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${\text{CoFe}}_{2}{\text{O}}_{4}/{\text{Fe}}_{3}{\text{O}}_{4}$ bilayers represent an unusual exchange-coupled system whose properties are due to the nature of the oxide-oxide superexchange interactions at the interface. In this work, we analyze the magnetization reversal behavior and magnetic interface structure using a variety of advanced techniques. Low-temperature magnetization, magnetotransport, and polarized neutron reflectivity measurements suggest that the magnetization configuration at the bilayer interface mimics that of an antiphase boundary found in spinel ferrite thin films, thus explaining the unique nature of the magnetic behavior.

International audience; An experimental and numerical study of the magnetization in (Fe 3 nm/Dy 2 nm) multilayers is presented. The samples were thermally evaporated under ultra-high vacuum at two different substrate temperatures, 320 and 570 K. In order to get the magnetization depth profile of these transition metal/ rare earth (TM/RE) multilayers, a fine investigation of the structural, chemical, and magnetic properties was carried out. The samples were studied by X-ray reflectivity (XRR), high resolution transmission electron microscopy (HRTEM), conversion electron Mö ssbauer spectrometry (CEMS), SQUID magneto-metry and polarized neutron reflectivity (PNR). Magnetization profiles were obtained by Monte Carlo simulations to support the PNR fits. The key role of the crystalline structure is emphasized by magnetic depth profile measurements performed using polarized neutron reflectometry. The antiparallel configuration of Fe and Dy layers' magnetizations was evidenced, as well as the perpendicular magnetic anisotropy (PMA), especially in the case of the sample prepared at 570 K.

We present an investigation of the structural and magnetic correlations during the remagnetization process of a gradient nanocrystalline multilayer with uniform uniaxial magnetic anisotropy by specular reflectometry and off-specular scattering of polarized neutrons with polarization analysis. The scattering data were analyzed within the distorted wave Born approximation (DWBA) including interfacial roughness correlations and lateral correlations of the spin misalignment. The magnetization reversal proceeds sequentially from the bottommost and thinnest ferromagnetic layers to the topmost and thickest ones. It is quantitatively explained within the random anisotropy model including dominant uniform uniaxial anisotropy. Two types of lateral spin misalignment were deduced, random and not random: The first one in all layers due to the random orientation of the grains, the second one in the layers with nonreversed magnetizations only and due to the applied field acting against the uniaxial anisotropy.

The alpha-beta magneto-structural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 K to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an alpha-beta phase coexistence and, more important, for the stabilization of the ferromagnetic alpha-phase at higher temperature than in bulk. We explain the premature appearance of the beta-phase at 275 K and the persistence of the ferromagnetic alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature., 4 pages, 3 figures, accepted for publication in Physical Review Letters

We have combined neutron scattering and piezoresponse force microscopy to study the relation between the exchange bias observed in CoFeB/BiFeO3 heterostructures and the multiferroic domain structure of the BiFeO3 films. We show that the exchange field scales with the inverse of the ferroelectric and antiferromagnetic domain size, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with the CoFeB. In view of these results we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3., Comment: submitted to Phys. Rev. Lett

We present two new effects in polarized neutron reflectivity. We show that we have a nonsymmetric spin-flip signal in reflectivity measurements on magnetic films when the external field is not negligible. This phenomenon is due to different Larmor precessions for the two spin states and has to be taken into account in some experiments. The second effect is still not understood, but we present results indicating that the specular reflection on a nonmagnetic surface can induce a neutron beam depolarization or rotation.

We consider the effect of an electrical current on a magnetic domain wall (DW) in conducting ferromagnets. When the current lines are at normal incidence onto the wall, two relevant physical effects, based on the Hall voltage and direct spin transfer, induce a pressure which is able to move DWs. When the current is not at normal incidence, we show that the Hall effect can generate a local charge on the wall which is subjected to a force from the electric field in the sample. The resulting pressure is quadratic in current, thus independent of its sign, and can dominate at high current densities and in materials with large resistivity and Hall angle. Hence, the effect is particularly relevant for magnetic semiconductors.

