Soma Das

Structural and magnetocaloric properties of La1−yNayMnO3 compounds prepared by microwave processing

Polycrystalline samples of sodium doped lanthanum manganites of the type La1−yNayMnO3 (0 < y ≤ 0.15) having nanometric crystallite size have been prepared by microwave processing. The Curie temperature TC of the prepared pellets is strongly dependent on the sodium content in the compound and ranges between 196 and 312 K. A detailed structural analysis has been performed on the prepared samples using the Rietveld FULLPROF program and important structural parameters are estimated. Close to TC, a significant change in magnetic entropy has been observed in all the studied samples. The maximum entropy change at 0.8 T field (µ0H) for the La0.90Na0.10MnO3 compound shows an enhancement by ~158% over that measured for a sample with Na content of 0.05 and is ~67% of that known for the prototype magnetic refrigerant (pure Gd). The maximum adiabatic temperature change (ΔTad) is estimated to be ~2 K. The temperature dependence of the magnetic entropy change for La1−yNayMnO3 compounds, measured under various magnetic fields, could be explained fairly well by including the contributions from the magnetoelastic and electron interaction in the expression for the Landau theory of phase transition.

Structural transitions and unusual magnetic behavior in Mn-doped Bi1−xLaxFeO3 perovskites

The effect of Mn doping on crystal structure and magnetic behavior of the polar antiferromagnetic and antipolar weak ferromagnetic phases of the Bi1−xLaxFeO3 system was investigated. Both reference compounds, Bi0.87La0.13FeO3 (space group R3c, tilting scheme (a−a−a−)] and Bi0.82La0.18FeO3 [space group Pnam, tilting scheme (a−a−c+)/(a−a−c−)], were shown to undergo the Mn doping induced structural phase transformation towards the average Imma (a−b0a−) symmetry. Magnetization measurements performed for the compounds with the close to maximal manganese concentration (y = 0.4) found complex temperature-dependent behavior that can be indicative of a spin-reorientation transition. Enhanced spontaneous magnetization characteristic of the Mn-enriched x = 0.13 and x = 0.18 samples at low temperature is consistent with a competitive character of the superexchange interactions peculiar to the Fe/Mn containing perovskite systems.

Morphology dependent magnetic properties of α-Fe2O3 nanostructures

Well crystalline α-Fe2O3 nanomaterials with a wide range of morphology variation have been successfully synthesized by solvothermal route. The synthesized products have been characterized for structural and morphological details by employing x-ray diffraction patterns, transmission electron microscopy, field emission scanning electron microscopy and energy dispersive x-ray spectroscopy. Various unique shapes of α-Fe2O3 nanocrystal have been modelled on the basis of their growth evolution. The effect of morphology of α-Fe2O3 nanocrystals on their magnetic behaviour has been studied by investigating temperature and field dependence of magnetization. The results are analyzed considering all the possible surface anisotropy and lattice strain evolved due to their surface structure. This comprehensive study of morphology dependent magnetic behaviour of α-Fe2O3 nanomaterials offers a better opportunity to tune the materials in the desired technological applications.

Mn doping-induced structural and magnetic transformations in the antiferroelectric phase of the Bi1−xNdxFeO3 perovskites

X-ray diffraction, differential scanning calorimetry, and magnetization measurements of the Bi0.825Nd0.175Fe1−yMnyO3 (y ≤ 0.3) compounds were carried out to follow the effect of Mn doping on the crystal structure and magnetic properties of the intermediate antiferroelectric and weak ferromagnetic phase of the Bi1−xNdxFeO3 perovskites. Suppression of the antipolar displacements typical of the parent B-site undoped compound followed by stabilization of the GdFeO3-type structure as well as decrease of the antipolar-to-nonpolar transition temperature were found in this series with increasing Mn content. Compositional variation of the spontaneous magnetization in the Bi0.825Nd0.175Fe1−yMnyO3 (y ≤ 0.3) system was shown to have a temperature-dependent character. At room temperature, a close to linear decrease of the spontaneous magnetization takes place with increase of the Mn content. At low temperatures, enhancement of the magnetization is observed with increasing the dopant concentration.

A-site disorder driven sharp field-induced transition and collapse of charge ordering in Sm1/2Ca1/2-xSrxMnO3

Our study on Sm1/2Ca1/2−xSrxMnO3 (x = 0, 1/6, 1/3, and 1/2) system shows an unusual consequence in magnetic field-induced transition to ferromagnetic metallic state and collapse of charge ordering in the intermediate compositions. Interestingly, the transition is sharp or step-like at low temperature that is observed at much lower field (∼75 kOe) for intermediate compositions than the end compositions (not visible up to 100 kOe). The atomic-scale local inhomogeneity/distortion arising from ionic mismatch due to doping is supposed to be critical behind this field-induced transition. The charge ordering temperature observed from magnetization and magnetoresistance studies decreases with increasing x which is consistent with the results for R1/2A1/2MnO3 (R = La − Sm, A = Ca, Sr) family.

