RT12 materials continue to attract attention due to their potential use as “rare-earth-lean” permanent magnets, but converting their promising intrinsic properties into practical high performance remains an elusive goal. Sophisticated experimental characterization techniques are providing unprecedented insight into the structure of these materials at the atomistic scale. Atomistic spin dynamics or micromagnetics simulations could help unravel the links between these structures and resultant magnet performance, but require input data describing the intrinsic magnetic properties. Here, first-principles calculations based on density-functional theory are used to determine these properties for two model interface structures which have been derived from recently reported high resolution electron microscopy images. One model structure is a stoichiometric twin formed by mirroring the RT12 structure in the (101) plane, and the other model structure is a “stacking fault” involving the insertion of a RT4 plane and a displacement along the [100] axis. Magnetic moments and crystal field coefficients have been calculated for the optimized structures. The interfaces modify the magnetic properties at the sub-nm scale. In particular, in the R-rich region of the “stacking fault”, the local easy axis of magnetization rotates by 49^∘ from its bulk direction, which may lead to reduced coercivity through the easier nucleation of reverse domains.

We present a theory describing the single-ion anisotropy of rare-earth (RE) magnets in the presence of point defects. Taking the RE-lean 1∶12 magnet class as a prototype, we use first-principles calculations to show how the introduction of Ti substitutions into SmFe_{12} perturbs the crystal field, generating new coefficients due to the lower symmetry of the RE environment. We then demonstrate that these perturbations can be described extremely efficiently using a screened point charge model. We provide analytical expressions for the anisotropy energy that can be straightforwardly implemented in atomistic spin dynamics simulations, meaning that such simulations can be carried out for an arbitrary arrangement of point defects. The significant crystal field perturbations calculated here demonstrate that a sample that is single phase from a structural point of view can nonetheless have a dramatically varying anisotropy profile at the atomistic level if there is compositional disorder, which may influence localized magnetic objects like domain walls or skyrmions.

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.

Solution processable metallic nanomaterials present a convenient way to fabricate conductive structures, which are necessary in all electronic devices. However, they tend to require post-treatments to remove the bulky ligands around them to achieve high conductivity. In this work, we present a method to formulate a post-treatment free conductive silver nanowire ink by controlling the type of ligands around the silver nanowires. We found that bidentate ligands with a rigid molecular structure were effective in improving the conductivity of the silver nanowire networks as they could maximize the number of linkages between neighboring nanowires. In addition, DFT calculations also revealed that ligands with good LUMO to silver energy alignment were more effective. Because of these reasons, fumaric acid was found to be the most effective ligand and achieved a large reduction in sheet resistance of 70% or higher depending on the nanowire network density. The concepts elucidated from this study would also be applicable to other solution processable nanomaterials systems such as quantum dots for photovoltaics or LEDs which also require good charge transport being neighboring nanoparticles.

Metals (Ni, Co) and hexaaminotriphenylene (HATP) molecules are evaporated separately in an ultrahigh vacuum onto the Au(111) surface. During an annealing step, monolayer single metal and bimetallic NixCo3-x(HITP)2 metal-organic frameworks (MOFs) are formed via an on-surface reaction of the organic molecules with the metals. The MOFs are characterized by scanning tunneling microscopy (STM), which reveals the hexagonal framework structures and growth processes. The pure Ni-3(HITP)2 and bimetallic NixCo3-x(HITP)2 frameworks have a similar isotropic island morphology, while the Co-3(HITP)(2) islands are ribbon shaped. Individual Ni and Co metal centers can be atomically identified by their different apparent heights in the STM images of the bimetallic framework. Density functional theory (DFT) calculations indicate that the apparent height difference is caused by electronic effects rather than the difference of topography. The STM images further reveal that in the mixed-metal MOFs the Ni and Co centers are distributed randomly.

