arXiv subjects

Churna Bhandari

Publications and source records attributed to Churna Bhandari.

15 recordsLinked to original sources

arXiv2024

Enhancing Stability, Magnetic Anisotropy, and Coercivity of $τ$-L$1_0$ MnAl: Machine Learning, $\textit{Ab Initio}$, and Micromagnetic Modeling

The binary manganese aluminium (MnAl) alloy with L$1_0$ crystal structure is a promising rare earth element-free permanent magnetic material because of its exceptional magnetic properties. However, experimentally synthesizing it in a stable bulk form is extremely challenging. Here, an alternative method of stabilizing the material is proposed and theoretically verified by partially substituting Mn and Al sites with Fe and Ni and identifying its enhanced phase stability, magnetic anisotropy, and coercivity from density functional theory (DFT), machine learning (ML) crystal graph convolution neural network (CGCNN), and micro-magnetic modeling. When considering a fixed ($50\%$)-Ni, the magnetic anisotropy increases with the increasing Fe content but decreases the formation energy. The calculated formation energies, elastic constants, and phonon frequencies demonstrate that all the binary and quaternary compositions are stable. Most importantly, the magnetic moment and magnetic anisotropy constants in $50\%$-Fe substituted composition (equiatomic phase) increase significantly compared to the MnAl. The predicted coercivity of the equiatomic phase is larger than the parent compound calculated by combining DFT computed parameters with micromagnetic simulations.

cond-mat.mtrl-sci↗

arXiv2024

Accurate Machine Learning Predictions of Coercivity in High-Performance Permanent Magnets

Increased demand for high-performance permanent magnets in the electric vehicle and wind turbine industries has prompted the search for cost-effective alternatives.Discovering new magnetic materials with the desired intrinsic and extrinsic permanent magnet properties presents a significant challenge to researchers because of issues with the global supply of rare-earth elements, material stability, and a low maximum magnetic energy product BH$_{max}$.While first-principle density functional theory (DFT) predicts materials' magnetic moments, magneto-crystalline anisotropy constants, and exchange interactions, it cannot compute coercivity ($H_c$).Although it is possible to calculate $H_c$ theoretically with micromagnetic simulations, the predicted value is larger than the experiment by almost an order of magnitude, due to the Brown paradox.To circumvent these, we employ machine learning (ML) methods on an extensive database obtained from experiments, DFT calculations, and micromagnetic modeling.The use of a large dataset enables realistic $H_c$ predictions for materials such as Ce-doped Nd$_2$Fe$_{14}$B, comparing favorably against micromagnetically simulated coercivities.Remarkably, our ML model accurately identifies uniaxial magneto-crystalline anisotropy as the primary contributor to $H_c$. With DFT calculations, we predict the Nd-site dependent magnetic anisotropy behavior in Nd$_2$Fe$_{14}$B, confirming that Nd $4g$-sites mainly contribute to uniaxial magneto-crystalline anisotropy, and also calculate Curie temperature (T$_{C}$).Both calculated results are in good agreement with experiment.The coupled experimental dataset and ML modeling with DFT input predict $H_c$ with far greater accuracy and speed than was previously possible using micromagnetic modeling.Further, we reverse-engineer the inter-grain exchange coupling with micromagnetic simulations by employing the ML predictions.

cond-mat.mtrl-sci↗

arXiv2023

Distinguishing erbium dopants in Y$_2$O$_3$ by site symmetry: \textit{ ab initio} theory of two spin-photon interfaces

We present a first-principles study of defect formation and electronic structure of erbium (Er)-doped yttria (Y$_2$O$_3$). This is an emerging material for spin-photon interfaces in quantum information science due to the narrow linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories and transducers. We calculate formation energies of neutral, negatively, and positively charged Er dopants and find the charge neutral configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the $C_2$ (more relevant for quantum memories) and $C_{3i}$ (more relevant for quantum transduction), we identify the former as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof (PBE) functional along with the Hubbard $U$ parameter and spin-orbit coupling (SOC), which yields a $\sim$ 6 $μ_B$ orbital and a $\sim$ 3 $μ_B$ spin magnetic moment, and 11 electrons in the Er $4f$ shell, confirming the formation of charge-neutral Er$^{3+}$. This standard density functional theory (DFT) approach underestimates the band gap of the host and lacks a first-principles justification for $U$. To overcome these issues, we performed screened hybrid functional (HSE) calculations, including a negative $U$ for the $4f$ orbitals, with mixing ($α$) and screening ($w$) parameters. These produced robust electronic features with slight modifications in the band gap and the $4f$ splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

cond-mat.mtrl-sci↗

arXiv2023

Giant magnetic and optical anisotropy in cerium-substituted M-type strontium hexaferrite driven by 4$f$ electrons

