We investigated the antiferromagnetic phase transition in the frustrated and multiferroic
hexagonal manganites h-YMnO3 (YMO) and h-(Y0.98Eu0.02)MnO3 (YEMO). Elastic neutron scattering
was used to study, in detail, the phase transition in YMO and YEMO under zero pressure and in
YMO under a hydrostatic pressure of 1.5 GPa. Under conditions of zero pressure, we found critical
temperatures of TN = 71.3(1) K and 72.11(5) K and the critical exponent 0.22(2) and b = 0.206(3),
for YMO and YEMO, respectively. This is in agreement with earlier work by Roessli et al. Under
an applied hydrostatic pressure of 1.5 GPa, the ordering temperature increased to TN = 75.2(5) K,
in agreement with earlier reports, while b was unchanged. Inelastic neutron scattering was used to
determine the size of the anisotropy spin wave gap close to the phase transition. From spin wave
theory, the gap is expected to close with a critical exponent, b0, identical to the order parameter b.
Our results indicate that the gap in YEMO indeed closes at TN = 72.4(3) K with b0 = 0.24(2), while
the in-pressure gap in YMO closes at 75.2(5) K with an exponent of b0 = 0.19(3). In addition, the low
temperature anisotropy gap was found to have a slightly higher absolute value under pressure.
The consistent values obtained for b in the two systems support the likelihood of a new universality
class for triangular, frustrated antiferromagnets
