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-31-109-ENG-38 European Commission (FP6) STREP NMP4-CT-2003-001516 (DYNASYNC)
Introduction Mössbauer spectroscopy Nuclear Resonant Scattering of SR part1
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
Iron Isotopes Table of nuclides http: / / atom.kaeri.re.kr/ 57 Fe probe layer substrate Other possible isotopes are 119 Sn, 181 Ta, 149 Sm, 153 Eu, … 57Fe
57 Fe isotope Nuclear properties: ∆ E = 4.66 neV E = 14.413 keV I = 3/2, µ = -0.155 µ n Excited level unstable Q = 0.16 b ( τ = 141.11 ns) I = 1/2, µ = 0.090 µ n Ground state (stable) Q = 0 b µ = × − = − = ⋅ 27 2 28 2 -16 5.05 10 1 10 6.58212 10 eV s h Am b m N Nat 57Fe
57 Co to 57 Fe Nuclear decay of 57Co -> 57Fe
Hyperfine Interactions Electric monopole term: 57 Fe 57 Fe 57 Fe Electron density at the nucleus Isolated depends on the chemical properties nucleus Isomer Isomer shift shift 57 Fe I = 3/2 57 Fe I = 1/2 Monopole term
Hyperfine Interactions Electric quadrupole term: Electric field gradient due to non-cubic environment: 57 Fe * tetragonal or hexagonal lattice, * surface, * impurity in neigbouring shell Isomer Isolated shift nucleus 57 Fe I = 3/2 57 Fe I = 1/2 Quadrupo le term
Hyperfine Interactions Magnetic dipole interaction: B hf 57 Fe + 3/2 B hf = + 33 T µ + 1/2 = − ⋅ ∆ E = 107 neV I = 3/2 H I B - 1/2 B µ = -0.155 µ n h I - 3/2 - 1/2 µ B = − E m I = 1/2 M I + 1/2 µ n = 5.05 10 -27 J/T 1 J = 6.2415 10 18 eV µ n = 31.52 10 -9 eV/T HFI Zeeman
Summary 57 Fe Electric monopole term: Electric quadrupole interaction: Magnetic dipole interaction: magnetic Quadrupole B hf = + 33 T splitting splitting ∆ E = 107 neV Isomer + 3/2 shift + 3/2, -3/2 + 1/2 - 1/2 + 1/2, -1/2 - 3/2 I = 3/2 ∆ E = 14.413 keV - 1/2 + 1/2, -1/2 I = 1/2 + 1/2 HFI summary
Introduction Mössbauer spectroscopy Nuclear Resonant Scattering of SR part2
Mössbauer spectroscopy Nuclear absorption : Nuclear emission : 14.4 keV I = 3/2 14.4 keV I = 3/2 57 Fe 57 Fe I = 1/2 I = 1/2 0 0 detector drive 0.2799 mm/s source ⎛ v ⎞ = + 1 ⎜ ⎟ E E 0 ⎝ ⎠ c Mossbaue r
Mössbauer spectroscopy Nuclear absorption : Nuclear emission : 14.4 keV I = 3/2 14.4 keV I = 3/2 57 Fe 57 Fe I = 1/2 0 I = 1/2 0 detector detector detector drive source absorber absorber velocity Mossbaue r
Mössbauer spectrum The Mössbauer spectrum depends on the strength of the magnetic field : magnetic splitting B = 33 T + 3/2 µ B + 1/2 = − E m M I - 1/2 - 3/2 - 1/2 B = 10 T + 1/2 MS
Mössbauer spectrum m = 1,0,-1 2 ( ) ⎡ ⎤ = − σ θ ϕ ρ 1 : 1 D , , Intensity I m m I m ⎣ ⎦ 1 1 2 2 , m coupling of two nuclear radiation probability in a angular momentum direction with respect to states the quantization axis + 3/2 + 1/2 I = 3/2 - 1/2 - 3/2 1 1 1 0 + 1 0 + - - = = = = Only six possible transitions = = m m m m m m ∆ ∆ ∆ ∆ ∆ ∆ - 1/2 I = 1/2 + 1/2 Sel rules
Information from Mössbauer spectra 2 ( ) ⎡ ⎤ = − 1 σ θ ϕ ρ : 1 D , , Intensity I m m I m ⎣ ⎦ 1 1 2 2 , m e.g. hyperfine field along the photon direction + 3/2 1 1 1 1 + + - - = = = = + 1/2 m m m m ∆ ∆ ∆ ∆ I = 3/2 - 1/2 - 3/2 1 1 1 0 + 1 0 + - - = = = = = = m m m m m m ∆ ∆ ∆ ∆ ∆ ∆ - 1/2 I = 1/2 B = + 33 T + 1/2 orientation M
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
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 B B k M k M or absorber absorber Info from M
µ B Determine the sign of B hf ? = − E m M I B = 33 T, i.e. M || -k B = - 33 T, i.e. M || k + 3/2 - 3/2 + 1/2 - 1/2 I = 3/2 - 1/2 + 1/2 - 3/2 + 3/2 1 1 1 1 1 1 1 0 + 0 1 0 + 0 + - + - - - = = = = = = = = = = = = m m m m m m m m m m m m ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ - 1/2 + 1/2 I = 1/2 + 1/2 - 1/2 ∆ m = +1 ∆ m = +1 ∆ m = -1 ∆ m = -1 ∆ m = +1 ∆ m = -1 ∆ m = +1 ∆ m = -1 Sign Bhf?