By performing density functional calculations, we find a giant magnetocrystalline anisotropy (MCA) constant in abundant element cerium (Ce) substituted M-type hexaferrite, in the energetically favorable strontium site, assisted by a quantum confined electron transfer from Ce to specific iron (2a) site. Remarkably, the calculated electronic structure shows that the electron transfer leads to the formation of Ce$^{3+}$ and Fe$^{2+}$ at the $2a$ site producing an occupied Ce($4f^1$) state below the Fermi level that adds a significant contribution to MCA and magnetic moment. A half Ce-substitution forms a metallic state, while a full substitution retains the semiconducting state of the strontium-hexaferrite (host). In the latter, the band gap is reduced due to the formation of charge transferred states in the gap region of the host. The optical absorption coefficient shows an enhanced anisotropy between light polarization in parallel and perpendicular directions. Calculated formation energies, including the analysis of probable competing phases, and elastic constants confirm that both compositions are chemically and mechanically stable. With successful synthesis, the Ce-hexaferrite can be a new high-performing critical-element-free permanent magnet material adapted for use in devices such as automotive traction drive motors.

cond-mat.mtrl-sci↗

arXiv2023

Dielectric Screening and Electric Field Control of Ferromagnetism at the CaMnO$_3$/CaRuO$_3$ Interface

Control of magnetism by an applied electric field is a desirable technique for the functionalization of magnetic materials. Motivated by recent experiments, we study the electric field control of the interfacial magnetism of CaRuO$_3$/CaMnO$_3$ (CRO/CMO) (001), a prototype interface between a non-magnetic metal and an antiferromagnetic insulator. Even without the electric field, the interfacial CMO layer acquires a ferromagnetic moment due to a spin-canted state, caused by the Anderson-Hasegawa double exchange (DEX) between the Mn moments and the leaked electrons from the CRO side. An electric field would alter the carrier density at the interface, leading to the possibility of controlling the magnetism, since DEX is sensitive to the carrier density. We study this effect quantitatively usingdensity-functional calculations in the slab geometry. We find a text-book like dielectric screening of the electric field, which introduces polarization charges at the interfaces and the surfaces. The extra charge at the interface enhances the ferromagnetism via the DEX interaction, while away from the interface the original AFM state of the Mn layers remains unchanged. The effect could have potential application in spintronics devices.

cond-mat.mtrl-sci↗

arXiv2021

Localized-delocalized crossover of spin-carriers and magnetization reversal in Co$_{2}$VO$_{4}$

Neutron diffraction, magnetization and muon spin relaxation measurements, supplemented by density functional theory (DFT) calculations are employed to unravel temperature-driven magnetization reversal (MR) in inverse spinel Co$_2$VO$_4$. All measurements show a second-order magnetic phase transition at $T_{\rm C} = 168$\,K to a collinear ferrimagnetic phase. The DFT results suggest the moments in the ferrimagnetic phase are delocalized and undergo gradual localization as the temperature is lowered below $T_{\rm C}$. The delocalized-localized crossover gives rise to a maximum magnetization at $T_{\rm NC} = 138$\,K and the continuous decrease in magnetization produces sign-change at $T_{\rm MR} \sim 65$\,K. Muon spectroscopy results support the DFT, as a strong $T_1$-relaxation is observed around $T_{\rm NC}$, indicating highly delocalized spin-carriers gradually tend to localization upon cooling. The magnetization reversal determined at zero field is found to be highly sensitive to the applied magnetic field, such that above $B\sim 0.25$\,T instead of a reversal a broad minimum in the magnetization is apparent at $T_{\rm MR}$. Analysis of the neutron diffraction measurements shows two antiparallel magnetic sub-lattice-structure, each belonging to magnetic ions on two distinct crystal lattice sites. The relative balance of these two structural components in essence determines the magnetization. Indeed, the order parameter of the magnetic phase on one site develops moderately more than that on the other site. Unusual tipping of the magnetic balance, caused by such site-specific magnetic fluctuation, gives rise to a spontaneous flipping of the magnetization as the temperature is lowered.

cond-mat.str-el↗

arXiv2021

Quantum-confined charge transfer that enhances magnetic anisotropy in lanthanum M-type hexaferrites

