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Register a new userOut- versus in-plane magnetic anisotropy of free Fe and Co nanocrystals: tight-binding and first-principles studies
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We report tight-binding (TB) and Density Function Theory (DFT) calculations of magnetocrystalline anisotropy energy (MAE) of free Fe (body centerd cubic) and Co (face centered cubic) slabs and nanocrystals. The nanocrystals are truncated square pyramids which can be obtained experimentally by deposition of metal on a SrTiO$_3$(001) substrate. For both elements our local analysis shows that the total MAE of the nanocrystals is largely dominated by the contribution of (001) facets. However, while the easy axis of Fe(001) is out-of-plane, it is in-plane for Co(001). This has direct consequences on the magnetic reversal mechanism of the nanocrystals. Indeed, the very high uniaxial anisotropy of Fe nanocrystals makes them a much better potential candidate for magnetic storage devices.
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Phase-separated semiconductors containing magnetic nanostructures are relevant systems for the realization of high-density recording media. Here, the controlled strain engineering of Ga$delta$FeN layers with Fe$_y$N embedded nanocrystals (NCs) textit{via} Al$_x$Ga$_{1-x}$N buffers with different Al concentration $0<x_mathrm{Al}<41$% is presented. Through the addition of Al to the buffer, the formation of predominantly prolate-shaped $varepsilon$-Fe$_3$N NCs takes place. Already at an Al concentration $x_mathrm{Al}$,$approx$,5% the structural properties---phase, shape, orientation---as well as the spatial distribution of the embedded NCs are modified in comparison to those grown on a GaN buffer. Although the magnetic easy axis of the cubic $gamma$-Ga$_y$Fe$_{4-y}$N nanocrystals in the layer on the $x_mathrm{Al} = 0%$ buffer lies in-plane, the easy axis of the $varepsilon$-Fe$_3$N NCs in all samples with Al$_x$Ga$_{1-x}$N buffers coincides with the $[0001]$ growth direction, leading to a sizeable out-of-plane magnetic anisotropy and opening wide perspectives for perpendicular recording based on nitride-based magnetic nanocrystals.
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 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.
Tilted off-plane magnetic anisotropy induces two unusual characteristic magnetotransport phenomena: extraordinary Hall effect in the presence of an in-plane magnetic field, and non-monotonic anisotropic magnetoresistance in the presence of a field normal to the sample plane. We show experimentally that these effects are generic, appearing in multiple ferromagnetic systems with tilted anisotropy introduced either by oblique deposition from a single source or in binary systems co-deposited from separate sources. We present a theoretical model demonstrating that these observations are natural results of the standard extraordinary Hall effect and anisotropic magnetoresistance, when the titled anisotropy is properly accounted for. Such a scenario may help explaining various previous intriguing measurements by other groups.
Solid solution BiFe1-xCoxO3 shows anti-ferromagnetic order and pyroelectric order, simultaneously. It has been known that BiFe1-xCoxO3 exhibits a structural phase transition between monoclinic and tetragonal phases as x increases. This kinds of transition is often called morphotoropic phase boundary, which is well known to take place in a representative piezoelectric oxide, PbZr1-xTixO3. In order to theoretically understand the piezoelectric property in BiFe1-xCoxO3, we performed ab-initio electronic-structure calculations and studied the structural stability, the magnetic property, and the electronic polarization by means of super-cell approach. It turns out that the large electric polarization and the particular pyramidal coordination suppress the response of the electric polarization under strain. A way to enhance the piezoelectric effect in BiFe1-xCoxO3 is proposed.
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