Pressure-induced spin fluctuations and spin reorientation in hexagonal manganites

The magnetic structures of hexagonal manganites YMnO3 and LuMnO3 have been studied by powder neutron diffraction up to 6 GPa in the temperature range 10-295 K. At ambient pressure, a triangular antiferromagnetic (AFM) state of a Gamma(1) irreducible representation is stable below T-N = 70 K in YMnO3. Upon the application of high pressure, a spin reorientation is induced and the triangular AFM structure evolves from Gamma(1) to Gamma(1) + Gamma(2) representations. On the other hand, in LuMnO3 the triangular AFM state of a Gamma(2) irreducible representation with T-N approximate to 90 K remains stable over the entire pressure range investigated. The ordered magnetic moment values decrease under pressure with dM/dP = - 0.35 mu B GPa(-1) in YMnO3 and - 0.08 mu(B) GPa(-1) in LuMnO3. Simultaneously, a considerable increase in diffuse scattering intensity was found in YMnO3, while it was much less pronounced for LuMnO3. Both features indicate the enhancement of spin fluctuations due to geometrical frustration effects and an increase in the volume fraction of the spin-liquid state coexisting with the ordered AFM phase. The characteristic spin correlation length is weakly affected by pressure. The relationship between the pressure-induced behaviour of magnetic structure and the structural characteristics of the quasi-two-dimensional (2D) triangular network formed by Mn and O ions in hexagonal RMnO3 is analysed.