Pyrochlore magnets, built from corner-sharing tetrahedra, serve as magnetic frameworks that attract extensive attention due to their exotic quantum spin states. Here we present K3Mn(MoO4)2Cl, a chemically tailored pyrochlore magnet derived from Na3Mn(CO3)2Cl, where Mn2+ ions form an ideal 3D frustrated network. Millimeter-sized single crystals of K3Mn(MoO4)2Cl were successfully achieved via a simple high-temperature self-flux method. Thermodynamic measurements reveal a sharp transition at 258 mK, yet only ∼15% of the expected Rln6 entropy is recovered, indicating pronounced spin fluctuations in this long-range ordered state. Remarkably, the ordered state is highly sensitive to perturbations, disappearing under a magnetic field of about 0.2 T, while magnetization along [111] shows a feature consistent with a near half-saturated state. Our results establish the newly discovered pyrochlore magnets as a chemically engineered pyrochlore platform, in which geometrical frustration and fragile magnetic order interplay, thereby offering new opportunities to explore emergent magnetic phenomena in three dimensions.

We present a brief review focusing on recent research progress in high-field electron spin resonance (ESR) spectrometer with a focus on the construction and application of a 25-Tesla high-field ESR system. The spectrometer is based on a water-cooled resistive magnet at the Steady High Magnetic Field Facility in Hefei, China. Sample temperatures can be regulated between 2 to 300K using a 4He variable-temperature insert cryostat. A broadband microwave source, covering a frequency range of 50 to 690GHz is achieved through a series of multiplier chain amplifiers, and signal detection is performed using an InSb bolometer. Key components of the spectrometer, including the probe, waveguide, and LabVIEW-based data acquisition software, are also described. The system’s performance was evaluated using the standard sample 2,2-diphenyl-1-picrlyhydrazyl. The spectrometer was primarily designed to support external users in investigating spin systems under extreme conditions—such as observing antiferromagnetic resonance in spin-ordered phases or detecting resonance signals in systems with large zero-field splitting, which are typically inaccessible to conventional X-band spectrometers. To illustrate its applicability, representative examples from materials science and condensed matter physics are provided.

We present a comprehensive study of the low-temperature magnetism and field-tuned thermodynamics of the layered Shastry-Sutherland compound Eu2MgSi2O7. Eu2+ ions (4 f7, S = 7/2) form a quasi-two-dimensional orthogonal-dimer network, realizing a large-spin Heisenberg system on a Shastry-Sutherland geometry that is prone to geometric frustration. Magnetic susceptibility and magnetization measurements reveal nearly isotropic Eu2+ moments with weak antiferromagnetic interactions. Specific-heat data uncover a primary long-range ordered state at TN1 = 0.8 K and a second anomaly near TN2 = 1.0 K that is rapidly suppressed by magnetic field, suggesting that it is more field sensitive and may not correspond to an independent thermodynamic phase transition. By separating the magnetic heat capacity, we determine the full magnetic-entropy surface Sm(T, B), which captures the smooth field-driven evolution from the ordered state to a correlated and eventually polarized paramagnetic regime. The resulting isentropic trajectories demonstrate efficient adiabatic demagnetization, allowing cooling from 1.5-3.1 K at 5 T to minimum temperatures of 0.21-0.50 K. Comparison with the benchmark refrigerant Gd3Ga5O12 shows that Eu2MgSi2O7 exhibits enhanced sub-Kelvin magnetic heat-storage capacity, reflecting its large accessible spin entropy and weak interactions. These results establish Eu2MgSi2O7 as a rare example of a Eu-based frustrated magnet that combines weak interactions, a well-resolved low-temperature ordered state, and tunable thermodynamics suitable for sub-Kelvin cryogenic applications.

