Abstract
Voltage-induced switching of magnetization, as opposed to current-driven spin transfer torque switching, can lead to a new paradigm enabling ultralow-power and high density instant-on nonvolatile magnetoelectric random access memory (MeRAM). To date, however, a major bottleneck in optimizing the performance of MeRAM devices is the low voltage-controlled magnetic anisotropy (VCMA) efficiency (change of interfacial magnetic anisotropy energy per unit electric field) leading in turn to high switching energy and write voltage. In this work, employing ab initio electronic structure calculations, we show that epitaxial strain, which is ubiquitous in MeRAM heterostructures, gives rise to a rich variety of VCMA behavior with giant VCMA coefficient (~1800 fJ V−1m−1) in Au/FeCo/MgO junction. The heterostructure also exhibits a strain-induced spin-reorientation induced by a nonlinear magnetoelastic coupling. The results demonstrate that the VCMA behavior is universal and robust in magnetic junctions with heavy metal caps across the 5d transition metals and that an electric-field-driven magnetic switching at low voltage is achievable by design. These findings open interesting prospects for exploiting strain engineering to harvest higher efficiency VCMA for the next generation MeRAM devices.
Highlights
Electric field (E-field) control of the magnetization vector via the magnetoelectric effect has sparked an explosion of technological and research interest due to its potential application in ultra-low power, highly-scalable, and non-volatile spin-based random access memory or MeRAM1–4
A linear voltage-controlled magnetic anisotropy (VCMA) was observed in Ta/Co40Fe40B20/MgO7 and in Pd/FePd/MgO8 tunnel junctions with β of −33 and +600 fJV−1m−1, respectively, where the convention of positive E-field corresponds to electron accumulation at the FM/I interface
Recent experiments on V/Fe/MgO revealed an asymmetric ∧-shape VCMA with giant β values of 1150 fJ V−1m−1 9, while a ∨-shape VCMA was observed in double-barrier MgO/FeB/MgO/Fe junctions with β = 100 fJ V−1m−1 10
Summary
The ab initio calculations have been carried out within the framework of the projector augmented-wave formalism[32], as implemented in the Vienna ab initio simulation package (VASP)[33,34]. The 〈110〉axis of MgO and Au are aligned with the 〈100〉axis of FeCo and the O atoms at the. FeCo/MgO interface are placed atop of Fe atoms. The iron atoms at the Fe/MgO and Fe/Au interfaces are denoted by Fe1 and Fe2, respectively [Fig. 1 inset]. The magnetic and electronic degrees of freedom and atomic z positions are relaxed in the presence of the E-field until the forces acting on the ions become less than ×10−3 eV/Å and the change in the total energy between two ionic relaxation steps is smaller than 10−6 eV. The plane-wave cutoff energy was set to 500 eV and a 15 × 15 × 1 Monkhorst-Pack k-mesh was used for the relaxation calculations.
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