Transport and magnetic properties have been studied in two sets of sol-gel prepared Pr 1/2 Sr 1/2 MnO 3 nanoparticles having average particle size of 30 nm and 45 nm. Our measurements suggest that the formation of charge ordered state is largely affected due to lowering of particle size, but the ferromagnetic transition temperature (T C ) remains unaffected.

Nanocrystalline Nd0.5Sr0.5MnO3 of average particle size 30 and 55 nm are prepared by the sol-gel technique. Transport, magnetotransport, magnetization, specific heat, and current-voltage characteristics studies have been performed on the samples. Our experimental results indicate that the ferromagnetic transition temperatures of the nanocrystalline samples decrease in comparison with the bulk form of the sample. However, the ferromagnetic to charge ordered antiferromagnetic transition does not occur down to 2 K. The properties of the nanoparticles are discussed in detail which will provide a deeper insight into the physics of charge ordering and related phenomena in the rare-earth based manganites especially in their nanocrystalline form.

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Nanocrystalline Pr0.65(Ca0.7Sr0.3)MnO3 show large magnetocaloric effect at their charge order transition temperature (TCO) as well as at the temperature at which the spontaneous destabilization of charge ordered state occurs (TM). In comparison to their polycrystalline bulk form, TM’s are substantially enhanced in the cases of nanocrystalline samples, whereas their TCO’s remain almost unchanged. Although there is no clear signature of charge order transition in the temperature dependence of magnetic susceptibility and resistivity for the sample with the lower particle size, a clear maxima due to charge order transition is visible in its temperature dependence of change in magnetic entropy.

We report the first observation of inverse magnetocaloric effect (IMCE) in hydrothermally synthesized single crystalline La0.5Sr0.5MnO3 nanowires. The core of the nanowires is phase separated with the development of double exchange driven ferromagnetism (FM) in the antiferromagnetic (AFM) matrix, whereas the surface is found to be composed of disordered magnetic spins. The FM phase scales with the effective magnetic anisotropy, which is directly probed by transverse susceptibility experiments. The surface exhibits a glassy behavior and undergoes spin freezing, which manifests as a positive peak (T(L) ~ 42 K) in the magnetic entropy change (-ΔS(M)) curves, thereby stabilizing the re-entrance of the conventional magnetocaloric effect. Precisely at T(L), the nanowires develop the exchange bias (EB) effect. Our results conclusively demonstrate that the mere coexistence of FM and AFM phases along with a disordered surface below their Néel temperature (T(N) ~ 210 K) does not trigger EB, but this develops only below the surface spin freezing temperature.

Some recent experimental studies show the invisibility of antiferromagnetic transition in the cases of manganites when their particle size is reduced to nanometer scale. In complete contrast to these cases, we have observed the signature of antiferromagnetic transition in the magnetocaloric properties of nanocrystalline La 0.125 Ca 0.875 MnO 3 of average particle size 70 and 60 nm similar to its polycrystalline bulk form. The system exhibit inverse magnetocaloric effect in its polycrystalline and nanocrystalline form. An extra ferromagnetic phase is stabilized at low temperature for the sample with particle size ∼ 60 nm.

A comprehensive study of the temperature (T) and magnetic field (H) dependence of magnetic entropy change (ΔS M) for different materials exhibiting inverse magnetocaloric effect (IMCE) is reported. We show that ΔS M follows a power law dependence of H (ΔS M ∼ H n , n is an exponent) for these compounds. In contrast to conventional magnetocaloric effect (CMCE), n is independent of H and T in the case of IMCE. As a result, a universal master curve can be constructed to describe ΔS M (T) of the IMCE systems for different H without rescaling temperature axis. This is completely different from that reported for CMCE, where the rescaling of temperature axis with the introduction of at least one reference temperature is needed for constructing a universal curve. The different universal behavior of IMCE is attributed to the constant value of n in any field and temperature, which is a generic feature of IMCE systems irrespective of their magnetic state and nature of phase transition. From the proposed phenomenological universal curve, one can extrapolate the magnetocaloric properties of IMCE systems in any temperature and magnetic field range, which would be helpful in designing controlled active magnetic refrigeration devices.

Observation of charge ordering in single crystalline and bulk polycrystalline systems of various rare-earth based manganites is well documented. However, there is hardly any manifestation of the same when the grain size is reduced to nanoscale. We have observed charge ordering in case of nanocrystalline Pr0.65Ca0.35MnO3 of average particle size 40 nm. This phenomenon is attributed to the primary role played by the martensitic character of the charge order transition in the material.