Ferromagnetic resonance in magnetic tunnel junctions under high dc biases
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Ferromagnetic resonance (FMR) is a standard spectroscopic technique which is used to probe the magnetodynamics of ferromagnetic materials in order to understand and improve performance of spintronics applications such as magnetic random-access memory (MRAM). In our experiments, we use rf microwave currents to excite FMR in magnetic tunnel junctions (MTJs) via spin-transfer torque (STT-FMR) that allows us to electrically detect magnetodynamics by means of a small rectified voltage which develops across the MTJ at resonance. The MTJ pillars used in this work have diameters on the order of 100 nm and consist of free and pinned CoFeB layers separated by a MgO barrier with typical tunneling magnetoresistances (TMRs) of about 100% at room temperature. As expected, the frequency-field relationship of the observed resonances can be well fitted by Kittel’s equation. However, as a function of the dc bias applied to the MTJ we observe an unexpected shift of the resonance field. This shift is symmetric about zero bias and may be a result of the out-of-plane voltage controlled magnetic anisotropy (VCMA) in the otherwise in-plane magnetized MTJ. In addition to the effective field due to VCMA, an out-of-plane field was produced by canting the applied field. A generalized angular dependent version of Kittel's equation revealed little influence of the out-of-plane applied field with respect to the effective VCMA field. Also, two measurement techniques for detecting FMR, rf amplitude modulation and applied magnetic field modulation are reviewed and compared.