$\alpha -{\mathrm{RuCl}}_3$ has attracted enormous attention since it has been proposed as a prime candidate to study fractionalized magnetic excitations akin to Kitaev's honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to $0.4\mathrm{K}$ in applied magnetic fields up to $9\mathrm{T}$ for fields parallel to the $ab$ plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to ${\mu }_0{H}_{\text{c}}\approx 6.9$ T. Above $H_{\text{c}}$, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point at $H_{\text{c}}$; this is supported by universal scaling behavior near $H_{\text{c}}$. Remarkably, the data also reveal the existence of a small characteristic energy scale well below 1 meV, above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a $J_1-K_1-{\mathrm{\Gamma }}_1-J_3$ honeycomb model.

Low-temperature neutron diffraction and NMR studies of field-induced phases in linarite are presented for magnetic fields $H\parallel b$ axis. A two-step spin-flop transition is observed, as well as a transition transforming a helical magnetic ground state into an unusual magnetic phase with sine-wave-modulated moments $\parallel H$. An effective ${\tilde{J}}_1\text{−}{\tilde{J}}_2$ single-chain model with a magnetization-dependent frustration ratio ${\alpha }_{\mathrm{eff}}=-{\tilde{J}}_2/{\tilde{J}}_1$ is proposed. The latter is governed by skew interchain couplings and shifted to the vicinity of the ferromagnetic critical point. It explains qualitatively the observation of a rich variety of exotic longitudinal collinear spin-density wave, ${\mathrm{SDW}}_p$, states ($9\ge p\ge 2$).

High-resolution thermal expansion and magnetostriction measurements were performed on single crystals of $\alpha -{\mathrm{RuCl}}_3$ in magnetic fields applied parallel to the Ru-Ru bonds. The length changes were measured in the direction perpendicular to the honeycomb planes. Our data show clear thermodynamic characteristics for the field-induced phase transition at the critical field ${\mu }_0{H}_{\mathrm{c}1}=7.8\left(2\right)\mathrm{T}$ where the antiferromagnetic zigzag order is suppressed. At higher fields, a kink in the magnetostriction coefficient signals an additional transition or crossover around ${\mu }_0{H}_{\mathrm{c}2}\approx 11\mathrm{T}$. The extracted Grüneisen ratio shows typical hallmarks for quantum criticality near $H_{\mathrm{c}1}$ but also displays anomalous behavior above $H_{\mathrm{c}1}$. We compare our experimental data with spin-wave calculations employing a minimal Kitaev-Heisenberg model in the semiclassical limit. Most of the salient features are in agreement with each other, however, the peculiar features in the region above $H_{\mathrm{c}1}$ cannot be accounted for in our semiclassical modeling and hence suggest a genuine quantum nature. We construct a phase diagram for $\alpha -{\mathrm{RuCl}}_3$ in a magnetic field along the Ru-Ru bonds, displaying a zigzag ordered state below $H_{\mathrm{c}1}$, a quantum paramagnetic regime between $H_{\mathrm{c}1}$ and $H_{\mathrm{c}2}$, and a semiclassical partially polarized state above $H_{\mathrm{c}2}$.

We report an optimized chemical vapor transport method to grow single crystals of (Mn_1−xNi_x)₂P₂S₆ where x = 0, 0.3, 0.5, 0.7, and 1. Single crystals up to 4 mm × 3 mm × 200 μm were obtained by this method. As-grown crystals are characterized by means of scanning electron microscopy and powder X-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group C2/m (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all principal crystallographic axes. Overall, the Néel temperature T_N is non-monotonous; with increasing Ni²⁺ doping, the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn₂P₂S₆ (x = 0) up to x = 0.5 and then increases again to 155 K for pure Ni₂P₂S₆ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn_1−xNi_x)₂P₂S₆ series. Transport studies under hydrostatic pressure on the parent compound Mn₂P₂S₆ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa.

In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba0.65Na0.35Fe2As2. The isothermal magnetic field (H ∥ c axis) dependent magnetization (M) loops exhibit a second peak (SP) or 'fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density Jc obtained from the M(H) loops is rather high, of the order of 10(6) A cm(-2). The analysis of the temperature- and field-dependent Jc implies that high Jc is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well as on the field sweep rate. The field for the first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field-dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggest that SP in M(H) is likely related to the crossover in creep dynamics from an elastic to a plastic mechanism. We have constructed the vortex phase diagram on the field-temperature plane.

The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi₂Te₄ and MnBi₄Te₇ benchmark the (MnBi₂Te₄)(Bi₂Te₃)_n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi₂Te₄ septuple layers (SLs). However, the QAHE realization is complicated in MnBi₂Te₄ and MnBi₄Te₇ due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi₂Te₃ quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi₆Te₁₀ (n = 2) with T_c ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi₆Te₁₀ system as perspective for the QAHE at elevated temperatures.