MnO3 Perovskite Manganite Research Articles

Interplay between charge and antiferromagnetic ordering in Bi0.6−xPrxCa0.4MnO3 (0≤x≤0.6) perovskite manganite

Structure, magnetic and transport properties of polycrystalline Bi0.6−xPrxCa0.4MnO3 (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) have been studied. Systematic substitution of Pr at Bi site induces an interesting interplay between the charge ordering and antiferromagnetism. The charge ordering temperature (TCO) decreases with increasing x. The antiferromagnetic (AFM) ordering temperature (TN) increases sharply at both the extremes but remains nearly constant from x=0.2 to 0.4. At temperatures lower than TN a transition to the glassy state is observed. The nature of this glass like state appears to be controlled by the Pr content, and at lower values of x this is akin to a spin glass, while at higher x it has a characteristic of cluster glass. The Pr doping also leads to enhancement in the magnetic moment. In the present work it has been proposed that the local lattice distortion induced due to size mismatch between the A-site cations and 6s2 character of Bi3+ lone pair electron is responsible for the observed magnetic and electrical properties.

On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La1− x Sr x MnO3 perovskite manganite materials with different compositions (x = 0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.

Dual imaging probes for magnetic resonance imaging and fluorescence microscopy based on perovskite manganite nanoparticles

The present study reveals the potential of magnetic nanoparticles based on the La0.75Sr0.25MnO3 perovskite manganite for magnetic resonance imaging (MRI). Moreover, it describes the development of the dual imaging probe where the magnetic cores are combined with a fluorescent moiety while the improved colloidal stability is achieved by a two-ply silica shell. At first, the magnetic cores of La0.75Sr0.25MnO3 are coated with a hybrid silica layer, comprising a covalently attached fluorescein moiety that is subsequently covered by a pure silica layer providing the enhanced stability. The detailed characterization of the intermediate and the final product reveals the importance of the complex two-ply shell. Viability tests show that the complete particles are suitable for biological studies. Internalization of the particles and their presence in intracellular vesicles are observed by fluorescence microscopy in different cell types. Further experiments prove no fatal interference with the vitality and insulin releasing ability of labeled pancreatic islets. Relaxometric measurements confirm high spin–spin relaxivities at magnetic fields of B0 = 0.5–3 T, while visualisation of in vitro labeled pancreatic islets by MRI is successfully tested.

Synthesis and on-line ultrasonic characterisation of bulk and nanocrystalline La0.68Sr0.32MnO3 perovskite manganite

La0.68Sr0.32MnO3 perovskite manganite samples were prepared using sonochemical reactor and solid state reaction technique. The ultrasonic velocity, attenuation and elastic moduli of samples were measured using ultrasonic through transmission method, at a fundamental frequency of 5MHz over a wide range of temperatures. The temperature dependence of the ultrasonic parameters shows an interesting anomaly in all the compositions. The observed dramatic softening and hardening in sound velocities or attenuation is related to phase transitions. The linear magnetostriction effect is more dominant in the perovskite than volume magnetostriction effect which is evident from the observed anomalous in both longitudinal and shear velocities and attenuation. Further, a decrease in grain size in the sintered sample leads to a shift in the ferromagnetic transition temperature (TC) from 375 to 370K.

The magnetic and hyperthermia studies of bare and silica-coated La0.75Sr0.25MnO3 nanoparticles

The magnetic nanoparticles of La0.75Sr0.25MnO3 perovskite manganite with a controlled size were prepared via sol–gel procedure, followed by thermal treatment and subsequent mechanical processing of the resulting raw product. The prepared materials were structurally studied by the XRD and TEM methods and probed by DC magnetic measurements. The nanoparticles of the mean crystallite sizes 11–40 nm exhibit T C in the range of ≈310–347 K and the sample possessing 20-nm crystallites was identified as the most suitable for hyperthermia experiments. In order to obtain a colloidally stable suspension and prevent toxic effects, the selected magnetic cores were further encapsulated into silica shell using tetraethoxysilane. The detailed magnetic studies were focused on the comparison of the raw product, the bare nanoparticles after mechanical processing and the silica-coated nanoparticles, dealing also with effects of size distribution and magnetic interactions. The heating experiments were carried out in an AC field of frequencies 100 kHz–1 MHz and amplitude 3.0–8.9 kA m−1 on water dispersions of the samples, and the generated heat was deduced from their warming rate taking into account experimentally determined thermal losses into surroundings. The experiments demonstrate that the heating efficiency of the coated nanoparticles is generally higher than that of the bare magnetic cores. It is also shown that the aggregation of the bare nanoparticles increases heating efficiency at least in a certain concentration range.

