Voltage-induced magnetization switching can lead to a new paradigm enabling ultralow-power and high density instant-on nonvolatile magnetoelectric random access memory (MeRAM) devices. Two major challenges for future MeRAM devices are to achieve large perpendicular magnetic anisotropy (PMA) and high voltage-controlled magnetic anisotropy (VCMA) coefficient of heavymetal/ferromagnet/insulator heterostructures (HM/FM/I). Employing ab initio electronic structure calculations we have investigated the effect of epitaxial strain and thickness of both FeCo and Ir layers as design parameters to optimize the PMA and VCMA of Ir/FeCo/MgO. We predict that the Ir cap layer can induce both large PMA and colossal VCMA efficiency which depend on the stain. More importantly, we predict that a single Ir cap monolayer gives rise to a VCMA efficiency one order of magnitude higher than that of thicker Ir layers. The underlying mechanism is the synergistic effects of the emergence of Ir local moments, the large Ir spin orbit coupling (SOC), and the large modulation of the magnetic anisotropy at the Ir/vacuum interface. These results provide useful guiding rules in the design of the next-generation of high performance MeRAM memory devices.

To date the realization of magnetoresistive RAM (MRAM) and magnetoelectric RAM (MeRAM) devices relies primarily on ultrathin ferromagnetic-based (FeCoB/MgO) magnetic tunnel junctions. On the other hand, the Heusler family of intermetallics is considered very promising for spintronic applications. Nevertheless, the voltage controlled magnetic anisotropy (VCMA) in ultrathin Heusler-based magnetic-tunnel junction stacks remains unexplored. Here, using the ferrimagnetic Heusler Mn 3 Ga as a prototype system, we report ab initio calculations of the electric field modulation of magnetism in the Ir/Mn 3 Ga/MgO heterostructure. The trilayer structures with one/three monolayers Ir cap and Mn-Mn termination exhibit large perpendicular magnetic anisotropy (PMA) in contrast to these with Mn-Ga termination which yield in-plane magnetization orientation. We predict giant VCMA coefficients whose magnitude and sign depend on both the interface termination and the Ir cap thickness. The underlying atomistic mechanism lies on the electric-field-induced shifts of the spin-orbit coupling energies of the spin-polarized Ir/d-orbitals with different orbital angular momentum symmetries. Our work paves the way for exploiting the unique magnetic properties of ferrimagnetic Heusler compounds for the next generation MeRAM devices.

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