The Mean-Field Theory in the Study of Ferromagnets and the Magnetocaloric Effect

H applied magnetic field β dependence of TC in volume λ mean-field exchange parameter K compressibility M magnetization α1 thermal expansion σ reduced magnetization T0 ordering temperature (no volume coupling) T temperature v volume χ magnetic susceptibility v0 volume (no magnetic interaction) N number of spins G Gibbs free energy J spin Msat saturation magnetization g gyromagnetic ratio S entropy μB Bohr magneton p pressure kB Boltzmann constant η Bean-Rodbell model parameter TC Curie temperature BJ Brillouin function (spin J) μe f f effective moment Hc critical field C Curie constant x fraction of ferromagnetic phase

Preparation and characterization of Pb1−x Sn x Te pseudobinary alloy semiconductors

Ap-type pseudo-binary alloy semiconductor, Pb0·3Sn0·7Te, has been prepared fromp-type specimens of PbTe and SnTe and lattice constants determined with an accuracy of 0.0001 nm. Vacuum annealing of Pb0·3Sn0·7 Te reveals two new x-ray powder diffraction lines bearing indices (444) and (800), while others become more sharp, CuKa-doublets get clearly resolved and the lattice constant is increased by ∼ 0·0002 nm. Slight deviation from Vegard’s law linearity is observed showing that the sample must be considered as ternary in nature. Thin films deposited on mica and glass substrates kept at room temperature are found to have a little higher SnTe content. The effective carrier concentration calculated from Hall measurements at room temperature is ∼ 3·4×1026 m−3.

Bandwidth Enhancement of Cylindrical Dielectric Resonator Antenna Using Thin Dielectric Layer Fed by Resonating Slot

Abstract In this paper Cylindrical Dielectric Resonator Antenna (CDRA) has been designed for X-band frequency range applications with slot feeding. Bandwidth of designed cylindrical dielectric resonator antenna has been enhanced by making the slot to be resonating and inserting very thin low permittivity dielectric layer between the slot and CDRA. Resonating slot excites the closely spaced HEM11δ and HEM21δ mode inside the antenna as well as provides the resonance at the upper side of desired X-band. Low profile low permittivity layer provides the excellent matching of these modes and shift the HEM21δ mode to higher frequency side at 11.25 GHz with better impedance matching. By combining all of three resonances, nearly 85 % increment in impedance Bandwidth has been obtained with reference to non resonating slot excited CDRA. Fractional impedance bandwidth for proposed design is 48 % and average Gain of 6 dB with more than 92 % radiation efficiency has been shown by the antenna throughout the desired band with broadside pattern.

Investigation of Binomial and Chebyshev Distribution on Dielectric Resonator Antenna Array

For wireless communication systems, the antenna is one of the most critical components for transmission as well as reception point of view. Good design of the antenna can relax system requirements and improve overall system performance. In addition to receiving or transmitting energy, an antenna in an advanced wireless system is usually required to optimize or accentuate the radiation energy in some directions and suppress it in others. Thus the antenna must also serve as a directional device in addition to a probing device. Many applications require radiation characteristics that may not be achievable by a single element. It may, however, be possible that an aggregate of radiating elements in an electrical and geometrical arrangement (an array) will result in the desired radiation characteristics [1]. The arrangement of the array may be such that the radiation from the elements adds up to give a radiation maximum in a particular direction or directions, minimum in others, or otherwise as desired. In this paper we concentrated on the electrical and geometrical properties of array elements that give a radiation maximum in a particular direction and minimum in others. We investigate the binomial and chebyshev distribution for linear and rectangular dielectric resonator antenna array.

Circularly Polarized Cylindrical Dielectric Resonator Antenna coupled by V-shaped resonating slot

In this paper circularly Polarized Dielectric Resonator Antenna for the Satellite frequency applications has been designed. With proper design the proposed Cylindrical DRA fed by centered V-shaped slot; is showing characteristics of Circular Polarization in the desired band. Two orthogonal arms of symmetrically centered V-shaped slot having same dimension excite the TM01δ modes at 9.75GHz as well as slots itself are resonating at 8.5 GHz. Crossing of λ/4 long micro strip line between both slots at an angle of 45° is generating the nature of circular polarization near the 8GHz frequency range. Axial Ratio bandwidth (AR ≤ 3 dB) of 9% (720MHz) and impedance bandwidth (S11 ≤ −10 dB) of 28% (2.47GHz) has been shown by the antenna which can be considered large axial ratio bandwidth for single and simple feed CDRA. Proposed antenna has shown average gain of more than 6 dBi in broadside direction with more than 95% radiation efficiency in desired band.