The Ba3Cu3In4O12 stands for unique topology of the magnetic subsystem. It consists of rotated by 90 degrees relative to each other "paper-chain" columns made of vertex-sharing (CuO4)-O-I and (CuO4)-O-II planar units. The overall pattern of the copper ions is that of a three-dimensional Shastry-Sutherland network. At high temperatures, the magnetic susceptibility follows the Curie-Weiss law with positive Weiss temperature indicating strong predominance of ferromagnetic coupling. At low temperatures, however, this compound exhibits a long-range antiferromagnetically ordered state that reaches saturation magnetization by a nontrivial succession of two spin-flop and two spin-flip transitions already in modest magnetic fields. We show that the ground state in Ba3Cu3In4O12 may be a three-dimensional orthogonal arrangement of the Cu2+ (S = 1/2) magnetic moments forming three virtually independent antiferromagnetic subsystems. In this arrangement, favored by anisotropic exchange interactions, the quantum fluctuations provide the coupling between three mutually orthogonalmagnetic subsystems resulting in an impressive "order by disorder" effect.

In this work, we study the interplay between the crystal structure and magnetism of the pyroarsenate alpha-Cu2As2O7 by means of magnetization, heat capacity, electron spin resonance, and nuclear magnetic resonance measurements as well as density functional theory (DFT) calculations and quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O chains in the crystal structure represent a realization of a quasi-one-dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with relevant pathways through nonmagnetic AsO4 tetrahedra. Owing to residual 3D interactions, antiferromagnetic long range ordering at T-N similar or equal to 10 K takes place. Application of the external magnetic field B along the magnetically easy axis induces the transition to a spin-flop phase at B-SF similar to 1.7 T (2 K). The experimental data suggest that substantial quantum spin fluctuations take place at low magnetic fields in the ordered state. DFT calculations confirm the quasi-one-dimensional nature of the spin lattice, with the leading coupling J(1) within the structural dimers. QMC fits to the magnetic susceptibility evaluate J(1) = 164 K, the weaker intrachain coupling J'(1)/J(1) = 0.55, and the effective interchain coupling J(ic1)/J(1) = 0.20.

In this work, we study the interplay between the crystal structure and magnetism of the pyroarsenate $\ensuremath{\alpha}$-Cu${}_{2}$As${}_{2}$O${}_{7}$ by means of magnetization, heat capacity, electron spin resonance, and nuclear magnetic resonance measurements as well as density functional theory (DFT) calculations and quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O chains in the crystal structure represent a realization of a quasi-one-dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with relevant pathways through nonmagnetic AsO${}_{4}$ tetrahedra. Owing to residual 3D interactions, antiferromagnetic long range ordering at ${T}_{\mathrm{N}}\ensuremath{\simeq}10$ K takes place. Application of the external magnetic field $B$ along the magnetically easy axis induces the transition to a spin-flop phase at ${B}_{\mathrm{SF}}\ensuremath{\sim}1.7$ T (2 K). The experimental data suggest that substantial quantum spin fluctuations take place at low magnetic fields in the ordered state. DFT calculations confirm the quasi-one-dimensional nature of the spin lattice, with the leading coupling ${J}_{1}$ within the structural dimers. QMC fits to the magnetic susceptibility evaluate ${J}_{1}=164$ K, the weaker intrachain coupling ${J}_{1}^{\ensuremath{'}}/{J}_{1}=0.55$, and the effective interchain coupling ${J}_{\mathrm{ic}1}/{J}_{1}=0.20$.

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.

The nature of a puzzling high temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high temperature ferromagnetism of vanadium oxide nanotubes.

We report a new peculiar effect of the interaction between a sublattice of frustrated quantum spin-1/2 chains and a sublattice of tunnelling pseudospin-1/2 centers (quantum electric dipoles) uniquely co-existing in the complex oxide Li2ZrCuO4. 7Li nuclear magnetic-, Cu{2+} electron spin resonance and a complex dielectric constant data reveal that the electric sublattice due to tunneling Li+ ions orders glass-like at Tg ~ 80 - 100 K yielding a spin site nonequivalency in the CuO2 chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom may strongly influence the properties of the spiral spin state in Li2ZrCuO4 that is close to a quantum critical point, in particular that strong quantum fluctuations and/or the glassy behavior of Li-derived dipoles might be responsible for the missing multiferroism present in other helicoidal magnets.