Fcc-Co Clusters in L10-FePt Matrix as Graded-Interface Magnetic Nanocomposite

Abstract

Hard magnetic nanocomposites, consisting of a fine mixture between a large magneto crystalline anisotropy phase and a large magnetization phase, are attractive materials both for integration of hardness-tuned magnets in microsystems and for building next-generation high performance permanent magnets for energy conversion technologies. Instead of relying on the hypothetical discovery of new hard magnetic compound, this approach is based on the mastering of the microstructure in a mixture of already known magnetic phases. Theoretical descriptions predicted a potential energy product of 1 MJ/m3, which is twice as large as the one of best Nd2Fe14B magnets produced today [1]. However, exploiting their full potential has appeared to be very challenging since the first reports on exchange spring structures in the 90’s. Indeed, calculations pointed out the absolute necessity to confine the softer phase in nano-sized grains, smaller than twice the domain wall width of the hard phase. In parallel, to maximize the magnetization, the hard phase volume fraction should be restricted to the minimum needed to develop enough coercivity. These requirements cannot be obtained with conventional process for mass scale material fabrication. In this context, investigations of model systems in films can give insights about the interplay between the nanoscale microstructure and the magnetic properties [2]. In particular, the degree of interdiffusion at the soft-hard interface is expected to significantly impact the magnetization reversal but is difficult to address experimentally.Here we focus on the impact of a graded soft-hard interface on magnetic properties. To do so, we recently prepared Co@FePt transition metal-based nanocomposite (NC) films from mass-selected low energy cluster beam deposition technique (MS-LECBD) of Co clusters, in situ embedded in FePt matrix independently produced by alternative electron gun evaporation on the same substrate [3,4,5]. The Co cluster inclusions can be selected in size prior to their deposition, between 1 and 10 nm. The cluster to matrix volume ratio is adjusted controlling the thickness of each Co, Fe, Pt individual layers. The as-prepared Co clusters have been observed by transmission electron microscopy (TEM) and found to crystallise in magnetically soft fcc phase. In this study, we focused on 8 nm Co clusters. The Co to FePt volume fraction was varied from 0 to 50%. Post-deposition annealing of the whole stack is a crucial step to drive the initial Fe and Pt multilayers to the high-magnetic anisotropy L10 phase. However, this inevitably comes with partial intermixing at the interface between the clusters and the matrix. We systematically compared the nanoclusters-based film with same stoichiometry and alloyed films (NF), obtained by sequential evaporation of Co, Fe, Pt thin and continuous layers.Local atomic structures have been thoroughly investigated using polarization-dependent hard x-ray absorption and element-specific spectroscopies (EXAFS, XLD). Transmission electron microscopy (TEM) equipped with energy-dispersive x-ray spectroscopy (EDX) was used to explore the local chemical profile of the composite films (figure 1). The macroscopic magnetic properties were measured in a Superconducting quantum interference device (SQUID) magnetometer together with local and element specific characterisations using circular dichroism (XMCD).We demonstrate the persistence of soft and hard regions and rule out the fully diluted hypothesis. Magnetic measurements (figure 2) showed a very different behaviour upon annealing between NF and NC films. Using a combinatorial approach, we investigated the coercive magnetic field decay when increasing the Co content. The magnetic coercivity is found to be systematically higher in the NC samples, over the whole range of studied composition. EXAFS spectra fitting and numerical modelling of the diffusion mechanism show that magnetically soft Co-rich inclusions remain after annealing, even at the high temperature required to obtain the L10 phase of the matrix. In the case of multilayer nanofilms with Co thin layers, the diffusion leads to homogeneous alloying. Local XMCD revealed the evolution of spin and orbital moments of Fe and Co as a function of the composition, further distinguishing the NC system with respect to fully homogeneous alloy.Studying these materials in model systems synthesised by nanofabrication routes provides interesting insights into the interplay between the microstructure and the magnetic performances in view to develop mass-scale production of such materials.This work is supported by the ANR collaborative project "SHAMAN": Soft in Hard MAgnetic Nanocomposites (2017-2020). **