Hitachi Improves Laminated Ferri Structure in Spin Injection MRAM

Nov 12, 2007 18:58 Motoyuki Ooishi, Nikkei Electronics

Hitachi Ltd and Tohoku University presented a research result on how to enhance the thermal stability of spin-transfer torque RAM (SPRAM), a spin injection MRAM under joint development.

The results were presented at the 52nd Annual Conference on Magnetism and Magnetic Materials (MMM), an international conference on magnetic recording that was held in Tampa, Florida.

SPRAM has a problem in that the thermal stability between the low resistance state and the high resistance state is degraded when the size of a TMR device is reduced. When the thermal stability is low, the transition between the two states may occur with a higher probability due to the influence of surrounding heat, etc.

This leads to the degradation of data retention performance at the time when no read current is passed through the device, resulting in more frequent erroneous writing during the passage of the read current.

Preferably, the Δ value (Eu/kT), an index of thermal stability of a TMR device, exceeds 60. Here, Eu, k and T denote the energy of magnetic body, Boltzmann constant and temperature, respectively.

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 Δ 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 Δ 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.

"The reduction of switching current density and the enhancement of Δ value are basically in a trade-off relationship," the company said. "Therefore, it is worthwhile to provide a technique to enhance the Δ value alone without increasing the switching current density."

According to Hitachi, the provision of the laminated ferri structure makes it possible to increase the Δ value to about 80, without using the perpendicular magnetization technology.

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