Nuclear resonant scattering of synchrotron radiation: a novel approach to the Mössbauer effect

Transcription

1 Nuclear resonant scattering of synchrotron radiation: a novel approach to the Mössbauer effect Johan Meersschaut Instituut voor Kern- en Stralingsfysica, Katholieke Universiteit Leuven, Belgium Johan.Meersschaut@fys.kuleuven.be C. L abbé, ( ) Instituut voor Kern- en Stralingsfysica, K.U.Leuven, Belgium W. Sturhahn, T.S. Toellner, E.E. Alp, Advanced Photon Source, Argonne National Laboratory J.S. Jiang, S.D. Bader, Materials Science Division, Argonne National Laboratory Fund for Scientific Research Flanders (F.W.O.-Vlaanderen) and the Inter-University Attraction Pole IUAP P5/1 Work at Argonne and the use of the APS was supported by U.S. DOE, BES Office of Science, under Contract No. W ENG-38 European Commission (FP6) STREP NMP4-CT (DYNASYNC)

2 Introduction Mössbauer spectroscopy Nuclear Resonant Scattering of SR part1

3 Motivation Site-selective magnetization measurements : - XMCD - element-specific scattering - study different materials independently - Mössbauer spectroscopy - Nuclear resonant scattering of synchrotron radiation - isotope selective - study specific sites within the material separately motivation

4 Iron Isotopes Table of nuclides 57 Fe probe layer substrate 57Fe Other possible isotopes are 119 Sn, 181 Ta, 149 Sm, 153 Eu,

5 57 Fe isotope Nuclear properties: Excited level unstable (τ = ns) E = kev E = 4.66 nev I = 3/2, µ = µ n Q = 0.16 b Ground state (stable) I = 1/2, µ = µ n Q = 0 b Nat 57Fe µ N = Am b= 10 m h = ev s

6 57Co -> 57Fe Nuclear decay of 57 Co to 57 Fe

7 Hyperfine Interactions Electric monopole term: 57 Fe 57 Fe 57 Fe Isolated nucleus 57 Fe Electron density at the nucleus depends on the chemical properties Isomer shift Isomer shift I = 3/2 57 Fe I = 1/2 Monopole term

8 Hyperfine Interactions Electric quadrupole term: 57 Fe Electric field gradient due to non-cubic environment: * tetragonal or hexagonal lattice, * surface, * impurity in neigbouring shell Isolated nucleus Isomer shift 57 Fe I = 3/2 57 Fe I = 1/2 Quadrupo le term

9 Magnetic dipole interaction: Hyperfine Interactions 57 Fe B hf H B µ = I B hi I = 3/2 µ = µ n + 3/2 + 1/2-1/2-3/2 B hf = + 33 T E = 107 nev E M µ B = I m I = 1/2-1/2 + 1/2 HFI Zeeman µ n = J/T 1 J = ev µ n = ev/t

10 Summary 57 Fe Electric monopole term: Electric quadrupole interaction: Magnetic dipole interaction: I = 3/2 Isomer shift Quadrupole splitting + 3/2, -3/2 + 1/2, -1/2 magnetic splitting B hf = + 33 T + 3/2 + 1/2-1/2-3/2 E = 107 nev E = kev -1/2 + 1/2, -1/2 I = 1/2 + 1/2 HFI summary

11 Introduction Mössbauer spectroscopy Nuclear Resonant Scattering of SR part2

12 Mössbauer spectroscopy Nuclear emission : Nuclear absorption : 14.4 kev 57 Fe I = 3/ kev 57 Fe I = 3/2 0 I = 1/2 0 I = 1/2 drive source detector mm/s E = E0 1+ v c Mossbaue r

13 Mössbauer spectroscopy Nuclear emission : Nuclear absorption : 14.4 kev 57 Fe I = 3/ kev 57 Fe I = 3/2 0 I = 1/2 0 I = 1/2 drive source absorber detector Mossbaue r velocity

14 Mössbauer spectrum The Mössbauer spectrum depends on the strength of the magnetic field : magnetic splitting B = 33 T + 3/2 + 1/2-1/2 E M µ B = I m -3/2-1/2 B = 10 T + 1/2 MS

