  Magnetic tunnel junctions (MTJs) are basic building blocks for devices such
as magnetic random access memories (MRAMs). The relevance for modern
computation of non-volatile high-frequency memories makes ac-transport
measurements of MTJs crucial for exploring this regime. Here we demonstrate a
frequency-mediated effect in which the tunnel magnetoimpedance reverses its
sign in a classical Co/Al{_2}O{_3}/NiFe MTJ, whereas we only observe a gradual
decrease of tunnel magnetophase. Such effects are explained by the capacitive
coupling of a parallel resistor and capacitor in the equivalent circuit model
of the MTJ. Furthermore, we report a positive tunnel magnetocapacitance effect,
suggesting the presence of a spin-capacitance at the two
ferromagnet/tunnel-barrier interfaces. Our results are important for
understanding spin transport phenomena at the high frequency regime, in which
the spin-polarized charge accumulation at the two interfaces plays a crucial
role.
