Tunnel magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) has recently attracted interest due to unique properties, such as large magnetic response, thermal stability, and robustness to the bias voltage. In this Letter, we report the sign inversion phenomenon of TMC observed with frequency modulation and dc voltage application to MgO-based MTJs at room temperature. A negative TMC is observed in the frequency region of about kHz due to the appearance of spin capacitance. By applying a dc voltage (a few hundred mV) in this frequency region, the spin flip is promoted in the parallel configuration of MTJs. This results in the observation of the sign inversion of TMC from negative to positive. These physical pictures can be well explained by the calculation based on the modified Debye-Fröhlich model. Our research offers a deeper understanding of AC spin transports, DC spin accumulation, equilibrium and non-equilibrium spin dynamics.

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