Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side (J1) and the diagonal (J2) of the square. From magnetization measurements J1 + J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows on cooling as the temperature approaches the transition to the magnetic ground state at Tc ≃ 42 K.

AbstractµSR and 7 Li NMR relaxation measurements in frustrated two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice are presented. It is found that both in Li 2 VOSiO 4 and Li 2 VOGeO 4 , spin dynamics at frequencies well below the Heisenberg exchange frequency are present. These dynamics are associated with the motions of walls separating coexisting collinear domains with a magnetic wave vector rotated by 90 o . *

Low-temperature specific heat, magnetic susceptibility, and zero-field muon spin resonance (microSR) measurements have been performed in the quasi-one-dimensional molecular helimagnetic compound Gd(hfac)3NITEt. The specific heat presents two anomalies at T(0)=2.19+/-0.02 K and T(N)=1.88+/-0.02 K, which both disappear upon the application of a weak magnetic field. Conversely, magnetic susceptibility and muSR data show the divergence of two-spin correlation functions only at T(N)=1.88+/-0.02 K. These results suggest an experimental validation of Villain's conjecture of a two-step magnetic ordering in quasi-one-dimensional XY helimagnets; i.e., the paramagnetic phase and the helical spin solid phase are separated by a chiral spin liquid phase, where translational invariance is broken without violation of rotational invariance.

An overview of the basic magnetic properties of vanadates which represent prototypes of frustrated two-dimensional S = 1/2 antiferromagnets on a square lattice is presented. It will be shown how information on the ground state sublattice magnetization, on the static uniform susceptibility and on the frustration driven lattice distortions can be achieved by means of NMR spectroscopy and magnetization measurements. The low-energy spin excitations, investigated by means of NMR and µSR relaxation measurements, will be analysed and the anomalous very-low-frequency dynamics, originating from the degeneracy of the ground state, discussed. Finally the effects of hydrostatic pressure on the degree of frustration of the vanadates will be addressed.

7 Li and 29 Si NMR, µSR and magnetization measurements in Li2V1−xOTixSiO4, for 0 ≤ x ≤ 0.2, are presented. The x = 0 compound is a prototype of frustrated two-dimensional Heisenberg antiferromagnet on a square-lattice with competing nearest (J1) and next-nearest (J2) neighbour exchange interactions. Ti 4+ (S=0) for V 4+ (S=1/2) substitution yields the spin dilution of the antiferromagnetic layers. The analysis of the magnetization and of the nuclear spin-lattice relaxation rate shows that spin dilution not only reduces the spin-stiffness by a factor ≃ (1 − x) 2 , but also causes the decrease of the effective ratio J2(x)/J1(x). Moreover, the sublattice magnetization curves derived from zero-field µSR measurements in the collinear phase point out that, at variance with non-frustrated two-dimensional Heisenberg antiferromagnets, spin dilution affects the low-temperature staggered magnetization only to a minor extent. This observation is supported also by the x dependence of the collinear ordering temperature. The results obtained for the Ti doped samples are discussed in the light of the results previously obtained in the pure x = 0 compound and in non-frustrated two-dimensional Heisenberg antiferromagnets with spin-dilution.