Defect induced ferromagnetism in Mn 3 Ga.

Ni-substituted Mn 3 Ga displays a weak ferromagnetism embedded in an antiferromagnetic phase. Upon field cooling, the alloy exhibits exchange bias and an open hysteresis loop, signifying kinetic arrest at room temperature. For the first time, a kinetic arrest is seen in a compound due to the first order transition of an embedded defect phase. A systematic study of crystal structure, local structure, and magnetic properties of Mn 3-x Ni x Ga (x = 0, 0.25) alloys reveal the origin of ferromagnetism
in Mn 2.75 Ni 0.25 Ga is due to the segregation of a Heusler-type environment around Ni in the cubic Mn 3 Ga matrix. Upon temper annealing at 400 ◦ C, these local structural defects around the Ni phase separate into a modulated ferromagnetic Ni-Mn-Ga Heusler phase. A strong interaction between the antiferromagnetic host and the
ferromagnetic defect phase gives rise to exchange bias. The first-order transition of the defect phase seems to be responsible for the observed kinetic arrest in Mn 2.75 Ni 0.25 Ga.