CePtPb hosts a distorted kagome lattice of Ce3+ ions, providing a clean platform to investigate how reduced local symmetry and strong spin-orbit coupling reshape frustrated magnetism. Magnetization, specific heat, and magnetocaloric effect measurements, combined with a symmetry analysis of the single-ion anisotropy, demonstrate that the local m2m site symmetry selects a nearly Ising-like Kramers doublet with easy axes lying within the ab plane. This results in three distinct in-plane Ising directions and an overall easy-plane anisotropy. The low-energy magnetic response is well captured by a three-sublattice Ising model, which quantitatively reproduces the saturation magnetization for arbitrary in-plane field orientations, including [110] and [11 & strns;0], as well as the ratio of the field-induced critical fields. For B H [110], the phase diagram exhibits two quantum critical points at Bc1 = 0.25 T and Bc2 = 0.55 T, arising from the sequential polarization of the three Ising sublattices. These results reveal that the system develops quasi-one-dimensional spin chains along the c axis, emerging from the nominally three-dimensional crystal structure composed of stacked kagome layers, and illustrate how reduced local symmetry can drive effective dimensional reduction in rare-earth Ising magnets.

Although magnetic systems with large spin quantum numbers are typically associated with classical magnetism, geometrical frustration can stabilize competing spin configurations, resulting in degenerate low-energy states and complex magnetic phases that emerge from the interplay between classical thermal fluctuations and residual quantum fluctuations. Here, a previously unreported S = 5/2 triangular lattice (TL) antiferromagnet, BaMnBe2(BO3)2F2, has been successfully designed based on a structural template and synthesized via the high-temperature flux method. The structure features magnetic layers built from equilateral TLs, where nearest-neighbor Mn2+ ions are separated by 4.63 Å within the ab plane and 7.80 Å along the c-axis. Zero-field specific heat measurements reveal two successive magnetic transitions at 0.57 and 0.72 K. Combined thermodynamic and magnetic susceptibility measurements, along with theoretical calculations, enable us to establish a magnetic phase diagram comprising multiple field-induced competing magnetic phases. Despite the large spin value of Mn2+, signatures of residual quantum fluctuations remain evident and influence the stabilization of these competing magnetic states. These results highlight BaMnBe2(BO3)2F2 as a large spin TL antiferromagnet for investigating frustrated magnetism and emergent quantum spin states.

Rare-earth-based Shastry-Sutherland lattice magnets have attracted considerable attention for uncovering exotic quantum phases and for magnetic refrigeration. This study reports on the magnetism and magnetocaloric effect of the Eu2+-based Shastry-Sutherland antiferromagnet Eu2ZnGe2OS6 with a large spin S = 7/2. Magnetic susceptibility and isothermal magnetization reveal antiferromagnetic interactions with only weak magnetic anisotropy. Zero-field-specific heat shows long-range magnetic order around TN = 0.25 K. Under applied magnetic fields, thermodynamic measurements identify three distinct phase regions and two quantum-critical points at low magnetic fields: Bc1 ≈ 0.30 T and Bc2 ≈ 0.65 T. Benefitting from strong quantum fluctuations near the quantum-critical points, the large saturation magnetic moment of the Eu2+ ions, the high magnetic ion density (42% by mass), and the low TN, we achieved a minimum temperature of 97 mK using a custom-built adiabatic demagnetization refrigerator. These findings demonstrate significant quantum fluctuations below 1 K and establish Eu2ZnGe2OS6 as a potential platform for exploring quantum spin states and for magnetic cooling applications in the sub-100 mK regime.

This corrects the article DOI: 10.1103/pywx-vxfh.

Large-spin (S) triangular lattice antiferromagnets exhibit both strong quantum fluctuations and high magnetic entropy, making them promising candidates for ultra-low temperature magnetic refrigeration. In such materials, an external magnetic field can significantly influence the system's Hamiltonian, leading to the emergence of distinct magnetically ordered ground states. Interestingly, at the critical field between two ordered phases, the spins can develop a highly degenerate magnetic ground state, giving rise to enhanced quantum fluctuations and a pronounced magnetocaloric effect. In this study, the magnetic phase diagram of the S = 7/2 triangular lattice antiferromagnet GdBO3 was established through measurements of specific heat, magnetization, and the magnetocaloric effect. The phase diagram reveals that the system exhibits four distinct ground states (phases I, II, III, and IV) under external magnetic fields. Notably, a 1/3 magnetization plateau is observed in phase II, as indicated by the magnetization curve. Due to strong quantum fluctuations at critical field Bc3 and the high density of magnetic Gd3+ ions, we achieved a minimum temperature of 50 mK, using a custom-designed adiabatic demagnetization refrigerator. Our findings reveal significant quantum fluctuations below 2 K, demonstrating GdBO3’s potential for millikelvin magnetic cooling applications

