M. Zaidi

Magnetic, magnetocaloric and critical behavior investigation of La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 (x, y = 0.0, 0.05 and 0.075) prepared by a sol–gel method

A systematic study on the magnetic, magnetocaloric and critical behavior properties of polycrystalline La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 prepared via a sol–gel method are studied. These compounds present a single magnetic transition from a ferromagnetic (FM) to paramagnetic (PM) phase with decreasing temperature. The critical exponents are estimated using various techniques, such as a modified Arrott plot, the Kouvel–Fisher method and critical isotherm analysis based on the data of static magnetic measurements recorded around the Curie temperature, TC. The estimated critical exponent values are found to be consistent and comparable to those predicted by the 3D-Ising model for x, y = 0.0 and by the mean field model for x, y = 0.05 and 0.075. We have confirmed the obtained critical exponents with the single scaling equation: M(μ0H,e) = eβf ± (μ0H/eβ+γ), where e = (T − TC)/TC is the reduced temperature. We have investigated the validity and usefulness of theoretical modeling in our compound La0.7Ca0.1Pb0.2Mn1−x−yAlxSnyO3 based on the mean-field analysis of the magnetic entropy change (−ΔSM) versus the magnetization data. For comparison, the MSp has been also deduced from the classical extrapolation of the Arrott plot. We obtain an excellent agreement between the spontaneous magnetization determined from the entropy change (−ΔSM vs. M2) and the Arrot curves (μ0H/M vs. M2), confirming the validity of the magnetic entropy change approach in order to estimate the spontaneous magnetization MSp in a ferromagnetic system.

Large magnetocaloric effect and critical behavior in La0.7Ba0.2Ca0.1Mn1−xAlxO3

The structural, magnetic and magnetocaloric properties of La0.7Ba0.2Ca0.1Mn1−xAlxO3 (0 ≤ x ≤ 0.1) perovskite manganite oxides have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement has shown that all of the samples under investigation crystallize with a rhombohedric structure in the Rc space group (N°167). Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all of the synthesized samples. In addition, the maximum magnetic entropy change (ΔSmaxM) for the x = 0 sample was found to reach ∼5.8 J kg−1 K−1 under an applied magnetic field of 5 T, which is sufficient for potential applications in magnetic refrigeration. For the same applied magnetic field (μ0H = 5 T), the relative cooling power (RCP) value is found to be 167 J kg−1. The critical properties of manganites La0.7Ba0.2Ca0.1Mn1−xAlxO3 (0 ≤ x ≤ 0.1) are investigated through various techniques, such as modified Arrott plots, Kouvel–Fisher methods and critical isotherm analyses around the paramagnetic–ferromagnetic phase transition (Tc). The values of critical exponents, derived from magnetic data using the above methods, yield 0.246 ≤ β ≤ 0.253, 1.01 ≤ γ ≤ 1.12 and 4.76 ≤ δ ≤ 4.9 with a Tc of 300–350 K. The exponent values are close to those expected for tricritical mean field model ferromagnets with short-range dipole–dipole interactions.

Analysis of Thermal Effects on Electrical Characterization of AlGaN/GaN/Si FAT-HEMTs

In this paper we report on the characteristics of Schottky contact behavior of AlGaN/GaN High Electron Mobility Transistors (HEMTs) on a Si Substrate. A variety of electrical techniques such as capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been used to characterize the diode. The behavior of the ideality factor n, the effective barrier height Φb, and the series resistance RS is studied with temperature. C-V measurements successively sweeping up and down the voltage from 0 V to 12 V(absolute value) have demonstrated a hysteresis phenomenon which is more pronounced when the temperature increases. This parasitic effect can be attributed to the presence of traps activated in the Schottky diode. The related deep levels defects which are responsible for parasitic effects, were characterized and extracted by the Deep Level Transient Spectroscopy (DLTS) technique that has been used in previous studies. The identification of these traps showed a correlation between DLTS and C-V measurements.

Static critical behavior of the ferromagnetic transition in La0.67Ba0.33–xZnxMnO3 manganites

ABSTRACT The critical properties of perovskite manganite La0.67Ba0.33−xZnxMnO3 around the paramagnetic–ferromagnetic phase transition are investigated through various techniques such as modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis. Through the nature of this transition was found to be in second order. The value of critical exponent, derived from the magnetic data using the Kouvel-Fisher method, yields , and 4.600.034.66 ± 0.06 with a TC of 259.98 ± 0.32 − 221.73 ± 0.05 K. The exponent values are close to those expected for three-dimensional Heisenberg ferromagnets with short-range magnetic interaction.