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Register a new userOut-of-plane magnetic anisotropy in bulk ilmenite $text{CoTiO}_3$
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Structural, electronic and magnetic properties of bulk ilmenite CoTiO$_3$ are analyzed in the framework of Density Functional Theory (DFT), using the Generalized Gradient Approximation (GGA) and Hubbard-corrected approaches. We find that the G-type antiferromagnetic (G-AFM) structure, which consists of antiferromagnetically coupled ferromagnetic $ab$ planes, is the ground-state of the system, in agreement with experiments. Furthermore, cobalt titanates present two critical temperatures related to the breaking of the inter- and intra-layer magnetic ordering. This would result in the individual planes remaining ferromagnetic even at temperatures above the Neel temperature. When spin-orbit coupling is included in our calculations, we find an out-of-plane magnetic anisotropy, which can be converted to an in-plane anisotropy with a small doping of electrons corresponding to about 2.5% Ti substitution for Co, consistent with experimental expectations. We thus present a disorder-dependent study of the magnetic anisotropy in bulk $text{CoTiO}_3$, which will determine its magnon properties, including topological aspects.
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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.
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.
The electronic and magnetic properties of individual Fe atoms adsorbed on the surface of the topological insulator Bi$_2$Te$_3$(111) are investigated. Scanning tunneling microscopy and spectroscopy prove the existence of two distinct types of Fe species, while our first-principles calculations assign them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray magnetic circular dichroism measurements and angular dependent magnetization curves reveals out-of-plane anisotropies for both species with anisotropy constants of $K_{text{fcc}} = (10 pm 4)$ meV/atom and $K_{text{hcp}} = (8 pm 4)$ meV/atom. These values are well in line with the results of calculations.
Recently, intriguing physical properties have been unraveled in anisotropic layered semiconductors with the in-plane anisotropy often originates directly from the low crystallographic symmetry. However, little has been known about the systems where the size effect dominates the anisotropy of electronic band structures. Here, applying both experiment and theory, we show that the anisotropic energy bands of monoclinic gallium telluride (GaTe) are determined by a strong bulk-surface interaction rather than geometric factors. Bulk electronic states are found to be the major contribution to the highest valence band, whose anisotropy is yet immune to surface doping by potassium atoms. Further analysis indicates the weakened bulk-surface interaction gives rise to an inverse anisotropy of hole effective masses and the strong interlayer coupling induces a direct-indirect-direct band gap transition at transfer from mono- to few-layer GaTe. Our results thus pave the way to future applications of anisotropic layered semiconductors in nanoelectronics and optoelectronics.
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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