The Phase Transition And Physics Mechanism In Mn-based Magnetic Alloys

The complex magnetic phase transition of Ti Ni Si-type Mn-based alloys has been widely investigated as one type of potential multi-functional material. And the original of magnetic phase transition has attracted increasing research attention in areas including spintronics, magnetostriction and magnetic refrigeration. In order to get a more clearly physical picture, we designed the material with multi-transition in magnetic character, and studied its magnetocaloric effect in application. Meanwhile, we research the origin of metamagnetism in Mn-based ternary alloys, which belong to Pnma group and in Ti Ni Si-type structure.First, the structure, magnetic phase transitions and magnetocaloric effects in Al-doped Co0.4Ni0.6Mn Ge compounds were investigated using X-ray diffraction and magnetization. With increasing temperature, the compound with x=0.03 undergoes four magnetic states: antiferromagnetic(AFM) in Ti Ni Si-type structure, ferromagnetic(FM) in Ti Ni Si-type structure, FM in Ni2In-type structure. And thus, three-step magnetic phase transitions were observed in this compound, i.e. a weak first order phase transition from AFM to FM in Ti Ni Si-type to FM Ni2In-type structure, and a following second order transition from FM to PM for Ni2In-type structure. Two continuous magnetic entropy changes have been observed in our studied system due to the last two magnetic phase transitions between two high magnetization states and the paramagnetic state.Secondly, density functional theory was used to study the relationship between magnetism and structure of Ti Ni Si-type Mn-based alloys. The magnetic states in these alloys largely rely on the value of Mn-Mn-Mn angle. There were two apparent critical bond angles being formed among ferromagnetic double exchange, anti-ferromagnetic super-exchange, and ferromagnetic super-exchange. Results indicated that the magnetic exchange interaction showed continuous transition in MMn X. The magnetism of MMn X was found to function on the basis of the relationship between bond angle and magnetic exchange interaction. The results provides a novel way of designing multi-functional materials made of MMn X with pointed magnetic exchange interaction. There is an active significance in designing of metamagnetism material.