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Register a new userEnhancement of perpendicular magnetic anisotropy and its electric field-induced change through interface engineering in Cr/Fe/MgO
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Added by
Anna Kozio{\\l}-Rachwa{\\l}
Publication date
2016
fields
Physics
and research's language is
English
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Recently, perpendicular magnetic anisotropy (PMA) and its voltage control (VC) was demonstrated for Cr/Fe/MgO (Physical Review Applied 5, 044006 (2016)). In this study, we shed a light on the origin of large voltage-induced anisotropy change in Cr/Fe/MgO. Analysis of the chemical structure of Cr/Fe/MgO revealed the existence of Cr atoms in the proximity of the Fe/MgO interface, which can affect both magnetic anisotropy (MA) and its VC. We showed that PMA and its VC can be enhanced by controlled Cr doping at the Fe/MgO interface. For Cr/Fe (5.9 {AA})/Cr (0.7 {AA})/MgO with an effective PMA of 0.8 MJ/m3, a maximum value of the voltage-controlled magnetic anisotropy (VCMA) effect of 370 fJ/Vm was demonstrated.
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Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high surface PMA. Moreover, the effective magnetic anisotropy has a strong tendency to go from in-plane to out-of-plane character as a function of Cr and V concentration favoring out-of-plane magnetization direction for ~1.5 nm thick Fe layers at impurity concentrations above 20 %. At the same time, spin polarization is not affected and even enhanced in most situations favoring an increase of tunnel magnetoresistance (TMR) values.
The electric field effect on magnetic anisotropy was studied in an ultrathin Fe(001) monocrystalline layer sandwiched between Cr buffer and MgO tunnel barrier layers, mainly through post-annealing temperature and measurement temperature dependences. A large coefficient of the electric field effect of more than 200 fJ/Vm was observed in the negative range of electric field, as well as an areal energy density of perpendicular magnetic anisotropy (PMA) of around 600 uJ/m2. More interestingly, nonlinear behavior, giving rise to a local minimum around +100 mV/nm, was observed in the electric field dependence of magnetic anisotropy, being independent of the post-annealing and measurement temperatures. The insensitivity to both the interface conditions and the temperature of the system suggests that the nonlinear behavior is attributed to an intrinsic origin such as an inherent electronic structure in the Fe/MgO interface. The present study can contribute to the progress in theoretical studies, such as ab initio calculations, on the mechanism of the electric field effect on PMA.
The structures of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures relating to the interface-induced perpendicular magnetic anisotropy (PMA) were investigated using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray magnetic circular dichroism. We found that Al atoms from the Co2FeAl layer significantly interdiffuse into MgO, forming an Al-deficient Co-Fe-Al/Mg-Al-O structure near the Co2FeAl/MgO interface. This atomic replacement may play an additional role for enhancing PMA, which is consistent with the observed large perpendicular orbital magnetic moments of Fe atoms at the interface. This work suggests that control of interdiffusion at ferromanget/barrier interfaces is critical for designing an interface-induced PMA system.
We investigated electronic structure and magnetic anisotropy in the Fe/MgO interface of magnetic metal and dielectric insulator under the Cr layer of small electronegativity, by means of the first-principles density functional approach. The result indicates that the interface resonance state gets occupied unlike a typical rigid band picture as the number of Fe layers decreases, finding large perpendicular anisotropies in the oscillating behavior for thickness dependence. We discuss scenarios of the two dimensional van Hove singularity associated with flat band dispersions, and also the accuracies of anisotropy energy in comparison with the available experimental data.
Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT-MRAM), which is non-volatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density functional theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising Fe(n)Co(m)Fe(n)/MgO storage layers with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m2, leveraging the interfacial perpendicular anisotropy of Fe/MgO along with a stress-induced bulk PMA discovered within bcc Co. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure Fe/MgO case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the Fe(3ML)Co(4ML)Fe(3ML) as a storage layer for MgO-based STT-MRAM cells. The large PMA in strained bcc Co is explained in the framework of Brunos model by the MgO-imposed strain and consequent changes in the energies of dyz and dz2 minority-spin bands.
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