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[en] Full text: Magnetic rare-earth (R.E.)/transition metal (T.M.) multilayered films have been extensively investigated during the last decade because they may exhibit strong perpendicular magnetic anisotropy, a large Kerr rotation angle and a high coercive field. The origin of these unusual properties is attributed to the artificial nanostructure that enhances the phenomena previously observed in rare earth-transition metal amorphous alloys. Our approach is based on the coupling of tomographic atom probe (T.A.P.) analysis with magnetometery measurements, in order to correlate the structural properties of the interfaces with the magnetic anisotropy properties. The samples are prepared by evaporation under ultrahigh vacuum, and studied by various investigation techniques: Moessbauer spectrometry, x-ray diffraction and reflectometry, SQUID magnetometery and T.A.P. The high resolution of the tomographic atom probe enables to get a 3D reconstruction of the analyzed material, and therefore to determine a concentration profile through the multilayer. The goal is then to explain the variation of the perpendicular anisotropy, as a function of preparation conditions, via the tomographic atom probe analysis. Refs. 3 (author)
[en] We describe the magnetism of CoPt clusters upon annealing-driven transition to chemically ordered L10-like phase. In one hand, X-ray magnetic circular dichroism (XMCD) investigations at each L2 3 edge, revealed a significant increase of both spin and orbital of Co and Pt magnetic moments after thermal ordering transition. In the other hand, reversely to the bulk alloys, a limited Magnetic Anisotropy Energy (MAE) enhancement has been obtained from SQUID magnetometry measurements on the same chemically ordered CoPt nanomagnets samples. In this paper, we relate such magnetic behaviour to element-specific dependence of the local atomic relaxations in nanoalloys, described from extended x-ray absorption fine structure (EXAFS) experiments
[en] In the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context 20 years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the low-energy cluster beam deposition (LECBD) technique. In this paper, we focus our attention on the typical case of Co clusters embedded in various matrices to study interface magnetic anisotropy and magnetic interactions as a function of volume concentrations, and on still current and perspectives through two examples of binary metallic 3d-5d TM (namely CoPt and FeAu) cluster assemblies to illustrate size-related and nanoalloy phenomena on magnetic properties in well-defined mass-selected clusters. The structural and magnetic properties of these cluster assemblies were investigated using various experimental techniques that include high-resolution transmission electron microscopy (HRTEM), superconducting quantum interference device (SQUID) magnetometry, and synchrotron techniques such as extended X-ray absorption fine structure (EXAFS) and X-ray magnetic circular dichroism (XMCD). Depending on the chemical nature of both NPs and matrix, we observe different magnetic responses compared to their bulk counterparts. In particular, we show how finite size effects (size reduction) enhance their magnetic moment and how specific relaxation in nanoalloys can impact their magnetic anisotropy.
[en] Starting from the theoretical results established in Tournus and Bonet (2010 ) to describe ZFC/FC (zero-field cooled/field cooled) susceptibility curves, we examine the limitations of the widely used two states model (or abrupt transition model) where the magnetic particles are supposed to be either fully blocked or fully superparamagnetic. This crude model appears to be an excellent approximation in most practical cases, i.e. for particle assembly with broad enough size distributions. We improve the usual model by taking into account the temperature sweep existing in experimental measurements. We also discuss a common error made in the use of the two states model. We then investigate the relation between the ZFC peak temperature and the particle anisotropy constant, and underline the strong impact of the size dispersion. Other useful properties of ZFC/FC curves are discussed, such as invariance properties, the reversibility of the FC curve and the link between the susceptibility curves and the magnetic anisotropy distribution. All these considerations lay solid bases for an accurate analysis of experimental magnetic measurements. - Research Highlights: →We apply the improved progressive crossover model to realistic particle assemblies. →The limitations of the usual abrupt transition model are examined. →This model is improved by taking into account the experimental temperature sweep. →A common error made in the simulation of ZFC/FC curves is discussed. →The link between the ZFC peak and the particle properties is investigated.
