New types of spintronics devices utilizing magnetization switching by current, such as Spin-Torque Transfer RAM (STT-MRAM) and spin-torque oscillators, have been intensely developed on MgO based Magnetic Tunnel Junctions (MTJs) with a large magneto-resistance ratio. MTJ is a sandwich of two magnetic layers separated by a thin nonmagnetic spacer. While the magnetization of the pinned layer is fixed due to the fabrication process, the magnetization direction of the free layer can be switched between the two states parallel and anti-parallel to the fixed magnetization direction.
Reducing the magnitude of the switching current density in MTJ structures is of considerable importance for the success of STT-MRAM. A MTJ with a composite free layer (C-MTJ) was earlier proposed (figure 1 left). The free magnetic layer of such a structure consists of two equivalent parts of half-ellipses separated by a narrow nonmagnetic spacer. The C-MTJs demonstrate a substantial decrease of the switching time and switching current as compared to the standard MTJ with the monolithic free layer.
We further performed the structural optimization of C-MTJs by means of extensive micromagnetic simulations and proposed a new structure of the composite free layer, C2-MTJ. In a C2-MTJ the free layer consists of the two ellipses with the major axes a/2 and b (a>2b) inscribed into a rectangle a x b (figure 1 right). This structure is easier to fabricate compared to the previous generation of C-MTJs. We investigated the switching statistics depending on the geometry. We found that the new C2-MTJ switches as fast as the previous one, which is twice as fast as the structure with a monolithic free layer, without loss of thermal stability. The narrow switching time distribution characteristic of C-MTJs is also preserved in C2-MTJs. Therefore, the newly proposed C2-MTJ offers greater potential for performance optimization of STT-MRAM devices.