The magnetic phase separation in single crystals of the ${\mathrm{P}\mathrm{r}}_{0.67}{\mathrm{C}\mathrm{a}}_{0.33}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ manganites is studied using polarized small angle neutron scattering. The measured spectra give a fractal dimension consistent with a configuration in ferromagnetic filaments of nanometric diameter. We argue here that localized charge carriers hop in a random walk fashion mediating a ferromagnetic "hopping exchange" which coexists with superexchange to create the filamentary phase separation. The arguments for this physical picture are validated by Monte Carlo simulations, where magnetism and transport are treated in a self-consistent manner.

Advanced nano-technology is exploited to produce a set of Fresnel neutron zone plates of large aperture and with similar and reproducible characteristics. These zone plates have been employed to test the performance of a stack of them. It is found that the stack of many zone plates can be effective in handling a neutron beam.

We present the operation of a matrix of Fresnel zone plates consisting of 900 individual zone plates. The individual efficiency of the zone plate lenses is close to 30% for λ =13.7 A. We discuss the use of such a matrix as an optical element for performing high-resolution neutron imaging (at a sub 50 μm resolution).

Our objective is to characterize surfaces and interfaces at the nanometric scale. This corresponds to very small volumes of matter of the order of a few micrograms. Thus, it is necessary to develop surface scattering techniques. The use of neutron reflection at grazing incidence increases the neutron interaction with the sample surface and makes such experiments feasible. It is possible to distinguish three grazing incidence scattering geometries (see Fig. 1): specular reflection, scattering in the incidence plane (off-specular scattering) and scattering perpendicular to the incidence plane. These different scattering geometries allow to probe different length scales and directions in the sample surface. Specular reflectivity (red line) allows to probe the structure along the depth in the film (3 nm ≤ ≤ 100 nm). Off-specular scattering measurements (blue plane) allows to probe in-plane surface features at a micrometric scale (600 nm ≤ ≤ 60 m). GISANS Grazing Incidence SANS (green plane) allows to probe in-plane surface features in the range 3 nm ≤ ≤ 100 nm. These different scattering geometries allow to probe a very wide range of length-scales , ranging from a few nm up to several m. Structures perpendicular and in-plane can be probed. In this chapter we will restrict ourselves to specular reflectivity. We will present the principle of the technique and some examples of measurements on real multilayer systems.

Magnetic interactions in aggregates of polyol-synthesized cobalt nanowires are studied by δM plots. Negative δM values are obtained as expected for magnetostatic coupling between physically isolated nanowires, without exchange interactions. Very weak interactions (down to δM = −0.05) are achieved by alignment and dispersion into polymeric matrices. We attribute this to the fact that during the fabrication of the composite materials, the nanowires reorganize so as to minimize the dipolar interactions. Since the δM-plots are sensitive to the nature of the demagnetized state, it is shown by micromagnetic simulations that the ac demagnetized state provides the best choice of a starting point since it ensures that macroscopic configurations consisting of single domain states of each wire are compared. The low interactions observed in these materials suggest that these composite materials are candidates for rare-earth free permanent magnets since demagnetization phenomena are minimized.

The maximum magnetic performance which can be expected from elongated single domain particle based permanent magnets is assessed. The results are derived using micromagnetic calculations to model the behavior of large bundles of aligned nanowires. In particular, we discuss the cases of Co and Fe nanowires and their packing fraction dependence of coercivity, which is the main limiting factor. We show that it is, in principle, feasible to achieve BHmax values close to 300 kJ/m3 in Co nanowires with a packing fraction p = 0.7 and close to 400 kJ/m3 at p = 0.85. The packing fraction limitations are essentially non-existing due to the intrinsic magnetocrystalline anisotropy of Co. On the other hand, if a low anisotropy material such as Fe could be produced in the form of fine, well crystallized wires it could yield a BHmax close to 200 kJ/m3 at an optimum p = 0.7. As the performance of iron nanowires is solely based on shape anisotropy it becomes coercivity limited above p = 0.35.

