An interplay of geometrical frustration and strong quantum fluctuations in a spin-1/2 triangular-lattice antiferromagnet (TAF) can lead to exotic quantum states. Here, we report the neutron-scattering, magnetization, specific heat, and magnetocaloric studies of the recently discovered spin-1/2 TAF Na ₂ BaCo(PO ₄ ) ₂ , which can be described by a spin-1/2 easy axis XXZ model. The zero-field neutron diffraction experiment reveals an incommensurate antiferromagnetic ground state with a significantly reduced ordered moment of about 0.54(2) μ _B /Co. Different magnetic phase diagrams with magnetic fields in the a b plane and along the easy c -axis were extracted based on the magnetic susceptibility, specific heat, and elastic neutron-scattering results. In addition, two-dimensional (2D) spin dispersion in the triangular plane was observed in the high-field polarized state, and microscopic exchange parameters of the spin Hamiltonian have been determined through the linear spin wave theory. Consistently, quantum critical behaviors with the universality class of d = 2 and ν z = 1 were established in the vicinity of the saturation field, where a Bose–Einstein condensation (BEC) of diluted magnons occurs. The newly discovered quantum criticality and fractional magnetization phase in this ideal spin-1/2 TAF present exciting opportunities for exploring exotic quantum phenomena.

Geometrical frustration and the enhancement of strong quantum fluctuations in two-dimensional triangular antiferromagnets can lead to various intriguing phenomena. Here, we studied the spin-1/2 triangular lattice antiferromagnet ${\mathrm{NdAuAl}}_4{\mathrm{Ge}}_2$. Thermodynamic and transport properties, such as magnetization and specific heat together with the resistivity measurements were performed. In zero field, two successive phase transitions were observed at $T_{\mathrm{N}1}=1.75\pm 0.02$ and $T_{\mathrm{N}2}=0.49\pm 0.02$ K, respectively. Under magnetic field, $\mathrm{XXZ}$-type anisotropy was revealed with the moments pointing along the easy $c$ axis. For $B\parallel c$, multiple field-induced states were observed, and the magnetic phase diagram was established based on the specific-heat and magnetization data. The temperature-dependent resistivity measurements indicate that ${\mathrm{NdAuAl}}_4{\mathrm{Ge}}_2$ is a good metal. It is very likely that both the long-range Ruderman-Kittel-Kasuya-Yosida interactions and the geometrical frustration play important roles in this case.

Rare-earth ($R$) kagome-lattice magnets offer an excellent platform to discover the novel magnetic phase as well as quantum phase transition tuned by nonthermal control parameters, while the experimental realizations remain largely unexplored. Here, we report the discovery of magnetic field ($B$) -induced anisotropic quantum phase transition in a distorted kagome antiferromagnet ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ with $T_{\mathrm{N}}\sim 0.32\mathrm{K}$. The isothermal magnetizations at 0.05 K exhibit the spin-flop-like metamagnetic crossover behaviors with different fractional magnetization anomalies for $B$ perpendicular ($B\left|\right|c$ axis) and parallel ($B\left|\right|a^{*}$ axis) to the kagome plane, respectively. In combination with the thermodynamic measurements, the field-temperature ($B\text{−}T$) phase diagrams for both field directions are constructed and reveal the existence of several field-induced magnetic states. Along the $c$ axis, a proximate quantum bicritical point is observed near the metamagnetic crossover, which separates the low-field antiferromagnetic (AFM) phase and the intermediate AFM phase; while, for $B\left|\right|a^{*}$, another intermediate magnetic phase (IAFM2) appears between the low-field AFM phase and the intermediate AFM (IAFM1) phase, giving rise to a tetracritical point. These results support the anisotropic field-induced metamagnetic quantum criticalities in ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$, making it a rare kagome antiferromagnet to investigate the quantum multicriticality driven by spin frustration.

The rare-earth perovskite ${\mathrm{TbScO}}_3$ has been widely used as a substrate for the growth of epitaxial ferroelectric and multiferroic thin films, while its detailed low-temperature magnetic properties were rarely reported. In this paper, we performed detailed magnetization, specific heat, and single crystal neutron scattering measurements, along with the crystalline electric field calculations to study the low-temperature magnetic properties of ${\mathrm{TbScO}}_3$. All our results suggest the magnetic ${\mathrm{Tb}}^{3+}$ has an Ising-like pseudo-doublet ground state at low temperatures. Due to the constrain of local point symmetry, these ${\mathrm{Tb}}^{3+}$ Ising moments are confined in the $ab$ plane with a tilt angle of $\phi =\pm {48}^{\mathrm{o}}$ to the $a$ axis. In zero field, the system undergoes an antiferromagnetic phase transition at $T_{\mathrm{N}}=2.53$ K, and forms a $G_x{A}_y$ noncollinear magnetic structure below $T_{\mathrm{N}}$. We find the dipole-dipole interactions play an important role to determine the magnetic ground state, which are also responsible for the quasi-one-dimensional magnetism in ${\mathrm{TbScO}}_3$. The significant anisotropic diffuse scatterings further confirm the quasi-one-dimensional magnetism along the $c$ axis. The magnetic phase diagram with the field along the easy $b$ axis is well established. In addition to the $G_x{A}_y$ antiferromagnetic state, there is an exotic field-induced phase emerged near the critical field $B_{\mathrm{c}}\simeq 0.7$ T, where three-dimensional magnetic order is suppressed but strong one-dimensional correlations may still exist.