Abstract
We study the tunnel magnetoresistance (TMR) effect and magnetocrystalline anisotropy in a series of magnetic tunnel junctions (MTJs) with $L1_1$-ordered fcc ferromagnetic alloys and MgO barrier along the [111] direction. Considering the (111)-oriented MTJs with different $L1_1$ alloys, we calculate their TMR ratios and magnetocrystalline anisotropies on the basis of the first-principles calculations. The analysis shows that the MTJs with Co-based alloys (CoNi, CoPt, and CoPd) have high TMR ratios over 2000$\%$. These MTJs have energetically favored Co-O interfaces where interfacial antibonding between Co $d$ and O $p$ states is formed around the Fermi level. We find that the resonant tunneling of the antibonding states, called the interface resonant tunneling, is the origin of the obtained high TMR ratios. Our calculation of the magnetocrystalline anisotropy shows that many $L1_1$ alloys have large perpendicular magnetic anisotropy (PMA). In particular, CoPt has the largest value of anisotropy energy $K_{\rm u} \approx 10\,{\rm MJ/m^3}$. We further conduct a perturbation analysis of the PMA with respect to the spin-orbit interaction and reveal that the large PMA in CoPt and CoNi mainly originates from spin-conserving perturbation processes around the Fermi level.
Highlights
Magnetic tunnel junctions (MTJs), in which an insulating tunnel barrier is sandwiched between ferromagnetic electrodes, have attracted considerable attention from the viewpoint of fundamental physics, and from their potential applications to various devices
To understand the origin of the high tunnel magnetoresistance (TMR) ratios, the bulk band structures of the electrodes and the barrier were first analyzed because the high TMR ratio in the well-known Fe/MgO/Fe(001) MTJ [14,15] was explained by the bulk band structures of Fe and MgO on the basis of the coherent tunneling mechanism [21,22]
If a similar mechanism holds for the present MTJs, the bulk band structures along the line in the Brillouin zone corresponding to the [111] direction should explain the high TMR ratios
Summary
Magnetic tunnel junctions (MTJs), in which an insulating tunnel barrier is sandwiched between ferromagnetic electrodes, have attracted considerable attention from the viewpoint of fundamental physics, and from their potential applications to various devices. The PMA is preferred for the different types of magnetization switching in MRAMs; the critical current for the switching in spin-transfer-torque MRAMs (STT-MRAMs) [1] can be reduced and the write error rate in voltage-controlled MRAMs [2] can be decreased To obtain both large PMA and high TMR ratios in MTJs, two types of approaches have been employed. Yakushiji et al [27] obtained PMA (Ku ∼ 0.5 MJ/m3) in Co/Pt(111) and Co/Pd(111) multilayers that have similar structures as L11 films All these studies indicate the potential of (111)-oriented MTJs with L11 alloys for large PMA; such MTJs have not been investigated both theoretically and experimentally in previous studies. A second-order perturbation analysis of the PMA with respect to the spin-orbit interaction (SOI) clarifies that the large PMA in CoPt and CoNi originates from the spin-conserving perturbation processes around the Fermi level
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