The magnetocrystalline anisotropy energy of atomically ordered L1_0 FeNi (the meteoritic mineral tetrataenite) is studied within a first-principles electronic structure framework. Two compositions are examined: equiatomic Fe_0.5Ni_0.5 and an Fe-rich composition, Fe_0.56Ni_0.44. It is confirmed that, for the single crystals modelled in this work, the leading-order anisotropy coefficient K_1 dominates the higher-order coefficients K_2 and K_3. To enable comparison with experiment, the effects of both imperfect atomic long-range order and finite temperature are included. While our computational results initially appear to undershoot the measured experimental values for this system, careful scrutiny of the original analysis due to Néel et al. [J. Appl. Phys. 35, 873 (1964)] suggests that our computed value of K_1 is, in fact, consistent with experimental values, and that the noted discrepancy has its origins in the nanoscale polycrystalline, multivariant nature of experimental samples, that yields much larger values of K_2 and K_3 than expected a priori. These results provide fresh insight into the existing discrepancies in the literature regarding the value of tetrataenite's uniaxial magnetocrystalline anisotropy in both natural and synthetic samples.

We present an inelastic neutron scattering study of the crystal electric field levels in the intermetallic ferrimagnets RECo$_{5}$ (RE = Nd and Y). In NdCo$_{5}$, measurements at $5~$K reveal two levels at approximately 28.9 and 52.9 meV. Crystal field calculations including the exchange field $B_{\textrm{exc}}$ from the Co sites account for both of these, as well as the spectrum at temperatures above the spin-reorientation transition at $\sim 280$~K. In particular, it is found that both a large hexagonal crystal field parameter $A_{6}^6\langle r^6 \rangle$ and $B_{\textrm{exc}}$ are required to reproduce the data, with the latter having a much larger value than that deduced from previous computational and experimental studies. Our study sheds light on the delicate interplay of terms in the rare-earth Hamiltonian of RECo$_5$ systems, and is therefore expected to stimulate further experimental and computational work on the broader family of rare-earth permanent magnets.

La(FexSi1-x)(13) and derived quaternary compounds are well-known for their giant, tunable, magneto-and barocaloric responses around a first-order paramagnetic-ferromagnetic transition near room temperature with low hysteresis. Remarkably, such a transition shows a large spontaneous volume change together with itinerant electron metamagnetic features. While magnetovolume effects are well-established mechanisms driving first-order transitions, purely electronic sources have a long, subtle history and remain poorly understood. Here we apply a disordered local moment picture to quantify electronic and magnetoelastic effects at finite temperature in La(FexSi1-x)(13) from first-principles. We obtain results in very good agreement with experiment and demonstrate that the magnetoelastic coupling, rather than purely electronic mechanisms, drives the first-order character and causes at the same time a huge electronic entropy contribution to the caloric response.

TiO2 has been identified as a promising electrontransportlayer in Si solar cells. Experiments have revealed that the Si:TiO2 interface undergoes structural changes depending on how itwas fabricated. However, less is understood about the sensitivityof electronic properties, such as band alignments, to these changes.Here, we present first-principles calculations of band alignmentsbetween Si and anatase TiO2, investigating different surfaceorientations and terminations. By calculating vacuum-level alignments,we observe a large band offset reduction of 2.5 eV for the O-terminatedSi slab compared to other terminations. Furthermore, a 0.5 eV increaseis found for the anatase (101) surface compared to (001). We comparethe band offsets obtained through vacuum alignment with four differentheterostructure models. Even though the heterostructure models containan excess of oxygen, their offsets agree well with vacuum-level alignmentsusing stoichiometric or H-terminated slabs, and the reduction in bandoffsets seen for the O-terminated Si slab is not observed. Additionally,we have investigated different exchange-correlation treatments includingPBE + U, postprocessing GW corrections, and the meta-GGArSCAN functional. We find that rSCAN provides more accurate band offsetsthan PBE, but further corrections are still required to achieve <0.5eV accuracy. Overall, our study quantifies the importance of surfacetermination and orientation for this interface.

Halide perovskite structures are revolutionizing the design of optoelectronic materials, including solar cells, light-emitting diodes, and photovoltaics when formed at the quantum scale. Four isolated sub-nanometer, or picoscale, halide perovskite structures formed inside ≈1.2-1.6 nm single-walled carbon nanotubes (SWCNTs) by melt insertion from CsPbBr3 and lead-free CsSnI3 are reported. Three directly relate to the ABX3 perovskite archetype while a fourth is a perovskite-like lamellar structure with alternating Cs4 and polyhedral Sn4 Ix layers. In ≈1.4 nm-diameter SWCNTs, CsPbBr3 forms Cs3 PbII Br5 nanowires, one ABX3 unit cell in cross section with the Pb2+ oxidation state maintained by ordered Cs+ vacancies. Within ≈1.2 nm-diameter SWCNTs, CsPbBr3 and CsSnI3 form inorganic-polymer-like bilayer structures, one-fourth of an ABX3 unit cell in cross section with systematically reproduced ABX3 stoichiometry. Producing these smallest halide perovskite structures at their absolute synthetic cross-sectional limit enables quantum confinement effects with first-principles calculations demonstrating bandgap widening compared to corresponding bulk structural forms.

