Nicola A. Spaldin

We use spin dynamics simulations to determine the origin of the unusual correlated diffuse scattering, characterized by half-moon shapes bridging the magnetic Bragg peaks, observed in the polarized elastic neutron scattering from manganese tungstate, MnWO 4 . We first fit a Heisenberg Hamiltonian with 12 nearest-neighbor exchange interactions and single-ion anisotropy to the experimental ground-state magnon dispersion. We then show via spin dynamics simulations that our model Hamiltonian both reproduces the experimentally observed half-moon features and captures their persistence into the paramagnetic regime. Moreover, we identify the three-dimensional, competing antiferromagnetic interactions driving this behavior. Our work complements earlier studies of half-moon-shaped signatures in pyrochlore and triangular structures, by providing insight into their origin and showing that the half moons in MnWO 4 result from three-dimensional competing exchange interactions, in the absence of the spin-liquid behavior with which they are usually associated.

We present a combined density-functional theory and dynamical mean-field theory (DFT+DMFT) study of the full structural phase space of rutile-based vanadium dioxide ( VO 2 ) , including also the less studied M2 and T phases, using an unconventional bond-centered orbital basis. The use of bond-centered orbitals allows us to treat all main phases of VO 2 , and the structural transitions between them, using one consistent approach with moderate computational cost and without prepattering of the structure into dimerized and undimerized V-V pairs. We obtain two distinct insulating states on the two different types of vanadium chains in the M2 phase, a singlet insulator on the dimerized chains and a Mott insulator on the zigzag-distorted chains, which, however, are strongly coupled in the M2 phase and thus the metal-insulator transition always occurs concomitantly for both types of sites. We also demonstrate that the M2 phase corresponds to a local energy minimum in the structural phase space of VO 2 , the stability of which, apart from the internal structural distortion, depends crucially on the unit cell strain relative to the undistorted rutile phase. Our calculations further indicate that the symmetry-distinct triclinic T phase corresponds electronically to either an M1- or an M2-type insulator with an abrupt transition as a function of distortion. Finally, we disentangle the effect of the dimerization and zigzag distortions by constructing hypothetical structures that contain only one site type, finding that the zigzag distortion strongly favors emergence of the Mott-insulating state, as a function of both distortion and on-site interaction.

We review the conditions that cause or prohibit non-relativistic spin splitting of the energy bands in antiferromagnets. We propose that the existence of spin splitting in magnetically ordered systems is the default scenario and outline the criteria that must be met to avoid it. We discuss some of the properties of those special antiferromagnets that succeed in preserving their spin degeneracy.

Roughness-insensitive surface magnetization is an intrinsic property of magnetoelectric antiferromagnets and is today understood as a manifestation of bulk magnetoelectric multipoles. Here, we provide an experimental quantification of the crystal-facet-dependent surface magnetization in the archetypical magnetoelectric antiferromagnet Cr 2 O 3 and show that the data are consistent with a quadrupolar component of the bulk magnetoelectric multipolization. Using quantitative nanoscale magnetometry on lithographically defined rectangular mesas that expose 30 distinct crystallographic facets, we quantify mesa stray magnetic fields, which arise from differences between the mesa top and side-facet magnetizations. The resulting dataset supports a linear map from surface normal to surface magnetization, from which we directly infer the bulk quadrupolar component within the multipolization framework. Our work provides systematic, facet-resolved evidence of roughness-insensitive surface magnetization, including in-plane components, and delivers a quantitative route to constraining bulk multipoles from surface stray-field data. Beyond addressing the modern theory of magnetoelectric multipolization, our approach offers a generally applicable method to quantify surface magnetizations in antiferromagnetic compounds and informs the choice of crystal cuts for future spintronics devices.

We present a density-functional-theory (DFT) study of the effects of oxygen vacancies on the structural and electronic properties of vanadium dioxide ( VO 2 ) . Our motivation is the reported suppression of the metal-insulator transition by oxygen vacancies and the lack of a clear consensus on its origin. We use the DFT + V method with a static intersite vanadium-vanadium interaction term, V , to calculate the properties of the oxygen-deficient metallic rutile and insulating monoclinic M1 phases of VO 2 on the same footing. We find that oxygen vacancies induce local distortions in the M1 phase, but do not destroy the dimerization usually associated with the insulating behavior. In spite of this, we find that the M1 phase becomes metallic as a result of the partial filling of the conduction band due to a rigid-band-like doping effect.

The development of magnetoelectric multiferroic materials, which combine and couple (ferro)magnetism and ferroelectricity in the same material, are discussed from a chemist's perspective. The chemical challenges that must be overcome to combine (ferro)magnetism and ferroelectricity are highlighted and the developments in crystal chemistry that have enabled identification of new multiferroics are outlined. The chemical applications of multiferroic materials are described and open questions that are particularly amenable to chemical solutions are discussed.

We present a systematic density-functional theory study of the effects of strain on the structural and electronic properties in vanadium dioxide ( VO 2 ), with particular emphasis on its effect on the relative stability of the metallic rutile and the insulating monoclinic M1 phases. We consider various strain conditions that can be related to epitaxial strain present in VO 2 films grown on different lattice planes. Our calculations confirm the dominant role of c -axis strain, i.e., along the direction of the V-V dimerization in the M1 phase. Our analysis suggests that this effect stems primarily from the weakening of the lattice stiffness, with the hopping amplitude along the c axis playing a minor role. We also confirm that, in strain scenarios that deform the basal plane, the c -axis strain still has a dominant effect on the phase stability.

We use the self-consistent harmonic approximation with machine learning interatomic potentials to calculate the effect of ^{18}O substitution on the properties of quantum paraelectric SrTiO_{3} (STO). We find that calculations including both quantum and anharmonic effects are able to reproduce the experimentally observed isotope effect, in which the replacement of ^{16}O by ^{18}O induces the ferroelectric state, and we demonstrate that the ferroelectric phase transition in ST^{18}O can be reproduced in a purely displacive description. We calculate the ferroelectric soft mode frequency as a function of volume, lattice parameters, and temperature for ST^{16}O and ST^{18}O and find that the phase space in which ST^{16}O shows quantum paraelectric behavior, while ST^{18}O becomes ferroelectric, is narrow. Our Letter shows that machine learning interatomic potentials enable temperature-dependent simulations that include quantum and anharmonic phonon effects and that quantitative prediction of the phase diagram in the case of STO is limited by the accuracy of the underlying electronic structure method.

We review the conditions that cause or prohibit non-relativistic spin splitting of the energy bands in antiferromagnets. We propose that the existence of spin splitting in magnetically ordered systems is the default scenario and outline the criteria that must be met to avoid it. We discuss some of the properties of those special antiferromagnets that succeed in preserving their spin degeneracy.