RESUMO
Magnetic perovskite films have promising properties for use in energy-efficient spintronic devices and magnetic refrigeration. Here, an epitaxial ferromagnetic La0.67Ba0.33Mn0.95Ti0.05O3 (LBMTO-5) thin film was grown on SrTiO3(001) single crystal substrate by pulsed laser deposition. High-resolution X-ray diffraction proved the high crystallinity of the film with tetragonal symmetry. The magnetic, magnetocaloric and magnetoresistance properties at different directions of the applied magnetic field with respect to the ab plane of the film were investigated. An in-plane uni-axial magnetic anisotropy was evidenced. The LBMTO-5 epilayer exhibits a second-order ferromagnetic-paramagnetic phase transition around 234 K together with a metal-semiconductor transition close to this Curie temperature (TC). The magnetic entropy variation under 5 T induction of a magnetic field applied parallel to the film surface reaches a maximum of 17.27 mJ/cm3 K. The relative cooling power is 1400 mJ/cm3 K (53% of the reference value reported for bulk Gd) for the same applied magnetic field. Giant magnetoresistance of about 82% under 5 T is obtained at a temperature close to TC. Defined as the difference between specific resistivity obtained under 5 T with the current flowing along the magnetic easy axis and the magnetic field oriented transversally to the current, parallel and perpendicular to the sample plane, respectively, the in-plane magneto-resistance anisotropy in 5 T is about 9% near the TC.
RESUMO
Epitaxial La0.67Ba0.33Ti0.02Mn0.98O3 (denoted as LBTMO hereafter) thin films of approximately 95 nm thickness were deposited by a pulsed laser deposition technique onto SrTiO3 (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 TC (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 (ΔSM) 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-1, somewhat lower than that of bulk Gd (410 J kg-1) 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 ΔSM in a wide range of fields and temperatures, thus confirming the order of the magnetic transition in this system. The magnetic entropy (ΔSM)max around TC is proportional to (µ0H/TC)2/3 in agreement with the mean-field theory, indicating the existence of long-range ferromagnetic interactions in epitaxial LBTMO thin films.
