Te6+/W6+ (d10/d0) substitution in B-site ordered double perovskites provides an effective route to tune lattice distortions and magnetic interactions. Here we present a comprehensive study of the Sr2Ni(Te1-xWx)O6 (0 <= x <= 1) system, combining X-ray and neutron powder diffraction with dc magnetic susceptibility measurements. W substitution drives a structural transition from a monoclinic (12/m) to a tetragonal (14/m) phase, accompanied by a continuous lattice contraction and a pronounced evolution of the Ni-O(1)-(Te/W) bond angle from a bent to a linear geometry, indicating the removal of octahedral tilting along the c direction. Magnetically, all compositions exhibit dominant antiferromagnetic interactions. The Weiss temperature shows a systematic decrease in the monoclinic phase with increasing W content, but exhibits a plateau-like behavior in the tetragonal phase, reflecting a clear correlation between the evolutions of the Ni-O-(Te/W) bond angles and the strength of magnetic exchange interactions. Notably, the intermediate compositions (x approximate to 0.2-0.4) show a strong suppression of the characteristic temperature scale, with a broad susceptibility hump around 25 K and an additional anomaly at T & lowast; similar to 8 K. This behavior is consistent with exchange disorder arising from the random distribution of nonmagnetic Te6+/W6+ cations, leading to competing interactions and short-range magnetic correlations. These results reveal how d10/d0 substitution simultaneously tunes lattice distortions and generates exchange disorder, providing a unified microscopic picture for the evolution of magnetic correlations in Sr2Ni(Te1-xWx)O6.

Near a quantum critical point (QCP), the low-temperature thermodynamics follows universal scaling laws. Using copper sulfate pentahydrate-a canonical spin-1/2 antiferromagnetic Heisenberg chain compound-we report the observation of a universal magnetocaloric effect (MCE) near a field-driven QCP. Remarkably, in the 1D quantum-critical regime, we obtain the universal magnetocaloric scaling function via adiabatic demagnetization measurements, which agrees with the analytical solutionof the critical 1D Fermi gas. This establishes the copper sulfate crystal as an ideal platform for studying quantum criticality and universal phenomena. Upon further cooling, our MCE and nuclear magnetic resonance measurements reveal a dimensional crossover to a 3D quantum-critical regime of the Bose-Einstein condensation (BEC) universality class, characterized by the scaling lawand a clear data collapse of the magnetic Grüneisen ratio with the 3D Bose-gas scaling function. Practically, this quantum-critical MCE enables cooling to 68.7 mK near the QCP and achieves a lowest temperature of 12.8 mK at zero field without the need for helium-3. Our work identifies a universal MCE in a common compound, establishing this magnon BEC system as a prototype quantum-critical coolant and a platform for next-generation millikelvin refrigeration.

Adiabatic demagnetization refrigeration (ADR) has attracted considerable attention as an effective approach to reach ultra-low temperatures required for fundamental physics and quantum technologies. Here we directly characterize the cryogenic magnetocaloric performance of the rare-earth-based double-perovskite oxide Gd2CuTiO6 (GCTO) through quasi-adiabatic demagnetization measurements. Magnetization measurements show no long-range magnetic transition above 1.8 K and indicate dominant antiferromagnetic (AFM) interactions, consistent with an AFM ordering temperature of TN approximate to 1.15 K reported previously. Notably, the isothermal magnetization M(H) at 1.8 K deviates from an ideal single-ion Brillouin response and is better described by a molecular-field correction for the Gd sublattice, suggesting correlated paramagnetism persisting above TN. In contrast to previous studies that inferred cooling performance from thermodynamic estimates, we directly validate the achievable sub-Kelvin cooling in GCTO through quasi-adiabatic measurements. In the quasi-ADR process starting from T0 similar to 2 K, demagnetization fields of 4, 6, and 9 T yield minimum temperatures of Tmin=761.5, 452.4, and 289.2 mK, respectively, well below TN. After complete removal of the magnetic field, the sample temperature remains highly stable for at least several tens of minutes, demonstrating a long hold time under quasi-adiabatic conditions. Moreover, the T(H) curves reveal a characteristic field scale around Hc similar to 1 T, implying a field-induced modification of the low-temperature magnetic-entropy landscape that is relevant to the cooling behavior during demagnetization. These results highlight GCTO as a promising magnetic refrigerant for sub-Kelvin ADR applications and underscore the role of correlated magnetism in optimizing cryogenic magnetocaloric performance.