Experimental results on neutron reflection from a magnetic film placed in an oscillating magnetic field are reported. We found evidence for neutron spin resonance in the film and observed a spatial beam-splitting. The beam-splitting finds its origin in the exchange of an energy quantum ω between the oscillating field and the neutron.

An experimental method is presented which allows to probe the shape of a magnetic domain wall. The method is based on the precession of the spin of neutrons traveling across the domain wall structure. This technique is experimentally demonstrated on a planar (half) Bloch wall created within the thickness of a soft magnetic film with in-plane crossed anisotropies.

We report the growth and magnetic properties of AFe2O4/Alq3/Co heterostructures (AFe, Co) containing a continuous Alq3 thin layer with a low roughness, down to a few nanometers, and we demonstrate the magnetic decoupling between AFe2O4 and Co electrodes showing the interest of these systems for spintronics devices.

We present the new polarized reflectometer for the investigation of surface magnetism, which has been built at the Laboratoire Leon Brillouin. This polarized neutron reflectometer with polarization analysis has been optimized for the study of small magnetic samples (1 cm2 range). We describe the new multilayer monochromator which has permitted us to obtain a wider wavelength distribution (∼4%) and the focusing neutron guide which has allowed the focalization of a 100 mm high beam onto a region 15 mm high. This new optical setup has allowed a large neutron flux increase, which is now of 2×105 n/cm2 s.

Planar waveguides can transform a conventional neutron beam into a very narrow but slightly divergent microbeam. Such microbeams could be used for the investigation of nanostructures with a high spatial resolution. We have investigated the tri-layer magnetic structure Fe/Co/Fe which transforms an unpolarized incident beam into a polarized microbeam. The polarized neutron reflectometer NREX with a horizontal sample plane was used for the waveguide characterization. Some practical aspects of microbeam extraction and polarization are discussed.

We have used polarized neutron reflectivity, X-ray diffraction, X-ray reflectivity and magneto-optical Kerr effect in polar configuration to study the properties of ultrathin Pt/Co/Pt films. Structures consisting of a 5-nm thick Pt buffer, 3-nm thick Co layer and 5-nm thick Pt cover layer were deposited onto (0001)-oriented Al2O3 substrate by the molecular beam epitaxy (MBE) method. Irreversible modifications of film properties, resulting from its illumination by single femtosecond laser pulses, of duration of 40 fs and wavelength of 800 nm, were observed and analyzed. As prepared films exhibited magnetization in-plane, but after laser irradiation, the direction of magnetization was rotated to out-of-plane state. Formation of Co–Pt alloy phase caused by quasi-uniform film irradiation was demonstrated by the results of X-ray and neutron scattering measurements. Moreover, polarized neutron and X-ray reflectivity data showed that after illumination Co was distributed mostly in the area of nominal Co ...

International audience; A new charge and spin density model and the corresponding refinement software were recently developed to combine X- ray and polarized neutron diffraction experiments [1]. This joint refinement procedure allows getting access to both charge and spin densitiy distributions by refining both spin up and down parameters for magnetic atoms. The paper is focused on the refinement procedure and its application to the case of an end-to-end azido double bridged copper(II) complex. The results of this joint refinement of X-ray and polarized neutron diffraction data are presented.

