The Magnetic Phase Transition and Universality Class of h-YMnO3 and h-(Y0.98Eu0.02)MnO3 Under Zero and Applied Pressure

Abstract

We investigated the antiferromagnetic phase transition in the frustrated and multiferroic hexagonal manganites h-YMnO 3 (YMO) and h-(Y 0.98 Eu 0.02 )MnO 3 (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 T N = 71.3 ( 1 ) K and 72.11 ( 5 ) K and the critical exponent 0.22 ( 2 ) and β = 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 T N = 75.2 ( 5 ) K, in agreement with earlier reports, while β 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, β ′ , identical to the order parameter β . Our results indicate that the gap in YEMO indeed closes at T N = 72.4 ( 3 ) K with β ′ = 0.24 ( 2 ) , while the in-pressure gap in YMO closes at 75.2(5) K with an exponent of β ′ = 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 β in the two systems support the likelihood of a new universality class for triangular, frustrated antiferromagnets.

Keywords

• critical exponent
• multiferroics
• spin-wave
• magnetic phase transition
• applied pressure
• inelastic neutron scattering