Polarised neutron reflectivity Frédéric OTT Laboratoire Léon Brillouin

ECNS’2003 Introductory Course Outline Reflectivity General principles Polarised neutrons Instrumentation Examples Superlattices Non colinear magnetism GMR Interface magnetism Resources

ECNS’2003 Introductory Course Specular reflectivity geometry x z k i  i  r k r q

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

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

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 :

ECNS’2003 Introductory Course Derivation of the reflectivity Schrödinger Helmoltz eq. Neutrons Eigenstates Interaction Propagation eq. Matrix formalism Continuity conditions

ECNS’2003 Introductory Course Optical index The optical index is defined as Snell’s law : ii  tr

ECNS’2003 Introductory Course Some values

ECNS’2003 Introductory Course Reflection on a substrate Vacuum « 0 » Substrate « s » Z = 0 Continuity conditions z 1 r t

ECNS’2003 Introductory Course Case of a non magnetic substrate

ECNS’2003 Introductory Course Reflection on a thin film deposited on a substrate (non magnetic case) d

ECNS’2003 Introductory Course Example Cu(500 Å)/Cr(90 Å) on silicon 2  /590 2  /90

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

ECNS’2003 Introductory Course Comparison nuclear and magnetic neutron scattering lengths.

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

ECNS’2003 Introductory Course Experimental set-up Guide field polariserflippersampleanalyserdetectorflipper B M 4 cross-sections

ECNS’2003 Introductory Course M // B Sample transfert matrix B M

ECNS’2003 Introductory Course B perpendicular to the layer No magnetic contrast B M

ECNS’2003 Introductory Course M makes an angle with B B M Sample transfert matrix

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

ECNS’2003 Introductory Course Field B parallel to the magnetisation

ECNS’2003 Introductory Course Fe thin film (30 nm) on a saphire substrate M // B

ECNS’2003 Introductory Course Spin-flip signal (M perp. B)

ECNS’2003 Introductory Course Fe thin film (30 nm) on a saphire substrate M perpendicular to B

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

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).

ECNS’2003 Introductory Course Roughness effects

ECNS’2003 Introductory Course Resolution effects Wavelength resolution Graphite monochromator : Multilayer monochromator : (not adjustable) Chopper (ToF) : adjustable Angular resolution Defined by the slits sizes

ECNS’2003 Introductory Course Resolution effects The resolution must be adjusted to be compatible with the studied sample Ni(10nm) on Silicon Practical limit

ECNS’2003 Introductory Course 2-axis spectrometer polariserflipper1 2 analysers detector Collimation slits sample White beam 1m Graphite monochromator 2 

ECNS’2003 Introductory Course Upgraded 2-axis spectrometer

ECNS’2003 Introductory Course ToF reflectometer : EROS

ECNS’2003 Introductory Course PNR range of studies Multilayers Superconductors Non colinear magnetism Interface magnetism

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.

ECNS’2003 Introductory Course Magnetic coupling FERRO ANTI - FERRO Non colinéaires J 1 > 0 J 1 < 0 J 1 > 0 et J 2 < 0

ECNS’2003 Introductory Course PNR on super-lattices Adapted from H. Zabel

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%

ECNS’2003 Introductory Course Example: magnetisation modulation La 0.3 Sr 0.7 MnO 3 film

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.

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 7K; 200K; 300K; 2.8mT

ECNS’2003 Introductory Course Evolution of the magnetic coupling as a function of the magnetic field AFM component disappears with the applied T (for 10K)

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

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 (2000) S. Nerger et al., Physica B, to be published.

ECNS’2003 Introductory Course Measurement in low field Quadratic coupling B = 1.2mT FeCo1FeCo2 Mn

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.

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

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

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

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

ECNS’2003 Introductory Course La 0.7 Sr 0.3 MnO 3 : Hysteresis cycle LSMO film (40 nm) deposited on a SrTiO 3 substrate

ECNS’2003 Introductory Course Reflectivity measurements. LSMO on MgO (68nm)LSMO on STO (56nm)

ECNS’2003 Introductory Course Fitting procedure Perfect system LSMO STO ou MgO More realistic model M3M3 M2M2 M1M1

ECNS’2003 Introductory Course Magnetic profile in a LSMO on STO

ECNS’2003 Introductory Course LSMO film (16nm) sur STO Spin asymmetry

ECNS’2003 Introductory Course Magnetisation variations (LSMO(16nm)/ STO)

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.

ECNS’2003 Introductory Course Bibliography A few recent general references H. Zabel et al, Physica B (2000) « 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) Other ressources www-llb.cea.fr/prism/PRISM.html All existing reflectometers :