This thesis focuses on the development of nanocomposites made from anisotropic cobalt nanowires / polymer for the manufacture of permanent magnets which do not contain any rare earth and the optimization of the magnetic properties of these composite materials.The preparation of single-domain cobalt nanowires (R ~ 6-10 nm and L ~ 250-350 nm) was performed by conventional thermal route and by microwave route. The films of composite aligned Co nanowires/polymer have been elaborated with very good magnetic properties (μ0Hc = 0.75T, Mr / Ms = 0.92). The (BH)max is 160 kJ/m3 which is in the range of SmCo magnets (~ 120-200 BHmax kJ/m3). The techniques of small angles neutron and X-ray small angle scattering (SANS and SAXS) were used for the characterization of anisotropic systems and dispersions. The wires in chloroform are better dispersed compared to other solvents and form aggregates smaller. For the films of composite, the aggregation of the nanowires is relatively denser in polystyrene than in poly (vinyl pyrrolidone). The quality of alignment is proportional to the amplitude of the applied field, even for very high fields. However, a better alignment does not automatically lead to a better coercivity. The interactions between nanowires were characterized by Henkel plots. The ΔM values are pretty low (ΔM, Cette thèse porte sur l’élaboration de nanocomposites anisotropes à base de nanofils de cobalt/polymères pour la fabrication d’aimants permanents qui ne contiennent pas de terres rares et l’optimisation des propriétés magnétiques de ces matériaux composites. La préparation de nanofils de cobalt mono-domaines (R~6-10 nm et L~250-350 nm) a été réalisée par voie thermique conventionnelle et par voie micro-onde. Des films de composites Co/polymère alignés ont été élaborés avec de très bonnes propriétés magnétiques (μ0Hc=0.75T, Mr/Ms=0.92). Le (BH)max est de 160 kJ/m3 qui est dans la gamme des aimants SmCo (BHmax~ 120-200 kJ/m3). Les techniques de diffusion de neutrons et de rayons-X aux petits angles (DNPA et DXPA) ont été utilisées pour la caractérisation des dispersions et des systèmes anisotropes. Les fils dans le chloroforme sont mieux dispersés par rapport aux autres solvants et forment des agrégats moins gros. Pour les films de composites, l’agrégation des nanofils est relativement plus dense dans le polystyrène que dans le poly(vinyl pyrrolidone). La qualité de l’alignement est proportionnelle à l’amplitude du champ appliqué même pour des champs très élevés. Cependant, un meilleur alignement ne conduit pas automatiquement à une meilleure coercivité. Les interactions entre des nanofils ont été caractérisées par Henkel plots. Les valeurs de ΔM sont faibles (ΔM

Magnetism of individual nano-objects is a very active research field thanks to the development of original synthesis routes and investigation tools. This thesis aims at probing the magnetism of nanowires synthesized via a pure chemical route, the polyol process. This process provides a large variety and an excellent crystallinity of the synthesized nanowires. This process allows to tune the diameter of the nanowires from 7nm to 20nm leading to coherent magnetization reversal in the nanowires. I show how the large shape and magnetocrystalline anisotropies provide large coercivities compared to magnetic nanowires synthesized via other routes. Furthermore, the oxidation of such objects has also been investigated. Magnetic measurements has revealed unsual temperature dependencies of both the coercive and exchange fields, emphasizing the role of the superparamagnetic fluctuations of the CoO antiferromagnetic grains in the Exchange Bias effect. Finally, this thesis also aims at developing Polarized Small Angle Neutron Scattering to probe magnetism in complex nano-objects. Such a technique has been ignored until now to study magnetic anisotropic nano-objects despite being well adapted. The key ingredient to carry through such a study is the perfect alignment of the nanowires. This is why, besides the nanowires synthesized via the polyol process, Polarized Small Angle neutron Scattering measurements have been performed on arrays of magnetic nanowires included in porous alumina membranes.; Le nanomagnétisme est actuellement un champ d'investigation très actif grâce aux développements de méthodes de synthèse et d'investigation originales. Cette thèse s'attache à sonder le magnétisme de nanofils magnétiques synthétisés par un procédé polyol. Ce procédé présente l'avantage de fournir un large éventail d'objets magnétiques anisotropes présentant une très bonne qualité cristalline. Les nanofils ainsi synthétisés présentent des diamètres variant de 7nm à 20nm, leur conférant un mode de renversement de l'aimantation cohérent. Cette thèse montre que les nanofils ainsi synthétisés ont des coercivités élevées comparées à celles de nanofils synthétisés par d'autres voies. Par ailleurs, les effets de l'oxydation de ces nanofils sur leurs propriétés magnétiques ont été étudiés. Les mesures magnétiques ont révélé une dépendance en température des champs d'échange et coercitif non reportée jusqu'à présent dans la littérature. Cela a permis de mettre en lumière le rôle prépondérant des fluctuations superparamagnétiques des grains antiferromagnétiques d'oxyde de cobalt dans le phénomène d'Exchange Bias. Enfin, cette thèse a aussi eu pour objectif de développer la technique de Diffusion de Neutrons Polarisés aux Petits Angles pour sonder le magnétisme de nanofils. Jusqu'à présent, cette technique a été surtout réservée à l'étude d'objets magnétiques isotropes. En effet, l'alignement des nanofils est crucial pour extraire des informations quantitatives d'une telle étude. C'est pour cela, qu'outre les nanofils synthétisés par procédé polyol, des nanofils inclus dans des matrices d'alumine poreuse ont aussi été étudiés par cette technique. Cette étude a ainsi montré la nécessité de prendre en compte le champ dipolaire- habituellement négligé- dans l'analyse des figures de diffusion.

