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Polarised neutron reflectivity Frédéric OTT Laboratoire Léon Brillouin fott@cea.fr
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ECNS’2003 Introductory Course Outline Reflectivity General principles Polarised neutrons Instrumentation Examples Superlattices Non colinear magnetism GMR Interface magnetism Resources
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ECNS’2003 Introductory Course Specular reflectivity geometry x z k i i r k r q
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ECNS’2003 Introductory Course Reflectivity measurements 2 ways of varying the scattering wave-vector Angular scan – 2 Time-of-flight k 0 q 1 q 2 k 1 q 1 q 2 k 2
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ECNS’2003 Introductory Course Neutron-matter interaction Optical approximation. Interaction neutron-nucleus Isotropic and ponctual Zeeman interaction Neutron spin – magnetic field Neutron-nucleusNeutron-magnetic field Neutron-magnetisation
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ECNS’2003 Introductory Course Limitation : planar magnetisation M // In the Born approximation : It can be shown that the magnetic interaction is sensitive only to the component of the magnetisation perpendicular to the scattering wave-vector Other approach The neutron spin interacts with B : For continuous thin films :
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ECNS’2003 Introductory Course Derivation of the reflectivity Schrödinger Helmoltz eq. Neutrons Eigenstates Interaction Propagation eq. Matrix formalism Continuity conditions
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ECNS’2003 Introductory Course Optical index The optical index is defined as Snell’s law : ii tr
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ECNS’2003 Introductory Course Some values www.neutron.anl.gov
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ECNS’2003 Introductory Course Reflection on a substrate Vacuum « 0 » Substrate « s » Z = 0 Continuity conditions z 1 r t
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ECNS’2003 Introductory Course Case of a non magnetic substrate
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ECNS’2003 Introductory Course Reflection on a thin film deposited on a substrate (non magnetic case) d
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ECNS’2003 Introductory Course Example Cu(500 Å)/Cr(90 Å) on silicon 2 /590 2 /90
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ECNS’2003 Introductory Course The neutron spin 1/2 particle ( associated magnetic moment µ n Interacts with the magnetic fields B ( aligned along z ) : Neutron in an eigenstate ( ) : stays in this state Quantified neutron along (Ox) ( ) : precession around B z
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ECNS’2003 Introductory Course Comparison nuclear and magnetic neutron scattering lengths.
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ECNS’2003 Introductory Course Polarised neutron reflectivity It is possible to polarise neutrons Manipulate the polarisation : neutron « flipper » Guide field Precession region spin up spindown H beam Guide field
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ECNS’2003 Introductory Course Experimental set-up Guide field polariserflippersampleanalyserdetectorflipper B + - + - + - - + M 4 cross-sections
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ECNS’2003 Introductory Course M // B Sample transfert matrix B + + - - M
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ECNS’2003 Introductory Course B perpendicular to the layer No magnetic contrast B + + - - M
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ECNS’2003 Introductory Course M makes an angle with B B + + - - M Sample transfert matrix
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ECNS’2003 Introductory Course Magnetic domains Neutron coherent illumination N vs magnetic domains sizes M If N < M then (R + + R - ) If N > M then no magnetic contrast B + + - -
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ECNS’2003 Introductory Course Field B parallel to the magnetisation
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ECNS’2003 Introductory Course Fe thin film (30 nm) on a saphire substrate M // B
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ECNS’2003 Introductory Course Spin-flip signal (M perp. B)
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ECNS’2003 Introductory Course Fe thin film (30 nm) on a saphire substrate M perpendicular to B
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ECNS’2003 Introductory Course Reflectivity geometry b 1 b 2 b 3 b 4 M 1 M 4 M 3 M 2 Substrate Incident beam
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ECNS’2003 Introductory Course Roughness effects Roughness at the atomic level : interdiffusion between the thin films, < 100 nm. Intermediate roughness ( de 0.1 µm à 50 µm). A large scale roughness ( > 50 µm).
