Magnetism in Geometrically Frustrated YMnO3 Under Hydrostatic Pressure   Studied with Implanted Muons
  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. 
T. Lancaster, S.J. Blundell, D. Andreica, M. Janoschek, B. Roessli,   S.N. Gvasaliya, K. Conder, E. Pomjakushina, M.L. Brook...