Basic Properties of Magnetic Shape-Memory Materials from First-Principles Calculations
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TRODUCTION
THE strong magnetoelastic interactions in the magnetic Heusler alloys are responsible for novel functional properties such as the magnetic-shape memory effects (MSMEs) and magnetocaloric effects (MCEs). Both effects have been highlighted recently in References 1–4. The discussion of the MCE and its relation to the MSME in ferromagnetic Heusler alloys[2] shows the development in this field since the pioneering work of Ullakko et al.[5] The magnetic field-induced (MFI) strain in an O-seven-layered Ni48.8Mn29.7Ga21.5 sample [at 300 K (27C)] has meanwhile reached 10 pct in a magnetic field of less than 1 T,[6] and a breakthrough in device development may be expected if one could still raise the martensitic transformation temperature MS and simultaneously change the high brittleness of the samples to more ductile behavior. So far, this problem has not been solved, and the development of better magnetic shape-memory alloys (MSMA) is needed. For instance, a large 7.6 pct MFI strain was observed in the martensitic phase (at room temperature) for the single variant Co47.5Ni22.5Ga30.0 under a static compressive stress of apporximately 12 MPa.[7] This shows that beside prototypical Ni-Mn-based materials, also other PETER ENTEL, University Professor, and ANTJE DANNENBERG, MARIO SIEWERT, HEIKE C. HERPER, MARKUS E. GRUNER, and DENIS COMTESSE, Research Assistants, are with the Faculty of Physics, University Duisburg-Essen, 47048 Duisburg, Germany. Contact e-mail: [email protected] HANS-JOACHIM ELMERS, University Professor, and MICHAEL KALLMAYER, Research Assistant, are with the Institute of Physics, Johannes-Gutenberg University, 55128 Mainz, Germany. Manuscript submitted March 30, 2011. Article published online August 4, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys like Co-Ni-Ga may serve as functional MSMA as discussed subsequently. In this article, we concentrate on the basic aspects of MSMA that can be discussed using first-principles calculations of the crystalline structure and magnetic and electronic properties. The peculiar features of MSMA are related to the large magnetocrystalline anisotropy energy, which develops when ferromagnetic cubic austenite undergoes a martensitic transformation to a tetragonal structure allowing a magnetic field to align martensitic variants with magnetic moments parallel to the external field. This gives rise to the observed MFI strain effects. From theoretical side, the particular interplay of magnetic and structural phase transformations in materials involving different order parameters is a highly interesting subject[8] that, based on the development of powerful computer platforms, has become accessible to parameter free zero-temperature calculations using density funtional theory. From free energy calculations retaining the most important elementary excitions in an approximate way or from Monte Carlo simulations, we can even determine MS and the Curie temperature TC; see, for example, the work on Ni2MnGa[9] and Heusler alloys that display the MCE.[10–12]
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