Directin situmeasurement of coupled magnetostructural evolution in a ferromagnetic shape memory alloy and its theoretical modeling

Abstract

Ferromagnetic shape memory alloys (FSMA) have shown great potential as active components in next generation smart devices due to their exceptionally large magnetic-fieldinduced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for off-stoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twin-reorientation in an FSMA single-crystal during the application of magnetic fields. In addition, high-resolution neutron scattering measurements and first-principles calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of Ni 2 Mn 1.14 Ga 0.86 . The results from these state-of-the-art experiments and calculations are self-consistently described within a phenomenological framework, which provided quantitative insights into the energetics of magnetostructural coupling in FSMAs.Based on the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is furthermore estimated to be ~150 kJ/m 3 .

Neutron Scattering as a Powerful Tool to Investigate Magnetic Shape Memory Alloys: A Review

“…They concluded that the magnetization becomes weaker as the temperature increases due to the decreasing magnetic ordering, and that there is no intermartensitic transformation in the investigated alloy. Pramanick et al [69] employed polarized neutron diffraction to establish a correlation between the rotation of magnetic moments and the twin-reorientation phenomena in a Ni 2 Mn 1.14 Ga 0.86 single crystal. Lázpita et al [70] performed polarized neutron diffraction measurements to determine the influence of the atomic positions within the crystal unit cell on the magnetic coupling between atoms in the austenitic phase of Ni 51.9 Mn 26.2 Ga 21.9 , and in the martensitic phase of Ni 52.6 Mn 26.9 Ga 20.5 , the latter presenting a non-modulated tetragonal phase.…”

Effects of magnetic frequency and the coupled magnetic-mechanical loading on a ferromagnetic shape memory alloy

“…Although the magnetic and mechanical properties of FSMA have been widely investigated in the past decades, the correlation between the macro-response and the micromagnetic domain evolution is still vague due to the limitation of experimental characterizations. For example, neutron scattering experiment has shown its great power in the study of FSMAs during the last decades [33][34][35][36], however, the experimental results are usually from statistical averages. Thereby, detailed microstructures in the associated experiments are not clear.…”