J. S. Amaral

The effect of magnetic irreversibility on estimating the magnetocaloric effect from magnetization measurements

We found that the anomalous magnetic entropy change peak obtained from magnetization measurements in some first-order magnetic phase transition materials may result from the usual data analysis procedure, which does not take into account magnetic irreversibility or mixed-phase regime. The deviations produced are comparable to anomalous effects discussed in the literature and may even exceed the theoretical limit. Our results show that this anomalous magnetic entropy change peak should not necessarily be interpreted as a consequence of the particular physics of the studied system. This also explains its absence in specific heat measurements.

On the Curie temperature dependency of the magnetocaloric effect

We investigate the magnetocaloric effect dependency on the most important microscopic parameters of ferromagnetic materials, such as the Curie temperature (TC), the spin value (J), and the magnetic field change (ΔH). Second- and first-order phase transition systems are considered, using the Bean-Rodbell model [C. P. Bean and D. S. Rodbell, Phys. Rev. 126, 104 (1962)] of magnetovolume interactions on the Weiss mean-field model [P. Weiss, J. Phys. Theory Appl. 6, 661 (1907)]. The magnetocaloric effect simulations show a surprising TC−2/3 linear dependence of the maximum entropy change (ΔSmmax), which is observed for all simulated systems. An approximate state equation establishing the dependence of ΔSmmax on TC, ΔH, J, and the magnetic atoms density (N) is presented. The dependence of maximum magnetic entropy change on TC−2/3 is validated by a wide set of experimental results of second- and first-order phase transition materials that are promising for magnetic refrigeration applications at room temperature.

A mean-field scaling method for first- and second-order phase transition ferromagnets and its application in magnetocaloric studies

We present a method based on mean-field theory, where from scaling of experimental magnetization data, the mean-field exchange parameter λ and the f function of the equation of state M=f[(H+Hexch)∕T] are directly determined. The scaling approach allows finding the dependence of Hexch on T or higher powers of M, which determine the order of the phase transition. Within the thermodynamics of the mean-field theory, the magnetocaloric effect is obtained, in a straightforward way, from the magnetic equation of state. Results are presented for first- and second-order magnetic phase transitions in manganites and metastable nanocrystalline metal alloy.

Age-related changes in renal expression of oxidant and antioxidant enzymes and oxidative stress markers in male SHR and WKY rats

Oxidative stress has been hypothesized to play a role in aging and age-related disorders, such as hypertension. This study compared levels of oxidative stress and renal expression of oxidant and antioxidant enzymes in male normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) at different ages (3 and 12 months). In the renal cortex of 3-month old SHR increases in hydrogen peroxide (H(2)O(2)) were accompanied by augmented expression of NADPH oxidase subunit Nox4 and decreased expression of antioxidant enzymes SOD1 and SOD3. A further increase in renal H(2)O(2) production and urinary TBARS was observed in 12-month old WKY and SHR as compared with 3-month old rats. Similarly, expressions of NADPH oxidase subunit p22(phox), SOD2 and SOD3 were markedly elevated with age in both strains. When compared with age-matched WKY, catalase expression was increased in 3-month old SHR, but unchanged in 12-month old SHR. Body weight increased with aging in both rat strains, but this increase was more pronounced in WKY. In conclusion, renal oxidative stress in 12-month old SHR is an exaggeration of the process already observed in the 3-month old SHR, whereas the occurrence of obesity in 12-month old normotensive rats may partially be responsible for the age-related increase in oxidative stress.

Handling mixed-state magnetization data for magnetocaloric studies—a solution to achieve realistic entropy behaviour

We present an approach to extract a realistic magnetic entropy value from non-equilibrium magnetization data near the transition temperature of a typical first-order system with a mixed-phase state, influenced by the phase transformation, which is responsible for large values reported, even higher than the theoretical limit. The effect of the mixed-phase state is modelled in the magnetization and its non-physical contribution is removed to obtain the magnetic entropy in accordance with calorimetric experiment and theoretical simulation. This approach gives a reliable estimation of the magnetic entropy value incorporating experimental non-equilibrium magnetization data and correcting the use of Maxwells relation.

High refrigerant capacity of PrNi5−xCox magnetic compounds exploiting its spin reorientation and magnetic transition over a wide temperature zone

The ferromagnetically coupled cobalt ion is observed to create a magnetocrystalline anisotropy in the PrNi5−xCox structure above a critical composition of x = 2. The competition of the anisotropy energies between Co and Pr sublattices gives rise to a spin reorientation (SR) phenomenon in PrNi5−xCox compounds at a low temperature (~150 K) which is then followed by a magnetic transition at a higher temperature. Co-doping has a strong influence on the Curie temperature, changing it from ~60 K (x = 1.95) to ~537 K (x = 3). The magnetic entropy change is associated with SR as well as a magnetic transition, and correspondingly a large full width at half maximum (δTFWHM) is obtained for this series of compounds. For example, the PrNi2.85Co2.15 compound presents δTFWHM = 166 K at a 1 T field. This series therefore has an appreciable relative cooling power, which makes this material a suitable magnetic refrigerant over a large temperature span.

