Orbital moment anisotropy in ultrathin FePt layers

Orateur : M. M. SOARES (European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France & Laboratório Nacional de Luz Síncrotron-LNLS, CP 6192, 13083-970 Campinas, Brazil)

Magnetic materials presenting large magnetocrystalline anisotropy energy (MAE) with perpendicular magnetization are of great interest in ultrahigh-density magnetic recording and spintronic devices. Systems with perpendicular magnetic anisotropy present lower critical current density for magnetization switching by spin-transfer torque in magnetic tunnel junctions [1]. FePt ordered alloy in the fct L10 phase, where monoatomic planes of Pt and Fe alternates along the c-axis, is one of the materials with largest MAE [2,3]. Adding to this fact its good resistance to corrosion and moderate Curie temperature (TC 750 K) makes L10 FePt the prime candidate for heat-assisted magnetic recording [2]. From a fundamental point of view, the origin of the MAE in FePt and other transition metal/noble metal alloys has been subject of intense theoretical research. For 3d transition metals, by using perturbation theory, Bruno [4] showed that the MAE should be directly proportional to the orbital moment anisotropy (OMA). Here [5] we investigate the OMA of Fe in epitaxial ultrathin (< 2 nm) FePt layers by angular dependent x-ray magnetic circular dichroism (XMCD) for two distinct epitaxial systems, CoO/FePt/Pt(001) and Pt/FePt/MnPt/Pt(001). Element selective hysteresis loops obtained from the XMCD signal [fig. 1-a] at Fe L3 edge are used to quantify the anisotropy constants, which are consistent with the corresponding chemical order parameter. A careful procedure for XMCD sum rules data analysis is set up to avoid experimental overestimation of the orbital magnetic moment [fig. 1-b]. The measured orbital moment of Fe is found comparable to theoretical values and shows a significant anisotropy. This OMA, experimentally observed here for the first time, is discussed in the light of the current theoretical understanding of magnetocrystalline anisotropy in ordered alloys.

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