Atomic ordering in nano-layered FePt M. Koz1owski a, * , R. Kozubski a , Ch. Goyhenex b , V. Pierron-Bohnes b , M. Rennhofer c , S. Malinov d a Interdisciplinary Centre for Materials Modelling, M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland b Institut de Physique et Chimie des Mate´riaux de Strasbourg, 23, rue du Loess, BP 43, F-67034 Strasbourg, France c Faculty of Physics, University of Vienna, Strudlhofg. 4, A-1090 Vienna, Austria d School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Stranmillis Road, Belfast BT9 5AH, UK article info Article history: Received 7 August 2008 Received in revised form 26 February 2009 Accepted 18 March 2009 Available online 21 April 2009 Keywords: A. Magnetic intermetallics D. Defects: antiphase domains D. Microstructure E. Simulations, atomistic E. Simulations, Monte Carlo abstract Monte Carlo simulation of chemical ordering kinetics in nano-layered L1 0 AB binary intermetallics was performed. The study addressed FePt thin layers considered as a material for ultra-high-density magnetic storage media and revealed metastability of the L1 0 c-variant superstructure with monoatomic planes parallel to the surface and off-plane easy magnetization. The layers, originally perfectly ordered in a c-variant of the L1 0 superstructure, showed homogeneous disordering running in parallel with a spon- taneous re-orientation of the monoatomic planes leading to a mosaic microstructure composed of a- and b-L1 0 -variant domains. The domains nucleated heterogeneously on the surface of the layer and grew discontinuously inwards its volume. Finally, the domains relaxed towards an equilibrium microstructure of the system. Two ‘‘atomistic-scale’’ processes: (i) homogeneous disordering and (ii) nucleation of the a- and b-L1 0 -variant domains showed characteristic time scales. The same was observed for the domain microstructure relaxation. The discontinuous domain growth showed no definite driving force and proceeded due to thermal fluctuations. The above complex structural evolution has recently been observed experimentally in epitaxially deposited thin films of FePt. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Properties of L1 0 -ordered FePt intermetallics The L1 0 -ordered FePt alloy (Fig. 1) due to its high magneto- crystalline anisotropy accompanied with high superstructure stability is perceived as a functional material for future magnetic data storage devices. Both above features provide excellent thermal stability of the magnetization direction [1,2]. There are several new technologies that involve L1 0 FePt: thin layers for improved conventional hard disc drives [3], or monodisperse FePt nano- particles [4,5] deposited with variety of techniques for patterned data storage media [6]. Despite the substantial development of deposition techniques effective application of the materials still faces a number of problems, one of which is that the monodisperse FePt particles show no atomic long-range order [7]. Although L1 0 ordering occurs on annealing, the treatment causes a parasitic effect of particle sintering and coalescence [8]. Recent results of both experimental studies and computer simulations are consistent about the fact that in the ordered L1 0 nano-particles or nano-layers free surface causes a decrease of the superstructure stability [9–11]. However, there is still an open discussion about the atomistic origin of the observed behaviour. In case of FePt recent MC simulations suggested the explanation either in terms of surface Pt precipitation [12] or in terms of surface-induced disorder [10]. 1.2. Previous results of MC simulations of chemical ordering in FePt Results of Monte Carlo simulations of ‘‘order–order’’ kinetics in the L1 0 bulk FePt as well as the preliminary observations of the process occurring in the related layers have been presented in our previous papers [13–15]. A lot of simulations following the methodology developed previously for Ni 3 Al [16] and applied to the bulk and homogeneously L1 0 -ordered FePt seemed to reveal two time scales in the ‘‘order– order’’ kinetics, with a very weak contribution of the short time scale [13]. The simulations recently repeated with averaging the results over 128 samples did not confirm the contribution of a fast relaxation component and definitely indicated one single time scale of the process. This result corroborates with a Phase-Field analysis [17] showing that only one time scale in the ordering kinetics in FePt corresponds to homogeneous ordering; the remaining two ones being related to phenomena connected with antiphase boundaries. * Corresponding author. Tel.: þ48 12 663 57 16; fax: þ48 12 633 70 86. E-mail address: kozlowski.miroslaw@gmail.com (M. Koz1owski). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2009.03.019 Intermetallics 17 (2009) 907–913