J Shobaki

Magneto-dielectric anisotropy in magnetic fluids determined from magneto-optical measurements

Abstract Magneto-optical measurements on magnetic fluids have been utilized to determine the magneto-dielectric anisotropy effect in magnetic fluids. Our results show that the dielectric constant, e| ∗ , seen by light polarized in the direction of the applied magnetic field increases with the field, while e ⊥ ∗ seen by light polarized normal to the field decreases with the field. Moreover, the results also show that e | ∗ exhibits a peak with temperature while e ⊥ ∗ exhibits a minimum. Our calculated results are in good agreement with those obtained using conventional impedance measurement techniques, with the advantage that this method is not affected by the electrode effect, parasitic impedances, skin depth and accuracy-related problems that are typical of conventional impedance measurement techniques.

Dissipative behavior and γ-irradiation of silver-sheathed Bi1.8Pb0.4Sr2Ca2Cu3Ox tape

Abstract Dissipative mechanisms in Bi 1.8 Pb 0.4 Sr 2 Ca 2 Cu 3 O x tapes were investigated using magnetoresistance measurements and V – I characteristics. A considerable broadening of the resistive transition in applied magnetic field was observed up to a field of 0.5 T. The temperature and magnetic field dependence of the resistance R ( T , B ) were fitted to the Arrhenius relation and from which the magnetic field dependence of the pinning energy U 0 ( B )∼ B − α with α ≈0.9 was derived. The variations of the critical current densities J c with temperature and magnetic field before and after γ-irradiation were also studied. The temperature dependence of J c ( T ) was fitted to the relation J c ( T )∼(1− T / T c ) n with n =1.9 and discussed in terms of thermally activated flux flow (TAFF). However, J c ( B ) results were explained based on the weak links, grain boundaries, and Josephson junctions in which it shows a power law behaviour J c ( B )∼ B −0.5 . At relatively low temperatures and magnetic fields, the critical current density was increased with a γ-irradiation up to a dose of 100 MR.

Mossbauer spectroscopic and crystal structure investigation of the Fe0.7-xTixAl0.3 alloy system

Abstract We describe a combination of Mossbauer spectroscopic and X-ray structural studies on the alloy system Fe0.7−xTixAl0.3 where x=0, 0.05, 0.1, 0.2, 0.3 and 0.35. The dependence of the Mossbauer hyperfine parameters and the lattice constant on x are determined, and a comparison is made to the known behavior of metallic Fe–Al–T alloys. X-ray diffraction patterns indicate that all the samples are single phase with body center cubic structure. The lattice constant increases linearly with increasing Ti concentration. Room temperature Mossbauer studies show magnetic ordering for small x and paramagnetic behavior for large x. The Mossbauer spectra were fitted by a distribution of magnetic hyperfine fields for small x and two singlets were added for large x. Analysis of the X-ray and Mossbauer results show that the Ti atoms preferentially occupy the Fe site. The average hyperfine field and isomer shift decreased with increasing Ti concentration. The results are related to local environment effects on the hyperfine interactions.

A scaling model for the optical anisotropy in magnetic fluids: dichroism and birefringence

A simple scaling model for dichroism and birefringence in magnetic fluids is proposed. This scaling is based on two physically measured quantities, i.e. the initial slope of the optical anistropy curve with magnetic field, and the saturation value of dichroism or birefringence. Using this model, the isothermal dichroism and birefringence are superimposed into a single function. This model is discussed and compared with one similar to that proposed for the magnetization of a superparamagnetic particle system.

Effect of γ-irradiation on polycrystalline Tl1Ba2Ca2Cu3Oy superconductor

Specimens of polycrystalline T1 1 Ba 2 Ca 2 Cu 3 O y superconductor (T1-1223) have been irradiated at room temperature by γ-rays. The variations of the normal state resistance, transition width and the critical current densities with γ-irradiation up to about 400 MR were studied. The normal state resistivity ρ(T) increases gradually up to a dose of 100 MR, whereas at higher doses, the effect of irradiation becomes rather negligible. The transition temperature T c decreases slightly and the transition temperature width AT broadens at doses below 100 MR with almost no change at higher doses up to 400 MR. At relatively low temperatures and low γ-doses, the critical current densities J c (T) increase with γ-irradiation, while the effect of γ-irradiation at temperatures very close to T c is rather negligible. Obtained results confirmed the high sensitivity of (T1-1223) superconductor to γ-irradiation at relatively low γ-doses. These results are discussed and explained in terms of the effect of γ-rays on the weak links and Josephson junctions between the grains.

Effect of gamma irradiation on the transport critical current density of Tl2Ba2Ca2Cu3O10 tapes

Specimens of Tl2Ba2Ca2Cu3O10 tapes were prepared using the powder in tube method. The voltage-current (V - I ) characteristics were used to study the temperature and magnetic field dependence of the critical current density Jc(T,B). Obtained results confirmed the high sensitivity of the superconducting tape to the applied magnetic field and are explained based on the weak links and Josephson junctions between the grains. The temperature dependence of Jc was also derived and discussed in terms of the thermally activated flux flow (TAFF). The effect of gamma irradiation on the critical current density Jc(T,B) was also studied up to a gamma dose of 400 MR. Gamma irradiation was found to increase Jc at relatively low temperature and magnetic field ranges. Therefore, gamma irradiation is a useful method to improve the superconductivity of the Tl2Ba2Ca2Cu3O10 tapes.

Magnetic and structural properties of GdFe2-xTix alloys

Abstract We report the structural and magnetic properties of GdFe 2− x Ti x alloys, where x =0, 0.05, 0.10, 0.15, 0.20, 0.30. X-ray diffraction shows that these compounds form the cubic Cu 2 Mg type structure. The lattice parameter ( a ) and the unit cell volume ( V ) are found to increase with increasing Ti concentration. Magnetization measurements using a SQUID magnetometer show that all samples studied are magnetically ordered. We find that the saturation magnetization increases with x at room temperature and at 5 K. This is due to the replacement of magnetic iron by non-magnetic titanium which decreases the Fe sublattice moment.

Numerical calculation of the temperature variation of the optical anisotropy in magnetic fluids

Abstract The temperature variation of the optical anisotropy in magnetic fluids has been calculated numerically. The calculations are based on the mono-dispersed particle model and the conversion of blocked particles into unblocked ones. The results have shown the basic features of the temperature variation of the optical anisotropy which were revealed experimentally. A basic feature that is seen in the calculated results, in agreement with experiment, is the existence of a peak in the optical anisotropy at a given temperature, Tm. Moreover, the variation in the value of Tm with concentration and applied field are in agreement with those found experimentally.

Temperature dependence of vortex flux pinning with γ-irradiation in polycrystalline Tl-2223

Hysteresis loop measurements of superconducting polycrystalline samples of Tl-2223 were carried out at various temperatures. Using the magnetization remanence (MR) as measurement of the maximum vortex flux pinning at each temperature, our results reveal that the vortex flux pinning forces show two distinct behaviours of MR with temperature, a linear behaviour with t at high temperatures and an exponential behaviour with tβ at low temperatures, where t = (1-T/Tc) and β is constant of order 8. The effect of γ-irradiation shows six-fold enhancement in the vortex pinning over all temperatures up to Tc. This increase in the pinning forces is attributed to the induced defects introduced by the γ-irradiation. The same enhancement in the pinning forces at all temperatures implies constant defects induced by γ-irradiation.