Abstract
Voltage control of magnetic anisotropy (VCMA) is one of the promising approaches for magnetoelectric control of magnetic tunnel junction (MTJ). Here, we systematically calculated the magnetic anisotropy (MA) and the VCMA energies in the well-known MTJ structure consisting of Fe/MgO interface with Cr buffer layer. In this calculation, we investigated an alloying between Fe and Cr and a strain effect. We used a spin density functional approach which includes both contributions from magnetocrystalline anisotropy energy (MCAE) originating from spin–orbit coupling and shape magnetic anisotropy energy from spin dipole–dipole interaction. In the present approach, the MCAE part, in addition to a common scheme of total energy, was evaluated using a grand canonical force theorem scheme. In the latter scheme, atom-resolved and k-resolved analyses for MA and VCMA can be performed. At first, we found that, as the alloying is introduced, the perpendicular MCAE increases by a factor of two. Next, as the strain is introduced, we found that the MCAE increases with increasing compressive strain with the maximum value of 2.2 mJ/m2. For the VCMA coefficient, as the compressive strain increases, the sign becomes negative and the absolute value becomes enhanced to the number of 170 fJ/Vm. By using the atom-resolved and k-resolved analyses, we clarified that these enhancements of MCAE and VCMA mainly originates from the Fe interface with MgO (Fe1) and are located at certain lines in the two dimensional Brillouin zone. The findings on MCAE and VCMA are fully explained by the spin-orbit couplings between the certain d-orbital states in the second-order perturbation theory.
Highlights
Magnetic random access memory (MRAM) is one type of random access memory, and has some advantages such as relatively fast read/write accessibility and unlimited endurance, compared to the other memories such as static and dynamic access memories and the storages such as NAND-type flash memory and hard disk drive
We investigated electronic structures in the metal oxide heterostructure of Fe/MgO interface with Cr buffer layer by means of the approach based on spin density functional theory (SDFT) [49]
The MA energy (MAE) for structure-I and structure-II are summarized in Table 1, including the magnetocrystalline anisotropy energy (MCAE) and shape magnetic anisotropy energy (SMAE) contributions
Summary
Magnetic random access memory (MRAM) is one type of random access memory, and has some advantages such as relatively fast read/write accessibility and unlimited endurance, compared to the other memories such as static and dynamic access memories and the storages such as NAND (not and)-type flash memory and hard disk drive. The switching of the magnetization is realized based on spin Hall effect [8,9] Since both the SOT- and STT-MRAMs work based on the flow of charge, the energy loss due to the ohmic dissipation cannot be avoided and becomes a problem in much smaller and more complex devices. In the same time, there may be some disadvantages, such as reductions in both TMR ratio and perpendicular MA, due to structural atomic-scale breakes at the interface between metal and oxide layers Another choice is to use alloying effect [42], underlayer effect [43,44], or strain effect [45,46]. We introduced two kinds of perturbation, namely alloying and strain These perturbations can effectively modify the electronic structure and eventually enhance the MA energy and VCMA coefficient. We systematically elucidate the possible origin of such enhancement by connecting the electronic structure near the Fermi level with the atom-resolved or k-resolved contributions
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