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We present a combined experimental and density functional theory study that characterizes the charge and spin density in NiX₂(3,5-lutidine)₄(X= Cl, Br and I). In this material, magnetic exchange interactions occur via Ni²⁺–halide...halide–Ni²⁺pathways, forming one-dimensional chains. We find evidence for weak halide...halide covalency in the iodine system, which is greatly reduced whenX= Br and is absent forX= Cl; this is consistent with the reported `switching-on' of magnetic exchange in the larger-halide cases. Our results are benchmarked against density functional theory calculations on [NiHF₂(pyrazine)₂]SbF₆, in which the primary magnetic exchange is mediated by F–H–F bridging ligands. This comparison indicates that, despite the largely depleted charge density found at the centre of halide...halide bonds, these through-space interactions can support strong magnetic exchange gated by weak covalency and enhanced by significant electron density overlapping that of the transition metal centres. 

Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states that are sensitive to the single-ion-anisotropy (SIA) energy ( $D$), and intrachain ( $J_0$) and interchain ( $J_{1,2}^{\prime }$) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of $\mathrm{Ni}\left(\mathrm{pyrimidine}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\left({\mathrm{NO}}_3\right)}_2$, a chain compound where the tilting of Ni octahedra leads to a twofold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at $T_{\text{N}}=2.3\mathrm{K}$and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find $J_0=5.107\left(7\right)\mathrm{K}, D=2.79\left(1\right)\mathrm{K},J_1^{\prime }=0.00\left(5\right)\mathrm{K}, J_2^{\prime }=0.18\left(3\right)\mathrm{K}$, and a rhombic anisotropy energy $E=0.19\left(9\right)\mathrm{K}$. Mean-field modeling reveals that the ground state structure hosts spin canting of $\varphi \approx 6.5^{\circ }$, which is not detectable above the noise floor of the elastic neutron diffraction data. Monte Carlo simulation of the powder-averaged magnetization, $M\left(H\right)$, is then used to confirm these Hamiltonian parameters, while single-crystal $M\left(H\right)$simulations provide insight into features observed in the data. Published by the American Physical Society2025 

We investigate the magnetic properties of $S=1$antiferromagnetic diamond-lattice, $\mathrm{Ni}{X}_2{\left(\mathrm{pyrimidine}\right)}_2$ $(X=\mathrm{Cl}, \mathrm{Br})$, hosting a single-ion anisotropy (SIA) orientation which alternates between neighboring sites. Through neutron diffraction measurements of the $X=\mathrm{Cl}$compound, the ordered state spins are found to align collinearly along a pseudo-easy axis, a unique direction created by the intersection of two easy planes. Similarities in the magnetization, exhibiting spin-flop transitions, and the magnetic susceptibility in the two compounds imply that the same magnetic structure and a pseudo-easy axis is also present for $X=\mathrm{Br}$. We estimate the Hamiltonian parameters by combining analytical calculations and Monte Carlo (MC) simulations of the spin-flop and saturation field. The MC simulations also reveal that the spin-flop transition occurs when the applied field is parallel to the pseudo-easy axis. Contrary to conventional easy-axis systems, there exist field directions perpendicular to the pseudo-easy axis for which the magnetic saturation is approached asymptotically and no symmetry-breaking phase transition is observed at finite fields. Published by the American Physical Society2024 

We present the magnetic and structural properties of [Cu(pyrazine)0.5(glycine)]ClO4 under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other S = 1/2 dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero field at a critical pressure of pc = 15.7(5) kbar. Pressure-dependent single-crystal x-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram.