Generalized-gradient-approximation study of the magnetic and cohesive properties of bcc, fcc, and hcp Mn

Accelerated crystal structure prediction of multi-elements random alloy using expandable features

Properties of solid-state materials depend on their crystal structures. In solid solution high entropy alloy (HEA), its mechanical properties such as strength and ductility depend on its phase. Therefore, the crystal structure prediction should be preceded to find new functional materials. Recently, the machine learning-based approach has been successfully applied to the prediction of structural phases. However, since about 80% of the data set is used as a training set in machine learning, it is well known that it requires vast cost for preparing a dataset of multi-element alloy as training. In this work, we develop an efficient approach to predicting the multi-element alloys' structural phases without preparing a large scale of the training dataset. We demonstrate that our method trained from binary alloy dataset can be applied to the multi-element alloys' crystal structure prediction by designing a transformation module from raw features to expandable form. Surprisingly, without involving the multi-element alloys in the training process, we obtain an accuracy, 80.56% for the phase of the multi-element alloy and 84.20% accuracy for the phase of HEA. It is comparable with the previous machine learning results. Besides, our approach saves at least three orders of magnitude computational cost for HEA by employing expandable features. We suggest that this accelerated approach can be applied to predicting various structural properties of multi-elements alloys that do not exist in the current structural database.

Ultra-low magnetic damping of a metallic ferromagnet

Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as damping strongly affects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials1-3. This presents a challenge for a broad range of applications in spintronics4 and spin-orbitronics that depend on materials and structures with ultra-low damping5,6. It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10-4, which is comparable to values reported only for ferrimagnetic insulators7,8. We explain this phenomenon by a unique feature of the band structure in this system: the density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms and a test of the theoretical predictions proposed by Mankovsky and colleagues3.

Magnetic properties of strained La2/3Sr1/3MnO3 perovskites from first principles

The critical temperature T(C) of ferromagnetic La(x)Sr(1-x)MnO3 (LSMO) can be controlled by distorting the crystal structure, as was reported by Thiele et al (2007 Phys. Rev. B 75 054408). To confirm these findings theoretically, we investigate the electronic as well as the magnetic ground state properties of La2/3Sr1/3MnO3 as a function of tetragonal lattice distortions, using a multiple-scattering Green function method. Within this approach, we calculate exchange coupling constants as well as the phase transition temperature from first principles. Comparing our findings with those for La2/3Sr1/3CoO3 (LSCO), we find that the decrease of T(C) is much stronger in LSMO than in LSCO. Our findings can be explained by the electronic structures and are also in accordance with the experiment. The computed decrease of TC with distortion is smaller than observed experimentally, a result that corroborates the importance of phonon contributions.

Polarization reduction in half-metallic Heusler alloys: the effect of point defects and interfaces with semiconductors

Half-metallic full-Heusler alloys represent a promising class of materials for spintronic applications. However, (i) intrinsic point defects in Heusler compounds can be detrimental with respect to their predicted 100% spin polarization at the Fermi level and (ii) when joined to mainstream semiconductors the presence of interface states-which destroys half-metallicity-can degrade their performance. Here, we present an overview of recent first-principles calculations performed to explore both these issues. In particular, we focus on ab initio FLAPW calculations performed for Co(2)MnGe and Co(2)MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well as non-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings show that Mn antisites, due to their low formation energies, can easily occur, in excellent consistency with experimental observations, and that they do not destroy half-metallicity. On the other hand, Co antisites, which also show a modest formation energy, give rise to defect states at the Fermi level. As for the [001]-ordered interfaces, we show that the strong hybridization in proximity to the junction gives rise to rather broad interface states that locally destroy half-metallicity. However, the bulk gaps (both in the minority spin channel for the Heusler alloy and for both spin channels in the semiconducting side) are fully recovered within a few layers away from the junction.

A review of the electronic and magnetic properties of tetrahedrally bonded half-metallic ferromagnets

The emergence of the field of spintronics has brought half-metallic ferromagnets to the centre of scientific research. A lot of interest was focused on newly created transition-metal pnictides (such as CrAs) and chalcogenides (such as CrTe) in the metastable zinc-blende lattice structure. These compounds were found to have the advantage of high Curie temperatures in addition to their structural similarity to semiconductors. Significant theoretical activity has been devoted to the study of the electronic and magnetic properties of these compounds in an effort to achieve a better control of their experimental behaviour in realistic applications. This paper is devoted to an overview of the studies of these compounds, with emphasis on theoretical results, covering their bulk properties (electronic structure, magnetism, stability of the zinc-blende phase, stability of ferromagnetism) as well as low-dimensional structures (surfaces, interfaces, nanodots and transition-metal delta-doped semiconductors) and phenomena that can possibly destroy the half-metallic property, like structural distortions or defects.

Ferromagnetism of MnAs studied by heteroepitaxial films on GaAs(001)

Thin epitaxial films of MnAs--promising candidates for the spin injection into semiconductors--are well known to undergo simultaneously a first-order structural and magnetic phase transition at 10-40 degrees C. The evolution of stress and magnetization of MnAs/GaAs(001), both measured quantitatively with our cantilever beam magnetometer at the coexistence region of alpha-MnAs and beta-MnAs, reveal an orthorhombically distorted unit cell of the ferromagnetic phase, which provides important clues on the origin of ferromagnetism in MnAs.