Iron-based hexaferrites are critical-element-free permanent magnet components of magnetic devices. Of particular interest is electron-doped M-type hexaferrite i.e., LaFe$_{12}$O$_{19}$ (LaM) in which extra electrons introduced by lanthanum substitution of barium/strontium play a key role in uplifting the magnetocrystalline anisotropy. We investigate the electronic structure of lanthanum hexaferrite using a \textit{localized} density functional theory which reproduces semiconducting behavior and identifies the origin of the very large magnetocrystalline anisotropy. Localized charge transfer from lanthanum to the iron at the crystal's $2a$ site produces a narrow $3d_{z^2}$ valence band strongly locking the magnetization along the $c$ axis. The calculated uniaxial magnetic anisotropy energies from fully self-consistent calculations are nearly double the single-shot values, and agree well with available experiments. The chemical similarity of lanthanum to other rare earths suggests that LaM can host for other rare earths possessing non-trivial $4f$ electronic states for, \textit{e.g.,} microwave-optical quantum transduction.

cond-mat.mtrl-sci↗

arXiv2020

A multiconfigurational study of the negatively charged nitrogen-vacancy center in diamond

Deep defects in wide band gap semiconductors have emerged as leading qubit candidates for realizing quantum sensing and information applications. Due to the spatial localization of the defect states, these deep defects can be considered as artificial atoms/molecules in a solid state matrix. Here we show that unlike single-particle treatments, the multiconfigurational quantum chemistry methods, traditionally reserved for atoms/molecules, accurately describe the many-body characteristics of the electronic states of these defect centers and correctly predict properties that single-particle treatments fail to obtain. We choose the negatively charged nitrogen-vacancy (NV$^-$) center in diamond as the prototype defect to study with these techniques due to its importance for quantum information applications and because its properties are well-known, which makes it an ideal benchmark system. By properly accounting for electron correlations and including spin-orbit coupling and dipolar spin-spin coupling in the quantum chemistry calculations, for the NV$^-$ center in diamond clusters, we are able to: (i) show the correct splitting of the ground (first-excited) triplet state into two levels (four levels), (ii) calculate zero-field splitting values of the ground and excited triplet states, in good agreement with experiment, and (iii) calculate the energy differences between ground and exited spin-triplet and spin-singlet states, as well as their ordering, which are also found to be in good agreement with recent experimental data. The numerical procedure we have developed is general and it can screen other color centers whose properties are not well known but promising for applications.

quant-ph↗

arXiv2019

Two dimensional electron gas in the $δ$-doped iridates with strong spin-orbit coupling: La$_δ$Sr$_2$IrO$_4$

Iridates are of considerable current interest because of the strong spin-orbit coupling that leads to a variety of new phenomena. Using density-functional studies, we predict the formation of a spin-orbital entangled two-dimensional electron gas (2DEG) in the $δ$-doped iridate La$_δ$Sr$_2$IrO$_4$, where a single SrO layer is replaced by a LaO layer. The extra La electron resides close to the $δ$-doped layer, partially occupying the $J_{\rm eff}= 1/2 $ upper Hubbard band and thereby making the interface metallic. The magnetic structure of the bulk is destroyed near the interface, with the Ir$_0$ layer closest to the interface becoming non-magnetic, while the next layer (Ir$_1$) continues to maintain the AFM structure of the bulk, but with a reduced magnetic moment. The Fermi surface consists of a hole pocket and an electron pocket, located in two different Ir layers (Ir$_0$ and Ir$_1$), with both carriers derived from the $J_{\rm eff}= 1/2 $ upper Hubbard band. The presence of both electrons and holes at the $δ$-doped interface suggests unusual transport properties, leading to possible device applications.

cond-mat.mtrl-sci↗

arXiv2018

Instability of the layered orthorhombic post-perovskite phase of SrTiO$_3$ and other candidate orthorhombic phases under pressure

While the tetragonal antiferro-electrically distorted (AFD) phase with space group $I4/mcm$ is well known for SrTiO$_3$ to occur below 105 K, there are also some hints in literature of an orthorhombic phase, either at lower temperature at high pressure. A previously proposed orthorhombic layered structure of SrTiO$_3$, known as the post-perovskite or CaIrO$_3$ structure with space group $Cmcm$ is shown to have significantly higher energy than the cubic or tetragonal phase and to have its minimum volume at larger volume than cubic perovskite. It thus cannot correspond to a high-pressure phase of SrTiO$_3$. We also study an alternative $Pnma$ phase obtained by two octahedral rotations about different axes. This phase is found to have slightly lower energy than the $I4/mcm$ phase in spite of the fact that its parent, in-phase tilted $P4/mbm$ phase is not found to occur. Alternative ferro-electric tetragonal and orthorhombic structures are also considered. These are not found to occur in our zero temperature calculations but have been suggested in previous literature to occur at higher temperature.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure and optical properties of Sr$_2$IrO$_4$ under epitaxial strain

