Commonly, the magnetostriction of a material is ascertained from macroscopic measurements on bulk samples. However, to truly understand their intrinsic microscopic behaviour, and allow direct comparison with theoretical models, it is necessary to measure structural strain at the atomic level. Commonly employed local probes, such as X-ray Absorption Spectroscopy (XAS), may obtain the absolute structural parameters on a scale of picometres, but lack the resolution to observe magnetostrictive strain, which is typically two or three orders of magnitude smaller. However, such measurements have recently become possible with the development of Differential XAS (DiffXAS) at the European Synchrotron Radiation Facility [1][2].

Here, we present the results of XAS and DiffXAS measurements obtained from the technologically important FePt system. Sputtered thin films of stoichiometric FePt were prepared in the disordered state and then annealed at temperatures in the range 400°C-600°C to produce the ordered high anisotropy L10 phase. Data analysis indicates that the magnetostriction coefficients of the as-deposited films are of the order of 100ppm in the cubic 100 direction and 250ppm in the 111 direction, and decrease systematically to approximately one fifth of those values as the annealing temperature is increased up to 600°C.

Furthermore, the chemical selectivity of DiffXAS has permitted the magnetostrictive strain to be quantified from both the perspective of the local structure surrounding the Fe site, and from that surrounding the Pt site. This has allowed the contributions from different bond types (Fe-Fe, Fe-Pt, Pt-Fe, and Pt-Pt) to be deconvolved from the bulk magnetostriction and assessed individually within the first two coordination shells around the absorbing atom.

References
[1] Pettifer, R. F. and Mathon, O. and Pascarelli, S. and Cooke, M. D. and Gibbs, M. R. J., Nature 435 78-81 (2005)
[2] Pascarelli, S. and Ruffoni, M. P. and Trapananti, A. and Mathon, O. and Aquilanti, G. and Ostanin, S. and Staunton, J. B. and Pettifer, R. F., Phys. Rev. Letters 99 237204 (2007)