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The recently discovered material Cs3Fe2Br9 contains Fe2Br9 bi-octahedra forming triangular layers with hexagonal stacking along the c axis. In contrast to isostructural Cr-based compounds, the zero-field ground state is not a nonmagnetic S = 0 singlet-dimer state. Instead, the Fe2Br9 bi-octahedra host semiclassical S = 5/2 Fe3+ spins with a pronounced easy-axis anisotropy along c, and interestingly, the intradimer spins are ordered ferromagnetically. The high degree of magnetic frustration due to (various) competing intradimer and interdimer couplings leads to a surprisingly rich magnetic phase diagram. The zero-field ground state is already reached via an intermediate phase, and the high-field magnetization and thermal expansion data for H parallel to c identify 10 different ordered phases. Among them are phases with constant magnetization of 1/3, respectively 1/2, of the saturation value, and several transitions are strongly hysteretic with pronounced length changes, reflecting strong magnetoelastic coupling.

The properties of transition metal compounds are largely determined by nontrivial interplay of different degrees of freedom: charge, spin, lattice, and also orbital ones. Especially rich and interesting effects occur in systems with orbital degeneracy. For example, they result in the famous Jahn-Teller effect, leading to a plethora of consequences for static and dynamic properties, including nontrivial quantum effects. In the present review, we discuss the main phenomena in the physics of such systems, paying central attention to the novel manifestations of those. After shortly summarizing the basic phenomena and their descriptions, we concentrate on several specific directions in this field. One of them is the reduction of effective dimensionality in many systems with orbital degrees of freedom due to the directional character of orbitals, with the concomitant appearance of some instabilities that lead in particular to the formation of dimers, trimers, and similar clusters in a material. The properties of such cluster systems, which are largely determined by their orbital structure, are discussed in detail, and many specific examples of those in different materials are presented. Another big field that has acquired special significance relatively recently is the role of the relativistic spin-orbit interaction. The mutual influence of this interaction and the more traditional Jahn-Teller physics is treated in detail in the second part of the review. In discussing all of these questions, special attention is paid to novel quantum effects.

Structural properties of CuAl2O4, which was recently argued to show unusual suppression of the Jahn–Teller distortions by the spin–orbit coupling, are investigated under pressures up to 6 GPa. Analysis of x-ray powder diffraction experiments shows that CuAl2O4 gets unstable and decomposes onto CuO and Al2O3 at pressures ∼6 GPa and temperature ∼1000 K. This finding is complemented by the density-functional theory + U + spin–orbit coupling calculations, which demonstrate that this instability is partially driven by a (relatively) large compressibility of strongly Jahn–Teller distorted CuO. © 2020 IOP Publishing Ltd Printed in the UK.

We unveil the microscopic origin of largely debated magnetism in the Mo3O8 quantum systems. Upon considering an extended Hubbard model at 1/6 filling on the anisotropic kagomé lattice formed by the Mo atoms, we argue that its ground state is determined by the competition between kinetic energy and intersite Coulomb interactions, which is controlled by the trimerisation of the kagomé lattice into the Mo3O13 clusters, and the sign of hopping parameters, specifying the electron localisation at such clusters. Based on first-principles calculations, we show that the strong interaction limit reveals a plaquette charge order with unpaired spins at the resonating hexagons that can be realised in LiZn2Mo3O8, and whose origin is solely related to the opposite signs of intracluster and intercluster hoppings, in contrast to all previous scenarios. On the other hand, both Li2InMo3O8 and Li2ScMo3O8 are demonstrated to fall into the weak interaction limit where the electrons are well localised at the Mo3O13 clusters. While the former is found to exhibit long-range antiferromagnetic order, the latter is more likely to reveal short-range order with quantum spin liquid-like excitations. Our results not only reproduce most of the experimentally observed features of the Mo3O8 systems, but will also help to describe various properties in other quantum cluster magnets. © 2021, The Author(s).

The silver ruthenium oxide AgRuO3 consists of honeycomb Ru25+O62- layers and can be considered an analogue of SrRu2O6 with a different intercalation. We present measurements of magnetic susceptibility and specific heat on AgRuO3 single crystals, which reveal a sharp antiferromagnetic transition at 342(3) K. The electrical transport in single crystals of AgRuO3 is determined by a combination of activated conduction over an intrinsic semiconducting gap of ≈100 meV and carriers trapped and thermally released from defects. From powder neutron diffraction data a Néel-type antiferromagnetic structure with the Ru moments along the c axis is derived. Raman spectroscopy on AgRuO3 single crystals and muon spin rotation spectroscopy on powder samples indicate a further weak phase transition or a crossover in the temperature range 125-200 K. The transition does not show up in the magnetic susceptibility, and its origin is argued to be related to defects but cannot be fully clarified. The experimental findings are complemented by density-functional-theory-based electronic structure calculations. It is found that the magnetism in AgRuO3 is similar to that in SrRu2O6, however, with stronger intralayer and weaker interlayer magnetic exchange interactions. © 2021 authors. Published by the American Physical Society.

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100-250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state. © 2021 American Physical Society.

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100–250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296–703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state.