15 Mössbauer spectrum ( ) 2 1 Intensity : = I1 1 m1 m I2m2 Dm, σ θ, ϕρ, coupling of two nuclear radiation probability in a angular momentum direction with respect to states the quantization axis + 3/2 m = 1,0,-1 I = 3/2 + 1/2-1/2-3/2 m = -1 m = 0 m = +1 m = -1 m = 0 m = +1 Only six possible transitions -1/2 I = 1/2 + 1/2 Sel rules

16 Information from Mössbauer spectra e.g. hyperfine field along the photon direction ( ) 2 1 Intensity : = I1 1 m1 m I2m2 Dm, σ θ, ϕρ, I = 3/2 + 3/2 + 1/2-1/2-3/2 m = -1 m = +1 m = -1 m = +1 m = -1 m = 0 m = +1 m = -1 m = 0 m = +1-1/2 I = 1/2 B = + 33 T + 1/2 orientation

17 Mössbauer spectra Mössbauer spectra on polycrystalline Fe powder : Random orientation of M, B = 33 T External magnetic field along photon µ 0 H = 1 T k B M Thin film magnetized perpendicular to photon MS

18 Information from Mössbauer spectra Mössbauer spectroscopy is sensitive to the direction of the hyperfine field the magnitude of the hyperfine field Can we determine the sign? M -k M k k M B or k B M absorber absorber Info from

19 Determine the sign of B hf? B = 33 T, i.e. M -k + 3/2 + 1/2-1/2-3/2 m = -1 m = 0 m = +1 m = -1 m = 0 m = +1-1/2 + 1/2 m = -1 m = +1 m = -1 m = +1 I = 3/2 I = 1/2 µ B EM = m I B = - 33 T, i.e. M k -3/2-1/2 + 1/2 + 3/2 m = +1 m = 0 m = -1 m = +1 m = 0 m = /2-1/2 m = +1 m = -1 m = +1 m = -1

20 Frauenfelder Frauenfelder Spectra are NOT sensitive to the sign of the magnetization vector Explanation : because the incident radiation is unpolarized the scattering process is not sensitive to the sign of B Solution : Use circularly polarized radiation

21 I = 3/2 I = 1/2 Use left circularly polarized source! B = 33 T + 3/2 + 1/2-1/2-3/2 B = - 33 T m = -1 m = 0 m = +1 m = 0 m = +1-1/2 + 1/2-3/2-1/2 + 1/2 + 3/2 + 1/2-1/2 m = -1 m = +1 m = 0 m = -1 m = +1 m = 0 m = -1 m = +1 m = +1 m = +1 m = +1

22 Practical implementation How to create circularly polarized radiation? with a monochromatic source (Mössbauer source) : - use a magnetized absorber whose 3 rd line coincides with the source line intensity intensity m = ±1 +1 source magnetized absorber (B k : M -k) B = 33 T energy (Γ) photons with helicity +1 are absorbed transmitted radiation is highly polarized with helicity -1

23 MS with circularly polarized radiation Instrum. Meth. B 119 (1996) 438 MS Szymanski

24 MS with left circularly polarized radiation Magnetized iron foil B = - 33 T k B k B m = m = Szymanski MS K. Szymanski, NATO ARW 02 proceedings

25 Information in time spectra The quantum beat pattern is the signature of the hyperfine interaction : - isomer shift ~ chemical environment of probe nuclei - electric field gradient ~ lattice symmetry around the probe nuclei - magnetic hyperfine field ~ magnetization properties The magnetic hyperfine field is related to the magnetization vector in Fe, e.g., the magnetization vector is opposite to the hyperfine field B M The quantum beat pattern is the signature of the magnetization vector!

