Annealing effects on the magnetic anisotropy of FePt/BaTiO3 bilayers

MARICEL RODRÍGUEZ; MARIANO CABABIE; AGUSTÍN LÓPEZ PEDROSO; LAURA STEREN; DIGO RUBÍ; ALEJANDRO BUTERA; JAVIER GÓMEZ; MARTÍN SIRENA

Boston

Encuentro; 2015 MRS Fall Meeting & Exhibit; 2015

Recent ab-initio calculations performed on FePt/BaTiO3 heterostructures remark on an important change of the magnetic anisotropy of FePt induced by the ferroelectric polarization of BaTiO3 [1]. M. Lee and coworkers found that the electronic structure of Ti is spin-polarized at the FePt/BaTiO3 interfaces and that this effect strongly depressed the magnetic anisotropy of the FePt. The change of the magnetic anisotropy was assigned to the hybridization state of Fe-Ti atoms. The simulations were performed considering an L10-ordered (P4/mmm) FePt alloy. In this phase, the FePt shows a high magnetic anisotropy.  However, it is well-known [2] that FePt thin films deposited by sputtering grow in a chemically disordered fcc crystallographic phase (A1). As expected, the as-deposited films display soft magnetic properties.  In order to examine the anisotropy change of the bilayers with the crystalline structure and underlayer-induced strains, we have investigated the magnetic and structural properties of  FePt/BaTiO3 bilayers of different thickness and submitted to different annealing treatments. For the study, a series of FePt tFePt/BaTiO3/Pt/Si(100) structures with  tBaTiO3=250nm and 20nm <tFePt< 60nm were grown by combining pulsed laser deposition for the ferroelectric and sputtering for the magnetic alloy, respectively. FePt reference films have been also fabricated in order to identify the effect of BaTiO3 strains onto the ordering process of the magnetic alloy during the thermal treatments. Post-deposition annealing treatments were performed at temperatures varying from 250ºC to 450ºC for 10 minutes in flowing Ar, in order to transform progressively the disordered FePt phase into the L10 [3]. The crystalline structure of the films was characterized by X-ray diffraction measurements while the magnetic properties of the samples have been investigated by magnetization vs. field loops. We observe in FePt/BaTiO3 bilayers a progressive transformation of the FePt disordered phase into the ordered L10 phase with a consequent increase of the magnetic anisotropy as the annealing temperature is increased. On the other hand, we do not measure significant changes in the reference layers.  These results would indicate that the BaTiO3 promotes the ordering process of the FePt alloy during annealing.  The correlation between the strains induced by the underlayer and the dynamic of the crystallographic phase transformation is also discussed.    [1] M. Lee et al, Journal of Applied Physics, 113 (2013). [2] Z. R. Dai et al, Nano Letters, 8, (2001). [3] C. L. Platt, Journal of Applied Physics, 92 (2002).