Effects of demagnetization on Magnetic-Field-Induced Strain and microstructural evolution in Ni-Mn-Ga Ferromagnetic Shape Memory Alloy by phase-field simulations

• Demagnetization effects on field-induced strain and microstructure in Ni-Mn-Ga are investigated by phase-field simulations.
• Demagnetization significantly affects both macro-behavior and microstructural evolution in Ni-Mn-Ga under an applied field.
• Demagnetization factor parallel to field-favored variant’s easy axis affects critical field, strain, twin boundary motion.
• Demagnetization factor parallel to field-unfavored variant’s easy axis affects domain wall motion during strain increase.
• Demagnetization affects behavior of Ni-Mn-Ga by influence of internal magnetic field on twin boundary and magnetic domain.

Demagnetization effect plays a vital role in ferromagnetic materials. However, its influence on Magnetic-Field-Induced Strain (MFIS) in Ni-Mn-Ga Ferromagnetic Shape Memory Alloy has not yet been fully quantified. This study investigates demagnetization effects on MFIS and microstructural evolution in Ni-Mn-Ga for different demagnetization factors (i.e. vector N) by phase-field simulations. The investigation reveals that demagnetization can exert substantial impact on both macroscopic and microscopic behavior of Ni-Mn-Ga through the influence of internal magnetic field on twin boundary movement and domain evolution. The switching field and the saturating field as well as their difference increase with Nf (component of N parallel to easy axis of field-favored martensitic variant). For larger Nf values, the increase of strain appears to be less sharp and more stable (hardening-like). The stable increase of strain is ascribed to the steady movement of twin boundary. The velocity of magnetic domain wall motion during MFIS process decreases as Nu (component of N parallel to easy axis of field-unfavored martensitic variant) increases and their relationship obeys the power law. However, Nu does not seem to affect strain behavior or twin boundary movement. The dependence of switching field and saturating field on Nf is established for application design purposes.

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