Magnetic and Magnetocaloric Properties of Monovalent Substituted La0.65M0.3M ′ 0.05MnO3 (M=Ba, Ca and M ′ =Na, Ag, K) Perovskite Manganites

The effects of monovalent doping in the A-site on the structural, magnetic and magnetocaloric properties in La0.65M0.3M′0.05MnO3 (M=Ba, Ca and M′=Na, Ag, K) powder perovskite manganites have been investigated. Our samples have been synthesized by the conventional solid state reaction at high temperature. X-ray diffraction analysis using the Rietveld refinement show that all our shynthetized samples are single phase and crystallize in the orthorhombic structure with Pbnm space group for the samples where the A-site is occupied by Ca and monovalent element and in the rhombohedral system with R3̄c space group for the samples where the A-site is occupied by Ba and monovalent element. Magnetization measurements versus temperature in a magnetic applied field of 50 mT indicate that all our investigated samples display a paramagnetic-ferromagnetic transition with decreasing temperature. A positive slope in the isotherm plots of M2 versus H/M is a clear indication of a second order phase transition from ferromagnetic to paramagnetic phase. From the measured magnetization data of La0.65M0.3M′0.05MnO3 (M=Ca, Ba and M′=Na, Ag, K) samples as a function of magnetic applied field, the associated magnetic entropy change, |ΔSM|, close to their respective Curie temperature and the relative cooling power RCP have been determined. The La0.65Ca0.3Ag0.05MnO3 compound has the highest |ΔSM| value of 5.16 J/Kg K at 245 K upon a magnetic field change of 5 T. The RCP values are comparable and almost the same for both series (about 70% of that of pure Gd), indicating that our compounds are suitable candidates to be used in magnetic refrigeration.

Magnetic field-induced transitions in La0.42Eu0.25Ca0.33MnO3 perovskite manganite

Electrical and magnetic transport properties of polycrystalline La0.42Eu0.25Ca0.33MnO3 manganite perovskite are reported here. Temperature dependence of electrical transport, in 0 and 0.6 T external magnetic fields, shows insulating behaviour down to the lowest measured temperatures. Application of 6 T external magnetic field induces two different transitions: (1) an insulator–metal transition around 110 K and (2) a low temperature upturn in resistance values at 30 K. These field-induced transitions suggest strong competition between various interactions present in these materials; this is further supported by the observation of hysteresis in electrical resistance in thermal cooling and warming cycles of measurement. The observed upturn in resistance values below 30 K is presumably due to the combined effect of weak localization, electron–electron and electron–phonon scattering. Magnetization measurements show a paramagnetic–AFM transition at 50 K, but a deviation from a straight line in the 1/M versus T is observed well above 170 K. These results manifest the competition between various possible interactions present in the system.

Electron doped (Sm1−x,Cax)MnO3 perovskite manganite as potential infrared thermochromic switch

We have synthesized (Sm1−x,Cax)MnO3 and related compositions to show their potential use as thermochromic switch in the infrared range. Depending on temperature or composition, the single phase is stabilized using the polyacrilamide gel process. Infrared transmittance was measured on selected samples in the 1,42–25μm range. Relative contrast up to 0.773 is measured in the 8–14μm range. Full opacity in the 8–14μm range occurs at temperatures between 273 and 300K, depending on the composition. The potential application of perovskites manganites as thermochromic infrared switch or coating is discussed.

151Eu Mössbauer Spectroscopy in La0.38Eu0.29Ca0.33MnO3

Electrical conductivity with and without magnetic field, d.c. magnetization and 151Eu Mossbauer studies were carried out in La0.38Eu0.29Ca0.33MnO3 perovskite manganite system. An insulating ground state is found throughout the temperature range with charge ordered (CO) state emerging at TCO ∼ 140 K, where as an external magnetic field of 6 T induces metal-insulator transition at ∼120 K. D.C. magnetization measurements show the antiferromagnetic (AFM) transition occurring at TN ≈ 48 K. The temperature dependent 151Eu Mossbauer measurements showed that the substituted Eu replaces La3+ in the 3+ charge state and a small magnetic moment gets induced at the Eu nucleus at low temperatures. The anomalous variation of the f- factor with temperature occurring around TN and TCO corroborates the occurrence of antiferromagnetic (AFM) and charge ordering (CO) transition, respectively.

Effect of Quenching on Magnetoresistance Properties in the Pr0.5Sr0.5MnO3 Perovskite Manganite

We report on the magnetization, resistivity and magnetoresistance (MR) measurements on polycrystalline Pr0.5Sr0.5MnO3. Quenching samples from 1400°C to room temperature in water (sample I) or in air (sample II) leads to different behaviors. Powder X-ray diffraction patterns for samples I and II could be indexed, respectively, in rhombohedral perovskite structure with R3c space group and in the orthorhombic one with Imma space group. Magnetization measurements show that both samples exhibit a paramagnetic–ferromagnetic transition at 280 K (sample I) and 265 K (sample II). At low temperature, sampleI presents a ferromagnetic spin-canted state, while sample II behaves as an antiferromagnet below 160 K. Resistivity and magnetoresistance studies show a net difference as a function of the quenching conditions.