We have synthesized high-quality single crystals of volborthite, a seemingly distorted kagome antiferromagnet, and carried out high-field magnetization measurements up to 74 T and ${}^{51}\mathrm{V}$ NMR measurements up to 30 T. An extremely wide $1/3$ magnetization plateau appears above 28 T and continues over 74 T at 1.4 K, which has not been observed in previous studies using polycrystalline samples. NMR spectra reveal an incommensurate order (most likely a spin-density wave order) below 22 T and a simple spin structure in the plateau phase. Moreover, a novel intermediate phase is found between 23 and 26 T, where the magnetization varies linearly with magnetic field and the NMR spectra indicate an inhomogeneous distribution of the internal magnetic field. This sequence of phases in volborthite bears a striking similarity to those of frustrated spin chains with a ferromagnetic nearest-neighbor coupling $J_1$ competing with an antiferromagnetic next-nearest-neighbor coupling $J_2$.

We present NMR measurements of a strong-leg spin-$1/2$ Heisenberg antiferromagnetic ladder compound $({\mathrm{C}}_7{\mathrm{H}}_{10}\mathrm{N}{)}_2{\mathrm{CuBr}}_4$ under magnetic fields up to 15 T in the temperature range from 1.2 K down to 50 mK. From the splitting of NMR lines, we determine the phase boundary and the order parameter of the low-temperature (three-dimensional) long-range-ordered phase. In the Tomonaga-Luttinger regime above the ordered phase, NMR relaxation reflects characteristic power-law decay of spin correlation functions as $1/T_1\propto T^{1/2K-1}$, which allows us to determine the interaction parameter $K$ as a function of field. We find that field-dependent $K$ varies within the $1<K<2$ range, which signifies attractive interaction between the spinless fermions in the Tomonaga-Luttinger liquid.

Total control over the electronic spin relaxation in molecular nanomagnets is the ultimate goal in the design of new molecules with evermore realizable applications in spin-based devices. For single-ion lanthanide systems, with strong spin–orbit coupling, the potential applications are linked to the energetic structure of the crystal field levels and quantum tunneling within the ground state. Structural engineering of the timescale of these tunneling events via appropriate design of crystal fields represents a fundamental challenge for the synthetic chemist, since tunnel splittings are expected to be suppressed by crystal field environments with sufficiently high-order symmetry. Here, we report the long missing study of the effect of a non-linear (C₄) to pseudo-linear (D_4d) change in crystal field symmetry in an otherwise chemically unaltered dysprosium complex. From a purely experimental study of crystal field levels and electronic spin dynamics at milliKelvin temperatures, we demonstrate the ensuing threefold reduction of the tunnel splitting.

We report O17 NMR measurements in the S=1/2\,(Cu2+) kagome antiferromagnet Herbertsmithite ZnCu3(OH)6Cl2 down to 45~mK in magnetic fields ranging from 2 to 12~T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-Kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments $\lesssim${}0.1$\mu${}B and gapped excitations. The H-T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field $\mu${}0Hc=1.55(25)\,\,T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagome Heisenberg antiferromagnet model.

Quantum spin liquids are exotic states of matter that form when strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of the magnetic interactions. Three-dimensional cases are especially challenging due to the significant reduction of the influence of quantum fluctuations. Here, we report the magnetic characterization of ${\mathrm{K}}_2{\mathrm{Ni}}_2({\mathrm{SO}}_4{)}_3$ forming a three-dimensional network of ${\mathrm{Ni}}^{2+}$ spins. Using density functional theory calculations, we show that this network consists of two interconnected spin-1 trillium lattices. In the absence of a magnetic field, magnetization, specific heat, neutron scattering, and muon spin relaxation experiments demonstrate a highly correlated and dynamic state, coexisting with a peculiar, very small static component exhibiting a strongly renormalized moment. A magnetic field $B\gtrsim 4\mathrm{T}$ diminishes the ordered component and drives the system into a pure quantum spin liquid state. This shows that a system of interconnected $S=1$ trillium lattices exhibits a significantly elevated level of geometrical frustration.

We present a direct NMR method to determine whether the interactions in a Tomonaga-Luttinger liquid (TLL) state of a spin-$1/2$ Heisenberg antiferromagnetic ladder are attractive or repulsive. For the strong-leg spin ladder compound $({\mathrm{C}}_7{\mathrm{H}}_{10}\mathrm{N}{)}_2{\mathrm{CuBr}}_4$ we find that the isothermal magnetic field dependence of the NMR relaxation rate $T_1^{-1}(H)$ displays a concave curve between the two critical fields bounding the TLL regime. This is in sharp contrast to the convex curve previously reported for a strong-rung ladder, $({\mathrm{C}}_5{\mathrm{H}}_{12}\mathrm{N}{)}_2{\mathrm{CuBr}}_4$. We show that the concavity and the convexity of $T_1^{-1}(H)$, which is a fingerprint of spin fluctuations, directly reflect the attractive and repulsive fermionic interactions in the TLL, respectively. The interaction sign is alternatively determined from an indirect method combining bulk magnetization and specific heat data.