MS-12-P-3201 Study of magnetic FeRh nanoalloys : structure and chemical order

With the increasing demand for ultra-high density magnetic recording, an important effort was put on the fabrication and control of magnetic nanoalloys. More recently, much attention has been paid to the remarkable magnetic properties of the FeRh alloy, for both fundamental and technological issues. Indeed, the FeRh alloy presents, in a very narrow range of composition (0.48<x_Rh<0.56), a remarkable magnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) state (Fig.1). This transition takes place at a temperature close to 370 K in the bulk, i.e. slightly higher than room temperature, which makes this alloy particularly attractive for applications as heat-assisted magnetic recording [1] or microelectronics [2].

From a structural point of view, the equiatomic FeRh alloy crystallizes in a CsCl (or B2) type chemically ordered body-centered cubic (bcc) structure [3-5]. Epitaxial growth of FeRh on MgO substrate requires a suitable lattice match. For α’ phase, it is achieved through an in-plane 45° rotation of the bcc cell with respect to the MgO unit cell since a_MgO=0.42 nm= √(2)a_α’ (Fig.2).

We will report the influence of the growth parameters on size, morphology and structure of the deposited nanostructures. In particular, we tried to answer the question of the existence of a critical size for chemical order and magnetic properties.

The aim is to optimize growth conditions, thus to obtain the chemically ordered α’ phase epitaxially ultra-thin films.

All the studied films were grown on MgO (001) substrates by dc magnetron co-sputtering from two element targets in ultra-high vacuum chamber with a low pressure. The films were grown at 550 °C with two different thicknesses, 50 and 100 nm. After deposition, some of the films were in-situ annealed at 700 °C for 6 hours.

The evolution of the morphological and structure characteristics was analyzed by high-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations of cross-sectional specimens. TEM experiments were performed in conventional and high-resolution mode (HRTEM) (Fig.3). However, the magnetic properties (studied by VSM) strongly depend on the precise composition, the growth process, the pressure or stress undergone by the alloy and of course the resulting structural phases.

[1] J.U. Thiele, S. Maat, J. Robertson, E. Fullerton, IEEE Transactions on 40 (4) (2004) 2537–2542.

[2] G. Ju, J. Hohlfeld, B. Bergman, R.J.M. van de Veerdonk, O.N. Mryasov, J.-Y. Kim, X. Wu, D.Weller, B. Koopmans, Physical Review

[3] M. Fallot, Annales de Physique (Paris) 10 (1938) 291.

[4] G. Shirane, C.W. Chen, P.A. Flinn, R. Nathans, Physical Review 131 (1) (1963) 183–190.

[5] G. Shirane, R. Nathans, C.W. Chen, Physical Review 134 (6A) (1964) A1547–A1553