Using a criterion given by Banerjee [Phys. Lett. 12, 16 (1964)] to distinguish first-order magnetic transitions from second-order ones, we demonstrate that not all the title perovskites exhibit a first-order transition at their Curie temperatures $T_C.$ We observe that, although ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ exhibits a first-order magnetic transition at $T_C,$ ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ exhibits a second-order one.

Using a sol‐gel method we have prepared monodispersive particles of La_0.67Ca_0.33MnO₃ in the range from 20 to 110 nm at temperatures from 540 to 1000 °C. A magnetoresistance above 10% was obtained in a field of 1 kOe for all the particles sizes. These results may be relevant for future applications.

We have performed electron spin resonance (ESR) and dc susceptibility measurements in Mn perovskites up to 1000 K. Assuming an effective Heisenberg-like interaction for ${\mathrm{Mn}}^{3+}\mathrm{-}{\mathrm{Mn}}^{4+}$ spin pairs, the dc susceptibility, ${\chi }_{\mathrm{dc}}\mathrm{}\left(T\right),$ is well described in the paramagnetic regime by the constant coupling approximation. Absolute determination of the ESR intensity indicates that all Mn spins contribute to the ESR line in the temperature range studied. The ESR linewidth can be described by $\Delta H_{\mathrm{pp}}\mathrm{}\left(T\right)=\Delta H_{\mathrm{pp}}\left(\infty \right)[C/T{\chi }_{\mathrm{dc}}\mathrm{}(T\left)\mathrm{}\right],$ thus presenting a universal behavior in a temperature scale normalized to $T_c.$ A single relaxation mechanism, related to spin-only interactions, explains the $T$ dependence of $\Delta H_{\mathrm{pp}}\mathrm{}\left(T\right)\mathrm{}$ for all the compounds studied: ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3,$ ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3,$ ${\mathrm{Pr}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3,$ and ${\mathrm{La}}_{0.67}{\mathrm{Pb}}_{0.33}{\mathrm{MnO}}_3.$ The dc susceptibility and the ESR linewidth and intensity all reflect the progressive importance of magnetic clustering below $\approx {2T}_c.$

We report in this work the characterization of La_0.67Ca_0.33MnO_3±δparticles synthesizedvia sol–gel technology starting from an aqueous solution of the metallic nitrates and using urea as gelificant agent. The gelification is assumed to happen through the formation of polynuclear species by condensation reactions between hydroxo complexes. Gels were decomposed at 250 °C and calcined for 3 h at temperatures ranging from 300 to 1000 °C. Complete crystallization takes place at ca. 600 °C. The powders were structurally characterized by X-ray diffraction and their structural parameters were calculated using the Rietveld method. The Mn^IV content of the several samples was determined to be higher than the stoichiometric 33%. TEM micrographs show elongated particles of which the polar (long) axis size increases from 40 to 300 nm as the calcination temperature increases. Magnetization and magnetoresistance studies are reported showing that the particles smaller than 80 nm behave as single magnetic domains while the large ones behave as multidomains. A magnetoresistance of 12% at 1 kOe was observed for all the particles synthesized by this sol–gel method.