Antiferromagnets (AFMs) are strong candidates for the future spintronic and memory applications largely because of their inherently fast dynamics and lack of stray fields, with Mn2Au being one of the most promising. For the numerical modeling of magnetic material properties, it is common to use ab initio methods, atomistic models, and micromagnetics. However, each method alone describes the physics within certain limits. Multiscale methods bridging the gap between these three approaches have been already proposed for ferromagnetic materials. Here we present a complete multiscale model of the AFM Mn2Au as an exemplar material, starting with results from ab initio methods going via atomistic spin dynamics (ASD) to an AFM Landau-Lifshitz-Bloch (AFM-LLB) model. First, bulk Mn2Au is modelled using a classical spin Hamiltonian constructed based on earlier first-principles calculations. Second, this spin model is used in the stochastic Landau-Lifshitz-Gilbert to calculate temperature-dependent equilibrium properties, such as magnetization and magnetic susceptibilities. Third, the temperature-dependent micromagnetic parameters are used in the AFM-LLB. We validate our approach by comparing the ASD and AFM-LLB models for three paradigmatic cases: (i) damped magnetic oscillations, (ii) magnetization dynamics following a heat pulse resembling pump-probe experiments, and (iii) magnetic domain wall motion under thermal gradients.

Nuclear fission technologies have the potential to play a significant role in the energy mix of a net-zero and sustainable society. However, to achieve the sustainability goal two significant challenges remain: efficient and sustainable fuel usage and the minimization of long-term nuclear waste. Civil nuclear molten salt systems and technologies offer the opportunity to address both, delivering future reactors at scale for efficient and effective power production and nuclear waste burnup. Potentially, both objectives could be fulfilled in one reactor system, which could significantly improve sustainability indices. The key to this innovation is demand driven development of a significantly reduced fuel cycle with enhanced proliferation resistance which offers further potential for improvement. To achieve these goals, a transformative approach for salt clean-up during molten salt reactor operation is proposed, by concentrating on the detection and removal of key neutron poisoning elements which prevent the reactor from long-term operation. To enable this highly innovative development work, a novel analysis of the evolving elementary fuel composition, their concentrations, and their criticality influence is now provided in this work. This, combined with consideration of the oxidation states of each of these elements then provides the basis for the selection of these key poisons and the development of advanced separation processes and process monitoring. This work also discusses the importance of the effective integration of physics and chemistry when systems modelling in achieving these system development goals.

Antiferromagnets (AFMs) are strong candidates for the future spintronic and memory applications largely because of their inherently fast dynamics and lack of stray fields, with Mn2Au being one of the most promising. For the numerical modelling of magnetic material properties, it is common to use ab-initio methods, atomistic models and micromagnetics. However, each method alone describes the physics within certain limits. Multiscale methods bridging the gap between these three approaches have been already proposed for ferromagnetic materials. Here, we present a complete multiscale model of the AFM Mn2Au as an exemplar material, starting with results from ab-initio methods going via atomistic spin dynamics (ASD) to an AFM Landau-Lifshitz-Bloch (AFM-LLB) model. Firstly, bulk is modelled using a classical spin Hamiltonian constructed based on earlier first-principles calculations. Secondly, this spin model is used in the stochastic Landau-Lifshitz-Gilbert (LLG) to calculate temperature-dependent equilibrium properties, such as magnetization and magnetic susceptibilities. Thirdly, the temperature dependent micromagnetic parameters are used in the AFM-LLB. We validate our approach by comparing the ASD and AFM-LLB models for three paradigmatic cases; (i) Damped magnetic oscillations, (ii) magnetization dynamics following a heat pulse resembling pump-probe experiments, (iii) magnetic domain wall motion under thermal gradients.

We investigate the use of programmable optical lattices for quantum simulation of Hubbard models, determining analytic expressions for the hopping and Hubbard U , finding that they are suitable for emulating strongly correlated systems with arbitrary structures, including those with multiple site basis and impurities. Programmable potentials are highly flexible, with the ability to control the depth and shape of individual sites in the optical lattice dynamically. Quantum simulators of Hubbard models with (1) arbitrary basis are required to represent many real materials of contemporary interest, (2) broken translational symmetry are needed to study impurity physics, and (3) dynamical lattices are needed to investigate strong correlation out of equilibrium. We derive analytic expressions for Hubbard Hamiltonians in programmable potential systems. We find experimental parameters for quantum simulation of Hubbard models with arbitrary basis, concluding that programmable optical lattices are suitable for this purpose. We discuss how programmable optical lattices can be used for quantum simulation of dynamical multi-band Hubbard models that represent complicated compounds, impurities, and non-equilibrium physics.

