Martensite Transition and Microscopic Magnetism of Epitaxial Ni2MnGa Films
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Martensite Transition and Microscopic Magnetism of Epitaxial Ni2MnGa Films Gerhard Jakob, Tobias Eichhorn, Michael Kallmayer, and Hans-Joachim Elmers Institute of Physics, University of Mainz, Staudinger Weg 7, Mainz, 55099, Germany ABSTRACT A magnetically induced shape memory effect in Ni2MnGa results in huge magnetostrictive effects of several percent. Using x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) we investigated element specific magnetic moments and electronic structure of single crystalline, (110) oriented Ni2MnGa films on a-plane Al2O3 substrates in the austenite and martensite state. The structural phase transition of the samples is evident from temperature dependent x-ray diffraction and magnetization measurements. The Ni XAS differ significantly for temperatures above and below the martensite transition in agreement with published ab-initio calculations. Using XAS in transmission geometry on our thin film samples we observe the corresponding reduction of the absorption feature as predicted by theoretical calculations. The XMCD analysis shows the orbital contribution of the Ni electrons to be responsible for the magnetic anisotropy. INTRODUCTION Magnetic shape memory materials can change their macroscopic shape due to application of a magnetic field. The magnetic anisotropy energy associated with the alignment of the magnetization with the crystallographic axes leads to hysteresis curves which differ in shape for different field directions. On application of a magnetic field, however, the crystallographic axes usually remain fixed. In shape memory materials the magnetic anisotropy energy is larger than the energy for movement of twin boundaries through the crystal. Rather than switching the magnetization direction to a hard axis direction, in Ni2MnGa the twin boundaries move and the crystallographic axes of the sample rearrange in order to have an easy axis in field direction. For tetragonal systems differences in the lengths of c- and a- axes lead to a magnetostrictive effect. In single crystals macroscopic length changes of up to 10% have been achieved [1,2]. The stoichiometric Ni2MnGa compound is at room temperature in a cubic L21-structure, consisting of four interpenetrating fcc-lattices. On cooling it undergoes around 200 K a martensite transition and the low temperature structure is to a first approximation tetragonal [3]. Only the low temperature phase can show the magnetic shape memory effect. However, small variations in sample stoichiometry can considerably shift the transition temperature, even well above room temperature [4]. How the martensite transition influences microscopically the magnetic and electronic structure is not fully clear yet. Ayuela et al. calculate the largest changes for Ni derived minority d-electron states [5]. The degenerated d-states in the cubic austenite phase split up in the lower symmetry martensite phase. According to their calculations this leads to a decreasing Ni magnetic moment at the martensite transition and to a c
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