First-principles study of static displacements in Fe-Pd magnetic shape-memory alloys
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1200-G04-04
First-principles study of static displacements in Fe-Pd magnetic shape-memory alloys M. E. Gruner Faculty of Physics and Center for Nanointegration, CeNIDE, University of Duisburg-Essen, 47048 Duisburg, Germany ABSTRACT This contribution reports static ionic displacements in ferromagnetic disordered Fe70Pd30 alloys obtained by relaxation of the ionic positions of a 108-atom supercell within the framework of density functional theory. Comparison with a simple statistical model based on Lennard-Jones pair interactions reveals that these displacements are significantly larger than can be explained by the different sizes of the elemental constituents. The discrepancies are presumably related to collective displacements of the Fe atoms. Corresponding distortions are experimentally observed for ordered Fe3Pt and predicted by first-principles calculations for all ordered Fe-rich L12 alloys with Ni group elements and originate from details of the electronic structure at the Fermi level. INTRODUCTION In disordered off-stoichiometric Fe70Pd30 and Fe3Pt as well as in Ni-Mn-Ga full Heusler alloys the so-called magnetic shape memory (MSM) effect is observed, allowing macroscopic strains of several percent to be achieved in realistic magnetic fields, which is of increasing technological interest for applications in microscale actuators [1–5]. However, especially for the Fe-based systems, the martensitic transition temperatures are still too low for practical usage. First-principles methods within the framework of density functional theory allow the determination of structural and electronic properties on the atomistic level and therefore ideally complement experimental investigations while the link to the electronic structure will help to understand the relevant properties and to predict improved materials. Within this contribution, a comparison of static displacements observed in stoichiometrically ordered Fe3Pd and disordered Fe-Pd alloys will be provided by means of first-principles calculations. The disorder is modeled implicitly within the coherent potential approximation (CPA) assuming the ideal lattice positions as well as explicitly within a supercell approach. While being computationally expensive, it is straightforward to allow for a relaxation of the atomic positions in the latter case. The relaxation patterns are compared to a simple Monte Carlo model calculation. THEORY The ab initio calculations were mainly carried out using the VASP code and PAW potentials [6,7] with 3d74s1 as valence for Fe and 4d95s1 for Pd, respectively, and a plane wave cutoff of 335 eV. For the disordered configurations, the atoms were distributed randomly over the sites. A cubic supercell with a size of 3x3x3 lattice constants (108 atoms) was used with a fixed lattice constant of a = 3.74 Å. Brillouin zone integration was performed on a 2x2x2 k-grid during relaxation using the Methfessel-Paxton method with a smearing parameter σ = 0.2 eV.
Structural optimizations were stopped if the forces were falling below 0.02 eV/Å. Afterwar
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