26 Information from Mössbauer spectra The Mössbauer spectrum is the signature of the hyperfine interaction : sensitive to the direction of the hyperfine field sensitive to the magnitude of the hyperfine field The hyperfine field is a measure for the magnetization vector : in Fe the magnetization vector is opposite to the hyperfine field Very simple! drive source detector Widely used to study magnetic properties of bulk materials. Unsufficient sensitivity (30 nm) to study nanostructures

27 Conversion Electron Mössbauer Spectroscopy Nuclear emission Nuclear absorption Internal conversion 14.4 kev 14.4 kev 14.4 kev 57 Fe 57 Fe + 57 Fe e Cems

28 Conversion Electron Mössbauer Spectroscopy Cems Conversion electron Mössbauer spectroscopy is sensitive enough to probe one monolayer

29 CEMS Example 1 Page ML Fe W(110) a) 2 nd monolayer from interface with Ag b) Interface monolayer with Ag c) Clean surface monolayer Magnetic hyperfine interaction B hf Isomer shift S Electric Quadrupole interaction ε Fe/W(110 )

30 Multilayer system: Fe/ 57 FeSi/Fe epitaxial CsCl-FeSi on Fe MBE growth 150 C Au-capping Nat Fe (40 Å) 57 Fe Si (x Å) Nat Fe (80 Å) Co-evaporated at a low rate (0.068 Å/s) MgO(001) FeSi structure

31 Conversion electron Mössbauer spectroscopy Quadrupole splitting + 3/2, -3/2 + 1/2, -1/2 + 1/2, -1/2 α-fe (22%) en strained B2-FeSi (78%) FeSi Cems

32 Strain relaxation in CsCl-FeSi B. Croonenborghs et al., Appl. Phys. Lett. 85 (2004) 200 X-ray diffraction strain

33 Conversion electron Mössbauer spectroscopy Epitaxially grown FePt L1 0 k B -3/2 Unpolarized source : -1/2 + 1/2 m = m = +1 m = 0 m = -1 m = +1 m = 0 m = /2 + 1/2-1/2 B = -28 T L10 FePt

34 Conversion electron Mössbauer spectroscopy Epitaxially grown FePt L1 0 k B Unpolarized source : Polarized source : m = m = MS

35 Perspectives Perspectives Mössbauer spectroscopy can be used to probe the local properties of materials (structural & magnetic) Conversion electron Mössbauer spectroscopy (CEMS) allows to study magnetic properties of monolayer thick nanostructures The radioactive source illuminates the whole sample: no spatial in-plane resolution Mössbauer spectroscopy or CEMS are difficult to perform under extreme conditions: low/high temperatures applied magnetic field high pressure, possibly in cryomagnets

36 Summary Hyperfine interactions: interaction between the nucleus and its environment (isomer shift, el. Quadr., magn dipole) Mössbauer spectroscopy can probe the hyperfine fields, yielding structural & magnetic information Mössbauer spectroscopy using a circularly polarized source Conversion electron Mössbauer spectroscopy (CEMS) allows to study the structural and magnetic properties of monolayer thin nanostructures Ag/Fe/W(110) Fe/FeSi/Fe L1 0 FePt Summary

37 Introduction Mössbauer spectroscopy Nuclear Resonant Scattering of SR part3

38 Perspectives NRS Mössbauer spectroscopy can be used to probe the local properties of materials (structural & magnetic) Conversion electron Mössbauer spectroscopy (CEMS) allows to study magnetic properties of monolayer thick nanostructures The radioactive source illuminates the whole sample: no spatial in-plane resolution Mössbauer spectroscopy or CEMS are difficult to perform under extreme conditions: low/high temperatures applied magnetic field high pressure, possibly in cryomagnets

39 motivation Nuclear resonant scattering Motivation : study material properties via the hyperfine interactions Mössbauer spectroscopy : as sample dimensions decrease source sample detector need for more brilliant sources Nuclear resonant scattering with synchrotron radiation : synchrotron orbit σ k sample detector - high brilliance + small beamsize (~ 10 µm) - linear polarization - pulsed time structure - broad energy bandwidth 50 ps ns

40 57 Fe isotope Nuclear properties: Excited level unstable (τ = ns) E = kev E = 4.66 nev I = 3/2, µ = µ n Q = 0.16 b Ground state (stable) I = 1/2, µ = µ n Q = 0 b Nat 57Fe µ N = Am b= 10 m h = ev s

41 Isolated nucleus : 57 Fe 14.4 kev I = 3/2 0 I = 1/2 Energy domain : Time domain : intensity nev intensity τ = 141 ns energy 14.4 kev (Γ) time (ns) exponential decay due to lifetime of excited state