Using first-principles calculations, based on disordered local moment (DLM) theory combined with the self-interaction corrected local spin density approximation (SIC-LSDA), we study magnetic correlations in the paramagnetic state of GdX (X=Cu, Zn, Ga, Ag, Cd, In, Au, Hg, and Tl) intermetallics and their alloys. The predicted magnetic orders and ordering temperatures that these correlations lead to are in overall good agreement with available experiments. The interactions between the Gd f -electron local moments are mediated by the valence electrons of the intermetallics which comprise both Gd and X d bands as well as sp bands. There are RKKY-like features such as dependence on the number of sp-valence electrons but other variations manifest themselves in the phase diagrams as regions of incommensurate magnetic ordering, the origin and range of which are related to the binding energies of the alloying anion d states, and their propensity to hybridize with the Gd states at the Fermi level.

Around discontinuous (first-order) magnetic phase transitions the strong caloric response of materials to the application of small fields is widely studied for the development of solid-state refrigeration. Typically strong magnetostructural coupling drives such transitions and the attendant substantial hysteresis dramatically reduces the cooling performance. In this context we describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant non-hysteretic magnetocaloric effect recently discovered in a Eu$_2$In compound. There is positive feedback between the magnetism of itinerant valence electrons and the ferromagnetic ordering of local $f$-electron moments, which appears as a topological change to the Fermi surface. The origin of this electronic mechanism stems directly from Eu's divalency, which explains the absence of a similar discontinuous transition in Gd$_2$In.

The unusual features of topological semimetals arise from its nontrivial band structure. The impact of strong electron correlations on the topological states remains largely unexplored in real materials. Here, we report the magnetotransport properties of YbCdSn single crystals. We found two fundamental experimental evidences of electron correlations through magnetic susceptibility and specific heat. The electron correlations in this compound lead to an intermediate valence state and enhance the effective mass of the charge carriers. This correlated state exhibits large nonsaturating magnetoresistance, low carrier density, magnetic field-induced metal-semiconductor-like crossover and a plateau in resistivity at low temperatures. This compound also shows a cusp-like magnetoconductivity at low magnetic field which indicates the presence of weak antilocalization effect. Our band structure calculations of Yb2+ state predict YbCdSn to be a topological nodal-line semimetal.

Using first-principles calculations, based on disordered local moment theory, combined with the self-interaction corrected local spin density approximation, we study magnetic interactions in GdX intermetallics for X=Cu, Zn, Ga, Cd, and Mg. Our predicted magnetic orders and ordering temperatures both at zero and other pressures agree well with experiments including the large increase in the Curie temperature of GdCd under pressure that is shown by our own experimental measurements. From our results it emerges that the Ruderman-Kittel-Kasuya-Yosida interaction on its own can not explain the observed behaviour under pressure, and that the magnetic ordering mechanism is strongly influenced by the occupations of both Gd and anion d-bands.

Magnetocrystalline anisotropy, the microscopic origin of permanent magnetism, is often explained in terms of ferromagnets. However, the best performing permanent magnets based on rare earths and transition metals (RE-TM) are in fact ferrimagnets, consisting of a number of magnetic sublattices. Here we show how a naive calculation of the magnetocrystalline anisotropy of the classic RE-TM ferrimagnet GdCo_{5} gives numbers that are too large at 0 K and exhibit the wrong temperature dependence. We solve this problem by introducing a first-principles approach to calculate temperature-dependent magnetization versus field (FPMVB) curves, mirroring the experiments actually used to determine the anisotropy. We pair our calculations with measurements on a recently grown single crystal of GdCo_{5}, and find excellent agreement. The FPMVB approach demonstrates a new level of sophistication in the use of first-principles calculations to understand RE-TM magnets.

The RCo5 family of rare-earth/transition metal permanent magnets has been the subject of decades of research, not only from the point of view of applications (e.g. R=Sm) but also fundamental theory, owing to their relatively simple crystal structure [1] (Fig. 1).

We present an investigation into the intrinsic magnetic properties of the compounds YCo5 and GdCo5, members of the RETM5 class of permanent magnets (RE = rare earth, TM = transition metal). Focusing on Y and Gd provides direct insight into both the TM magnetization and RE-TM interactions without the complication of strong crystal field effects. We synthesize single crystals of YCo5 and GdCo5 using the optical floating zone technique and measure the magnetization from liquid helium temperatures up to 800 K. These measurements are interpreted through calculations based on a Green's function formulation of density-functional theory, treating the thermal disorder of the local magnetic moments within the coherent potential approximation. The rise in the magnetization of GdCo5 with temperature is shown to arise from a faster disordering of the Gd magnetic moments compared to the antiferromagnetically aligned Co sublattice. We use the calculations to analyze the different Curie temperatures of the compounds and also compare the molecular (Weiss) fields at the RE site with previously published neutron scattering experiments. To gain further insight into the RE-TM interactions, we perform substitutional doping on the TM site, studying the compounds RECo4.5Ni0.5, RECo4Ni, and RECo4.5Fe0.5. Both our calculations and experiments on powdered samples find an increased/decreased magnetization with Fe/Ni doping, respectively. The calculations further reveal a pronounced dependence on the location of the dopant atoms of both the Curie temperatures and the Weiss field at the RE site.

