Magnetic susceptibility, Cu NQR, and high-filed magnetization have been measured in polycrystalline $\mathrm{SrCu}{}_2(\mathrm{BO}{}_3{)}_2$ having a two-dimensional (2D) orthogonal network of Cu dimers. This cuprate provides a new class of 2D spin-gap system $(\Delta =30\mathrm{}\mathrm{K})$ in which the ground state can be solved “exactly.” Furthermore, in the magnetization, two plateaus corresponding to $\frac{1}{4}$ and $\frac{1}{8}$ of the full Cu moment were first observed for 2D quantum spin systems.

Besides being an ancient pigment, ${\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_6$ is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap, creating a gas of bosonic spin triplet excitations. The topology of the spin lattice makes ${\mathrm{B}\mathrm{a}\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_6$ an ideal candidate for studying the Bose-Einstein condensation of triplet excitations as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures.

The effect of strontium substitution on structural, magnetic, and dielectric properties of a multiferroic Y-type hexaferrite (chemical formula Ba_2−xSr_xMg₂Fe₁₂O₂₂ with 0 ≤ x ≤ 2) was investigated. Y-type hexaferrite phase formation was not affected by strontium substitution for barium, in the range 0 ≤ x ≤ 1.5, confirmed by x-ray diffraction and Raman spectroscopy measured at room temperature. Two intermediate magnetic spin phase transitions (at tempertures T_I and T_II) and a ferrimagnetic-paramagnetic transition (at Curie temperature T_C) were identified from the temperature dependence of the magnetic susceptibility. Magnetic transition temperatures (T_I, T_II, and T_C) increased with increasing strontium content. Magnetic hysteresis measurements indicated that by increasing strontium concentration, the coercivity increases, while the saturation magnetization decreases. The ⁵⁷Fe NMR spectrum of the Y-type hexaferrite measured at 5 K and in zero magnetic field showed remarkable differences compared to that of other hexaferrites due to their different number of tetrahedral and octahedral iron sites. The temperature and frequency dependence of the dielectric permittivity evidenced broad peaks with frequency dispersion in correspondence of the Curie temperature.

We present an overview of solid-state NMR studies of endohedral H₂-fullerene complexes, including ¹H and ¹³C NMR spectra, ¹H and ¹³C spin relaxation studies, and the results of ¹H dipole–dipole recoupling experiments. The available data involves three different endohedral H₂-fullerene complexes, studied over a wide range of temperatures and applied magnetic fields. The symmetry of the cage influences strongly the motionally-averaged nuclear spin interactions of the endohedral H₂ species, as well as its spin relaxation behaviour. In addition, the non-bonding interactions between fullerene cages are influenced by the presence of endohedral hydrogen molecules. The review also presents several pieces of experimental data which are not yet understood, one example being the structured ¹H NMR lineshapes of endohedral H₂ molecules trapped in highly symmetric cages at cryogenic temperatures. This review demonstrates the richness of NMR phenomena displayed by H₂-fullerene complexes, especially in the cryogenic regime.

Han purple (${\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}$) is not only an ancient pigment, but also a valuable model material for studying Bose-Einstein condensation of magnons in high magnetic fields. Using precise low-temperature structural data and extensive density-functional calculations, we elucidate magnetic couplings in this compound. The resulting magnetic model comprises two types of nonequivalent spin dimers, in excellent agreement with the $^{63,65}\mathrm{Cu}$ nuclear magnetic resonance data. We further argue that leading interdimer couplings connect the upper site of one dimer to the bottom site of the contiguous dimer, and not the upper-to-upper and bottom-to-bottom sites, as assumed previously. This finding is verified by inelastic neutron scattering data and implies the lack of frustration between the layers of spin dimers in ${\mathrm{BaCuSi}}_{2}{\mathrm{O}}_{6}$, thus challenging existing theories of the two-dimensional-like Bose-Einstein condensation of magnons in this compound.