  Voltage-controlled magnetic anisotropy (VCMA) offers an emerging approach to
realize energy-efficient magnetization switching in spintronic devices such as
magnetic random access memories (MRAMs). Here, we show that manipulating the
condensed states, i.e., introducing quantum well (QW) can significantly
influence the VCMA in a Cr/Fe-QW/MgAl2O4 based magnetic tunnel junction (MTJ).
Only for the MTJ with an even number of Fe atomic layers, we observed a novel
A-shaped VCMA curve for a particular QW state, where magnetic anisotropy energy
(MAE) reaches a local maximum at zero bias and reduces when applying both
positive and negative bias, i.e., a novel bi-polar VCMA effect. Our ab initio
calculations demonstrate that the QW states give an additional contribution to
perpendicular magnetic anisotropy (PMA), which can explain not only the
A-shaped VCMA but also the Fe-layer-number parity dependence of VCMA. The
present study suggests that the QW-modulated VCMA should open a new pathway to
design VCMA-assisted MRAM.