Two-dimensional van der Waals ferromagnet Fe$_3$GeTe$_2$ (FGT) holds a great potential for applications in spintronic devices, due to its high Curie temperature, easy tunability, and excellent structural stability in air. Theoretical studies have shown that pressure, as an external parameter, significantly affects its ferromagnetic properties. In this study, we have performed comprehensive high-pressure neutron powder diffraction (NPD) experiments on FGT up to 5 GPa, to investigate the evolution of its structural and magnetic properties with hydrostatic pressure. The NPD data clearly reveal the robustness of the ferromagnetism in FGT, despite of an apparent suppression by hydrostatic pressure. As the pressure increases from 0 to 5 GPa, the Curie temperature is found to decrease monotonically from 225(5) K to 175(5) K, together with a dramatically suppressed ordered moment of Fe, which is well supported by the first-principles calculations. Although no pressure-driven structural phase transition is observed up to 5 GPa, quantitative analysis on the changes of bond lengths and bond angles indicate a significant modification of the exchange interactions, which accounts for the pressure-induced suppression of the ferromagnetism in FGT.

Bose-Einstein condensation (BEC), a macroscopic quantum phenomenon arising from phase coherence and bosonic statistics, has been realized in quantum magnets. Here, we report the observation of a universal magnetocaloric effect (MCE) near a BEC quantum critical point (QCP) in copper sulfate crystal (CuSO_4 · 5H_2O). By conducting magnetocaloric and nuclear magnetic resonance measurements, we uncover a field-driven BEC QCP, evidenced by the universal scaling law T_c ∝ (B_c - B)^2/3 and the perfect data collapse of the magnetic Grüneisen ratio. Thermal excitation triggers a dimensional crossover to a 1D quantum-critical regime, where the MCE scaling strictly matches the universality class of 1D Fermi gases. Notably, the quantum-critical MCE enables cooling down to 12.8 mK without helium-3, with very fast thermal relaxation rate that is critical for high cooling power. This work demonstrates the universal MCE in magnon BEC systems, using a common copper sulfate compound as a paradigmatic example, and paves the way for next-generation sub-Kelvin cooling.

Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates Na2BaCo(PO4)2 with J eff = 1/2 and Na2BaNi(PO4)2 with S = 1, the magnetic properties of spin-5/2 antiferromagnet Na2BaMn(PO4)2, their classical counterpart, are comprehensively investigated experimentally. DC magnetization and specific heat measurements on polycrystalline samples indicate two successive magnetic transitions at T N1 approximate to 1.13 K and T N2 approximate to 1.28 K, respectively. Zero-field neutron powder diffraction measurement at 67 mK reveals a Y-like spin configuration as its ground-state magnetic structure, with both the ab plane and c-axis components of the Mn2+ moments long-range ordered. The incommensurate magnetic propagation vector k shows a dramatic change for the intermediate phase between T N1 and T N2 , in which the spin state is speculated to change into a collinear structure with only the c-axis moments ordered, as stabilized by thermal fluctuations. The successive magnetic transitions observed in Na2BaMn(PO4)2 are in line with the expectation for a triangle-lattice antiferromagnet with an easy-axis magnetic anisotropy.

