Spin polaron and bistability in ferromagnetic semiconductor quantum structures

Abstract

This work shows that the in-plane localization of a hole confined in a ferromagnetic semiconductor quantum well (QW) can lead to significant energy gain if spontaneous easy-plane magnetization is mediated by the mechanisms other than itinerant carriers. The hole spin normal to the QW plane reorients the in-plane magnetization of the ferromagnetic layer at the location of polaron formation, resulting in an exchange potential with a discrete level of localization. A flexible model that incorporates the magnetization gradient term, as well as magnetic anisotropy, is proposed. In contrast to the calculations of magnetic polaron in the paramagnetic semiconductors, the energy of spin polaron in a ferromagnetic semiconductor is almost independent of the temperature in a wide range below the critical temperature of phase transition. Our calculation also demonstrates the existence of bistability in the hole state when the structure consists of appropriate ferromagnetic and nonmagnetic QWs separated by a finite barrier. Hence, a memory element that can be scaled down to a single hole may be achieved through polaron formation.