Gilbert damping for STT-MRAM
The spin-transfer torque magneto-resistive access memory (STT-MRAM) is a new technology that allows to interleave logic and memory, similarly to the human mind!
At the heart of every spintronic device, such as the STT-MRAM, stand the losses of spin angular momentum to the lattice. These losses are described by the Gilbert damping.
One way of quantifying these losses is by measuring ferromagnetic resonance (FMR) response. This well-established technique operates in the frequency domain, and has been used for many years. Alternatively, the impulse response can be measured in the time domain. One method, which operates in the time domain and is especially suitable for measuring atomically thin magnetic films, is known as the time-resolved magneto-optical Kerr effect (TRMOKE).
In principle, the FMR and the TRMOKE methods should result in the same value of Gilbert damping.
So far, there have been two approaches to interpret the results of the TRMOKE measurement. The first makes use of numerical models while in the second, which is commonly used, an effective damping parameter is used to describe the dynamics. At high enough magnetic fields the effective damping parameter should converge to the values of the Gilbert damping. The limit at which this convergence happens however, is not well defined.
In the work presented in [PRB 92, 224402], my colleagues and I explained why the Gilbert damping coefficient obtained using the effective damping method often results in a higher value than the one obtained using the FMR measurement. Moreover, we developed a new time-domain approach which is analogous to the well-established frequency domain method. We further showed that in order that the effective damping method give the correct result for the Gilbert damping, the measurement should be carried out at non-practical high magnetic fields of several Teslas for typical samples.
The method we described is now used widely by the industry in the search for suitable STT-MRAM low-damping materials.
Frequency (FMR) and time (TRMOKE) domain measurements at the Spintronic Lab
FMR (left) and TRMOKE (measurements)