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

Abstract

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.

Classical Spin Liquid or Extended Critical Range in h - YMnO3 ?

“…1(a)], separated by interlayer Y and O ions. h-YMnO 3 has a Curie-Weiss temperature of θ CW ¼ −545 K [6] and a magnetic phase transition to a three-dimensionally ordered antiferromagnetic state at T N ¼ 71 K [9,10]. Its magnetic interactions are dominantly an antiferromagnetic Heisenberg nearest-neighbor exchange, J ¼ 2.4 meV, with a weaker easy-plane anisotropy within the hexagonal plane with D=J ≈ 0.13 and even weaker interlayer interactions, J z =J ≈ 0.06 [11], thus making h-YMnO 3 highly two-dimensional and TLHA-like.…”

“…1(f)], a spin wave is seen as a steep parabolic shape that has its bottom at the Γ 0 point, at ℏω ≈ 1.7 meV ¼ Δ. The spin-wave gap Δ is due to the single-ion anisotropy of h-YMnO 3 [10,27,28]. However, the intensity below Δ is not associated with any spin wave, and we recognize this important feature as the same diffuse signal as above T N , although it is narrowed below T N .…”

“…3(a)] shows two magnon modes at 2.2 meV and at 5.2 meV [11,30]. With increasing temperatures, the magnons soften [10], but, in addition, diffuse intensity appears. At 75 K, just above T N , quasielastic, critical fluctuations are seen close to the three-dimensional phase transition.…”

“…Anomalous values have also been reported for the critical exponent β in h-YMnO 3 [9,10] and other TLHA materials, such as VCl 2 [37,38], that do not correspond to any of the well-known universality classes. We speculate that this is caused by the presence of the two-dimensional, dynamic correlated disorder which moves the phase transition beyond the simple Landau regime.…”

Quantum Beams for New Aspects in Magnetic Materials and Magnetism

“…Other interesting magnetic systems are the triangular frustrated magnets. In [6], neutron scattering was used to determine the critical exponent β describing the universal behavior of magnetization near the Curie temperature. The measurements were performed in YMnO 3 in undoped and doped material both under ambient and high pressure.…”