Fluctuations and correlations in a frustrated S=1/2 square lattice with competing ferromagnetic and antiferromagnetic interactions studied by muon-spin relaxation

Zero and longitudinal field mu SR measurements in Pb2VO(PO4)(2) and BaCdVO(PO4)(2), two prototypes of the frustrated S=1/2 square lattice model with competing ferromagnetic and antiferromagnetic interactions, are presented. Both systems are observed to undergo a phase transition to a long-range magnetic order at T-N similar or equal to 3.46 K, for Pb2VO(PO4)(2), and at T-N similar or equal to 0.99 K, for BaCdVO(PO4)(2). In Pb2VO(PO4)(2) both the temperature dependence of the order parameter and the longitudinal relaxation rate above TN are consistent with a two-dimensional XY model, as it is found for Sr2CuO2Cl2. On the other hand, for BaCdVO (PO4)(2), which lies very close to the magnetically disordered region of the phase diagram where a bond-nematic order was predicted, a peculiar logarithmic increase in the relaxation is observed above T-N. In both systems a rather broad distribution of internal fields at the muon sites is noticed below T-N. The origin of this distribution is discussed in the light of the mu SR experiments already performed on S=1/2 frustrated antiferromagnets on a square lattice.