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MMM 2022

November 07, 2022

Minneapolis, United States

Magnetic Field Detection Using Spin Torque Nano Oscillator Combined with Magnetic Flux Concentrators

In this work, we report the design of a spin torque nano-oscillator (STNO) combined with magnetic flux concentrators (MFCs) to improve the sensitivity of detecting magnetic fields, where the STNO consists of two out-of-plane (OP) ferromagnetic layers separated by a MgO insulating layer (see Fig. 1(a)) and the MFCs placed next to STNO are permalloy. We carried out micromagnetic simulations on the spin-transfer torque (STT) induced magnetic dynamics in the free layer of STNO. Based on the Landau-Lifshitz-Gilbert (LLG) equation with STT term, the time evolved magnetization precession of the free layer is plotted in Fig. 1(b). First, we show that the ferromagnetic resonance frequency (FMR) of the free layer magnetization precession can be tuned by the charge current density and external magnetic fields. In addition, the FMR as a function of external magnetic field strength is also studied on STNO with and without MFCs, see Fig. 1(c). We observed a general suppression of the resonance frequency due to the damping effect of the magnetization induced by MFCs placed at different distances to the STNO (see Fig. 1(d)). The FMR shift of the STNO free layer magnetization as function of external magnetic field is summarized in Fig 1. (e). By placing MFCs next to STNO, the lowest detectable magnetic field strength is enhanced from 10 μT to 10 nT. It is concluded that MFCs improve the sensitivity of STNO to external fields due to the distortion caused by the magnetization damping. The results presented in this work could inspire the optimal design of STNO and MFC-based ultra-low magnetic field sensors.

Fig. 1. (a) Schematic view of STNO with a diameter of 80 nm. Charge current is defined as +J and applied in the positive direction. (b) Evolution of the free layer magnetization resonating in-plane from the initial condition x,y,z> = (0, 0, 1). (c) Schematic view of MFCs placed next to STNO. (d) The distance between MFCs and STNO affects the resonance frequency. J=5e10 A/m2. (e) FMR shift of the magnetization induced by an external magnetic field for an STNO with and without MFCs, where LOD stands for limit of detection. J=5e10 A/m2, MFCs are placed at 70 nm to the STNO.

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