Abstract
Fe/MgO-based Magnetic Tunnel Junctions (MTJs) are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite size effects and temperature dependent magnetic properties which must be carefully controlled for practical applications. In this letter, we investigate the electronic, structural and magnetic properties of MgO/Fe/MgO sandwiches using first principles calculations and atomistic spin modeling based on a fully parameterized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin spiral ground state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layer-wise temperature dependence of the magnetization. The different temperature dependences of the interface and bulk-like layers results in an unexpected non-monotonic temperature variation of the effective magnetic anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing new insight when interpreting experimental data of nanoscale MTJs.
Highlights
The control of perpendicular magnetocrystalline anisotropy (PMCA) at ferromagnetic transition–metal-insulator interfaces is of paramount importance in the manufacture of spintronic devices, such as perpendicular magnetic tunnel junctions [1,2,3] and tunneling anisotropic magnetoresistive systems [4]
Our results reveal the role of single-ion and two-ion anisotropy contributions, as well as the long-range exchange interactions in the ground-state configurations of two different bcc-Fe thicknesses sandwiched by two MgO(001) regions: Á Á Á MgO=nFeFe=MgO Á Á Á
We show that the lack of Fe out-of-plane symmetry and dissimilar in-plane lattice constants compared to the Fe bulk have a drastic effect on the magnetic properties at the Fe=MgO interface, leading to an exchange anisotropy which provides a dominant contribution to the total PMCA
Summary
The control of perpendicular magnetocrystalline anisotropy (PMCA) at ferromagnetic transition–metal-insulator interfaces is of paramount importance in the manufacture of spintronic devices, such as perpendicular magnetic tunnel junctions [1,2,3] and tunneling anisotropic magnetoresistive systems [4]. It has been demonstrated that other 3d transition metal elements show increased PMCA even if their spin-orbit coupling is weak [10,11,12] Such is the case of Fe-based thin films at MgO(001) interfaces, where the Fe dz2 − O pz hybridization at the interfaces results in enhanced PMCA [13]. Detailed understanding of the magnetization dynamics and temperature-dependent magnetic properties, that of the magnetization and anisotropy, require a multiscale approach based on atomistic spin dynamics [18] with ab initio parametrization This is especially important given the localization of the magnetic anisotropy at the interface and the possibility of noncollinear spin structures. The temperature dependence of the anisotropy is shown to be nonmonotonic, driven by the different temperature dependences of the single-ion and two-ion anisotropies and leads to a significant reduction in the Curie temperature of the system as observed experimentally
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