Task 3: Gilbert damping

3.1-A Gilbert damping in half metal Heusler alloy films (Oogane, Mizukami – Sendai)

From recent theoretical and experimental researches on spin torque transfer switching, the magnitude of the critical current density JC0 is considered to be proportional to the damping constant (G) and saturation magnetization (MS), and it is inversely proportional to the spin injection efficiency. Thus one needs materials showing low G, low MS, and high-spin polarization. According to theoretical models, the local band structure determines the damping properties. The underlying microscopic process for the relaxation of magnetization dynamics is transfer of angular momentum from the electronic system to the lattice occurring via inter- and intra- band transitions near the Fermi level. Half-metallic materials thus offer an ideal basis for tailoring the damping properties towards small G value because the energy gap for minority electrons closes the spin-flip channel as one of the important transfer mechanisms. Such a situation is considered to be realized in the some types of the Heusler alloys, which shows a very low damping constant as previously reported by our group. While the mechanism of damping in Heusler alloys is poorly understood, much investigation is required. Thus, the central goals of this experimental project are a better understanding of the fundamental origin and a control of the damping in half-metallic Heusler alloy films. Finally, we will realize high quality ultra-thin films of Heusler alloy films showing low-damping constant.

3.3-A Theoretical study of Gilbert damping in half metal Heusler alloy films (Sakuma, Tsuchiura – Sendai)

One of the serious problems of MTJ using the Heusler alloy film is the strong temperature dependence of TMR ratio, that is, remarkable decrease of MR ratio with increasing temperature.  To improve the MR ratio of this system, it is of great importance to make clear of the mechanism of such strong temperature dependence.  Thus, one of the aims of this project is to calculate the exchange constants or the exchange stiffness constants of Heusler alloy films especially around the interfaces of MTJ. The goal of this subject is to propose microscopic structure improving the MR ratio at room temperature.  Another aim of this project is to provide microscopic description of the Gilbert damping of both bulk and multilayer systems.  The microscopic origin of the Gilbert damping is still unclear as well as that of the temperature dependence of MR ratio.  Based on the theory developed by Kamberský,1)  we extend the theory to investigate the damping at the interface and will perform the first principles calculation of the Gilbert damping of Heusler alloys and MTJs in order to support the development of current induced spin transfer torque in MRAM.