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The J1-J2 model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J2/J1. We use bulk magnetometry and polarized neutron scattering to determine J1 and J2 unambiguously for two materials in a new family of vanadium phosphates, Pb2VO(PO 4 ) 2 and SrZnVO(PO 4 ) 2 , and we find that they have ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.

A study of magnetic properties in the insulating spinel ZnCrl 6Gao 404 (chromium ions in octahedral sites) has been made by carrying out low-field dcand ac-susceptibility measurements as well as neutron diffraction, Mossbauer, and EPR experiments, which all give evidence of spin-glass-like behavior. The topological frustration, connected with the high ground-state degeneracy of the antiferromagnetic spinel octahedral sublattice, is the origin of the spin-glass ordering. Strong nearestneighbor antiferromagnetic interactions are dominant in the system, leading to short-range magnetic order which appears at temperatures much higher than the freezing temperature defined by the acsusceptibility cusp [T(v=17 Hz}=2.50 K is frequency dependent]. The thermoremanent magnetization does not vanish at Tf, suggesting the existence of uncompensated clusters blocked above Tf.A paramagnetic behavior is observed below T=1 K, providing evidence for the presence of unfrozen spins below Tf (entropic clusters).

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