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Register a new userMagnetism study on a frustration-free spatially anisotropic $S$ = 1 square lattice antiferromagnet Ni[SC(NH$_2$)$_2$]$_6$Br$_2$
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Magnetism of the $S$ = 1 Heisenberg antiferromagnets on the spatially anisotropic square lattice has been scarcely explored. Here we report a study of the magnetism, specific heat, and thermal conductivity on Ni[SC(NH$_2$)$_2$]$_6$Br$_2$ (DHN) single crystals. Ni$^{2+}$ ions feature an $S$ = 1 rectangular lattice in the $bc$ plane, which can be viewed as an unfrustrated spatially anisotropic square lattice. A long-range antiferromagnetic order is developed at $T rm_N =$ 2.23 K. Below $Trm_N$, an upturn is observed in the $b$-axis magnetic susceptibility and the resultant minimum might be an indication for the $XY$ anisotropy in the ordered state. A gapped spin-wave dispersion is confirmed from the temperature dependence of the magnetic specific heat. Anisotropic temperature-field phase diagrams are mapped out and possible magnetic structures are proposed.
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We report structural and magnetic properties of the spin-$frac12$ quantum antiferromagnet Cu[C$_6$H$_2$(COO)$_4$][C$_2$H$_5$NH$_3$]$_2$ by means of single-crystal x-ray diffraction, magnetization, heat capacity, and electron spin resonance (ESR) measurements on polycrystalline samples, as well as band-structure calculations. The triclinic crystal structure of this compound features CuO$_4$ plaquette units connected into a two-dimensional framework through anions of the pyromellitic acid [C$_6$H$_2$(COO)$_4$]$^{4-}$. The ethylamine cations [C$_2$H$_5$NH$_3]^+$ are located between the layers and act as spacers. Magnetic susceptibility and heat capacity measurements establish a quasi-two-dimensional, weakly anisotropic and non-frustrated spin-$frac12$ square lattice with the ratio of the couplings $J_a/J_csimeq 0.7$ along the $a$ and $c$ directions, respectively. No clear signatures of the long-range magnetic order are seen in thermodynamic measurements down to 1.8,K. However, the gradual broadening of the ESR line suggests that magnetic ordering occurs at lower temperatures. Leading magnetic couplings are mediated by the organic anion of the pyromellitic acid and exhibit a non-trivial dependence on the Cu--Cu distance, with the stronger coupling between those Cu atoms that are further apart.
We report the crystal growth and structural and magnetic properties of quasi two-dimensional $S=1/2$ quantum magnet Cu[C$_6$H$_2$(COO)$_4$][H$_3$N-(CH$_2$)$_2$-NH$_3$]$cdot$3H$_2$O. It is found to crystallize in a monoclinic structure with space group $C2/m$. The CuO$_4$ plaquettes are connected into a two-dimensional framework in the $ab$-plane through the anions of [C$_6$H$_2$(COO)$_4$]$^{4-}$ (pyromellitic acid). The [H$_3$N-(CH$_2$)$_2$-NH$_3$]$^{2+}$$cdot$3H$_2$O groups are located between the layers and provide a weak interlayer connection via hydrogen (H...O) bonds. The temperature dependent magnetic susceptibility is well described by $S=1/2$ frustrated square lattice ($J_1-J_2$) model with nearest-neighbor interaction $J_1/k_{rm B} simeq 5.35$ K and next-nearest-neighbor interaction $J_2/k_{rm B} simeq -0.01$ K. Even, our analysis using frustrated rectangular lattice ($J_{1a,b}-J_2$) model confirms almost isotropic nearest-neighbour interactions ($J_{rm 1a}/k_{rm B} simeq 5.31$ K and $J_{rm 1b}/k_{rm B} simeq 5.38$ K) in the $ab$-plane and $J_2/k_{rm B}simeq-0.24$ K. Further, the isothermal magnetization at $T=1.9$ K is also well described by a non-frustrated square lattice model with $J_1/k_{rm B} simeq 5.2$ K. Based on the $J_2/J_1$ ratio, the compound can be placed in the N{e}el antiferromagnetic state of the $J_1 - J_2$ phase diagram. No signature of magnetic long-range-order was detected down to 2 K.
[Ni(HF$_2$)(3-Clpyridine)$_4$]BF$_4$ (NBCT) is a one-dimensional, $S = 1$ spin chain material that shows no magnetic neutron Bragg peaks down temperatures of 0.1 K. Previous work identified NBCT to be in the Haldane phase and near a quantum phase transition as a function of $D/J$ to the large-$D$ quantum paramagnet phase (QPM), where $D$ is the axial single-ion anisotropy and $J$ is the intrachain superexchange. Herein, inelastic neutron scattering results are presented on partially deuterated, $^{11}$B enriched NBCT polycrystalline samples in zero magnetic field and down to temperatures of 0.3 K. Comparison to density matrix renormalization group calculations yields $D/J = 1.51$ and a significant rhombic single-ion anisotropy $E$ ($E/D approx 0.03$, $E/J approx 0.05$). These $D$, $J$, and $E$ values place NBCT in the large-$D$ QPM phase but precipitously near a quantum phase transition to a long-range ordered phase.
The crystal structures of Ni$X_2$(pyz)$_2$ ($X$ = Cl (textbf{1}), Br (textbf{2}), I (textbf{3}) and NCS (textbf{4})) were determined at 298~K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN$_4$$X_2$ units that are bridged by pyz ligands. The 2D layered motifs displayed by textbf{1}-textbf{4} are relevant to bifluoride-bridged [Ni(HF$_2$)(pyz)$_2$]$Z$F$_6$ ($Z$ = P, Sb) which also possess the same 2D layers. In contrast, terminal $X$ ligands occupy axial positions in textbf{1}-textbf{4} and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5~K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and $g$ factor of textbf{2}, textbf{3} and textbf{4} are measured by electron spin resonance where no zero--field splitting was found. The magnetism of textbf{1}-textbf{4} crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsed-field magnetization, magnetic susceptibility and $T_textrm{N}$ of textbf{2} and textbf{4}. Magnetization curves for textbf{2} and textbf{4} calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. textbf{3} is characterized as a three-dimensional antiferromagnet with the interlayer interaction ($J_perp$) slightly stronger than the interaction within the two-dimensional [Ni(pyz)$_2$]$^{2+}$ square planes ($J_textrm{pyz}$).
Complex systems with coexisting polarity, chirality and incommensurate magnetism are of great interest because they open new degrees of freedom in interaction between different subsystems and therefore they host a plethora of intriguing physical properties. Here we report on optical properties and lattice and spin dynamics of Ni$_2$InSbO$_6$ single crystals studied with the use of polarized optical microscopy and micro-Raman spectroscopy in the temperature range 10-300 K. Ni$_2$InSbO$_6$ crystallizes in a polar structure described by the noncentrosymmetric space group R3 and two types of structural domains were visualized due to natural optical activity of opposite chirality. Raman tensor elements of most A and E phonons along with their symmetry were determined. The manifestation of LO-TO splitting was observed for the A modes. By tracking the temperature dependencies of phonon frequencies the well pronounced spin-phonon interaction was observed for several modes below and above the Neel transition temperature TN = 76 K. In antiferromagnetic phase a wide excitation centred at 247 cm-1 was detected and assigned to the two-magnon mode and this value was used for estimating exchange parameters through linear spin-wave theory calculations.
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