Zero and longitudinal field measurements in and , two prototypes of the frustrated 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 , for , and at , for . In both the temperature dependence of the order parameter and the longitudinal relaxation rate above are consistent with a two-dimensional model, as it is found for . On the other hand, for , 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 . In both systems a rather broad distribution of internal fields at the muon sites is noticed below . The origin of this distribution is discussed in the light of the experiments already performed on frustrated antiferromagnets on a square lattice.
- Received 13 March 2009
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