Structural, magnetic and magnetocaloric effects in epitaxial LaBaTiMnO ferromagnetic thin films grown on 001-oriented SrTiO substrates.

Epitaxial LaBaTiMnO (denoted as LBTMO hereafter) thin films of approximately 95 nm thickness were deposited by a pulsed laser deposition technique onto SrTiO (STO) (001) substrates. High-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM) investigations revealed that the films are epilayers with a four-fold symmetry around the [001] direction. Cross-sectional TEM and the presence of Pendellosung fringes in the XRD profiles demonstrate smooth interfaces. The STO substrate induces an in-plane compressive strain, which leads to a slight tetragonality of the film structure. The epilayers exhibit paramagnetic-to-ferromagnetic phase transitions at the Curie temperature T (286 K), close to room temperature. The magnetization easy axis lies in the film plane along the [100] direction of the (001) substrate. The magnetic entropy change (ΔS) associated with the second-order magnetic phase transition was determined via magnetization measurements in the temperature range between 210 and 350 K under different magnetic fields. The relative cooling power (RCP) of this film is about 220 J kg, somewhat lower than that of bulk Gd (410 J kg) for a 50 kOe field change, making the LBTMO ferromagnetic thin films a promising candidate for micro/nanomagnetic refrigeration around room temperature. The proposed universal curve provides a simple method for extrapolating ΔS in a wide range of fields and temperatures, thus confirming the order of the magnetic transition in this system. The magnetic entropy (ΔS) around T is proportional to (μH/T) in agreement with the mean-field theory, indicating the existence of long-range ferromagnetic interactions in epitaxial LBTMO thin films.