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Register a new userNMR study of the spin correlations in the $S=1$ armchair chain Ni$_2$NbBO$_6$
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We report our nuclear magnetic resonance (NMR) study on the structurally spin chain compound Ni$_2$NbBO$_6$ with complex magnetic coupling. The antiferromagnetic transition is monitored by the line splitting resulting from the staggered internal hyperfine field. The magnetic coupling configuration proposed by the first-principle density functional theory (DFT) is supported by our NMR spectral analysis. For the spin dynamics, a prominent peak at $Tsim35$ K well above the N{e}el temperature ($T_Nsim20$ K at $mu_0H=10$ T) is observed from the spin-lattice relaxation data. As compared with the dc-susceptibility, this behavior indicates a antiferromagnetic coupling with the typical energy scale of $sim3$ meV. Thus, the Ni$_2$NbBO$_6$ compound can be viewed as strongly ferromagnetically coupled armchair spin chains along the crystalline $b$-axis. These facts place strong constraints to the theoretical model for this compound.
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Detailed ${}^{31}$P nuclear magnetic resonance (NMR) measurements are presented on well-characterized single crystals of antiferromagnetic van der Waals Ni$_2$P$_2$S$_6$. An anomalous breakdown is observed in the proportionality of the NMR shift $K$ with the bulk susceptibility $chi$. This so-called $K$$-$$chi$ anomaly occurs in close proximity to the broad peak in $chi(T)$, thereby implying a connection to quasi-2D magnetic correlations known to be responsible for this maximum. Quantum chemistry calculations show that crystal field energy level depopulation effects cannot be responsible for the $K$$-$$chi$ anomaly. Appreciable in-plane transferred hyperfine coupling is observed, which is consistent with the proposed Ni$-$S$-$Ni super- and Ni$-$S$-$S$-$Ni super-super-exchange coupling mechanisms. Magnetization and spin$-$lattice relaxation rate ($T_1^{-1}$) measurements indicate little to no magnetic field dependence of the Neel temperature. Finally, $T_1^{-1}(T)$ evidences relaxation driven by three-magnon scattering in the antiferromagnetic state.
We report an optimized chemical vapor transport method to grow single crystals of (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ where x = 0, 0.3, 0.5, 0.7 & 1. Single crystals up to 4,mm,$times$,3,mm,$times$,200,$mu$m were obtained by this method. As-grown crystals characterized by means of scanning electron microscopy, and powder x-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group $C2/m$ (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all crystallographic principal axes. Overall, the Neel temperature TN is non-monotonous, with increasing $Ni^{2+}$ doping the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn$_2$P$_2$S$_6$ (x = 0) up to x = 0.5, and then increases again to 155 K for pure Ni$_2$P$_2$S$_6$ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ series. Transport studies under hydrostatic pressure on the parent compound Mn$_2$P$_2$S$_6$ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa
[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.
We report the signatures of dynamic spin fluctuations in the layered honeycomb Li$_3$Cu$_2$SbO$_6$ compound, with a 3$d$ S = 1/2 $d^9$ Cu$^{2+}$ configuration, through muon spin rotation and relaxation ($mu$SR) and neutron scattering studies. Our zero-field (ZF) and longitudinal-field (LF)-$mu$SR results demonstrate the slowing down of the Cu$^{2+}$ spin fluctuations below 4.0 K. The saturation of the ZF relaxation rate at low temperature, together with its weak dependence on the longitudinal field between 0 and 3.2 kG, indicates the presence of dynamic spin fluctuations persisting even at 80 mK without static order. Neutron scattering study reveals the gaped magnetic excitations with three modes at 7.7, 13.5 and 33 meV. Our DFT calculations reveal that the next nearest neighbors (NNN) AFM exchange ($J_{AFM}$ = 31 meV) is stronger than the NN FM exchange ($J_{FM}$ = -21 meV) indicating the importance of the orbital degrees of freedom. Our results suggest that the physics of Li$_3$Cu$_2$SbO$_6$ can be explained by an alternating AFM chain rather than the honeycomb lattice.
The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO$_2$, which consists of a distorted lattice of Mn$^{3+}$ spins with single-ion anisotropy. Physical property measurements on CuMnO$_2$ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to $sim$25 meV above TN= 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.
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