Equilateral triangular lattices (TLs) with antiferromagnetic interactions are ideal templates for inducing magnetic frustration, offering a platform to explore exotic quantum spin states that are crucial for advancing quantum science and technology. However, few examples meet these structural criteria, and even fewer realize these fascinating physical properties. We report four novel equilateral TL magnets realized through a crystal-symmetry-protected molecular-brick strategy. By deliberately selecting a high-symmetry nonmagnetic structural template and incorporating magnetic molecular building blocks, magnetic ions are geometrically constrained by three-fold rotational symmetry within the a-b plane, enforcing an ideal equilateral TL and turning on magnetism in a nonmagnetic structural template. Magnetic susceptibility and specific heat measurements reveal a long-range magnetic ordering in BaCoBe2(BO3)2F2 around 180 mK, while BaNiBe2(BO3)2F2 remains disordered at 140 mK. Furthermore, crystal orbital Hamilton population analysis shows that different molecular bricks exhibit distinct bonding characteristics, resulting in diverse magnetic properties. Our results demonstrate that the molecular-brick chemical strategy offers valuable insights into the investigation of geometrically frustrated magnets. (c) 2026 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.

Interactions of collective excitations often lead to rich emergent phenomena in many-particle quantum systems. In ordered magnets, the elementary excitations are spin waves (magnons), which obey Bose-Einstein statistics. Similar to the Cooper pairs in superconductors, magnons can be paired into bound states under attractive interactions. Even more interestingly, the Zeeman coupling to a magnetic field acts as a chemical potential that can tune the particle density through a quantum critical point (QCP), beyond which a ``hidden order'' is predicted to exist. However, experimental confirmation of this QCP and the associated new state of matter remain elusive. Here we report direct observation of the Bose-Einstein condensation (BEC) of the two-magnon bound state in Na$_2$BaNi(PO$_4$)$_2$. Comprehensive thermodynamic measurements confirmed the existence of a two-dimensional BEC-QCP at the saturation field. Inelastic neutron scattering experiments were performed to accurately establish the magnetic exchange model. An exact solution of the model found stable 2-magnon bound states that were further confirmed by an electron spin resonance (ESR) experiment, demonstrating that the QCP is due to the pair condensation and the phase below saturation field is the long-sought-after spin nematic (SN) phase.

Rare-earth-based triangular lattice antiferromagnets, with strong quantum fluctuations and weak magnetic interactions, can often retain large magnetic entropy down to very low temperatures, making them excellent candidates for magnetic refrigeration at ultra-low temperatures. These materials exhibit a substantial magnetocaloric effect (MCE) due to enhanced spin fluctuations, particularly near quantum critical points, which leads to significant changes in magnetic entropy. This study reports on the crystal growth, structure, magnetism, and MCE of a Gd-based triangular lattice material, GdBO3, characterized by a large spin quantum number (S = 7/2). Successive phase transitions (T1 = 0.52 K, T2 = 0.88 K, and T3 = 1.77 K) were observed in zero-field specific heat measurements. Furthermore, thermal dynamic analysis under external magnetic fields identified five distinct phase regions and three quantum critical points for GdBO3. Due to its broad specific heat features and the high density of magnetic Gd3+ ions, we achieved a minimum temperature of 50 mK near the field-induced quantum critical point, using a custom-designed GdBO3-based adiabatic demagnetization refrigerator. Our findings reveal significant quantum fluctuations below 2 K, demonstrating GdBO3's potential for milli-Kelvin magnetic cooling applications.

Spin excitation continua observed in neutron scattering studies are often considered to be strong evidence of quantum spin liquid formation. In a disorder-free magnetic compound with a quantum spin liquid ground state, the elementary excitation is no longer the conventional spin waves (magnons). Instead, the magnons fractionalize into spinons, producing a characteristic two-spinon continuum. However, it remained uncertain whether a clean, ordered antiferromagnet could exhibit a continuous spectrum similar to that of a quantum spin liquid. This paper presents evidence of a spin excitation continuum in the magnetically ordered state of Na2BaCo(PO4)2, where free spinons are absent. This challenges the interpretation of such a continuum as a definitive signature of a quantum spin liquid in new material studies.

