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Inelastic neutron scattering for temperatures below 30 K from a powder of LiZn2Mo3O8 demonstrates this triangular-lattice antiferromagnet hosts collective magnetic excitations from spin 1/2 Mo3O13 molecules. Apparently gapless (Delta < 0.2 meV) and extending at least up to 2.5 meV, the low energy magnetic scattering cross section is surprisingly broad in momentum space and involves one third of the spins present above 100 K. The data are compatible with the presence of valence-bonds involving nearest-neighbor and next-nearest-neighbor spins forming a disordered or dynamic state.
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Low-dimensional magnetic materials with spin-$frac{1}{2}$ moments can host a range of exotic magnetic phenomena due to the intrinsic importance of quantum fluctuations to their behavior. Here, we report the structure, magnetic structure and magnetic properties of copper(II) thiocyanate, Cu(NCS)$_2$, a one-dimensional coordination polymer which displays low-dimensional quantum magnetism. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, $^{13}$C magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy, and density functional theory (DFT) investigations indicate that Cu(NCS)$_2$ behaves as a two-dimensional array of weakly coupled antiferromagnetic spin chains ($J_2 = 133(1)$ K, $alpha = J_1/J_2 = 0.08$). Powder neutron-diffraction measurements confirm that Cu(NCS)$_2$ orders as a commensurate antiferromagnet below $T_mathrm{N} = 12$ K, with a strongly reduced ordered moment (0.3 $mu_mathrm{B}$) due to quantum fluctuations.
We introduce the idea of emergent lattices, where a simple lattice decouples into two weakly-coupled lattices as a way to stabilize spin liquids. In LiZn2Mo3O8, the disappearance of 2/3rds of the spins at low temperatures suggests that its triangular lattice decouples into an emergent honeycomb lattice weakly coupled to the remaining spins, and we suggest several ways to test this proposal. We show that these orphan spins act to stabilize the spin-liquid in the $J_1-J_2$ honeycomb model and also discuss a possible 3D analogue, Ba2MoYO6 that may form a depleted fcc lattice.
By applying density functional theory, we find strong evidence for an itinerant nature of magnetism in two families of iron pnictides. Furthermore, by employing dynamical mean field theory with continuous time quantum Monte Carlo as an impurity solver, we observe that the antiferromagnetic metal with small magnetic moment naturally arises out of coupling between unfrustrated and frustrated bands. Our results point to a possible scenario for magnetism in iron pnictides where magnetism originates from a strong instability at the momentum vector ($pi$, $pi$, $pi$) while it is reduced by quantum fluctuations due to the coupling between weakly and strongly frustrated bands.
LaCoO3 (LCO) nanoparticles were synthesized and their magnetic and structural properties were examined using SQUID magnetometery and neutron diffraction. The nanoparticles exhibit ferromagnetic long-range order beginning at T_C approximately 87K that persists to low temperatures. This behavior is contrasted with the ferromagnetism of bulk LCO, which also starts at T_C approximately 87K but is suppressed below a second transition at T_o approximately 37K, due to a structural phase transition. The ferromagnetism in both systems is attributed to the tensile stress from particle surfaces and impurity phase interfaces. This stress locally increases the Co-O-Co bond angle gamma, and competes with the thermal contraction of the lattice. It has recently been shown that LCO loses long-range ferromagnetic order when gamma decreases below the critical value gamma_c = 162.8 degrees. Consistent with this model, we show that gamma in nanoparticles remains larger than gamma_c at low temperatures, likely a consequence of all spins being in close proximity to surfaces or interfaces.
We report a new peculiar effect of the interaction between a sublattice of frustrated quantum spin-1/2 chains and a sublattice of pseudospin-1/2 centers (quantum electric dipoles) uniquely co-existing in the complex oxide Li2ZrCuO4. 7Li nuclear magnetic-, Cu2+ electron spin resonance and a complex dielectric constant data reveal that the sublattice of Li+-derived electric dipoles orders glass like at Tg ~ 70 K yielding a spin site nonequivalency in the CuO2 chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom might strongly influence the properties of the spiral spin state in Li2ZrCuO4 that is close to a quantum ferromagnetic critical point. In particular that strong quantum fluctuations and/or the glassy behavior of electric dipoles might renormalize the exchange integrals affecting this way the pitch angle of the spiral as well as be responsible for the missing multiferroicity present in other helicoidal magnets.
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