Anomalous Magnetic Ordering in Epitaxial TbMnO3 Ultrathin Films

Orthorhombic c-axis-oriented TbMnO3 films of 24~200 nm thickness were grown epitaxially on SrTiO3 (001) single crystal substrates by using pulsed laser deposition technique. The temperature (2~300 K) dependence of magnetization data were obtained on a quantum design superconducting quantum interference device. When the magnetic field was applied in the a-b plane, apart from the three magnetic ordering transitions at low temperatures (10 K, 21 K and 43 K), the magnetization evidently displayed an anomalous antiferromagnetic ordering around 156 K, which was related to the film thickness seriously. A broad transition peak at the same temperature was also observed when the applied field was along the c axis direction. The anomalies at high temperature were not observed in bulk TbMnO3 materials. These results were discussed in terms of unexpected magnetic ordering of the Mn3+ moments induced by the interface strain between the films and the substrates.