Magnetic Order and Spin Dynamics in Ferroelectric HoMnO${}_3$

Abstract

Hexagonal HoMnO${}_3$ is a frustrated antiferromagnet (T${}_N$=72 K) ferroelectric (T${}_C$=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin wave dispersion for the S=2 Mn${}^{3+}$ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy $D$ that is 0.093 meV above the spin reorientation transition at 40 K, and 0.126 meV below. For $H\parallel c$ the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho${}^{3+}$ ions are ordered magnetically.

To be published in Physical Review Letters