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One of the most important issues in modern condensed matter physics is the realization of fractionalized excitations, such as the Majorana excitations in the Kitaev quantum spin liquid. The 3d-based Kitaev candidate Na2Co2TeO6 is promising to realize such kind of spin liquid phase under external magnetic fields. Here, we first established a complete phase diagram comprising an intermediate magnetically disordered phase sandwiched by the zigzag ordered phase and the polarized trivial phase by the in-plane magnetic torque measurements. Then, the observations, including the restoration of the crystalline point group symmetry in the angle-dependent torque data and the coexisting spinons and magnons from the inelastic neutron scattering data, provide strong evidence that this disordered phase is a field induced quantum spin liquid with partially polarized spins. Our variational Monte Carlo simulation with the effective K-J1-Γ-Γ′-J3 model agrees well with the experimental data and further supports this conclusion.

In this paper, the synergistic effects of Ce4+ ions incorporation into the (Bi2O2)2+ layer and Mo6+ ions at the Nb site on the electrical properties of CaBi2−xCexNb2−xMoxO9 high-temperature piezoelectric ceramics were studied. The appearance of pseudo-tetragonal phase owing to the introduction of Ce4+ ions is beneficial to the polarization switching, and thus, an enhanced piezoelectric response (d33 = 19 pC/N) was obtained in the x = 0.03 sample. Both the ferroelectric and dielectric properties of the doped samples are enhanced. Moreover, the reduction of the oxygen vacancies of the samples resulting from Mo6+ cations could also facilitate the improvement of electrical properties. As a result, a high Curie temperature (TC = 941 °C) and good electrical properties (Pr =14.3 µC/cm2, ρ = 1.1 × 105 Ω cm at 600 °C) were obtained in Ce/Mo-co-substituted CaBi2Nb2O9 ceramics. The results are rather important to explore the piezoelectric ceramics used in high-temperature environment.

$$\hbox {0.7BiFeO}_{{3}}{-}0.3(\hbox {Ba}_{0.85}\hbox {Ca}_{0.15})\hbox {TiO}_{{3}}$$ (BF–BCT) ceramics were synthesized with a conventional solid-state reaction method. The electrical properties of the BF–BCT ceramics depend on the milling time via a planetary ball-milling technique. The results show that the sample prepared by milling for 40 h possesses a rather large remanent polarization of $$52 \,\upmu \hbox {C}/\hbox {cm}^{2}$$ and this value is superior to that of Non-Pb BF-based ceramics at room temperature. Moreover, the values of remanent polarization were confirmed by a PUND measurement to eliminate the contribution of leakage current. We attributed the large remanent polarization to the decrease in oxygen vacancies confirmed by X-ray photoelectron spectroscopy results. Moreover, the BF–BCT ceramics exhibit the dielectric relaxation process caused by the diffusion of oxygen vacancies inside ceramics.

A novel ternary solid solution (1-y)((1-x)BiFeO3–xBi0.5Na0.5TiO3)–yCaTiO3 (0 ≤ x ≤ 0.3, 0.15 ≤ y ≤ 0.25) is designed to present a rhombohedral–orthorhombic morphotropic phase boundary (MPB) with enhanced ferroelectricity, canted ferromagnetism, and room-temperature magnetoelectric responses. The addition of Bi0.5Na0.5TiO3 can generate a distinct MPB region and make BiFeO3–CaTiO3-based ceramics more denser. Therefore, superior ferroelectric properties with the remanent polarization of 37µC/cm2 can be obtained in the x = 0.3, y = 0.15 sample with the presence of rhombohedral–orthorhombic MPB. However, weak ferromagnetism is observed in both the rhombohedral and orthorhombic phases due to a destruction of the spiral spin structure of BiFeO3. The MPB with canted ferromagnetism is constructed. The maximum magnetoelectric interaction constant is found in the MPB compositions with x = 0.2, y = 0.2 which is related with the multiferroic phase boundary.

Stable material properties against large and frequent temperature variations are strongly desired in modern industry. Here we report the combination of low thermal expansion, high thermal conductivity and good mechanical properties in Hf0.87Ta0.13Fe2Cux, all of which are well retained from ambient down to cryogenic temperatures. Cu partially substitutes for Hf/Ta atoms, causing a continuous suppression of negative thermal expansion. For x ≥ 1.25 the Cu precipitates remarkably improves the overall thermal conductivity and compressive strength of the compounds. The excellent comprehensive performance covering such a broad temperature window suggests the wide applications of current composites, for example in cryogenic engineering.

Stable material properties against large and frequent temperature variations are strongly desired in modern industry. Here we report the combination of low thermal expansion, high thermal conductivity and good mechanical properties in Hf0.87Ta0.13Fe2Cux, all of which are well retained from ambient down to cryogenic temperatures. Cu partially substitutes for Hf/Ta atoms, causing a continuous suppression of negative thermal expansion. For x ≥ 1.25 the precipitation of Cu phase remarkably improves the overall thermal conductivity and compressive strength of the compounds. The excellent comprehensive performance covering such a broad temperature window suggests the wide applications of current composites, for example in cryogenic engineering.

We investigate the effect of Jahn-Teller (JT) active Cu2+ ions on the structural, magnetic, and ferroelectric properties of Co1–xCuxCr2O4 (0 ≤ x ≤ 0.4). All the samples can be indexed with a cubic lattice with space group Fd 3 ¯ m at room temperature. All the samples undergo a paramagnetic-ferrimagnetic transition at TC and spiral ferrimagnetic ordering at TS. The magnetization measurements manifest that magnetization reversal occurs in the x = 0.2 and 0.3 samples. The x = 0.3 sample possesses a rather large polarization of 14 μC/m2, which is 7 times in magnitude compared with the CoCr2O4 single crystal. The analysis of the infrared active phonon modes implies that two T 1 u(IR) optical phonon modes are gradually split into a doublet at around 40 K, implying the lowering of lattice symmetry. The presence of a low-symmetry tetragonal phase becomes polar and enhances the electric polarization of Co0.7Cu0.3Cr2O4.

We have investigated the structural, magnetic, and thermal transport properties of Co 1 − x Ni x Cr 2 O 4 ( 0 ≤ x ≤ 1) to check the evolution of structural and magnetic properties upon the introduction of the Jahn–Teller active ion Ni 2 +. The x ≤ 0.95 samples can be indexed with a cubic lattice with the space group Fd3m, whereas the x = 0.98 sample undergoes a cubic-tetragonal structural transition, and the x > 0.98 samples belong to a tetragonal lattice with the space group I41-amd at room temperature. All these samples undergo a paramagnetic–ferrimagnetic transition at T C. A spiral ferrimagnetic ordering at T S is suppressed when the amount of Ni is increased, and the abnormal magnetization at T S for the x = 0.6 and 0.8 samples is not pronounced. The measurements of magnetic hysteresis reveal that the x = 0.4 and 0.6 samples possess a rather large coercivity H C compared with the x = 0 and 1 samples. Specifically, the x = 0.4 and 0.6 samples undergo a short-range antiferromagnetic transition above T C. The magnetic properties of the Ni-doped samples can be understood in terms of the magnetic structure model consisting of longitudinal and transverse magnetic sublattices.