arXiv subjects

V. S. Amaral

Publications and source records attributed to V. S. Amaral.

23 records · Page 2Linked to original sources

arXiv2004

Relevance of a magnetic moment distribution and scaling law methods to study the magnetic behavior of antiferromagnetic nanoparticles

In antiferromagnetic nanoparticles magnetization the linear component, $χ_{AF}$H superposed to the saturation one usually complicates the fit of experimental data. We present a method based on scaling laws to determine the variation of $χ_{AF}$ with temperature and to find the temperature dependence of the average magnetic moment $<μ>$, without any assumption on both the magnetization dependence on field or the moment distribution function, whose relevance can also be estimated. We have applied this method to ferritin and found that $<μ>$ decreases with increasing temperature and that a distribution function cannot be ignored. The fit with Langevin magnetization law and lognormal moment distribution functions yielded parameters close to those estimated with the scaling method. We also show that in general if the distribution is ignored, and a single particle moment $μ_p$ is used, $μ_p$ presents an artificial systematic increase with temperature. This calls the attention to the necessity of evaluating the effect of a size distribution before concluding about the physical nature of the parameters variation.

cond-mat.mtrl-sci↗

arXiv2003

Influence of charge ordering on the magnetocaloric effect of (Pr,Ca)MnO$_3$ manganites

In the present work we analyze the influence of the charge-ordering on the magnetocaloric effect of Pr$_{1-x}$Ca$_x$MnO$_3$ manganites. The results for the samples with $x<$0.30 present the usual ferromagnetic behavior, peaking at the Curie temperature T$_C$. In contrast, for the samples above the onset concentration for the charge-ordering ($x\sim$0.30), an anomalous magnetic entropy change was observed below the charge-ordering temperature $T_{CO}$, persisting for lower temperatures. This effect is associated to a positive contribution to the magnetic entropy change due to charge-ordering, which is superimposed to the negative contribution from the spin ordering. We found that the charge-ordering component peaks at $T_N$ and is negligible above $T_{CO}$. Moreover, around T$^\star$ (temperature below which the insulator-metal transition induced by magnetic field becomes completely irreversible, where the system remains in such state even after the external magnetic field has been removed), we found extremely large values for the magnetic entropy change, establishing a `colossal' magnetocaloric effect on CMR manganites.

cond-mat.mtrl-sci↗

arXiv2003

Magnetic phase diagram for a non-extensive system: Experimental connection with manganites

In the present paper we make a thorough analysis of a classical spin system, within the framework of Tsallis nonextensive statistics. From the analysis of the generalized Gibbs free energy, within the mean-field approximation, a para-ferromagnetic phase diagram, which exhibits first and second order phase transitions, is built. The features of the generalized, and classical, magnetic moment are mainly determined by the values of $q$, the non-extensive parameter. The model is successfully applied to the case of La$_{0.60}$Y$_{0.07}$Ca$_{0.33}$MnO$_3$ manganite. The temperature and magnetic field dependence of the experimental magnetization on this manganite are faithfully reproduced. The agreement between rather "exotic" magnetic properties of manganites and the predictions of the $q$-statistics, comes to support our early claim that these materials are magnetically nonextensive objects.

cond-mat.stat-mech↗

arXiv2002

NMR evidence for an inhomogeneous transition between the ferromagnetic and antiferromagnetic ground states in Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ manganites

The low temperature behavior of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ manganites are known to undergo a transition at $x\sim$0.3, from an insulating ferromagnetic state, at low Ca concentration, to an insulating antiferromagnetic state. Above the onset concentration for the charge-ordering effect ($x\sim 0.3$), a metal-insulator transition induced by an external magnetic field is also observed and related to the collapse of the charge-ordered state. In this paper we show that the ferro-antiferromagnetic transition takes place in an inhomogeneous way: around the critical concentration the sample is a mixture of ferromagnetic and antiferromagnetic regions. The zero-field NMR measurements show that a fraction of the ferromagnetic regions is in a fast hopping regime, which suggests that a small fraction of the sample could be metallic, even in zero field. This fraction of fast hopping regions is maximal at $x=$0.3, which is also the concentration for which the insulator-metal transition has been observed in the smallest field.

cond-mat.mtrl-sci↗

arXiv2002

Magnetic behavior of a non-extensive S-spin system: possible connections to manganites

We analyzed the magnetic behavior of a S-spin system within framework of the Tsallis nonextensive statistics, employing the normalized approach. Unusual properties on magnetization, entropy and susceptibility emerge, as a consequence of nonextensivity. We further show that the nonextensive approach can be relevant to the field of manganites, materials which exhibit long-range interactions and fractality, two basic ingredients for nonextensivity. Our results are in qualitative agreement to experimental data in La$_{0.67}$Ca$_{0.33}$MnO$_3$ and Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.95}$Ga$_{0.05}$O$_3$ manganites.

cond-mat.stat-mech↗