This review tries to establish what is the current understanding of the rare-earth monopnictides and monochalcogenides from first principles. The rock salt structure is assumed for all the compounds in the calculations and wherever possible the electronic structure/properties of these compounds, as obtained from different ab initio methods, are compared and their relation to the experimental evidence is discussed. The established findings are summarised in a set of conclusions and provide outlook for future study and possible design of new materials.

We explain a profound complexity of magnetic interactions of some technologically relevant gadolinium intermetallics using an ab initio electronic structure theory which includes disordered local moments and strong f-electron correlations. The theory correctly finds GdZn and GdCd to be simple ferromagnets and predicts a remarkably large increase of Curie temperature with a pressure of +1.5 K kbar(-1) for GdCd confirmed by our experimental measurements of +1.6  K kbar(-1). Moreover, we find the origin of a ferromagnetic-antiferromagnetic competition in GdMg manifested by noncollinear, canted magnetic order at low temperatures. Replacing 35% of the Mg atoms with Zn removes this transition, in excellent agreement with long-standing experimental data.

Using first principles calculations, we have investigated f-electron delocalization and oxidation in the actinide dioxides under pressure. Whilst UO2 is found on the verge of an insulator to metal transition at the equilibrium volume, increasingly larger pressures are required to delocalize f-electrons in NpO2, PuO2, and AmO2, respectively 49, 112, and 191GPa. Compared to this broad range of pressures, the experimentally observed structural transitions, in all four dioxides, occur between 30 and 40GPa, which leads us to conclude that the associated volume collapse is not due to f-electron delocalization. In contrast, oxidation of the dioxides is found to be linked to the degree of f-electron localization, but it emerges that for naturally occurring pressures (<10GPa), higher oxides only exist for UO2.

The electronic structure of EuO under pressure is studied using the self-interaction corrected local spin density approximation. EuO, which at ambient conditions crystallizes in the NaCl (B1) structure, is predicted to undergo an isostructural insulator to metal transition at 48 GPa. This transition is associated with a change of valence from a divalent to an intermediate valent state, with the resulting effective valency of 2.35. The pressure range between 48 and 70 GPa is characterized by the competition between an intermediate valent B1 structured phase and a CsCl (B2) structured phase where both the divalent and intermediate valence configurations are in play. Eventually, at pressures above 70 GPa, the intermediate valent B2 phase prevails. The effective Eu valence in the B2 intermediate valence phase is around 2.28, i.e., a decrease in effective valence occurs. This scenario is in line with the reentrant valence behavior observed in recent pressure experiments.

Using first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R3+) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R2+) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 → B2 transition that occurs first, followed by the R2+ → R3+ valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.

We present an efficient methodology for the computational design of structural, electronic and magnetic properties of complex magnetic oxides. Our approach combines several complementary first-principles methods rooted in density functional theory. The optimization of atomic positions and the optimization of the crystalline structure is achieved by means of pseudopotential methods, known for yielding precise total energies and forces. The obtained structural information constitutes an input for calculations of electronic and magnetic properties using the Korringa-Kohn-Rostoker and Linear Muffin-Tin Orbital methods. The resulting Kohn-Sham Green function is utilized in various applications, including the description of transport, spectroscopic properties and magnetism. This multi-code approach is particularly well suited for strongly correlated systems and therefore enables a faithful description of complex oxides. In this highlight, we illustrate the strengths of the methodology on examples of electronic and magnetic properties of transition metal monoxides, diluted magnetic oxides and magnetic oxidic surfaces.

The ground state electronic structures of the actinide oxides AO, A2O3 and AO2 (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations using the self-interaction corrected local spin-density approximation. Our study reveals a strong link between preferred oxidation number and degree of localization. The ionic nature of the actinide oxides emerges from the fact that those oxides where the ground state is calculated to be metallic do not exist in nature, as the corresponding delocalized f-states favour the accommodation of additional O atoms into the crystal lattice.

We present the results of an ab initio study of the electronic structure of 140 rare earth compounds. Specifically, we predict an electronic phase diagram of the entire range of rare earth monopnictides and monochalcogenides, composed of metallic, semiconducting and heavy fermion-like regions and exhibiting valency transitions brought about by a complex interplay between ligand chemistry and lanthanide contraction. The calculations exploit the combined effect of a first-principles methodology, which can adequately describe the dual character of electrons, itinerant versus localized, and high-throughput computing made possible by the increasing available computational power. Our findings, including the predicted 'intermediate valent' compounds SmO and TmSe, are in excellent overall agreement with the available experimental data. The accuracy of the approach, proven e.g. through the lattice parameters calculated to within ∼1.5% of the experimental values, and its ability to describe localization phenomena in solids, makes it a competitive atomistic simulation approach in the search for and design of new materials with specific physical properties and possible technological applications.