42 Nucleus embedded in lattice: I = 3/2 M1 I = 1/2 Energy domain : Time domain : intensity µev intensity energy 14.4 kev (Γ) time (ns) quantum beats due to hyperfine splitting of nuclear states

43 Information in time spectra The quantum beat pattern is the signature of the hyperfine interaction : sensitive to the direction of the hyperfine field in-plane synchrotron orbit σ k

44 Information in time spectra The quantum beat pattern is the signature of the hyperfine interaction : sensitive to the direction of the hyperfine field in-plane out-of-plane

45 Information in time spectra The quantum beat pattern is the signature of the hyperfine interaction : sensitive to the direction of the hyperfine field B k x σ B σ B k intensity energy (Γ) intensity energy (Γ) intensity energy (Γ) intensity intensity intensity time (ns) time (ns) time (ns) sensitive to orientation of B in-plane and out-of-plane

46 Information in time spectra The quantum beat pattern is the signature of the hyperfine interaction : sensitive to the direction of the hyperfine field sensitive to the magnitude of the hyperfine field B = 33 T B = 11 T intensity 100 E 100 E energy (Γ) intensity energy (Γ) intensity intensity quantum beat ~ cos( E t / ħ) time (ns) time (ns) beat frequency ~ magnitude of B

47 applications Applications for magnetism Thus, nuclear resonant scattering can be used to probe the magnetic properties of materials Examples : - measurement of spin rotation in exchange-coupled bilayers - measurement of spin orientation in nanoscale islands

48 Application: exchange springs depth-selective measurement of spinrotation in exchange-coupled bilayers : soft magnet hard magnet with uniaxial anisotropy Fe FePt H exchange spring insert an 57 Fe probe layer : M scattering plane 11 nm 20 mm 0.7 nm 57 Fe R. Rohlsb R. Röhlsberger et al., Phys. Rev. Lett. 89 (2002)

49 Applications scattering plane 11 nm 20 mm 0.7 nm 57 Fe rotation angle ( ) Ag H = 160 mt Fe FePt H = 240 mt H = 500 mt depth (nm) R. Rohlsb R. Röhlsberger et al., Phys. Rev. Lett. 89 (2002)

50 Application: nanoscale islands measurement of nanoscale islands Fe/W(110): 2 nm 57 Fe 1 atomic step W(110) coverage of 0.57 monolayer perpendicular spin orientation in Fe islands below 100 K Fe/W(110 R. Röhlsberger et al., Phys. Rev. Lett. 86 (2001) 5597

51 Polarized radiation Nuclear resonant scattering permits to retrieve detailed magnetic information There is one restriction : two opposite directions of M give exactly the same time spectrum!

52 Spectra are NOT sensitive to the sign of the magnetization vector Explanation : because the incident radiation is linearly polarized the scattering process is not sensitive to the sign of B intensity m = energy (Γ) B k B -k With linearly polarized radiation the same nuclear transitions are excited for opposite directions of the hyperfine field B

53 Sign of the hyperfine field How can one measure the sign of the hyperfine field (or magnetization)? Use circularly polarized radiation : 100 B k 100 B -k intensity intensity m = energy (Γ) 20 m = energy (Γ) depending on the sign of B, different transitions are excited Circ polarization

54 Even with circularly polarized radiation 100 B k 100 B -k intensity intensity m = energy (Γ) 20 0 m = energy (Γ) if transitions for two opposite field directions are symmetric around E 0 the quantum beat is the same for both field directions 100 B k B -k 100 intensity intensity time (ns) time (ns)

55 Trick Break the symmetry by adding an extra single-line transition at E E 0 intensity SL B k intensity SL B -k 20 m = energy (Γ) 20 0 m = energy (Γ) clear difference between two time spectra B k B -k intensity intensity time (ns) time (ns)

56 Practical implementation Extra single-line transition can be achieved by adding a single-line reference sample to the beam B k single-line reference magnetic sample the resonances in reference and sample are excited coherently intensity SL B k Extra single line 20 m = energy (Γ)