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ECNS’2003 Introductory Course Roughness effects
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ECNS’2003 Introductory Course Resolution effects Wavelength resolution Graphite monochromator : Multilayer monochromator : (not adjustable) Chopper (ToF) : adjustable Angular resolution Defined by the slits sizes
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ECNS’2003 Introductory Course Resolution effects The resolution must be adjusted to be compatible with the studied sample Ni(10nm) on Silicon Practical limit
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ECNS’2003 Introductory Course 2-axis spectrometer polariserflipper1 2 analysers detector Collimation slits sample White beam 1m Graphite monochromator 2
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ECNS’2003 Introductory Course Upgraded 2-axis spectrometer
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ECNS’2003 Introductory Course ToF reflectometer : EROS
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ECNS’2003 Introductory Course PNR range of studies Multilayers Superconductors Non colinear magnetism Interface magnetism
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ECNS’2003 Introductory Course Polarised Neutron Reflectivity Allows the study of the magnetic configuration of a multilayer system: access to the magnetisation amplitude and direction in each layer. Determination of in-depth magnetic profiles Absolute measurement of the magnetic moment in µ B per f.u. (sum of the spin and orbital moment) But sensitivity only to the in-plane moment. Resolution of the order of 0.1µB (better on simple systems) No sensitivity to the substrate para/dia-magnetism. No absorption, no phenomenological parameter, absolute normalisation.
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ECNS’2003 Introductory Course Magnetic coupling FERRO ANTI - FERRO Non colinéaires J 1 > 0 J 1 < 0 J 1 > 0 et J 2 < 0
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ECNS’2003 Introductory Course PNR on super-lattices Adapted from H. Zabel
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ECNS’2003 Introductory Course Modulated structures Example of a modulation of period 10 nm in a layer of thickness 200nm in YBCO/STO Index variation of 2% only : Difference of density stœchiométrie variation Magnetisation modulation de l’aimantation YBCO Y: 10% Ba: 12% Cu: 30% O: 49%
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ECNS’2003 Introductory Course Example: magnetisation modulation La 0.3 Sr 0.7 MnO 3 film
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ECNS’2003 Introductory Course Exchange coupling in super-lattices [GaMnAs/GaAs] Super-lattice (GaMnAs) m /(GaAs) n where 8<m<16 et 4<n<8 Mn doping 6-7% Magnetisation of 0.03 T (27kA/m) No antiferromagnetic coupling is observed.
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ECNS’2003 Introductory Course Magnetic ordering in multilayers [ Fe/Si ] n K. Fronc (Polish. Acad. Sc.) GaAs//[Fe(2.4nm)Si(1.2nm)] n magnetic AF order at 300K non collinear coupling at 200K @ 7K; 20mT@ 200K; 20mT @ 300K; 2.8mT
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ECNS’2003 Introductory Course Evolution of the magnetic coupling as a function of the magnetic field AFM component disappears with the applied field @1.5 T (for 10K)
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ECNS’2003 Introductory Course Spin-valves structures E. Kentzinger, S. Nerger, U. Rücker, J. Voigt, O.H. Seeck, Th. Brückel (Forschungszentrum Jülich, Germany) More academic structure AsGa Ag FeCo Au Mn
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ECNS’2003 Introductory Course In a saturating field of 0.5T Fe 0.5 Co 0.5 /Mn (8A°)/ Fe 0.5 Co 0.5 Moment of 2.4 µB/atom in Fe 0.5 Co 0.5 Moment in manganese of 0.8µ B /atom! Theoretically predicted in FeCo alloys (not observed in Fe alone) E. Kentzinger et al., Physica B 276-278 (2000) S. Nerger et al., Physica B, to be published.
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ECNS’2003 Introductory Course Measurement in low field Quadratic coupling B = 1.2mT FeCo1FeCo2 Mn
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ECNS’2003 Introductory Course GMR optimisation Typical GMR structure SiO 2 // Ta/ NiFe/ CoFe/ Cu/ CoFe/ MnPt/ Ta Aim to to optimize GMR sensors used in high density tape recording PNR magnetometry characterize the system magnetically in a saturating field (thickness and amplitudes of the magnetic moments). sweep the field H or the temperature T but restrict the measurement to a few points of the reflectivity curve Adjust these points by letting vary only the amplitude and direction of the magnetic moments in the multilayer model.
