Anisotropic magnetostructural transition in epitaxial Mn–Ni–Co–Ti Heusler alloy thin film

Heusler alloys are distinctive functional materials related to the phase transitions due to the strong magnetic and structural coupling. By the epitaxial strain from the rigid substrates, anisotropic magnetostructural transition (MST) might be observed in different crystalline orientations, leading to novel properties and functions. In this work, 62 nm thick (001)-oriented Mn49.1Ni34.0Co9.6Ti7.3 films are epitaxially grown on (001) MgO substrates. Clear MST is observed accompanied by a broad transition hysteresis from the thermomagnetic (M–T) curves with an out-of-plane (OP) magnetic field, while the transition hysteresis is absent under an in-plane (IP) magnetic field. It is related to the main lattice distortion occurring in OP orientation during the martensitic transformation because of the kinetic arrest of the IP structure by the MgO substrate. The spin glass (SG) phase is observed in IP orientation, which is lacking in OP orientation and the bulk. Clear exchange bias (EB) is observed in both the IP and OP directions. OP EB originates from exchange coupling between the ferromagnetic (FM) and antiferromagnetic phases, while IP EB is partially contributed by the exchange coupling between the SG and FM phases. It is ascribed to the establishment of metastable spin configuration in IP orientation with sufficient surrounding spins, but stable spin configuration in OP orientation due to the limited number of surrounding spins, which has been confirmed by Monte-Carlo simulation results. Our results provide a novel strategy for the modification of the physical properties of Heusler alloys and the design of novel magnetic devices.