Magnetic and Electronic Properties of Heusler Alloy Films Investigated by X-Ray Magnetic Circular Dichroism

Abstract

We have investigated the magnetic properties of epitaxial Heusler alloy films using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism∈dex{x-ray!magnetic circular dichroism} (XMCD) in the transmission (TM) and in the surface sensitive total electron yield (TEY) mode. We have investigated Ni_2MnGa based shape memory alloys and half-metallic Co2Cro.6Feo.4 films. Single crystalline Ni2MnGa(110)/Al2O3(1120) and Ni2MnGa(100)/MgO(100) films show a martensitic transition from a cubic high temperature phase to a martensitic low-temperature phase at 250–275 K as concluded from magnetometry and x-ray diffraction. The martensitic transition of this Heusler compound is shifted in films on Al2O3 to higher temperatures Tm=276 K compared to the bulk value of 200 K. A remarkable change of the Ni x-ray absorption spectra occurs at Tm indicating specific changes of the electronic structure. The observed changes are in agreement with theoretical predictions. The orbital to spin momentum ratio of the Ni moment increases significantly on entering the martensite state thus explaining the macroscopic increase of magnetic anisotropy. The spin and orbital magnetic moments of Co2Cro.6Feo.4 films are similar to values measured for the bulk materials of the corresponding compounds. Interface properties can severely deviate from the bulk properties. We have investigated the interfaces of Co2Cro.6Feo.4 and Ni2MnGa Heusler alloy films and Al cap layers. At elevated temperatures and at rough surfaces the deposited Al severely reacts with the surface of a Heusler alloy indicated by changes of the absorption spectra. Compositional deviations at the interface as detected by XAS can also severely influence magnetic interface properties. Micro-spectroscopy using photoemission electron microscopy reveals an Al surface reaction proceeding inhomogeneously with reaction nuclei separated on a micron length scale.