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.

The magnetocrystalline anisotropy energy of atomically ordered L10 FeNi (the meteoritic mineral tetrataenite) is studied within a first-principles electronic structure framework. Two compositions are examined: equiatomic Fe0.5Ni0.5 and an Fe-rich composition, Fe0.56Ni0.44. It is confirmed that, for the single crystals modeled in this work, the leading-order anisotropy coefficient K1 dominates the higher-order coefficients K2 and K3. To enable comparison with experiment, the effects of both imperfect atomic long-range order and finite temperature are included. While our computational results initially appear to undershoot the measured experimental values for this system, careful scrutiny of the original analysis due to Néel et al. [J. Appl. Phys. 35, 873 (1964)] suggests that our computed value of K1 is, in fact, consistent with experimental values, and that the noted discrepancy has its origins in the nanoscale polycrystalline, multivariant nature of experimental samples, that yields much larger values of K2 and K3 than expected a priori. These results provide fresh insight into the existing discrepancies in the literature regarding the value of tetrataenite’s uniaxial magnetocrystalline anisotropy in both natural and synthetic samples.

Nd$_2$Fe$_{14}$B's unsurpassed, hard magnetic properties for a wide range of temperatures result from a combination of a large volume magnetization from Fe and a strong single-ion anisotropy from Nd. Here, using finite temperature first-principles calculations, we focus on the other crucial roles played by the Fe atoms in maintaining the magnetic order on the Nd sublattices, and hence the large magnetic anisotropy, and directly generating significant uniaxial anisotropy at high temperatures. We identify effective spins for atomistic modelling from the material's interacting electrons and {quantify pairwise and higher order, non-pairwise magnetic interactions among them. We find the Nd spins couple most strongly to spins on sites belonging to two specific Fe sublattices, 8$j_1$, 8$j_2$. Moreover the Fe 8$j_1$ sublattice also provides the electronic origin of the unusual, nonmonotonic temperature dependence of the anisotropy of Y$_2$Fe$_{14}$B.} Our work provides atomic-level resolution of the properties of this fascinating magnetic material.

Abstract The power conversion efficiency of solar cells is strongly impacted by an unwanted loss of charge carriers occurring at semiconductor surfaces and interfaces. Here the use of ion‐charged oxide nanolayers to enhance the passivation of silicon surfaces via the field effect mechanism is reported. The first report of enhanced passivation from rubidium and cesium ion‐charged oxide nanolayers is provided. The charge state and formation energy of ion‐charged silicon dioxide are calculated from first principles. Ion embedding is demonstrated and exploited to control the interface population of carriers and minimize electron‐hole pair recombination. The passivation quality directly improves with charge concentration, yet excess ions can produce detrimental interface states. An optimal ionic charge concentration of ≈1.5 × 1012 q cm−2 is deduced, and a recombination velocity and current density as low as 2.8 cm s−1 and 7.8 fA cm−2 are achieved at the Si‐SiO2 interface. Maximized charge is shown to provide efficiency improvements as high as 0.7% absolute. This work provides a unique route to enhance passivation without compromising the film synthesis, thus retaining the antireflection and hydrogenation film properties. As such, ion‐charged dielectrics provide complementary paths for surface and interface optimization in future single‐junction and tandem solar cells.

We present MARMOT , a hybrid Python / FORTRAN implementation of the disordered local moment picture within multiple scattering density-functional theory. MARMOT takes atom-centred, scalar-relativistic potentials and constructs an effective medium (within the coherent potential approximation) to describe the disordered magnetic moment orientations at finite temperature. By solving the single-site scattering problem fully relativistically, spin–orbit effects are included, allowing the magnetocrystalline anisotropy to be calculated. Magnetic transition temperatures, spin and orbital moments, the density-of-states, and analytical parameterizations of the magnetic potential energy surface can also be calculated. Here, we describe the theory and practical implementation of MARMOT , and demonstrate its use by calculating Curie temperatures, magnetizations and anisotropies of bcc Fe, GdFe 2 and YCo 5 .