[en] The structural and magnetic properties of (Fe tFenm/DytDynm) multilayers deposited at 320K by thermal evaporation in ultra-high vacuum (UHV) have been investigated by conversion electron 57Fe Mossbauer spectrometry (CEMS) at 300K. These measurements enabled to determine in which conditions the perpendicular magnetic anisotropy (PMA) is optimised as a function of the layers thickness. The Fe thickness, tFe, is varied between 1 and 4nm, and the Dy thickness, tDy, equals 1 or 2nm. The Mossbauer spectrometry measurements reveal that the PMA is the largest when tFe is the smallest, i.e., when the multilayers are structurally close to an Fe-Dy amorphous alloy with modulation of composition
[en] We report the synthesis and characterization of CoPt nanoparticles, using X-ray magnetic circular dichroism (XMCD) at the Co L2,3 edges. Clusters are produced in ultra-high vacuum conditions, following a physical route, and embedded in non-metallic matrices: MgO and amorphous carbon (a-C). In MgO, Co atoms are partially oxidized, which goes with a μL/μS enhancement. On the contrary, a-C appears as a very suitable matrix. In particular, annealing of CoPt cluster embedded in a-C is able to promote L 10 chemical order, without alteration of the sample. This transformation, which has been directly evidenced by transmission electron microscopy observations, is accompanied by a striking augmentation of μS, μL and the μL/μS ratio of Co. The presence of Pt leads to an enhanced Co magnetic moment, as compared to Co bulk, even for the chemically disordered alloy. Moreover, the high value of 1.91μB/at. measured for μS is unusual for Co and must be a signature of chemical order in CoPt alloy nanoparticles
[en] Highly uniform (8 nm) bare maghemite nanoparticles were pressed into a disc with a volume fraction close to the characteristic filling factor of random-close-packed ensembles of spherical particles. We review the ideal superspin glass behaviour exhibited by this material, including an onset of the absorption component of the ac susceptibility at the freezing transition as sharp (in a normalized temperature scale) as those observed in atomic spin glasses, narrow memory dips in the zero-field cooled magnetization, and a spin-glass characteristic field-dependence of the susceptibility. Critical exponents were extracted from static and dynamic scaling analysis
[en] CoPt nanoparticles are widely studied, in particular for their potentially very high magnetic anisotropy. However, their magnetic properties can differ from the bulk ones and they are expected to vary with the particle size. In this paper, we report the synthesis and characterization of well-defined CoPt nanoparticle samples produced in ultrahigh vacuum conditions following a physical route: the mass-selected low energy cluster beam deposition technique. This approach relies on an electrostatic deviation of ionized clusters which allows us to easily adjust the particle size, independently from the deposited equivalent thickness (i.e. the surface or volume particle density in a sample). Diluted samples made of CoPt particles, with different diameters, embedded in amorphous carbon are studied by transmission electron microscopy and superconducting interference device magnetometry, which gives access to the magnetic anisotropy energy distribution. We then compare the magnetic properties of two different particle sizes. The results are found to be consistent with an anisotropy constant (including its distribution) which does not evolve with the particle size in the range considered. - Highlights: → Samples of mass-selected CoPt nanoparticles are synthesized by an original physical method. → The magnetic properties of two different particle sizes are compared. → The anisotropy constant (including its dispersion) does not evolve in the range considered. → These results illustrate some invariance properties of ZFC curves.
[en] (Fe 3 nm/Dy 2 nm) multilayers deposited at 300 and 600 K have been investigated by SQUID magnetometry. These samples were deposited by thermal evaporation under ultra-high vacuum on (1 1 1) Si substrates. Their magnetic hysteresis loops show that the magnetic properties strongly depend on the deposition temperature, since the perpendicular magnetic anisotropy is much higher for the sample prepared at 600 K. In order to understand this phenomenon, a fine investigation of the structure and chemistry of the interfaces has to be carried out. As a first step, a (Dy/Fe) multilayer has been deposited on a preformed tungsten tip and analysed by tomographic atom probe (TAP). A three-dimensional reconstruction of the layers has been obtained that enabled the interfaces between iron and dysprosium to be observed at the atomic scale