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Register a new userA 4-fold-symmetry hexagonal ruthenium for magnetic heterostructures exhibiting enhanced perpendicular magnetic anisotropy and tunnel magnetoresistance
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An unusual crystallographic orientation of hexagonal Ru with a 4-fold symmetry emerging in epitaxial MgO/Ru/Co2FeAl/MgO heterostructures is reported, in which an approximately Ru(02-23) growth attributes to the lattice matching among MgO, Ru, and Co2FeAl. Perpendicular magnetic anisotropy of the Co2FeAl/MgO interface is substantially enhanced as compared with those with a Cr(001) layer. The MTJs incorporating this structure gave rise to the largest tunnel magnetoresistance for perpendicular MTJs using low damping Heusler alloys. The 4-fold-symmetry hexagonal Ru arises from an epitaxial growth with an unprecedentedly high crystal index, opening a unique pathway for the development of perpendicular anisotropy films of cubic and tetragonal ferromagnetic alloys.
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While the effects of lattice mismatch-induced strain, mechanical strain, as well as the intrinsic strain of thin films are sometimes detrimental, resulting in mechanical deformation and failure, strain can also be usefully harnessed for applications such as data storage, transistors, solar cells, and strain gauges, among other things. Here, we demonstrate that quantum transport across magnetic tunnel junctions (MTJs) can be significantly affected by the introduction of controllable mechanical strain, achieving an enhancement factor of ~2 in the experimental tunneling magnetoresistance (TMR) ratio. We further correlate this strain-enhanced TMR with coherent spin tunneling through the MgO barrier. Moreover, the strain-enhanced TMR is analyzed using non-equilibrium Greens function (NEGF) quantum transport calculations. Our results help elucidate the TMR mechanism at the atomic level and can provide a new way to enhance, as well as tune, the quantum properties in nanoscale materials and devices.
We investigated perpendicular magnetic anisotropy (PMA) and related properties of epitaxial Fe (0.7 nm)/MgAl2O4(001) heterostructures prepared by electron-beam evaporation. Using an optimized structure, we obtained a large PMA energy ~1 MJ/m3 at room temperature, comparable to that in ultrathin-Fe/MgO(001) heterostructures. Both the PMA energy and saturation magnetization show weak temperature dependence, ensuring wide working temperature in application. The effective magnetic damping constant of the 0.7 nm Fe layer was ~0.02 using time-resolved magneto-optical Kerr effect. This study demonstrates capability of the Fe/MgAl2O4 heterostructure for perpendicular magnetic tunnel junctions, as well as a good agreement with theoretical predictions.
Using first-principles calculations, we elucidate microscopic mechanisms of perpendicular magnetic anisotropy (PMA)in Fe/MgO magnetic tunnel junctions through evaluation of orbital and layer resolved contributions into the total anisotropy value. It is demonstrated that the origin of the large PMA values is far beyond simply considering the hybridization between Fe-3d$ and O-2p orbitals at the interface between the metal and the insulator. On-site projected analysis show that the anisotropy energy is not localized at the interface but it rather propagates into the bulk showing an attenuating oscillatory behavior which depends on orbital character of contributing states and interfacial conditions. Furthermore, it is found in most situations that states with $d_{yz(xz)}$ and $d_{z^2}$ character tend always to maintain the PMA while those with $d_{xy}$ and $d_{x^2-y^2}$ character tend to favor the in-plane anisotropy. It is also found that while MgO thickness has no influence on PMA, the calculated perpendicular magnetic anisotropy oscillates as a function of Fe thickness with a period of 2ML and reaches a maximum value of 3.6 mJ/m$^2$.
We report the observation of anomalies in the longitudinal magnetoresistance of tensile-strained (Ga,Mn)As epilayers with perpendicular magnetic anisotropy. Magnetoresistance measurements carried out in the planar geometry (magnetic field parallel to the current density) reveal spikes that are antisymmetric with respect to the direction of the magnetic field. These anomalies always occur during magnetization reversal, as indicated by a simultaneous change in sign of the anomalous Hall effect. The data suggest that the antisymmetric anomalies originate in anomalous Hall effect contributions to the longitudinal resistance when domain walls are located between the voltage probes. This interpretation is reinforced by carrying out angular sweeps of $vec{H}$, revealing an antisymmetric dependence on the helicity of the field sweep.
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.
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