  The rare-earth ferroelectromagnet YMnO3 consists of weakly coupled triangular
layers of S=2 spins. Below T_N ~ 70 K muon-spin relaxation data show two
oscillatory relaxing signals due to magnetic order, with no relaxing signal
resolvable (which would be expected for different coexisting spin
distributions). The transition temperature T_N increases with applied
hydrostatic pressure, even though the ordered moment decreases. These results
suggest that pressure increases both the exchange coupling between the layers
and the frustration within the layers.
