Hitachi and the university focused on a structure called laminated ferri as the most likely candidate for a high thermal resistive TMR device. While the existing TMR device uses a CoFeB single layer for the free layer, the laminated ferri has a free layer with a structure in which a thin metal (Ru) is sandwiched by two layers of magnetic films (CoFeB) having opposite magnetization directions. This structure can provide equivalently large anisotropy fields and thereby enhances the thermal stability.
Hitachi and the university investigated how to enhance the thermal stability while reducing the current density at the time of switching with the use of the laminated ferri structure. As a result, they discovered the ? value can be made larger when the two CoFeB layers constituting the free layer have the same thickness and that the value increases when the total thickness of the two layers is larger.
Moreover, the current density at the time of switching can be maintained substantially constant even though the total thickness of the two CoFeB layers is increased, said the company. For example, the ? value can be made larger when each of the two CoFeB layers has a thickness of 2.6nm and the Ru layer has a thickness of 0.8nm, compared to the case where the thicknesses of the CoFeB and Ru layers are set to 1.6nm and 0.8nm, respectively.
According to Hitachi, the provision of the laminated ferri structure makes it possible to increase the ? value to about 80, without using the perpendicular magnetization technology.