57 Practical implementation How to create circularly polarized radiation? with a monochromatic source (Mössbauer source) : - use a magnetized absorber whose 3 rd line coincides with the source line with a broadband source (synchrotron radiation) : - use an X-ray phase retarder linearly polarized circularly polarized 45 Phase retarder single crystal with the diffraction planes inclined at 45 Bragg reflection involves both a σ and π component offset the crystal from exact Bragg condition a phase retardation between the σ and π components is induced tune offset angle for maximal degree of circular polarization

58 To measure the sign of the hyperfine field (magnetization vector) one has to use circularly polarized radiation and an additional single-line reference sample Now the full magnetization information can be retrieved : the magnitude of the magnetization vector the direction of the magnetization vector the sign of the magnetization vector One can perform nuclear resonant magnetometry : measure magnetization curves as a function of the external field

59 Fe/Cr Interlayer coupling in Fe/Cr multilayers Fe/Cr multilayers : Fe Cr Depending on the Cr layer thickness, the Fe magnetization vectors will align : under 0 or 180 : bilinear coupling under 90 : biquadratic coupling

60 5-layers Study influence of growth mechanism on interlayer coupling : quintalayer samples grown on MgO(100) Fe 4 nm 4 nm 4 nm 1.1 nm 1.1 nm Cr strong AF coupling expected standard magnetization measurement : molecular beam epitaxy magnetron sputtering M/Ms µ 0 H (T) M/ Ms µ 0 H (T)

61 Iso enrichment In order to study the interlayer coupling in detail : measure the magnetization of 1 Fe layer selectively use the isotope-selectivity of nuclear resonant scattering 57 Fe 56 Fe 56 Fe buried 57 Fe layer grown on thick substrate isotopic enrichment does not change the magnetic properties of the sample Measurement yields the magnetization vector of the central Fe layer

62 Set-up 3ID Experiment at APS beamline 3-ID kev C (111) SS foil H undulator premono high-resolution monochromator phase retarder reference multilayer sample detector

63 Time spectra Sample grown with molecular beam epitaxy on MgO(100) : Nuclear resonant magnetometry M/Ms µ 0 H (T)

64 NRM sputtered Sample grown with magnetron sputtering on MgO(100) : M / Ms µ 0 H (T) at zero field, the central magnetization vector is NOT antiparallel!!

65 Retrieve quantitative values for coupling angle : ϕ coupl angle θ ϕ ϕ : angle of outer magnetization vectors θ : angle of central magnetization vector nuclear resonant magnetometry : standard magnetometry : M/Ms µ 0 H (T) central Fe layer M/M S = cos θ M/Ms µ 0 H (T) all Fe layers M/M S = (2cos ϕ + cos θ)/3

66 Retrieve quantitative values for coupling angle : ϕ coupl angle θ ϕ ϕ : angle of outer magnetization vectors θ : angle of central magnetization vector From the combination of nuclear resonant and standard magnetometry : 180 θ ϕ ( o ) µ 0 H (T) at zero field : θ ϕ = 162 ± 4 non-collinear coupling!!

67 Nuclear Resonant scattering of SR Nuclear resonant scattering with circularly polarized radiation and an additional single-line reference sample permits to retrieve the full magnetic information This allows to perform nuclear resonant magnetometry We measured a layer-selective magnetization curve in [Fe(5.0nm)/Cr(1.1nm)] 3 and found - bilinear coupling for MBE-grown samples - non-collinear coupling for sputtered samples we attribute the existence of non-collinear coupling to extrinsic properties of the multilayer which are determined by the preparation conditions C. L abbé et al., Phys. Rev. Lett. 93 (2004)

68 Summary Origin of quantum beats in time-domain Sensitivity to the direction of B Examples: exchange system FePt/Fe using isotopic marker layer low-temperature spin state in sub-monolayer Fe/W(110) How and why to introduce circularly polarized radiation into nuclear resonant scattering of synchrotron radiation additional single-line reference sample Example: interlayer coupled Fe/Cr/Fe/Cr/Fe quintalayer with isotope selective hysteresis curve comparison MBE vs sputtered samples Summary

69 Conclusion Applications for magnetism Mössbauer spectroscopy can be used to probe the magnetic properties of materials (including homogeneous ultrathin films) Nuclear resonant scattering of synchrotron radiation allows to measure magnetization curves of specific parts as a function of the external magnetic field or under extreme conditions