  The paper investigates the systematic dependencies of the anisotropy field
and the strength of spin-orbit (SO) interaction on gate voltage in Ta/FeB/MgO
nanomagnets. Our findings reveal an intriguing opposite polarity in the
gate-voltage dependencies of the anisotropy field and the coefficient of SO
interaction across all studied nanomagnets. This opposite polarity indicates
that the gate-voltage modulation of spin-orbit interaction is not the primary
contributor to the voltage-controlled magnetic anisotropy (VCMA) effect.
Instead, the gate-voltage modulation of magnetization emerges as the most
probable candidate, given its polarity aligns with the observed modulation of
anisotropy. The modulation of magnetic anisotropy is influenced by two major
contributions of opposite polarities, which effectively counterbalance each
other and reduce the overall VCMA effect. Optimizing the balance between these
contributions could potentially lead to a substantial enhancement of the VCMA
effect. Our measurements did not detect any modulation of the in-plane
component of spin accumulation by the gate voltage.
