Correlation between Atomic Ordering and Magnetic Properties in L10 Alloys INVITED

Abstract

Magnetic binary alloys with a CuAu-I type chemically ordered face centered tetragonal (fct) structure, mainly known as L10 structure, have attracted a special attention in the last decades due to their peculiar physical properties and excellent chemical stability, which arise from the particular arrangement of the atoms that alternate in composition along the c-axis direction of the fct unit cell [1]. Among the L10 alloys, ferromagnetic (e.g. FePt, FePd, CoPt, CoPd, NiPt, MnAl, FeNi) and antiferromagnetic (e.g. MnPt, MnPd, MnNi, CrPt) nanostructured materials are of particular interest in view of their potential applications in many technological fields. The extraordinary high uniaxial magnetic anisotropy of L10 FePt(Pd) and CoPt makes such alloys of particular importance for next generation ultra-high density magnetic recording media, magnetic hyperthermia and catalysis. High-anisotropy L10 MnAl and FeNi alloys are excellent and cost-effective alternative to materials containing rare-earths and other critical elements for both permanent magnets and spintronic applications.Many studies have been carried out to find the optimum conditions for the preparation of L10-ordered alloys that are commonly obtained by post-deposition high-temperature treatments for a few hours in order to favour the conversion from the chemically disordered fcc (A1) phase into the chemically ordered fct (L10) phase.However, in the last decade advanced chemical synthesis strategies have been proposed, allowing highly-ordered L10 magnetic alloys to be synthesized at a much milder conditions than usual; this is the case of the Pre-ordered Precursor Reduction (PPR) method [2] where the desired MPt phase (M=Fe, Co, Ni) is obtained by a thermal decomposition in a reductive atmosphere of a pre-ordered M(H2O)6PtCl6 crystalline precursor salts consisting of alternating planes of pure Me and Pt atoms, which mimic the atomic structure in the alloy (fig.1).Among the characterization techniques applied to investigate the structural properties at the local-scale, XAS is an effective tool to probe the chemical environment around an absorber element, and to get information on the average structural features of the materials, thus being complementary to conventional X-ray diffraction techniques. Due to its peculiar characteristics, i.e. selectivity and high sensitivity, Extended X-ray Absorption Fine Structure (EXAFS) analysis represents the main technique to investigate the local properties in many systems whose behaviour is strongly affected by the atomic arrangement, as in the case of the L10 chemically ordered alloys, where the degree of chemical order influences significantly the final magnetic properties.In this paper, recent studies based on a combination of ex-situ and in-situ X-ray Absorption Spectroscopy (XAS) experiments suitably set-up to describe the local environment around the metals during the synthesis processes of different L10 alloys are presented. In particular, by comparing PPR-synthetised L10 alloys by means of EXAFS, X-ray diffraction results, TGA-DTA analysis and magnetic measurements, we could firstly formulate a general and comprehensive explanation of the process at the basis of the thermal treatment that brings to the reduced alloys with such a high ordering degree (S>0.9). This allows a greater control on the synthesis method, which can be exploited, by properly choosing the starting salt, to synthesize other binary alloys, such as L10-FeNi, L10-MnAl and L10-MnPt that are of great interest for many technological applications but extremely challenging to be obtained. **