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 Röhlsberger Deutsches Elektronen-Synchrotron DESY, Hamburg
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
Nuclear Resonant The 14.4 keV nuclear Forward Scattering of resonance of 57 Fe Synchrotron Radiation t 0 = 141 ns, G 0 = 4.7 neV Pulsed broadband excitation of hyperfine-split nuclear levels w 1 w 2 w 3 Temporal beats The beat pattern is a fingerprint of the magnetic structure of the sample:
Magnetic Hyperfine Interaction Energy dependence Directional dependence à Dipole emission characteristics, described by vector spherical harmonics Magnetization direction D m = -1 D m = 0 D m = +1 From: Phys. Rev. B 67, 245412 (2003) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 4
Magnetic Hyperfine Interaction Directional dependence From: Phys. Rev. B 67, 245412 (2003) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 5
Electric Hyperfine Interaction Directional dependence From: R. Röhlsberger, Springer Tracts in Modern Physics, Vol. 208 (2005) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 6
Diffraction as Method of Structure Determination Intensity |A(q)| 2 Array of slits A(x) Diffraction pattern Illumination by a spatially extended beam Position x Momentum transfer q Array of resonances A(E) Intensity |A(t)| 2 Illumination by an energetically extended beam Beat Energy E pattern Time t Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 7
Optical Properties in the X-ray Regime Example: Fe Fe K-edge Index of refraction n = 1 − δ δ ≈ 10 − 5 − 10 − 6 for λ = 0.1nm n = 1 − 2 πρ Scattering amplitude ( f-Z)/r 0 f 2 k 0 Critical angle of Si Pd total reflection 2 δ ϕ c = Ralf Röhlsberger | Workshop on NRS and Data Analysis | ANL/APS, 16 – 19 November 2017 | Page 8
X-ray Optical Properties of Matter (Example: 57 Fe) 57 Fe: nuclear resonance Fe K-edge 0.5 µ eV Scattering amplitude f/r 0 G 0 e E 0 = 14.4 keV G 0 = 4.7 neV t 0 = 141 ns g Ralf Röhlsberger | Workshop on NRS and Data Analysis | ANL/APS, 16 – 19 November 2017 | Page 9
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
Temporal beats and magnetization direction The temporal beat pattern sensitively depends on the orientation of the Magnetization M relative to the Photon wavevector k 0 à Use isotopic probe layers to investigate the depth dependence of magnetic properties Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 11
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) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 12
Exchange-Coupled Bilayers Ferromagnet/Antiferromagnet Hard magnet/Soft magnet What happens at the interface ? Exchange spring Magnetic hysteresis Magnetic hysteresis minor loop Exchange bias major loop Ralf Röhlsberger | Workshop on NRS and Data Analysis | ANL/APS, 16 – 19 November 2017 | Page 13
The Spin Structure of Magnetic Bilayers Fe on FePt Exchange-Spring magnets Soft – magnetic Fe Hard – magnetic FePt with uniaxial anisotropy Exchange coupling at the • interface: Parallel alignment of Fe and FePt moments Fe With increasing distance from • the interface: Coupling becomes weaker External field H induces spiral • FePt magnetization Return to parallel alignment for • H = 0 Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 14
Imaging the Internal Spin Structure of Exchange-Spring Magnets Time spectra of nuclear resonant scattering 0.7 nm 57 Fe 20 mm = 160 mT 3 nm Ag 10 nm Fe 30 nm FePt R. Röhlsberger et al., PRL 89, 237201 (2002)
160 mT 240 mT 500 mT Domain wall compression with increasing external field Fe on FePt Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 16
Simulation of Exchange-Spring Layer Systems E. Fullerton et al. PRB 58, 12193 (1998) Exchange Anisotropy Dipolar energy Divide the layer system into N sublayers of thickness d Minimize the Depth profiling of magnetic properties magnetic free energy Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 17
Depth Dependence of the Oxide Phase RR, H. Thomas, K. Schlage, T. Klein, J. Magn. Magn. Mater. 282, 329 (2004) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 18
New magnetic order in Fe/Fe-oxide heterostructures Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 19
(1) New magnetic order in Fe/Fe-oxide heterostructures 56 Fe M 1 k 0 57 Fe H=70mT 56 Fe-oxide M 1 k 0 H=70mT M 1 M 2 M 2 k 0 M 1 H=70mT Dq = 60 o - 70 mT field Dq = 85 o - in remanence Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 20 20
(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 Eddy current problem at high frequencies : Possibilities with new materials? native FeO x (1nm) Oxide / insulating spacer layer: à efficient eddy current damping 1 Fe(2nm) Fe/Fe-oxide Heterostructures: N times Materials with high magnetisation and low electrical conductivity 1 G.S.D Beach and A.E. Berkowitz, PRL 91, 267201 (2003) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 21
Fe/Fe-Oxide Multilayer: Magnetic Superstructure Iron layers probed by 57 Fe B=70 mT Electronic reflectivity 56 Fe-oxide 23 Å 56 Fe 10.6 Å 57 Fe Nuclear reflectivity k 0 15x B Superstructure peak k 0 • Pure magnetic origin B • Canted moments of Fe-layers B Th. Diederich et al., PRB 76, 54401 (2007) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 22
Fe/Fe-oxide Multilayers Coupling strength and magnetization reversal ! B B (mT) -1200 -20 20 1200 Th. Diederich, S. Couet, and RR, Phys. Rev. B 76, 54401 (2007) What is the origin of the magnetic coupling ? à Investigate the nature of the buried native oxide ! Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 23
Non collinear coupling in the system Fe/Fe-oxide/Fe The strong coupling between the Fe layers excludes an RKKY type of interlayer coupling à Coupling is mediated by an antiferromagnetically ordered oxide (at room temperature) !! Ideal systems, 2 possibilities: Real system, roughness: Spin frustration at the interfaces FM coupling AFM coupling QUESTION: How can the native oxide be Non collinear coupling antiferromagnetically ordered at room temperature? Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 24
Evolution of the native oxide during growth An in-situ XAS study Reference spectra Oxidation of 8 Å Fe at 5x10 -5 mbar 1 Langmuir (L) = 10 -6 Torr s Mixture of phases at saturation Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 25
Chemistry of the free native oxide Fitting XANES : Linear combination of reference spectra allows to extract the relative concentration of the different species Some metallic iron exists even at saturation Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 26
Chemistry of the buried native oxide Deposition of iron on its native oxide S. Couet et al., Phys. Rev. Lett. 101, 056101 (2008) Fe deposition Oxidation Complete reduction to an FeO like buried oxide!! Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 27
The Fe/Fe-Oxide Story: Summary (Doctoral thesis of Sebastien Couet, Hamburg, 2008) • 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 of the Fe layers [1] S. Couet et al., Phys. Rev. Lett. 101 , 056101 (2008) [2] S. Couet et al., Phys. Rev. Lett. 103 , 097201 (2009) Ralf Röhlsberger | Workshop on NRS and Data Analysis | APS/ANL, 16 – 19 Nov 2017 | Page 28
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