Magnetic two-dimensional short-range order in hexagonal manganites

Hexagonal manganites REMnO3 consist of layers of two-dimensional connected distorted MnO5 bipyramids separated by distorted layers of RE, leading to frustrated Mn3+-O2--Mn3+ in-plane exchange paths (superexchange) and Mn3+-O2--O2--Mn3+ interplane exchange paths (super-superexchange). We report single-crystal diffraction experiments (RE=Ho,Er,Y) indicating magnetic 2D short-range order up to temperatures of T=150 K (T(N)similar to74 K), and SQUID magnetization data (RE=Y) indicating short-range order up to at least 250 K. Fits of a model based on an extended mean-field theory to the single-crystal data show in-plane exchange about 3 orders of magnitude stronger than the interplane exchange, explaining the unusual strong deviation from the Curie-Weiss law of the SQUID magnetization data. (C) 2003 American Institute of Physics.