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 Frauenfelder Frauenfelder
Use left circularly polarized source! B = 33 T B = - 33 T + 3/2 - 3/2 + 1/2 - 1/2 I = 3/2 - 1/2 + 1/2 - 3/2 + 3/2 1 1 1 1 1 1 1 0 + 0 1 0 + 0 + - + - - - = = = = = = = = = = = = m m m m m m m m m m m m ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ ∆ - 1/2 + 1/2 I = 1/2 + 1/2 - 1/2 ∆ m = +1 ∆ m = +1 ∆ m = +1 ∆ m = +1 Use circ
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 ∆ m = ±1 100 source 80 intensity 60 40 20 0 magnetized absorber (B || k : M || -k) 100 80 intensity 60 +1 40 20 B = 33 T 0 -100 -50 0 50 100 energy ( Γ ) photons with helicity +1 are absorbed transmitted radiation is highly polarized with helicity -1
MS with circularly polarized radiation Instrum. Meth. B 119 (1996) 438 MS Szymanski
MS with left circularly polarized radiation Magnetized iron foil B = - 33 T B B k k ∆ m = +1 ∆ m = -1 +1 -1 +1 +1 -1 -1 MS Szymanski K. Szymanski, NATO ARW’02 proceedings
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 !
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 detector detector drive source Very simple! Widely used to study magnetic properties of bulk materials. Unsufficient sensitivity (30 nm) to study nanostructures Mos info
Conversion Electron Mössbauer Spectroscopy Nuclear absorption Internal conversion Nuclear emission 14.4 keV 14.4 keV 14.4 keV 57 Fe 57 Fe 57 Fe + e- 0 0 0 Cems
Conversion Electron Mössbauer Spectroscopy Conversion electron Mössbauer spectroscopy is sensitive enough to probe one monolayer Cems
CEMS Example 1 Page 2491 20 ML Fe W(110) 2 nd monolayer from interface with a) 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 )
57 FeSi/Fe Multilayer system: Fe/ 57 FeSi/Fe Multilayer system: Fe/ epitaxial CsCl-FeSi on Fe 150°C MBE growth Au-capping Nat Fe (40 Å) Co-evaporated at a low rate 57 Fe Si (x Å) 50 50 (0.068 Å/s) Nat Fe (80 Å) MgO(001) FeSi structure
Conversion electron Mössbauer spectroscopy + 3/2, -3/2 + 1/2, -1/2 Quadrupole splitting + 1/2, -1/2 α -Fe (22%) en strained B2-FeSi (78%) FeSi Cems
Strain relaxation in CsCl- -FeSi FeSi Strain relaxation in CsCl B. Croonenborghs et al. , Appl. Phys. Lett. 85 (2004) 200 X-ray diffraction strain
Conversion electron Mössbauer spectroscopy Epitaxially grown FePt L1 0 B k - 3/2 - 1/2 Unpolarized source : + 1/2 + 3/2 ∆ m = -1 +1 -1 +1 1 1 1 1 0 0 + - + - = = = = = = m m m m m m ∆ ∆ ∆ ∆ ∆ ∆ + 1/2 - 1/2 B = -28 T L10 FePt
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