Topological transport properties of ferromagnetic and antiferromagnetic materials

Magnetic materials are of fundamental importance to the welfare of our society since they find use, among others, in energy harvesting applications. The rapid technological development requires the generation of new environment friendly methods to power up novel devices, therefore, thermoelectric materials become vital for future applications. A particular class of magnetic materials that can be used in thermoelectric devices is those with nontrivial band topology, such as Weyl and nodal line semimetals, that have recently attracted intensive attention due to their interesting properties. The anomalous Hall (Nernst) effect, being the generation of a transverse spin polarized charge current as a response to a longitudinal charge current (thermal gradient), was initially associated to ferromagnets. Recently however, it was demonstrated that collinear and non-collinear antiferromagnets can induce finite values, making them interesting for novel applications. These findings though, not only challenged the current understanding of the theoretical background but also the conditions of their existence, being till nowadays pending problems. In this work, a computational framework to construct maximally localized Wannier functions in an automatic way is provided and subsequently used to calculate the anomalous Hall and Nernst conductivities of ferromagnetic and non-collinear antiferromagnetic intermetallic compounds, with a high success rate of 92%. Detailed symmetry analysis is performed in order to reveal the vanishing anomalous Hall and Nernst conditions in certain ferromagnetic and antiferromagnetic compounds. It is demonstrated that the large values of anomalous Hall and Nernst conductivities are due to the presence of Weyl nodes, nodal lines and small gap areas and that they can further be tuned by means of external stimuli, leading to further enhancement of the anomalous Hall and Nernst conductivities. In the future, the automated Wannier function workflow can be used to construct the maximally localized Wannier functions of any 3d transition-metal based system, with or without the inclusion of spin-orbit interaction with minimum human intervention and external stimuli can be used to further enhance the topological transport properties of a compound

Enhanced anomalous Nernst effects in ferromagnetic materials driven by Weyl nodes

Based on high-throughput first-principles calculations, we evaluated the anomalous Hall and anomalous Nernst conductivities of 266 transition-metal-based ferromagnetic compounds. Detailed analysis based on the symmetries and Berry curvatures reveals that the origin of singular-like behaviour of anomalous Hall/Nernst conductivities can be mostly attributed to the appearance of Weyl nodes or nodal lines located in the proximity of the Fermi energy, which can be further tailored by external stimuli such as biaxial strains and magnetic fields. Moreover, such calculations are enabled by the automated construction of Wannier functions with a success rate of 92%, which paves the way to perform accurate high-throughput evaluation of the physical properties such as the transport properties using the Wannier interpolationComment: 9 pages, 4 figure

Multifunctional Antiperovskites driven by Strong Magnetostructural Coupling

Based on density functional theory calculations, we elucidated the origin of multifunctional properties for cubic antiperovskites with noncollinear magnetic ground states, which can be attributed to strong isotropic and anisotropic magnetostructural coupling. 16 out of 54 stable magnetic antiperovskites M$_3$XZ (M = Cr, Mn, Fe, Co, and Ni; X = selected elements from Li to Bi except for noble gases and 4f rare-earth metals; and Z = C and N) are found to exhibit the $\Gamma_{4g}$/$\Gamma_{5g}$ (i.e., characterized by irreducible representations) antiferromagnetic magnetic configurations driven by frustrated exchange coupling and strong magnetocrystalline anisotropy. Using the magnetic deformation as an effective proxy, the isotropic magnetostructural coupling is characterized, and it is observed that the paramagnetic state is critical to understand the experimentally observed negative thermal expansion and to predict the magnetocaloric performance. Moreover, the piezomagnetic and piezospintronic effects induced by biaxial strain are investigated. It is revealed that there is not a strong correlation between the induced magnetization and anomalous Hall conductivities by the imposed strain. Interestingly, the anomalous Hall/Nernst conductivities can be significantly tailored by the applied strain due to the fine-tuning of the Weyl points energies, leading to promising spintronic applications.Comment: 11 pages, 5 figure

The anomalous Hall effect in non-collinear antiferromagnetic Mn3_{3}NiN thin films

We have studied the anomalous Hall effect (AHE) in strained thin films of the frustrated antiferromagnet Mn$_{3}$NiN. The AHE does not follow the conventional relationships with magnetization or longitudinal conductivity and is enhanced relative to that expected from the magnetization in the antiferromagnetic state below $T_{\mathrm{N}} = 260$\,K. This enhancement is consistent with origins from the non-collinear antiferromagnetic structure, as the latter is closely related to that found in Mn$_{3}$Ir and Mn$_{3}$Pt where a large AHE is induced by the Berry curvature. As the Berry phase induced AHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of the chemically flexible Mn$_{3}A$N structure

