Marwène Oumezzine

Effect of strontium deficiency on the structural, magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3−xMnO3 (0 ≤ x ≤ 0.15) perovskites

We have investigated the effect of nominal strontium deficiency on the structure, magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3−xMnO3 (x = 0, 0.10 and 0.15) perovskites. The nanocrystalline samples were prepared by the sol–gel-based Pechini method. Rietveld refinement of the X-ray diffraction patterns, shows the formation of single-phase compositions with rhombohedral symmetry (space group Rc, no. 167). Raman spectra at room temperature reveal a gradual change in phonon modes with increasing nominal strontium deficiency. All the samples undergo paramagnetic–ferromagnetic (PM–FM) transition. The Curie temperature decreases linearly with increasing x and changes from 355 K for x = 0 to 280 K for x = 0.15. Arrott plot analyses and a universal curve method were applied for studying the order of the magnetic transition in this system, found to be of second order. As strontium deficiency content increased further, peak entropy values were seen to decline. However, a simultaneous broadening of the ΔSmaxM peaks led to enhanced relative cooling power (RCP) in the Sr-deficiency samples of up to 22% over that of La0.65Eu0.05Sr0.3MnO3. La0.65Eu0.05Sr0.15MnO3 exhibits the largest RCP value of 283 J kg−1 at 280 K among the compounds investigated up to 5 T applied field. Through these results, La0.65Eu0.05Sr0.3−xMnO3 materials are strongly suggested for use as active refrigerants for magnetic refrigeration technology near and above room temperature.

Structural, magnetic, magneto-transport properties and Bean–Rodbell model simulation of disorder effects in Cr3+ substituted La0.67Ba0.33MnO3 nanocrystalline synthesized by modified Pechini method

Nanocrystalline powders (around 100 nm) of La0.67Ba0.33CrxMn1−xO3 (x ≤ 0.17) perovskites have been synthesized by the sol–gel based Pechini method at low temperatures. The structure, resistivity, magnetization, and magneto-transport properties were systematically investigated as a function of Cr doping. Rietveld refinement of fitted and observed X-ray diffractions patterns shows the formation of single-phase compositions with rhombohedral symmetry (space group Rc, no. 167). Magnetization measurements confirm a transition from a paramagnetic to ferromagnetic phase. An increase in resistivity and a decrease in the metal–semiconductor transition and Curie temperatures was observed as a consequence of Cr doping. A simple phenomenological model, describing the competition between the PM-semiconductor and FM-metallic phases, was used to elucidate the temperature dependence of the resistivity with and without an applied magnetic field, which agrees quantitatively with experimental observations. Moreover, upon Cr doping, the polaron activation energy was found to increase due to the localization of carriers. Based on the Bean–Rodbell model, we studied the chromium inducing disorder effects of second-order phase transition of the system La0.67Ba0.33CrxMn1−xO3, which was confirmed by the η parameter value (η < 1). We applied the model to the magnetization data of the samples for x = 0.10 and x = 0.15. We showed excellent agreement between measurements and simulated data. The results account for the random replacement of Mn3+ by Cr3+, which induces more disorder in the system, resulting in an increase in the disorder parameter and the fluctuation of the spin.

Structural, magnetic and magnetocaloric effects in epitaxial La0.67Ba0.33Ti0.02Mn0.98O3 ferromagnetic thin films grown on 001-oriented SrTiO3 substrates

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.

Structure, transition temperature, and magnetoresistance of titanium-doped lanthanum barium manganite epilayers onto STO 001 substrates

We have developed a thin film structure with a maximum magnetoresistance effect (MRE) at room temperature, which is one of the operating requirements for many applications. It is shown that La0.67Ba0.33Ti0.02Mn0.98O3 epilayers obtained by pulsed laser deposition onto (001) SrTiO3 single crystal substrates exhibit the highest MRE, ΔR/R(H)≈150% or ΔR/R(0)≈60% under 5 T, at 300 K, a temperature near to the corresponding Curie temperature (TC). Both doping with a tiny amount of titanium and induced stress due to lattice mismatch between the thin film and the substrate contribute to a decrease in TC as compared to the pristine compound and therefore to the decrease in the temperature where the highest MRE is recorded.