Mechanism of the exchange-bias field in ferromagnetic and antiferromagnetic bilayers

Exchange-bias field phenomena in ferromagnetic and antiferromagnetic bilayers have been investigated by means of the Monte Carlo calculations within the framework of the classical Heisenberg model. The calculations for a binary alloy composed of magnetic and nonmagnetic atoms such as a disordered gamma-phase antiferromagnetic layer show a noncollinear spin structure. In addition, the calculated magnetization process gives a loop shift due to the unidirectional exchange-bias field. In other words, the noncollinear spin structure caused by the geometric frustrations in the antiferromagnetic layer is responsible for the magnetization loop shift. On the other hand, a collinear spin structure formed in an ordered L1(0)-type antiferromagnetic alloy results in only the enhancement of coercivity of the ferromagnetic layer. Introducing the multidomains into the ordered L1(0)-type antiferromagnetic layer, however, the magnetization loop shift of the ferromagnetic layer is evidently developed by the geometrically frustrated spins induced at the magnetic domain boundaries.