Origin of the large voltage-controlled magnetic anisotropy in a Cr/Fe/MgO junction with an ultrathin Fe layer: First-principles investigation

Voltage-controlled magnetic anisotropy (VCMA) has attracted broad interest due to its high efficiency in switching magnetization. Large VCMA was experimentally observed in Cr/Fe/MgO junction with ultrathin Fe layer [Nozaki et al., Phys. Rev. Appl. 5, 044006 (2016)], whose underlying mechanism was still not clear however. The Cr/Fe/MgO/Fe magnetic tunnel junction (MTJ) is also well known for its quantum-well (QW) states and as-induced spin-dependent resonant tunneling [Greullet et al., Phys. Rev. Lett. 99, 187202 (2007)]. Here, in order to uncover the relation between the large VCMA and the QW states, we developed a k-resolved VCMA calculation method combined with the second-order perturbation theory to investigate it. We find the VCMA coefficient reaches -297 fJ/V m matching well with the previous experiment with three monolayers (MLs) of Fe. The coefficient oscillates strongly and even changes its sign with increasing the number of Fe MLs. Comparing the k-resolved VCMA with the Fermi surface of the interfacial Fe atom, the screening charges theory for VCMA was verified. For 2-9 MLs Fe, interestingly, the QW states of Delta(1) electron at the Gamma point provide large (no) contribution to the VCMA with odd (even) MLs. Moreover, the change of the orbital-resolved Fermi surface at the interfacial Fe atom also plays an important role on VCMA oscillation, which as well as the QW states results in the largest VCMA for 3-ML Fe. Our results deepen the understanding of the large VCMA in the Cr/Fe/MgO junction, which would be helpful to design a practical MTJ with large VCMA.