Fundamental Investigation of Ferromagnetic Shape Memory Alloys: A New Perspective
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Fundamental Investigation of Ferromagnetic Shape Memory Alloys: A New Perspective Matthew R. Sullivan1, Daniel A. Ateya1, Steven Pirotta1, Ashish A. Shah1, G. H. Wu2, and Harsh Deep Chopra1* 1 Thin Films & Nanosynthesis Laboratory, Materials Program, Mechanical and Aerospace Engineering Department, State University of New York at Buffalo, Buffalo, NY, USA 2 State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ABSTRACT In the present study, the evolution of micromagnetic structure and microstructure is studied in-situ both as a function of temperature and applied magnetic field, using single crystal Fe-Pd and Ni-Mn-Ga Heusler alloys. Through the development of a novel technique called ‘Magnetic Transition Spectrum’ to study temperature dependent domain dynamics, the relative sequence of micromagnetic reconfiguration with respect to the martensitic transformation has been determined for the first time. Results show that the FSMAs may be viewed as magnetic mosaics, a new perspective, which is also more amenable to modeling the physical properties of these alloys. Finally, the concept of magnetic mosaics has been used to synthesize a novel class of materials with engineered magnetic anisotropies, and is briefly discussed. INTRODUCTION Ferromagnetic shape memory alloys (FSMAs) belong to the class of complex correlated systems whose physical properties depend on the interaction between three or more energy regimes. Thus in the case of FSMAs, interaction between thermal, magnetic and elastic energy regimes gives rise to a rich variety of phenomena, such as the magnetic shape memory effect [1], thermo-elastic shape memory effect [2], and the magneto-caloric effect [3]. Therefore FSMAs may be referred to as multi-functional materials as opposed to functional materials (such as thermo-elastic SMAs, piezoelectric or magnetostrictive materials). The martensite transition in SMAs or FSMAs results in transformation of the homogeneous high temperature austenite phase into heterogeneous low temperature martensite phase. Here ‘heterogeneity’ refers to the formation of fine twins, which are differently oriented variants of the low temperature, lower symmetry martensite phase. As a result, whereas the ferromagnetic austenite phase has a spatially well defined magnetocrystalline anisotropy axes throughout the volume of the crystal, the direction of the magnetocrystalline anisotropy in the martensite phase varies from one twin plate to another. However, little is known about the temperature dependent evolution of the micromagnetic structure due to the redistribution of the magnetocrystalline anisotropy axis resulting from the structural transformation, as well as its response to an applied magnetic field. Such an understanding is not only of fundamental interest, but is also of practical importance in designing low switching field FSMAs (in addition to anisotropy, the switching field has contributions from magneto-elastic coupling and coercivity, and i
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