – not magnetic

  • Fe deposition leads to a FeO-like

structure – not magnetic [1]

  • FM order in the metal stabilizes

AFM order in the oxide at room temperature [2]

  • Exchange coupling at both

interfaces of the oxide leads to a non-collinear arrangement

  • f the Fe layers

(Doctoral thesis of Sebastien Couet, Hamburg, 2008)

[2] S. Couet et al., Phys. Rev. Lett. 103, 097201 (2009) [1] S. Couet et al., Phys. Rev. Lett. 101, 056101 (2008)

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 29

Spin-structured multilayers: A new class of materials for precision spintronics

GMR

Rotation angle

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 30

Oblique incidence deposition allows to induce a strong and adjustable magnetic anisotropy in ultrathin magnetic films. Origin thin films waviness thick films tilted grains

Tuning the Magnetic Anisotropy of Ultrathin Films via OID

OID = oblique incidence deposition

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 31

Spin-structured multilayers: A new class of materials for precision spintronics Magnetic Hardening of a Single Iron Layer

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 32

Applicable also to multilayer systems with crossed easy axis?

Magnetic Hardening of a Single Iron Layer

Spin-structured multilayers: A new class of materials for precision spintronics

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 33

Probing Spin Structures in Multilayers via Nuclear Resonant Diffraction

Arbitrary in-plane spin structures via oblique-incidence deposition à Magnetic order does not rely on interlayer coupling à No limitation to prepare vertical stacking profile.

[57Fe/C]13 Superstructure Bragg-peaks Reflectivity Non-resonant Nuclear-resonant Two magnetic sublattices

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 34

Tailored Sequential Layer Switching via polar deposition angle

Tuning of MR via Oblique Incidence Deposition

q

  • K. Schlage et al., Adv. Funct. Mater. 26, 7423 (2016)

Co Co

sensitive to field strength and sign!

Crossed Magnetization Axes via azimuthal deposition angle

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 35

q

  • field-dependent switching characteristics

signal shape: peak or plateau

  • with crossed magnetization axes:

sensitive to frequency and sense of rotation 8mT 22mT

Application: Precision Rotational Sensing

  • extremely sharp switching

European Patent EP 2 846 334 A1 (2015), pending

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL 16 – 19 Nov 2017 | Page 36

Determination of magnetic dynamics via nuclear resonant scattering

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL 16 – 19 Nov 2017 | Seite 37

Nuclear Resonant Scattering for Magnonics

Spin manipulation functional spin devices Magnetic resonances magnetic storage à switching times, energy losses

Spintronics Magnonics

Spin current manipulation & generation Spin wave manipulation & generation

www.hitachigst.com

  • Y. Tserkovnyak et al. , Phys. Rev. Lett. 88,

117601 (2002)

m

Spin Pumping

Ispin

j

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL 16 – 19 Nov 2017 | Seite 38

Spin excitations in magnetic microstructures

Thin film system

permalloy (Ni8057Fe20) stripline hrf

Irf M

Excitation of uniform spin precession à Kittel mode at ferromagnetic resonance

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL 16 – 19 Nov 2017 | Seite 39

Experimental Setup

k

KB Mirror High frequency generator

Pole shoes

k

Thin film system

APD detector

k

GaAs substrate stripline 10 µm k0

j

hrf insulation (HSQ) 170 nm permalloy (Ni8057Fe20) 13 nm Au 30 nm 800 µm

KB mirror focus 10 x 10 µm² matches typical sample size

0.25 ° grazing incidence Setup at beamline P01 of PETRA III

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Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL 16 – 19 Nov 2017 | Seite 40

Spin trajectory determination

micromagnetic simulations with MicroMagnum software

Determination of the opening angle dj dj Power sweep Frequency sweep Field sweep

  • L. Bocklage et al., Phys. Rev. Lett. 114, 147601 (2015)
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Summary

(3) Spin-structured multilayers: A new class of materials for precision spintronics

GMR

Antiferromagnetic native oxide mediates non- collinear coupling of ferromagnetic layers Tuning of magnetoresistance via oblique-angle deposition

(1) Spin structure of exchange-spring magnets

57Fe isotopic probe layers reveal the magnetic

depth profile of exchange-coupled layers

160 mT

(2) Magnetic order in Fe/Fe-oxide heterostructures (4) Spin-trajectories in thin films under FMR conditions

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The DESY research group Magnetism and Coherent Phenomena (MCP)

May 2017

Johann Haber RR Lars Bocklage Andrey Siemens Pavel Alexeev Kai Schlage Svenja Willing Cornelius Strohm Jakob Gollwitzer Christian Adolff Carolin Behncke Sven Velten Guido Meier Jade Comfort Tatiana Gurieva Not on picture: Sven Velten