New Functional Magnetic Shape Memory Alloys from First-Principles Calculations
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1200-G04-01
New Functional Magnetic Shape Memory Alloys from First-Principles Calculations Peter Entel1, Mario Siewert1, Antje Dannenberg1, Markus E. Gruner1 and Manfred Wuttig2 1 Faculty of Physics and CeNIDE, University of Duisburg-Essen, 47048 Duisburg, Germany 2 Department of Materials Science, University of Maryland, College Park, MD 20742, U.S.A. ABSTRACT An overview is given of new ferromagnetic Heusler alloys like Ni-Co-(Al, Ga, Zn), CoNi-(Al, Ga, Zn), Fe-Ni-(Al, Ga, Zn) and Fe-Co-(Al, Ga, Zn), which are compared with today's mostly investigated systems such as Ni-Mn-Z (Z = Al, Ga, In, Sn, Sb). The investigations are based on first-principles as well as Monte Carlo calculations. For some new systems, the simulations of atomic structure and magnetic and electronic properties allow to predict higher Curie and martensitic transformation temperatures than those of prototypical Ni-Mn-Z materials. Some of the new materials may be distinguished for devices which exploit the magnetic shape memory effect. Interestingly, in general, all off-stoichiometric alloys display competing antiferromagnetic correlations, which may be important for devices using the magnetocaloric effect. The Curie temperatures are obtained from Monte Carlo simulations using magnetic exchange parameters from ab initio calculations while the structural instability is inferred from local minima in the ab initio total energy curves as a function of the tetragonal distortion. The manifestation of phonon softening as a precursor of structural transformations is present in the austenitic phase of most of the calculated ferromagnetic shape-memory alloys. However, quite remarkably, we find that phonon softening is absent in a few systems such as Co2NiGa. INTRODUCTION Smart materials, particularly shape memory alloys and magnetic shape memory materials have become an reactivated field of research since the discovery that twin variants of the martensitic phase can be reoriented and redistributed by an external magnetic field giving rise to magnetic field-induced (MFI) strain in an unstressed single crystal of Ni2MnGa [1, 2]. The MFI strain effect has meanwhile reached about 10% in a magnetic field of less than 1 T [3]. Since then, intensive experimental work and theoretical modeling have been undertaken and a series of review articles can be found in the literatures highlighting different aspects of the magnetic shape memory effect (MSME) or the magnetocaloric effect (MCE) which occurs in the same alloys at slightly different compositions [4-14]. An early model of magnetization process and MFI strain in magnetic shape memory alloys (MSMA) was proposed by O'Handley who suggested that depending on the value of the magnetocrystalline anisotropy energy compared with the Zeeman energy, the magnetization mechanism is either caused by twin boundary motion in case of strong anisotropy or by magnetization rotation in case of weak anisotropy [15, 16]. It is now generally accepted that the mechanism of MFI strain involves transitions between variants of the tetrago
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