By performing experiment and model studies on a triangular-lattice dipolar magnet KBaGd(BO_3)_2 (KBGB), we find the highly frustrated magnet with a planar anisotropy hosts a strongly fluctuating dipolar spin liquid (DSL), which originates from the intriguing interplay between dipolar and Heisenberg interactions. The DSL constitutes an extended regime in the field-temperature phase diagram, which gets lowered in temperature as field increases and eventually ends with an unconventional quantum critical point (QCP) at B_c≃ 0.75 T. Based on dipolar Heisenberg model calculations, we identify the DSL as a Berezinskii-Kosterlitz-Thouless (BKT) phase with emergent U(1) symmetry. Due to the tremendous entropy accumulation that can be related to the strong BKT and quantum fluctuations, unprecedented magnetic cooling effects are observed in the DSL regime and particularly near the QCP, making KBGB a superior dipolar coolant to commercial Gd-based refrigerants. We establish the phase diagram for triangular-lattice dipolar quantum magnets where emergent symmetry plays an essential role, and provide a basis and opens an avenue for their applications in sub-Kelvin refrigeration.

Magnetic double perovskite (DP) compounds provide a fertile playground to explore interesting electronic and magnetic properties. By complementary macroscopic characterizations, neutron powder diffraction measurements, and first-principles calculations, we have performed comprehensive studies on the magnetic ordering in the DP compound Sr2NiWO6. It is found by neutron diffraction to order magnetically in a collinear type-II antiferromagnetic (AFM) structure in a tetragonal lattice with k = (0.5, 0, 0.5) below T-N = 56K. In the ground state, the ordered moment of the spin-1 Ni2+ ions is determined to be 1.9(2) mu(B), indicating a significant quenching of the orbital moment. The Ni2+ moments in Sr2NiWO6 are revealed to cant off the c axis by 29.2 degrees, which is well supported by the first-principles magnetic anisotropy energy calculations. Furthermore, the in-plane and out-of-plane next-nearest-neighbor superexchange couplings (J(2) and J(2c)) are found to play a dominant role in the spin Hamiltonian of Sr2NiWO6, which accounts for the stabilization of the type-II AFM structure as its magnetic ground state.

The ground-state magnetic structures of the Eu^{2+} spins in recently discovered RbEu(Fe_{1−x}Ni_{x})_{4}As_{4} superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase in Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease in the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the c axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor RbEu(Fe_{0.95}Ni_{0.05})_{4}As_{4} and the superconducting ferromagnet RbEu(Fe_{0.93}Ni_{0.07})_{4}As_{4}. Such a helical structure transforms into a purely collinear ferromagnetic structure for nonsuperconducting RbEu(Fe_{0.91}Ni_{0.09})_{4}As_{4}, with all the Eu^{2+} spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the Eu^{2+} spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent Eu^{2+} layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida interaction.

The ground-state magnetic structures of the Eu2+ spins in recently discovered RbEu(Fe1-xNix)(4)As-4 superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase in Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease in the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the c axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor RbEu(Fe0.95Ni0.05)(4)As-4 and the superconducting ferromagnet RbEu(Fe0.93Ni0.07)(4)As-4. Such a helical structure transforms into a purely collinear ferromagnetic structure for nonsuperconducting RbEu(Fe0.91Ni0.09)(4)As-4, with all the Eu2+ spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the Eu2+ spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent Eu2+ layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida interaction.

Na4−δNiTeO6 is a rare example in the transition-metal tellurate family of realizing an S = 1 spin-chain structure. By performing neutron powder diffraction measurements, the ground-state magnetic structure of Na4−δNiTeO6 is determined. These measurements reveal that below TN ∼ 6.8(2) K, the Ni2+ moments form a screwed ferromagnetic (FM) spin-chain structure running along the crystallographic a axis but these FM spin chains are coupled antiferromagnetically along the b and c directions, giving rise to a magnetic propagation vector of k = (0, 1/2, 1/2). This zigzag magnetic order is well supported by first-principles calculations. The moment size of Ni2+ spins is determined to be 2.1(1)μB at 3 K, suggesting a significant quenching of the orbital moment due to the crystalline electric field (CEF) effect. The previously reported metamagnetic transition near HC ∼ 0.1 T can be understood as a field-induced spin-flip transition. The relatively easy tunability of the dimensionality of its magnetism by external parameters makes Na4−δNiTeO6 a promising candidate for further exploring various types of novel spin-chain physics.