We study the modification of the electronic structure in the strong spin-orbit coupled Sr$_2$IrO$_4$ by epitaxial strain using density functional methods. Structural optimization shows that strain changes the internal structural parameters such as the Ir-O-Ir bond angle, which has an important effect on the band structure. An interesting prediction is the $Γ- $X crossover of the valence band maximum with strain, while the conduction minimum at M remains unchanged. This in turn suggests strong strain dependence of the transport properties for the hole doped system, but not when the system is electron-doped. Taking the measured value of the $Γ-X$ separation for the unstrained case, we predict the $Γ- $X crossover of the valence band maximum to occur for the tensile epitaxial strain $e_{xx} \approx 3\%$. A minimal tight-binding model within the $J_{\rm eff} = 1/2$ subspace is developed to describe the main features of the band structure. The optical absorption spectra under epitaxial strain are computed using density-functional theory, which explains the observed anisotropy in the optical spectra with the polarization of the incident light. We show that the optical transitions between the Ir (d) states, which are dipole forbidden, can be explained in terms of the admixture of Ir (p) orbitals with the Ir (d) bands.

cond-mat.mtrl-sci↗

arXiv2018

Spin-orbit coupling induced magnetic anisotropy and large spin wave gap in $\rm Na Os O_3$

The role of spin-orbit coupling and Hund's rule coupling on magnetic ordering, anisotropy, and excitations are investigated within a minimal three-orbital model for the $5d^3$ compound $\rm Na Os O_3$. Asymmetry between the magnetic moments for the $xy$ and $xz,yz$ orbitals, arising from the hopping asymmetry generated by the $\rm Os O_6$ octahedral tilting and rotation, together with the weak correlation effect, are shown to be crucial for the large SOC induced magnetic anisotropy and spin wave gap observed in this compound. Due to the intrinsic SOC-induced changes in the electronic densities under rotation of the staggered field, their coupling with the orbital energy offset is also found to contribute significantly to the magnetic anisotropy energy.

cond-mat.str-el↗

arXiv2018

Effects of the structural distortion on the electronic band structure of {\boldmath $\rm Na Os O_3$} studied within density functional theory and a three-orbital model

Effects of the structural distortion associated with the $\rm OsO_6$ octahedral rotation and tilting on the electronic band structure and magnetic anisotropy energy for the $5d^3$ compound NaOsO$_3$ are investigated using the density functional theory (DFT) and within a three-orbital model. Comparison of the essential features of the DFT band structures with the three-orbital model for both the undistorted and distorted structures provides insight into the orbital and directional asymmetry in the electron hopping terms resulting from the structural distortion. The orbital mixing terms obtained in the transformed hopping Hamiltonian resulting from the octahedral rotations are shown to account for the fine features in the DFT band structure. Staggered magnetization and the magnetic character of states near the Fermi energy indicate weak coupling behavior.

cond-mat.str-el↗

arXiv2017

All-electron quasi-particle self-consistent $GW$ band structures for SrTiO$_3$ including lattice polarization corrections in different phase

The electronic band structure of SrTiO$_3$ is investigated in the all-electron QS$GW$ approximation. Unlike previous pseudopotential based QS$GW$ or single-shot $G_0W_0$ calculations, the gap is found to be significantly overestimated compared to experiment. After putting in a correction for the underestimate of the screening by the random phase approximation in terms of a 0.8$Σ$ approach, the gap is still overestimated. The 0.8$Σ$ approach is discussed and justified in terms of various recent literature results including electron-hole corrections. Adding a lattice polarization correction (LPC) in the ${\bf q}\rightarrow0$ limit for the screening of $W$, agreement with experiment is recovered. The LPC is alternatively estimated using a polaron model. We apply our approach to the cubic and tetragonal phases as well as a hypothetical layered post-perovskite structure and find that the LDA (local density approximation) to $GW$ gap correction is almost independent of structure.

cond-mat.mtrl-sci↗

arXiv2017

Lattice polarization effects on the screened Coulomb interaction $W$ of the GW approximation

In polar insulators where longitudinal and transverse optical phonon modes differ substantially, the electron-phonon coupling affects the energy-band structure primarily through the long-range Fröhlich contribution to the Fan term. This diagram has the same structure as the $GW$ self-energy where $W$ originates from the electron part of the screened coulomb interaction. The two can be conveniently combined by combining electron and lattice contributions to the polarizability. Both contributions are nonanalytic at the origin, and diverge as $1/q^2$ so that the predominant contribution comes from a small region around $q{=}0$. Here we adopt a simple estimate for the Fröhlich contribution by assuming that the entire phonon part can be attributed to a small volume of $q$ near $q{=}0$. We estimate the magnitude for $\mathbf{q}{\rightarrow}0$ from a generalized Lyddane-Sachs-Teller relation, and the radius from the inverse of the polaron length scale. The gap correction is shown to agree with Fröhlich's simple estimate $-α_Pω_L/2$ of the polaron effect.

cond-mat.mtrl-sci↗