Magnetism of individual nano-objects is a very active research field thanks to the development of original synthesis routes and investigation tools. This thesis aims at probing the magnetism of nanowires synthesized via a pure chemical route, the polyol process. This process provides a large variety and an excellent crystallinity of the synthesized nanowires. This process allows to tune the diameter of the nanowires from 7nm to 20nm leading to coherent magnetization reversal in the nanowires. I show how the large shape and magnetocrystalline anisotropies provide large coercivities compared to magnetic nanowires synthesized via other routes. Furthermore, the oxidation of such objects has also been investigated. Magnetic measurements has revealed unsual temperature dependencies of both the coercive and exchange fields, emphasizing the role of the superparamagnetic fluctuations of the CoO antiferromagnetic grains in the Exchange Bias effect. Finally, this thesis also aims at developing Polarized Small Angle Neutron Scattering to probe magnetism in complex nano-objects. Such a technique has been ignored until now to study magnetic anisotropic nano-objects despite being well adapted. The key ingredient to carry through such a study is the perfect alignment of the nanowires. This is why, besides the nanowires synthesized via the polyol process, Polarized Small Angle neutron Scattering measurements have been performed on arrays of magnetic nanowires included in porous alumina membranes.; Le nanomagnétisme est actuellement un champ d'investigation très actif grâce aux développements de méthodes de synthèse et d'investigation originales. Cette thèse s'attache à sonder le magnétisme de nanofils magnétiques synthétisés par un procédé polyol. Ce procédé présente l'avantage de fournir un large éventail d'objets magnétiques anisotropes présentant une très bonne qualité cristalline. Les nanofils ainsi synthétisés présentent des diamètres variant de 7nm à 20nm, leur conférant un mode de renversement de l'aimantation cohérent. Cette thèse montre que les nanofils ainsi synthétisés ont des coercivités élevées comparées à celles de nanofils synthétisés par d'autres voies. Par ailleurs, les effets de l'oxydation de ces nanofils sur leurs propriétés magnétiques ont été étudiés. Les mesures magnétiques ont révélé une dépendance en température des champs d'échange et coercitif non reportée jusqu'à présent dans la littérature. Cela a permis de mettre en lumière le rôle prépondérant des fluctuations superparamagnétiques des grains antiferromagnétiques d'oxyde de cobalt dans le phénomène d'Exchange Bias. Enfin, cette thèse a aussi eu pour objectif de développer la technique de Diffusion de Neutrons Polarisés aux Petits Angles pour sonder le magnétisme de nanofils. Jusqu'à présent, cette technique a été surtout réservée à l'étude d'objets magnétiques isotropes. En effet, l'alignement des nanofils est crucial pour extraire des informations quantitatives d'une telle étude. C'est pour cela, qu'outre les nanofils synthétisés par procédé polyol, des nanofils inclus dans des matrices d'alumine poreuse ont aussi été étudiés par cette technique. Cette étude a ainsi montré la nécessité de prendre en compte le champ dipolaire- habituellement négligé- dans l'analyse des figures de diffusion.