L. E. Hueso

Tuning of the magnetocaloric effect in La0.67Ca0.33MnO3−δ nanoparticles synthesized by sol–gel techniques

We report the effect of progressive reduction of the annealing temperature on the magnetocaloric effect (MCE) of La2/3Ca1/3MnO3−δ nanoparticles synthesized by the sol–gel technique. With this method, we are able to obtain particle diameters ranging form 60 to 500 nm. The peak in the MCE at the ferromagnetic to paramagnetic phase transition is strongly reduced as annealing temperature does, due to loss of the intrinsic first-order magnetic phase transition. This opens up a new way in which to tune the intrinsic properties of mixed-valence manganites, most of which are associated with the first-order character of the magnetic transition.

Intergranular magnetoresistance in nanomanganites

In this paper we present some of the most important magnetic and transport properties of mixed-valence manganite nanoparticles. The samples were prepared by a sol–gel method, which allows us to control particle size and, in this way, to obtain new properties of the archetypal ferromagnetic- metallic compound La2/3Ca1/3MnO3. Magnetic properties allow us to present a model for the nanoparticles based on an ideal inner core and an outer shell in which the magnetism is modified by oxygen non-stoichiometry, vacancies and stress. The experimental results obtained from the electrical transport properties, namely increasing intergranular magnetoresistance (MR) with reducing particle size, tuning of intrinsic colossal MR and low-temperature electrostatic blocking effects, seem to support the proposed model.

Tuning of colossal magnetoresistance via grain size change in La0.67Ca0.33MnO3

In this article, we show how colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La0.67Ca0.33MnO3 via changing grain size by means of a sol-gel method. Below a critical diameter (150 nm), CMR disappears, but large intergrain MR remains even well above Tc (1.2Tc for ≈95 nm particles). Possible explanation for this effect involves single magnetic domain behavior in samples annealed at low temperature.

Drop of magnetocaloric effect related to the change from first- to second-order magnetic phase transition in La2/3(Ca1−xSrx)1/3MnO3

From data of initial magnetization isotherms, the magnetic entropy change ΔSM of the series of ferromagnetic perovskites La2/3(Ca1−xSrx)1/3MnO3, with x=0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1, has been measured near their Curie temperatures. The results go from 3.7 Ju200akg−1u200aK−1 for La2/3Ca1/3MnO3 to 1.5 Ju200akg−1u200aK−1 for La2/3Sr1/3MnO3. Nevertheless, the evolution of ΔSM with x is not monotonic, it shows a steep decrease between x=0.05 and x=0.15. This point corresponds to a tolerance factor t=0.92, at which the system changes from orthorhombic (Pbnm) to rhombohedral (R3c) structure, and the magnetic phase transition from first to second order. Provided that rhombohedral symmetry forbids static long-range cooperative Jahn–Teller distortions, present in orthorhombic samples, we interpret our results as the strong increase of lattice effects in them. The anomalous thermal expansion at the Curie temperature, due to these lattice effects, is thought to underlie the jump in ΔSM with x.

Low field magnetoresistance effects in fine particles of La0.67Ca0.33MnO3 perovskites

Abstract In this work magnetic and magnetotransport experimental data in well-characterized small particles of La 0.67 Ca 0.33 MnO 3 are presented. Grain size reduction leads to a larger resistivity and a decrease in metal–insulator transition temperature. Intrinsic colossal magnetoresistance (CMR) is destroyed while intergranular one is promoted to larger values. This low field MR can be explained taking into account magnetization data through spin-polarized tunneling model, which ensures an acceptable first-order fit between both magnitudes. Finally, low-temperature resistivity upturn present in small particle size samples can be understood in terms of an electrostatic barrier between grains.

Magnetoresistance in manganite'alumina nanocrystalline composites

Magnetotransport properties of manganite/insulator composites are studied in this article. By merging the half metallic character of La2/3Ca1/3MnO3 and the mixed composition with alumina grains dispersed in the structure we have been able to increase three times intergranular magnetoresistance around the percolation threshold. The transport properties have been studied employing a theoretical model for ferromagnetic–insulator systems and a two channel equation in order to reproduce the behavior of resistivity in the whole temperature range. The percolation theory is introduced to try to understand and improve the fascinating properties of these mixed systems.

Large magnetocaloric effect in manganites with charge order

In this work, we report the magnetocaloric effect (|ΔSM|), around the charge/orbital ordering transition in the mixed valent manganite Nd0.5Sr0.5MnO3. The magnitude of |ΔSM| around this first-order transition is around three times larger than that obtained around the second-order transition (ferromagnetic-metallic-to-paramagnetic-insulator) in the same compound. Actually, the magnetocaloric response around the charge-order transition is comparable to pure Gd, the rare earth with the highest magnetocaloric effect. The possibility of an easy tuning of the charge-order transition temperatures in doped manganites opens a way of investigation materials usable in magnetic refrigerators.

Influence of the grain-size and oxygen stoichiometry on magnetic and transport properties of polycrystalline La0.67Ca0.33MnO3±δ perovskites

Abstract Effects of grain size and oxygen stoichiometry in sol–gel and ceramics samples of La 0.67 Ca 0.33 MnO 3± δ have been studied. Grain size (90xa0nm versus >10xa0μm) does not change either the Curie temperature ( T C ) or the metal–insulator transition temperature ( T M–I ), but great differences in low-field magnetoresistance are observed. These differences have been explained by a spin polarized tunneling model for conduction between grains. Oxygen content ( δ ) could be changed in La deficient samples via an electrochemical method at room temperature (i.e., without changing grain size). We observe great differences in T M–I when decreasing δ , while T C remains unchanged. Broken ways between Mn ions leads to zones with ferromagnetic behavior, but inhibits conductivity.

Coexistence of paramagnetic-charge-ordered and ferromagnetic-metallic phases in La0.5Ca0.5MnO3 evidenced by electron spin resonance

Throughout a complete electron spin resonance (ESR) and magnetization study of La0.5Ca0.5MnO3, we discuss about the nature of the complex phase-segregated state established in this compound below T∼210u200aK. Between TN⩽T⩽TC, the ESR spectra shows two lines characteristic of two different magnetic phases. From the resonance field (Hr) derived for each line, we argue that the incommensurate-charge-ordering phase (ICO) which coexists with ferromagnetic–metallic (FMM) clusters in this temperature interval, is mainly paramagnetic and not antiferromagnetic. The FMM/ICO ratio can be tuned with a relatively small field, which suggests that the internal energy associated with those phases is very similar. Below TN, there is an appreciable ferromagnetic (FM) contribution to the magnetization and the ESR spectra indicates the presence of FM clusters in an antiferromagnetic matrix (canted). Our results show that ESR could be a very useful tool to investigate the nature of the phase-separated state now believed to play a...

Strong ferro-antiferromagnetic competition and charge ordering in Pr0.67Ca0.33MnO3

Abstract In this article we present d.c. resistivity, a.c. impedance, magnetization and dielectric permittivity measurements in Pr0.67Ca0.33MnO3. A rise in the imaginary part of impedance was detected at the charge ordering transition temperature (T=225xa0K). This indicates an intrinsic capacitive behavior of the sample in the charge ordering state. The a.c. impedance is presented here as a valid and easy tool to characterize the charge ordering transition.