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ECNS’2003 Introductory Course Hysteresis cycle A B C D E F G The easy axis of the AF layer is perpendicular to the easy axis of the free layer
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ECNS’2003 Introductory Course PNR magnetometry Magnetisation of the different layers as a function of the applied field D (+1mT) E (1.5mT) G (6mT) A (6mT) B (0.5mT) C (-4mT) CoFe NiFe CoFe MnPt Cu
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ECNS’2003 Introductory Course Spin injection materials Magnetite (DRECAM/SPCSI, J.B. Moussy et al) Fabrication of all oxide magnetic junctions Combination of Al 2 O 3, Fe 2 O 3, Fe 3 O 4 layers Magnetite Fe 3 O 4 is a potential candidate as spin-injector material Typical structure : A 2 O 3 //Fe 2 O 3 /Fe 3 O 4 /Al 2 O 3 /Fe 3 O 4 BUT often a partial or total transformation of the Fe 2 O 3 into Fe 3 O 4 occurs (not visible during the deposition process using XPS or RHEED) Collaboration : P. Bayle-Guillemaud, P. Warin, DRFMC-SP2M, CEA-Grenoble HRTEM [111] [1-10] [11-2] -Al 2 O 3 (0001) -Fe 2 O 3 (0001) Fe 3 O 4 (111) 2.5 nm
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ECNS’2003 Introductory Course PNR characterisation Neutron reflectivity allows to very quickly check the presence or absence of Fe 2 O 3 layer (by using the magnetic contrast) Information on the magnetic moments : the transformed layer has a reduced magnetic moment Al 2 O 3 M = 2.5 µB/f.u. M = 1.75 µB/f.u. The transformation process is not yet understood
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ECNS’2003 Introductory Course La 0.7 Sr 0.3 MnO 3 : Hysteresis cycle LSMO film (40 nm) deposited on a SrTiO 3 substrate
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ECNS’2003 Introductory Course Reflectivity measurements. LSMO on MgO (68nm)LSMO on STO (56nm)
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ECNS’2003 Introductory Course Fitting procedure Perfect system LSMO STO ou MgO More realistic model M3M3 M2M2 M1M1
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ECNS’2003 Introductory Course Magnetic profile in a LSMO on STO
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ECNS’2003 Introductory Course LSMO film (16nm) sur STO Spin asymmetry
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ECNS’2003 Introductory Course Magnetisation variations (LSMO(16nm)/ STO)
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ECNS’2003 Introductory Course Conclusion Applications Multilayers Non colinear magnetism Interface magnetism Determination of magnetic profiles with a depth resolution: access to the magnetisation amplitude and direction in each layer. Determination of in-depth magnetic profiles Absolute measurement of the magnetic moment in µ B per f.u. (sum of the spin S and orbital moment L ) But sensitivity only to the in-plane moment. Resolution of the order of 0.1µB (better on simple systems) No sensitivity to the substrate para/dia-magnetism. No absorption, no phenomenological parameter, absolute normalisation. “low” flux.
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ECNS’2003 Introductory Course Bibliography A few recent general references H. Zabel et al, Physica B 276-278 (2000) 17-21. « Neutron Reflectometry on magnetic thin films » H. Zabel et al, J. Phys.: Condens. Matter 15 (2003) S505-S517. Polarized neutron reflectivity and scattering studies of magnetic heterostructures. G.P. Felcher, J. Applied Physics 87 (2000) 5431 Neutron reflectometry as a tool to study magnetism G. Fragneto-Cusani, J. Phys. : Condens. Matter 13 (2001) 4973- 4989 Other ressources www-llb.cea.fr/prism/PRISM.html http://www.neutron.anl.gov/software.html All existing reflectometers : http://www.studsvik.uu.se/research/NR/reflect.htm
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