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We report the CoII-substitution effect on a cyanido-bridged three-dimensional FeII spin-crossover network, Fe2[Nb(CN)8](4-pyridinealdoxime)8·2H2O. A series of iron–cobalt octacyanidoniobate, (FexCo1−x)2[Nb(CN)8](4-pyridinealdoxime)8·zH2O, was prepared. In this series, the behavior of FeII spin-crossover changes with the CoII concentration. As the CoII concentration increases, the transition of the spin-crossover becomes gradual and the transition temperature of the spin-crossover shifts towards a lower temperature. Additionally, this series shows magnetic phase transition at a low temperature. In particular, (Fe0.21Co0.79)2[Nb(CN)8](4-pyridinealdoxime)8·zH2O exhibits a Curie temperature of 12 K and a large coercive field of 3100 Oe.

We report results on the structural and microwave properties and magnetic phase transitions in polycrystalline Sr3Co2Fe24O41 hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave absorbing material. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves revealed a change in the magnetization behavior at 293 K. The reflection losses in the 1–20 GHz range of the Sr3Co2Fe24O41 powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in both the absence and presence of a magnetic field. In the latter case, a dramatic rise in the attenuation was observed. The microwave reflection losses reached the maximum value of 32.63 dB at 17.29 GHz in the Ku-band. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the appearance of new reflection losses maxima. At a fixed thickness tm of the composite, the attenuation peak frequency can be adjusted to a certain value either by changing the filling density or by applying an external magnetic field.

Magnetism plays a key role in modern technology and stimulates research in several branches of condensed matter physics. Although the theory of classical magnetism is well developed, the demonstration of a widely tunable experimental system has remained an elusive goal. Here, we present the realization of a large-scale simulator for classical magnetism on a triangular lattice by exploiting the particular properties of a quantum system. We use the motional degrees of freedom of atoms trapped in an optical lattice to simulate a large variety of magnetic phases: ferromagnetic, antiferromagnetic, and even frustrated spin configurations. A rich phase diagram is revealed with different types of phase transitions. Our results provide a route to study highly debated phases like spin-liquids as well as the dynamics of quantum phase transitions.

Layered van der Waals 2D magnetic materials are of great interest in fundamental condensed-matter physics research, as well as for potential applications in spintronics and device physics. We present neutron powder diffraction data using new ultrahigh-pressure techniques to measure the magnetic structure of Mott-insulating 2D honeycomb antiferromagnetFePS3at pressures up to 183 kbar and temperatures down to 80 K. These data are complemented by high-pressure magnetometry and reverse Monte Carlo modeling of the spin configurations. As pressure is applied, the previously measured ambient-pressure magnetic order switches from an antiferromagnetic to a ferromagnetic interplanar interaction and from 2D-like to 3D-like character. The overall antiferromagnetic structure within theabplanes, ferromagnetic chains antiferromagnetically coupled, is preserved, but the magnetic propagation vector is altered fromk=(0,1,12)tok=(0,1,0), a halving of the magnetic unit cell size. At higher pressures, coincident with the second structural transition and the insulator-metal transition in this compound, we observe a suppression of this long-range order and emergence of a form of magnetic short-range order which survives above room temperature. Reverse Monte Carlo fitting suggests this phase to be a short-ranged version of the original ambient-pressure structure—with the Fe moment size remaining of similar magnitude and with a return to antiferromagnetic interplanar correlations. The persistence of magnetism well into the HP-II metallic state is an observation in contradiction with previous x-ray spectroscopy results which suggest a spin-crossover transition.

Recent demonstrations of moiré magnetism, featuring exotic phases with noncollinear spin order in the twisted van der Waals (vdW) magnet chromium triiodide CrI 3 , have highlighted the potential of twist engineering of magnetic (vdW) materials. However, the local magnetic interactions, spin dynamics, and magnetic phase transitions within and across individual moiré supercells remain elusive. Taking advantage of a scanning single-spin magnetometry platform, here we report observation of two distinct magnetic phase transitions with separate critical temperatures within a moiré supercell of small-angle twisted double trilayer CrI 3 . By measuring temperature-dependent spin fluctuations at the coexisting ferromagnetic and antiferromagnetic regions in twisted CrI 3 , we explicitly show that the Curie temperature of the ferromagnetic state is higher than the Néel temperature of the antiferromagnetic one by ~10 K. Our mean-field calculations attribute such a spatial and thermodynamic phase separation to the stacking order modulated interlayer exchange coupling at the twisted interface of moiré superlattices. van der Waals magnets can be arranged into twisted heterostructures, with the twisting leading to the formation of new magnetic phases. Here, Li, Sun, and coauthors show via NVcentre based magnetometry small angle twisted double trilayer CrI 3 exhibits a co-existing, hybrid magnetic phase with distinct phase transition temperatures.