Unveiling the (De)coupling of magnetostructural transition nature in magnetocaloric R5Si2Ge2 (R 5Tb, Gd) materials

We present a detailed study on the magnetization under high magnetic fields of Tb5Si2Ge2 and Gd5Si2Ge2 compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the TC of each structure (M and O(I)) experimentally and found that the TC of the O(I) phase can be directly obtained by extrapolating the TC curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R5(SixGe1−x)4 (R = Tb, Gd) compounds.

Structural, magnetic, magneto-transport properties and Bean–Rodbell model simulation of disorder effects in Cr3+ substituted La0.67Ba0.33MnO3 nanocrystalline synthesized by modified Pechini method

Nanocrystalline powders (around 100 nm) of La0.67Ba0.33CrxMn1−xO3 (x ≤ 0.17) perovskites have been synthesized by the sol–gel based Pechini method at low temperatures. The structure, resistivity, magnetization, and magneto-transport properties were systematically investigated as a function of Cr doping. Rietveld refinement of fitted and observed X-ray diffractions patterns shows the formation of single-phase compositions with rhombohedral symmetry (space group Rc, no. 167). Magnetization measurements confirm a transition from a paramagnetic to ferromagnetic phase. An increase in resistivity and a decrease in the metal–semiconductor transition and Curie temperatures was observed as a consequence of Cr doping. A simple phenomenological model, describing the competition between the PM-semiconductor and FM-metallic phases, was used to elucidate the temperature dependence of the resistivity with and without an applied magnetic field, which agrees quantitatively with experimental observations. Moreover, upon Cr doping, the polaron activation energy was found to increase due to the localization of carriers. Based on the Bean–Rodbell model, we studied the chromium inducing disorder effects of second-order phase transition of the system La0.67Ba0.33CrxMn1−xO3, which was confirmed by the η parameter value (η < 1). We applied the model to the magnetization data of the samples for x = 0.10 and x = 0.15. We showed excellent agreement between measurements and simulated data. The results account for the random replacement of Mn3+ by Cr3+, which induces more disorder in the system, resulting in an increase in the disorder parameter and the fluctuation of the spin.

Age-related changes in the renal dopaminergic system and expression of renal amino acid transporters in WKY and SHR rats

This study examined age-related changes in renal dopaminergic activity and expression of amino acid transporters potentially involved in renal tubular uptake of l-DOPA in Wistar Kyoto (WKY) and spontaneously hypertensive rats. Aging (from 13 to 91 weeks) was accompanied by increases in systolic blood pressure (SBP) in both WKY and SHR. The sum of urinary dopamine and DOPAC and the urinary dopamine/l-DOPA ratio were increased in aged SHR but not in aged WKY. The urinary dopamine/renal delivery of l-DOPA ratio was increased in both rat strains with aging. LAT2 abundance was increased in aged WKY and SHR. The expression of 4F2hc was markedly elevated in aged SHR but not in aged WKY. ASCT2 was upregulated in both aged WKY and SHR. Plasma aldosterone levels and urinary noradrenaline levels were increased in aged WKY and SHR though levels of both entities were more elevated in aged SHR. Activation of the renal dopaminergic system is more pronounced in aged SHR than in aged WKY and is associated with an upregulation of renal cortical ASCT2 in WKY and of LAT2/4F2hc and ASCT2 in SHR. This activation may be the consequence of a counter-regulatory mechanism for stimuli leading to sodium reabsorption.

Loss of oxidative stress tolerance in hypertension is linked to reduced catalase activity and increased c-Jun NH2-terminal kinase activation

Hypertension is accompanied by increased levels of reactive oxygen species, which may contribute to progressive renal injury and dysfunction. Here we tested the hypothesis that sensitivity to exogenous hydrogen peroxide (H(2)O(2)) is enhanced in immortalized renal proximal tubular epithelial cells from spontaneously hypertensive rats (SHR) compared to normotensive control Wistar Kyoto rats (WKY). We found that SHR cells were more sensitive to H(2)O(2)-induced cell death than WKY cells. Lower survival in SHR cells correlated with increased DNA fragmentation, chromatin condensation, and caspase-3 activity, indicating apoptosis. H(2)O(2) degradation was slower in SHR than in WKY cells, suggesting that reduced antioxidant enzyme activity might be the basis for their increased sensitivity. In fact, catalase activity was downregulated in SHR cells, whereas glutathione peroxidase activity was similar in both cell types. We next examined whether MAPK signaling pathways contributed to H(2)O(2)-mediated apoptosis. Inhibition of c-Jun NH(2)-terminal kinase (JNK) with SP600125 partially rescued H(2)O(2)-induced apoptosis in WKY but not in SHR cells. In addition, p54 JNK2 isoform was robustly phosphorylated by H(2)O(2), this effect being more pronounced in SHR cells. Together, these results suggest that the survival disadvantage of SHR cells upon exposure to H(2)O(2) stems from impaired antioxidant mechanisms and activated JNK proapoptotic signaling pathways.