Defect induced ferromagnetism in MnGa.

Ni-substituted MnGa displays a weak ferromagnetism embedded in an antiferromagnetic (AF) 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 Mn3-xNiGa (= 0, 0.25) alloys reveal the origin of ferromagnetism in MnNiGa is due to the segregation of a Heusler-type environment around Ni in the cubic MnGa matrix. Upon temper annealing at 400C, these local structural defects around the Ni phase separate into a modulated ferromagnetic (FM) Ni-Mn-Ga Heusler phase. A strong interaction between the AF host and the FM 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 MnNiGa.