The magnetic ground state of geometrically frustrated antiferromagnets has attracted great research interest due to the possibility to realize novel quantum magnetic states such as a quantum spin liquid. Here we present a comprehensive magnetic characterization of DyTa7O19 with an ideal two-dimensional triangular lattice. DyTa7O19 exhibits c-axis single-ion magnetic anisotropy. Although long-range magnetic order is not observed down to 100 mK under zero field, by applying a small magnetic field (similar to 0.1 T), a magnetically ordered state with net magnetization of Ms/3 below Tm = 0.14 K is identified (Ms denotes the saturated magnetization). We argue that this state is an up-up-down magnetic structure phase driven by the dipole-dipole interactions between Isinglike spins of Dy3+ in a two-dimensional triangular lattice, since its ordering temperature and temperature-field phase diagram can be well explained by the theoretical calculations based on dipolar interactions. DyTa7O19 could be viewed as a rare material platform that realizes pure Ising-like dipolar interaction in a geometrically frustrated lattice.

Discovery of new states of matter is a key objective in modern condensed matter physics, which often leads to revolutionary technological advancements such as superconductivity. Quantum spin nematic, a “hidden order” that evades conventional magnetic probes, is one such state. Na_2BaNi(PO_4)_2 is a potential spin nematic material, suggested by the observation of a two-magnon Bose-Einstein condensation from above the saturation field. However, direct confirmation of the spin nematicity remains elusive. This Letter presents inelastic neutron scattering spectra from the putative spin nematic phases of Na_2BaNi(PO_4)_2, revealing low-energy quadrupole waves that are absent in the neighboring conventional magnetic phases. A spin-one model quantitatively captures the full details of the spin excitation spectra across all low-temperature phases, providing direct evidence of the spin nematic orders. Additionally, we show evidence of the three-magnon continuum and two-magnon bound states in the 1/3-magnetization plateau, revealing condensation of the two-magnon bound state as the origin of the low-field spin nematic supersolid phase.

Dimerized quantum magnets provide fertile ground for exploring novel quantum phases arising from the condensation of magnon excitations. Here, we investigate the quantum dimer magnet Cu(en)2SO4 using specific heat, magnetocaloric effect, and electron spin resonance (ESR) measurements, combined with infinite projected entangled pair state (iPEPS) simulations. Our results establish a dimer-singlet ground state in Cu(en)2SO4 with intradimer coupling J 10.9 K. A magnetic field applied along the c axis drives two quantum phase transitions at Bc1 6.43 T and Bc2 11.73 T, enclosing a dome-shaped antiferromagnetic phase below Tc 0.65 K. This ordered state is naturally interpreted as a result of triplon Bose-Einstein condensation (BEC), with the excitation gap closing and reopening across the critical fields. ESR spectra reveal not only sharp magnon resonances, but also broad continuumlike features, pointing to unconventional spin dynamics that may arise from triplet excitations. iPEPS simulations reproduce the experimental critical fields and confirm a stripe-type spin configuration with ordering wave vector at the P point, establishing Cu(en)2SO4 as a clean platform for studying magnon BEC and field-tuned quantum criticality.

Sr2IrO4, a prototypical Jeff = 1/2 square lattice system, is widely studied for novel physics. While transition metal substitution effects are well explored, rare-earth doping at the Ir4+ site is less explored. Here we present a detailed magnetization and neutron diffraction study on Sr2Ir1-xTbxO4 (x = 0.11). The system tends to adopt an in-plane antiferromagnetic configuration with a "+ + ++" stacking sequence. Strong spin fluctuations persist down to the lowest measured temperature, consistent with the concave shape order parameter. A pronounced magnetic diffuse scattering rod develops along the L direction at low temperature, indicating the formation of short-range magnetic correlations with a characteristic length of similar to 24(2) & Aring;. This work provides a new pathway to exotic quantum phases in the strongly spin-orbit-coupled iridates.