R. Bah

Magnetic properties of La0.6Sr0.4MnO3 thin films on SrTiO3 and buffered Si substrates with varying thickness

La0.60Sr0.40MnO3 (LSMO) thin films of varying thickness from 12 to 55 nm were deposited using the pulsed-laser deposition technique onto single-crystalline SrTiO3 (STO) and STO-buffered Si substrates. The Tc of LSMO films grown on STO-buffered Si substrates decreases faster than films directly grown on STO with decreasing film thickness. The LSMO/STO film with thickness of 55 nm shows Tc at about 360 K, which is close to the bulk value, whereas Tc LSMO film on STO-buffered Si film of similar thickness is reduced to 320 K. This difference is attributed to the strain and interfacial disorders in LSMO film on STO/Si. The film surface morphology is influenced by the film thickness. Oxygenation of LSMO films on STO-buffered Si affects the Tc minimally but improved the overall magnetization of the films due to better oxygenation, which is also the case for postannealing the sample at elevated temperatures. The thermomagnetic history effects observed in LSMO films of STO-buffered Si indicate the presence of inho...

Synthesis and magnetic characterizations of manganite-based composite nanoparticles for biomedical applications

We report chemically synthesized highly crystalline lanthanum strontium manganite (LaSrMnO3) and Eu-doped Y2O3 and their composites. The synthesis yields nanoparticles of size 30–40nm. Magnetic measurements performed on nanoparticles and composites show magnetic transition at about 370K with a superparamagnetic behavior at room temperature. The ferromagnetic resonance studies of the nanoparticles show large linewidth due to surface strains. The composite nanoparticles also display luminescent behavior when irradiated with ultraviolet light. The manganites as well their composite with the luminescent nanoparticles may be very useful for biomedical applications.

Effect of diamagnetic dilution and non-stoichiometry on ESR spectra in manganites

ESR spectra are studied in a series of diamagnetically diluted manganites, single crystals of LaGa1?xMnxO3, in a broad range of the Mn ion concentration, . With increase in Mn concentration, a complex multi-line spectrum, typical for low Mn concentrations, evolves to an exchange-narrowed several-?or single-line spectrum at x>0.05. The temperature behaviour of the ESR line-width in the range of 80?300?K is found to be very different in the materials with high and intermediate concentration of Mn ions: a slight growth of the line-width is observed in the former case, while a steep line narrowing occurs at x = 0.05 and?0.1 with increase in temperature. Similar temperature dependence of the line-width is also observed for an additional ESR line arising in the non-stoichiometric oxygen-deficient LaMnO3?? manganite. We explain the experimental data by taking into account the effect of the thermo-activated Jahn?Teller reorientation process on spin dynamics in Mn clusters, and estimate its activation energy as 50?100?meV.

Ferromagnetic resonance studies on colossal magnetoresistance films: Effects of homogeneity and light illumination

Ferromagnetic resonance (FMR) in the dependence on magnetic field orientation, and effect of the laser light on the magnetic ordering were studied in colossal magnetoresistance films, La1−xSrxMnO3 and La1−xBaxMnO3. The FMR linewidth increased with the decrease in temperature in La1−xSrxMnO3 films, and was nearly temperature independent in La1−xBaxMnO3, that was attributed to the different degree of magnetic homogeneity of the films. Layered structures La1−xBaxMnO3 on silicon with intermediate layer of SrTiO3 were characterized by FMR as highly homogeneous. From the orientation and temperature dependences of the FMR spectra, the effective magnetization, and effective anisotropy fields were estimated. The laser light illumination resulted in the change of FMR line shape and intensity, and was interpreted in terms of optically induced magnetic disordering and effective heating of the spin system.

Ferromagnetic semiconductor material based on mechanochemical mixture of polyaniline polystyrene and Fe nanoparticles

We report on mechanochemical synthesis and magnetic and electric properties of organic polymeric semiconductor that contains paramagnetic and ferromagnetic particles. As starting materials we used polyaniline (emeraldine base), polystyrene, FeCl3, and elemental sulfur. These components were chosen for the purpose of obtaining semiconductor polymer composite containing magnetic iron nanoparticles and having flexible thermomechanical properties. The presence of nanoparticles was evidenced by scanning electron microscopy. The value of conductivity of the Fe-doped polymer composite was increased by several orders of magnitude compared to the initial undoped polyaniline. Temperature dependence of the conductivity shows semiconductor character of the composite material.

Magnetic transitions in manganese benzoquinone coordination polymers

Coordination polymers exhibiting magnetic properties can be considered as promising materials for magnetic applications. We obtained such materials by means pf mechanochemical incorporation of binuclear metal complexes Mn–O–Mn into polystyrene. Incorporation of binuclear manganese complexes was achieved by using substituted benzoquinones (organic acceptors) as intermediate ligands that react with manganese acetylacetonate and provide new coordination and stabilization of manganese in different valence states. Magnetic transitions in this system are due to reversible disproportionation process Mn3+–O–Mn3+⇔Mn2+–O–Mn4+ in the binuclear complex. The structure of the binuclear complex was obtained using ab initio calculations.

Spin dynamics and internal motion in magnetically dilute manganites probed by EPR

EPR spectra of a series of single crystal samples of LaGa1−xMnxO3 manganites were studied in a broad range of temperatures (20–300 K) and compositions (0.02≤x≤1). Evolution of the spectra under the action of exchange interactions of Mn3+ ions leads to the formation of a single Lorentzian line for x≥0.2. The results confirm the transition from antiferromagnetic to ferromagnetic spin ordering during diamagnetic dilution of the manganite with gallium, beginning with x=0.8. For x=0.1, the EPR spectra exhibit an anomalous broadening and splitting on cooling of the samples, which is interpreted as manifestations of a thermoactivated internal motion with a characteristic energy of about 50 meV. The results are compared with published data on the nuclear spin-lattice relaxation of gallium. The model of thermoactivated reorientations of the electron orbitals of Mn3+ ions subject to the Jahn-Teller effect is discussed.

Equilibrium geometries and electronic structure calculations of divalent lead Pb(II) complexes with paramagnetic organic ligands

During the past several decades enormous effort has been dedicated to experimental and theoretical studies of metal-radical organic complexes. Equilibrium geometries and electron energy structure of divalent lead Pb(II) complexes with ortho-semiquinone radicals have been calculated by generalized gradient approximation method (GGA) within density functional theory (DFT). Optical absorption spectra were calculated within random phase approximation (independent particles picture). Predicted substantial modification of the absorption spectra in visible infrared regions is attributed to the atomic configuration changes and to modifications of electronic energy due to the metal-radical coupling. The results of the calculations are discussed in comparison with available experimental data on electron spin resonance spectra of Pb(II) with paramagnetic ortho-semiquinone ligands.