CCDCGAN: Inverse design of crystal structures

Autonomous materials discovery with desired properties is one of the ultimate goals for modern materials science. Applying the deep learning techniques, we have developed a generative model which can predict distinct stable crystal structures by optimizing the formation energy in the latent space. It is demonstrated that the optimization of physical properties can be integrated into the generative model as on-top screening or backwards propagator, both with their own advantages. Applying the generative models on the binary Bi-Se system reveals that distinct crystal structures can be obtained covering the whole composition range, and the phases on the convex hull can be reproduced after the generated structures are fully relaxed to the equilibrium. The method can be extended to multicomponent systems for multi-objective optimization, which paves the way to achieve the inverse design of materials with optimal properties.Comment: 14 pages, 3 figure

Tunable anomalous Hall and Nernst effects in MM′X compounds

Based on first-principles calculations, the anomalous Hall conductivity (AHC) and anomalous Nernst conductivities (ANCs) of the XMnP (X = Ti, Zr, Hf) compounds are evaluated, and the possibility to tailor such properties in compounds susceptible to changing the magnetization directions is also investigated. We observe large changes in the calculated AHC and ANC for different magnetization directions that are originating from changes in the band structure all over the whole Brillouin zone. Our study gives a promising clue on engineering magnetic intermetallic compounds for tunable transverse thermoelectric applications

Giant Anomalous Hall and Nernst Conductivities in Magnetic All‐d Metal Heusler Alloys

All‐d Heuslers are a category of novel compounds combining versatile functionalities such as caloric responses and spintronics with enhanced mechanical properties. Despite the promising transport properties (anomalous Hall (AHC) and anomalous Nernst (ANC) conductivities) shown in the conventional Co₂XY Heuslers with p‐d hybridization, the all‐d Heuslers with only d‐d hybridization open a new horizon to search for new candidates with outstanding transport properties. In this work, the AHC and ANC are evaluated for thermodynamically stable ferro/ferri‐magnetic all‐d‐metal regular Heusler compounds based on high‐throughput first‐principles calculations. It is observed that quite a few materials exhibit giant AHCs and ANCs, such as cubic Re₂TaMn with an AHC of 2011 S cm⁻¹, and tetragonal Pt₂CrRh with an AHC of 1966 S cm⁻¹ and an ANC of 7.50 A m⁻¹K⁻¹. Comprehensive analysis on the electronic structure reveals that the high AHC can be attributed to the occurrence of the Weyl nodes or gapped nodal lines in the neighborhood of the Fermi level. The correlations between such transport properties and the number of valence electrons are also thoroughly investigated, which provides a practical guidance to tailor AHC and ANC via chemical doping for transverse thermoelectric applications

Tailoring the anomalous Hall effect in the noncollinear antiperovskite Mn3GaN

Based on first-principles calculations, we investigated the topological transport properties of Mn$_3$GaN with coplanar noncollinear magnetic structures. The intrinsic anomalous Hall conductivity (IAHC) displays a significant dependence with respect to the in-plane magnetization direction between the $\Gamma_{5g}$ and $\Gamma_{4g}$ magnetic configurations, where large anomalous Nernst effect (ANE) can be induced by tailoring the magnetization direction. Moreover, we observed strong piezospintronic effect in Mn$_3$GaN, where large IAHC can be induced by moderate epitaxial strain. Symmetry analysis reveals that for both cases, the nonzero IAHC is originated from the spin-orbit coupling instead of the noncollinear magnetic configurationsComment: 5 pages, 2 figure

Enhanced anomalous Nernst effects in ferromagnetic materials driven by Weyl nodes

Based on high-throughput (HTP) first-principles calculations, we evaluated the anomalous Hall and anomalous Nernst conductivities of 266 transition-metal-based ferromagnetic compounds. Detailed analysis based on the symmetries and Berry curvatures reveals that the origin of singular-like behavior of anomalous Hall/Nernst conductivities can be mostly attributed to the appearance of Weyl nodes or nodal lines located in the proximity of the Fermi energy, which can be further tailored by external stimuli such as biaxial strains and magnetic fields. Moreover, such calculations are enabled by the automated construction of Wannier functions with a success rate of 92%, which paves the way to perform accurate HTP evaluation of the physical properties such as the transport properties using the Wannier interpolation

High-throughput screening of Half-antiperovskites with a stacked kagome lattice

Half-antiperovskites (HAPs) are a class of materials consisting of stacked kagome lattices and thus host exotic magnetic and electronic states. We perform high-throughput calculations based on density functional theory (DFT) and atomistic spin dynamics (ASD) simulations to predict stable magnetic HAPs M$_3$X$_2$Z$_2$ (M = Cr, Mn, Fe, Co, and Ni; X is one of the elements from Li to Bi except noble gases and 4$f$ rare-earth metals; Z = S, Se, and Te), with both thermodynamical and mechanical stabilities evaluated. Additionally, the magnetic ground states are obtained by utilizing DFT calculations combined with the ASD simulations. The existing spin frustration in an AFM kagome lattice manifests as competing behavior of the in-plane FM and AFM couplings. For a total number of 930 HAP compositions considered, we have found 23 compounds that are stabilized at non-collinear antiferromagnetic (AFM) state and 11 compounds that possess ferromagnetic (FM) order