Damping in Free Layers of Spin-Transfer-Torque Magnetic Memory at Elevated Temperatures

In spin-transfer-torque (STT) magnetic random-access memory (MRAM), both switching current and switching time depend on the damping of the free layer. It is known that the magnetic damping constant (& alpha;) varies with temperature; because STT MRAM cells operate well above room temperature, there is an urgent need to determine and understand damping properties in STT MRAM free layers at elevated temperatures. This paper reports on comparative studies of high-temperature frequency-dependent ferromagnetic resonance (FMR) in two STT MRAM free layers: (i) a low-& alpha; free layer with split W layers, and (ii) a conventional free layer with a single W layer. Comprehensive FMR measurements and analyses show that (1) the damping constant of the low-& alpha; free layer is always smaller than that in the conventional free layer, as the temperature increases from 300 to 520 K; (2) the damping constant increases monotonically with temperature in both the free layers; and (3) the damping increase is relatively modest in the low-& alpha; free layer, from about 0.0035 to about 0.0079, but is more pronounced in the conventional free layer, from about 0.0092 to about 0.028. These results can be interpreted in terms of the temperature dependence of the damping produced by spin-flip magnon-electron scattering and the difference of the Curie temperatures in the two free layers. They clearly establish the technological advantage of the low-& alpha; free layer over the conventional free layer from the perspective of the STT MRAM application. In addition, the high temperature FMR data also yield important information about the temperature dependence of the effective perpendicular anisotropy field and the origin of the inhomogeneous line broadening in the free layers.