Growth and Characterization of Hexagonal Lu-Fe-O Multiferroic Thin Films

In the quest for new types of information processing and storage, complex oxides stand out as one of the most promising material classes. The multiple functionalities of complex oxides naturally arise from the delicate energy balance between the various forms of order (structural, electronic, magnetic). In particular, multiferroic and magnetoelectric oxides which simultaneously exhibit more than one type of ferroic orders have many advantages over existing materials. Widespread practical applications will require a single-phase multiferroic material with a transition temperature that lies considerably above room temperature, large electric and magnetic polarizations, and strong coupling between ferroic orders.

Recently, multiferroic LuFe₂O₄ has attracted great interest because it has relatively high transition temperatures, large polarization, high magnetic coercivity, and the strong magnetoelectric coupling. Compared to the large amount of effort to study bulk LuFe₂O₄, there are only a couple of reported attempts to grow LuFe₂O₄ thin films, presumably due to difficulties in the sample preparation. In this work, a comprehensive growth diagram of Lu-Fe-O compounds on MgO (111) substrates using pulsed laser deposition is constructed based on extensive growth experiments. The LuFe₂O₄ phase can only be grown in a small range of temperature and O₂ pressure conditions. An understanding of the growth mechanism of Lu-Fe-O compound films is offered in terms of the thermochemistry at the surface. Superparamagnetism is observed in the LuFe₂O₄ film and is explained in terms of the effect of the impurity hexagonal LuFeO₃ (h-LuFeO₃) phase and structural defects.

In addition to LuFe₂O₄, we also succeeded in growing hexagonal-LuFeO₃ (h-LuFeO₃) epitaxial films in single crystalline form on either insulating or metallic substrates using pulsed laser deposition (PLD). H-LuFeO₃ thin films exhibit hysteresis in piezoresponse force microscopy (PFM) measurements indicative of ferroelectricity, and simultaneously show antiferromagnetic order, with both properties coexisting at room temperature.