MnBi is remarkable for having a magnetocrystalline anisotropy (MCA) that increases with temperature. This unusual behavior has been attributed to the thermal expansion of the lattice and, more recently, to an anisotropic vibrational free energy. However, the effect of magnetic fluctuations on the MCA has not yet been quantified. Here, first-principles density-functional theory calculations based on the disordered local moment picture (DFT-DLM) are used to calculate the MCA of MnBi in the presence of magnetic disorder. The MCA is obtained from the magnetic torque, calculated as a function of magnetization angle and temperature T. At fixed ionic positions, the MCA decays monotonically with increasing T. The DFT-DLM torques provide access to the individual anisotropy constants kappa(l) (which parametrize the relation between magnetic energy and magnetization angle), and their dependence on order parameter m. The lowest order constant kappa(2) follows single-ion-like behavior at low T but decays as m(4) as T increases, while the higher-order constants deviate strongly from single-ion predictions. Zero-T calculations show that a spin reorientation transition can be triggered by removing 0.1 electrons, resulting in c-axis magnetization once 0.25 electrons have been removed. The zero-temperature calculations are cross-validated using an alternative implementation of DFT based on wave functions, plane waves, and pseudopotentials.

A holistic approach for studying both the nature of atomic order and finite-temperature magnetostrictive behavior in the binary alloy Galfenol (${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}, 0\ensuremath{\le}x\ensuremath{\le}0.25$) is presented. The phase behavior is studied via atomistic modeling with inputs from ab initio calculations, and the ordered phases of interest at nonstoichiometric concentrations are verified to exhibit $B2$- and ${D0}_{3}$-like order. The finite-temperature magnetoelasticity of these phases, in particular the magnetoelastic constant ${B}_{1}$, is obtained within the same ab initio framework using disordered local moment theory. Our results provide an explanation for the origin of the experimentally observed peak and subsequent fall in the material's magnetostriction at $x\ensuremath{\sim}0.19$, which has been disputed. In addition, we show that it is possible to enhance the magnetostriction of ${D0}_{3}\ensuremath{-}{\mathrm{Fe}}_{3}\mathrm{Ga}$ by removing a small fraction of electrons from the system, suggesting that a Fe-Ga-Cu or Fe-Ga-Zn alloy could exhibit greater magnetostrictive properties than Galfenol.

Dielectric-silicon interfaces are becoming ever more important to device performance. Charge inside a surface dielectric layer is neutralized in Si leading to an accumulation or inversion layer of free carriers. Additionally, states at the interface are occupied by charges via Shockley-Read-Hall carrier statistics. It is accepted that the density of interface charge near midgap, which can only reach a concentration as high as the density of states, Dit, has a minor effect on band bending compared to the charges in the dielectric for a well passivated interface. Here, we show that it is the state density near the band edge what plays the major role. We conclude this by comparing our measurements with device modelling of a Si/SiO2 interface. We measure the wafer sheet resistance while applying various amounts of positive charge to the passivating dielectric on an n-type Si wafer, and then reproduce the measured resistance values using simulations. This modelling indicates that Dit at midgap has indeed a minor effect on sheet resistance change, while the total amount of tail states has a significant impact on the distribution of induced carriers. We test this model to detect the amount of acceptor-like states at the band-tails of oxide passivated silicon with different processing. We discuss and analyse the limitations of this technique. While we report on the Si/SiO2 interface due to its relevance in photovoltaics, our method can be used to study the properties of other semiconductor-dielectric interfaces. As such this work is of importance across various optoelectronic devices.

We present pressure-dependent magnetization measurements carried out in the domain of the spin reorientation transitions (SRTs) of a NdCo5 single crystal. The application of a hydrostatic pressure leads to a shift in the SRTs to higher temperatures. This shift is found to be very sensitive to pressure, with the SRT temperatures increasing at a rate of ≈17 K/GPa. To explain the experimental results, we have also performed first-principles calculations of the SRT temperatures for different applied strains, which corroborate the experimental findings. The calculations attribute the pressure dependence of the SRTs to a faster weakening of the Co contribution to the magnetocrystalline anisotropy with pressure compared to the Nd contribution.