We investigate rare-earth magnetic Weyl semimetals through first-principles simulations, analyzing the connection between incommensurate magnetic order and the presence of Weyl nodes in the electronic band structure. Focusing on PrAlSi, NdAlSi, and SmAlSi, we demonstrate that the reported helical ordering does not originate from the nesting of topological features at the Fermi surface or the Dzyaloshinskii-Moriya interaction. Instead, the helical order arises from frustrated isotropic short-range superexchange between the $4f$ moments facilitated by $pd$ hybridization with the main group elements. Employing a spin Hamiltonian with isotropic exchange and single-ion anisotropy we replicate the experimentally observed helical modulation.
For many years, density-functional-based calculations for the total energies of substitutionally disordered alloys have been based upon the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA). However, as a result of the single-site nature of the KKR-CPA, such calculations do not take into account important local environmental effects such as charge correlations (the Madelung energy) and chemical short-range order (SRO). Here the above approach is generalized by combining the recently developed Korringa-Kohn-Rostoker nonlocal coherent-potential approximation with density functional theory, showing how these effects may be systematically taken into account. As a first application of the theory, total energy calculations for the bcc ${\mathrm{Cu}}_{50}{\mathrm{Zn}}_{50}$ solid solution are presented, showing how the total energy varies as a function of SRO. The fcc ${\mathrm{Cu}}_{60}{\mathrm{Pd}}_{40}$ and ${\mathrm{Cu}}_{77}{\mathrm{Ni}}_{23}$ systems are also investigated.
For many years the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) has been widely used to describe the electronic structure of disordered systems based upon a first-principles description of the crystal potential. However, as a single-site theory the KKR-CPA is unable to account for important environmental effects such as short-range order (SRO) in alloys and spin fluctuations in magnets, amongst others. Using the recently devised KKR-NLCPA (where NL stands for nonlocal), we show how to remedy this by presenting explicit calculations for the effects of SRO on the electronic structure of the bcc Cu_{50}Zn_{50} solid solution.
We describe a `disordered local moment' (DLM) first-principles electronic structure theory which demonstrates that tricritical metamagnetism can arise in an antiferromagnetic metal due to the dependence of local moment interactions on the magnetisation state. Itinerant electrons can therefore play a defining role in metamagnetism in the absence of large magnetic anisotropy. Our model is used to accurately predict the temperature dependence of the metamagnetic critical fields in CoMnSi-based alloys, explaining the sensitivity of metamagnetism to Mn-Mn separations and compositional variations found previously. We thus provide a finite-temperature framework for modelling and predicting new metamagnets of interest in applications such as magnetic cooling.
Recently, we outlined a scheme to investigate the effects of both short-ranged and long-ranged compositional order on the magnetocrystalline anisotropy of alloys from a first-principles electronic structure point of view [Phys. Rev. Lett. 82, 5369 (1999)] and showed that in the ${\mathrm{Co}}_{0.5}{\mathrm{Pt}}_{0.5}$ alloy compositional order enhances the magnitude of magnetocrystalline anisotropy energy (MAE) by some two orders of magnitude. Here we describe our scheme in detail and study some more transition metal alloys. In the ${\mathrm{Co}}_{0.25}{\mathrm{Pt}}_{0.75}$ alloy we find the perfect ${L1}_{2}$ structure to be magnetically soft whereas imposition of directional order greatly enhances its MAE. We also present the effect of lattice distortion (tetragonalization) on MAE on the same footing and find that in the ${\mathrm{Co}}_{0.5}{\mathrm{Pt}}_{0.5}$ alloy it accounts for only about 20% of the observed enhancement, thus confirming that compositional order is the major player in this effect. Tetragonalization of the lattice has also a modest effect on the MAE of the ${\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}$ alloy. We also examine the electronic effects which underpin the directional chemical order that is produced by magnetic annealing of permalloy which we study within the same framework.
Abstract We study the phase behaviour of the Al x CrFeCoNi high-entropy alloy. Our approach is based on a perturbative analysis of the internal energy of the paramagnetic solid solution as evaluated within the Korringa-Kohn-Rostoker formulation of density functional theory, using the coherent potential approximation to average over disorder. Via application of a Landau-type linear response theory, we infer preferential chemical orderings directly. In addition, we recover a pairwise form of the alloy internal energy suitable for study via atomistic simulations, which in this work are performed using the nested sampling algorithm, which is well-suited for studying complex potential energy surfaces. When the underlying lattice is fcc, at low concentrations of Al, depending on the value of x , we predict either an L1 2 or D0 22 ordering emerging below approximately 1000 K. On the other hand, when the underlying lattice is bcc, consistent with experimental observations, we predict B2 ordering temperatures higher than the melting temperature of the alloy, confirming that this ordered phase forms directly from the melt. For both fcc and bcc systems, chemical orderings are dominated by Al moving to one sublattice, Ni and Co the other, while Cr and Fe remain comparatively disordered. On the bcc lattice, our atomistic modelling suggests eventual decomposition into B2 NiAl and Cr-rich phases. These results shed light on the fundamental physical origins of atomic ordering tendencies in these intriguing materials.
The optimal amount of dysprosium in the highly magnetostrictive rare-earth compounds Tb$_{1-x}$Dy$_x$Fe$_2$ for room temperature applications has long been known to be $x$=0.73 (Terfenol-D). Here, we derive this value from first principles by calculating the easy magnetization direction and magnetostriction as a function of composition and temperature. We use crystal field coefficients obtained within density-functional theory to construct phenomenological anisotropy and magnetoelastic constants. The temperature dependence of these constants is obtained from disordered local moment calculations of the rare earth magnetic order parameter. Our calculations find the critical Dy concentration required to switch the magnetization direction at room temperature to be $x_c$=0.78, with magnetostrictions $\lambda_{111}$=2700 and $\lambda_{100}$=-430~ppm, close to the Terfenol-D values.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