Characterization of a quantum spin liquid Theoretical predictions of the existence of an exotic state of matter called a quantum spin liquid have been supported by experimental evidence, but many fundamental questions about this novel state remain unanswered. Quantum spin liquids can occur in an insulator with strongly interacting magnetic units when the usual symmetry-breaking magnetically ordered state at low temperatures is avoided as a result of quantum fluctuations. An advanced magnetic probing technique, called muon spin relaxation, has now been used to study the molecular layered system κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , widely regarded as a prime example of a quantum spin liquid. Using implanted muons as a sensitive local magnetic probe, the authors show that magnetic order can be recovered in this system by applying a very small magnetic field. A complex phase diagram is revealed for the spin liquid, which demonstrates the delicate balance between the spin liquid and magnetic phases. A quantum spin liquid is an intriguing exotic state of matter involving a magnetic system in which a magnetically ordered ground state at low temperatures is avoided despite strong interactions between magnetic units, due to quantum fluctuations. Experimental evidence for the existence of such a state has become available, but there are many fundamental questions about this novel state. This study uses an advanced magnetic probing technique, called muon spin rotation, to study a molecular layered system that is widely regarded as a prime candidate for quantum spin liquid. A complex magnetic phase diagram for this system is determined and characteristic critical properties of the spin liquid are measured, thereby providing important new insights into this exotic state of matter. A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins 1 . Recently, materials have become available showing persuasive experimental evidence for such a state 2 . Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment 3 . This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators 4 . The layered molecular system κ-(BEDT-TTF) 2 Cu 2 (CN) 3 is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers 5 that provides a prime example of a spin liquid formed in this way 6 , 7 , 8 , 9 , 10 , 11 . Despite a high-temperature exchange coupling, J , of 250 K (ref. 6 ), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7 , 8 ). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose–Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.

Transition metal takes charge The introduction of 'foreign' elements into transition-metal oxides (called chemical doping) can change the valence state of the metal's cations and hence modify the physical properties of the material as a whole. These changes can be dramatic, for example causing high-temperature superconductivity in copper oxides and colossal magnetoresistance in manganese oxides. Youwen Long et al . have identified an oxide system, the perovskite LaCu 3 Fe 4 O 12 , in which changes in valence state occur when charge is shuffled between different cations (iron and copper) in the host structure, rather than via doping. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe 3.75+ state (partnered with fairly common Cu 2+ ions) to one possessing rare Cu 3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, the material contracts slightly on being warmed through the transition. The temperature sensitivity of this effect makes it a strong candidate for technological applications. This paper identifies an oxide system where changes in valence state occur as a result of charge being shuffled between different cations in the host structure, rather than via doping, this charge transfer being sensitive to temperature. As a result, the material can be reversibly transformed from one possessing iron in an unusually high Fe3.75+ state to one possessing rare Cu3+ ions. These changes are reflected in the magnetic and electronic properties of the material and, intriguingly, are accompanied by negative thermal expansion. Changes of valence states in transition-metal oxides often cause significant changes in their structural and physical properties 1 , 2 . Chemical doping is the conventional way of modulating these valence states. In ABO 3 perovskite and/or perovskite-like oxides, chemical doping at the A site can introduce holes or electrons at the B site, giving rise to exotic physical properties like high-transition-temperature superconductivity and colossal magnetoresistance 3 , 4 . When valence-variable transition metals at two different atomic sites are involved simultaneously, we expect to be able to induce charge transfer—and, hence, valence changes—by using a small external stimulus rather than by introducing a doping element. Materials showing this type of charge transfer are very rare, however, and such externally induced valence changes have been observed only under extreme conditions like high pressure 5 , 6 . Here we report unusual temperature-induced valence changes at the A and B sites in the A-site-ordered double perovskite LaCu 3 Fe 4 O 12 ; the underlying intersite charge transfer is accompanied by considerable changes in the material’s structural, magnetic and transport properties. When cooled, the compound shows a first-order, reversible transition at 393 K from LaCu 2+ 3 Fe 3.75+ 4 O 12 with Fe 3.75+ ions at the B site to LaCu 3+ 3 Fe 3+ 4 O 12 with rare Cu 3+ ions at the A site. Intersite charge transfer between the A-site Cu and B-site Fe ions leads to paramagnetism-to-antiferromagnetism and metal-to-insulator isostructural phase transitions. What is more interesting in relation to technological applications is that this above-room-temperature transition is associated with a large negative thermal expansion.

An array of single nonmagnetic metallic split rings can be used to implement a magnetic resonance, which arises from an inductor-capacitor circuit (LC) resonance, at 100-terahertz frequency. The excitation of the LC resonance in the normal-incidence geometry used in our experiments occurs through the coupling of the electric field of the incident light to the capacitance. The measured optical spectra of the nanofabricated gold structures come very close to the theoretical expectations. Additional numerical simulations show that our structures exhibit a frequency range with negative permeability for a beam configuration in which the magnetic field couples to the LC resonance. Together with an electric response that has negative permittivity, this can lead to materials with a negative index of refraction.

Large magnetic-field-induced strains 1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field 1 , 2 , 3 . These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy 4 . We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni 45 Co 5 Mn 36.7 In 13.3 single crystal.

Negative refraction is currently achieved by driving the magnetic permeability and electrical permittivity simultaneously negative, thus requiring two separate resonances in the refracting material. The introduction of a single chiral resonance leads to negative refraction of one polarization, resulting in improved and simplified designs of negatively refracting materials and opening previously unknown avenues of investigation in this fast-growing subject.