paper

Fluctuations and correlations in a frustrated S=1/2 square lattice with competing ferromagnetic and antiferromagnetic interactions: a muSR study

arXiv:0904.3618 · doi:10.1103/PhysRevB.79.224432

Abstract

Zero and longitudinal field $I^¼$ SR measurements in Pb $_{2}$ VO(PO $_{4}$ ) $_{2}$ and BaCdVO(PO $_{4}$ ) $_{2}$ , 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 $T_{N} ≃ 3.46$ K, for Pb $_{2}$ VO(PO $_{4}$ ) $_{2}$ , and at $T_{N} ≃ 0.99$ K, for BaCdVO(PO $_{4}$ ) $_{2}$ . In Pb $_{2}$ VO(PO $_{4}$ ) $_{2}$ both the temperature dependence of the order parameter and the longitudinal relaxation rate above $T_{N}$ are consistent with a two-dimensional XY model. On the other hand, for BaCdVO(PO $_{4}$ ) $_{2}$ , which lies very close to the magnetically disordered region of the phase diagram where a bond-nematic order was predicted, a peculiar logarithmic increase of 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 $I^¼$ SR experiments already performed on $S = 1/2$ frustrated antiferromagnets on a square lattice.

6 pages, 6 figures