Magnetization dynamics of an in-plane magnetized synthetic ferrimagnetic free layer submitted to spin-transfer torques and applied field

An equilibrium stability analysis of an in-plane magnetized synthetic ferrimagnetic free layer (SyF) submitted to multiple spin-transfer-torque (STT) influences and to applied field has been carried out. Analytical expressions for the frequency and linewidth of the excitations were derived, in the case of strong coupling between the two layers compared to the anisotropy field. The expression of the critical current for the onset of excitations in the SyF was then calculated. Above the critical current, the destabilized mode, acoustic or optical, is found to depend on the applied field direction, but also on the asymmetry of the two layers composing the SyF. A strong thickness asymmetry reduces the critical current at zero field, and allows to select the destabilized mode. The critical current of an in-plane SyF with mutual spin torque between the layers and without reference layer was computed. For a symmetric SyF, the optical mode is the only mode that can be destabilized by STT.