S.K. Ghatak

Giant magnetoimpedance in Ag-doped La0.7Sr0.3MnO3

The resistive and reactive parts of the magnetoimpedance of sintered ferromagnetic samples of La0.7Sr0.3−xAgxMnO3 (x=0.05,0.25) have been measured at room temperature (<Tc) over a frequency interval from 1kHzto15MHz and in the presence of magnetic field up to 4kOe. The field dependence of relative change in resistance [ΔR∕R(0)] is small in the kilohertz region but increases strongly for higher frequency of excitation. The maximum value of [ΔR∕R(0)] at H=4kOe and for ω=15MHz is around 70%. On the contrary the corresponding change in reactance [ΔX∕X(0)] has less frequency sensitivity, and the maximum occurs at ω≈1MHz. The magnetoimpedance is negative for all frequencies. The normalized magnetoimpedance δZ as defined by [Z(H)−Z(0)]∕[Z(0)−Z(4kOe)] when plotted against scaled field H∕H1∕2 is found to be frequency independent; H1∕2 is the field where δZ is reduced to half its maximum. A phenomenological formula for magnetoimpedance, Z(H), in a ferromagnetic material, is proposed based on the Pade approximant. T...

Effect of Rotating Acoustic Stimulus on Heart Rate Variability in Healthy Adults

Acoustic stimulus can modulate the Autonomic Nervous System. However, previous reports on this topic are conflicting and inconclusive. In this study we have shown, how rotating acoustic stimulus, a novel auditory binaural stimulus, can change the autonomic balance of the cardiac system. We have used Heart rate Variability (HRV), an indicator of autonomic modulation of heart, both in time and frequency domain to analyze the effect of stimulus on 31 healthy adults. A decrease in the heart rate accompanied with an increase in SD and RMSSD indices on linear analysis was observed post-stimulation. In the Poincaré Plot, Minor Axis (SD1), Major Axis (SD2) and the ratio SD12 (SD1/SD2) increased after the stimulation. Post stimulus greater increment of SD12 with higher lag numbers of (M) beat to beat intervals, when compared to pre stimulus values, resulted in increased curvilinearity in the SD12 vs. Lag number plot. After stimulation,value of exponent alpha of Dretended Flactuation Analysis of HRV was found to be decreased. From these characteristic responses of the heart after the stimulus, it appears that rotating acoustic stimulus may be beneficial for the sympathovagal balance of the heart.

Magnetic behaviour of soft ferromagnetic materials

Abstract The initial M−H behaviour of two soft ferromagnetic Fe40Ne40B20 and Co75Si15B10 amorphous alloys is measured under different tensile stresses. It is observed that the data when plotted in terms of reduced variables (M/Ms and H/Hc) collapse into a single curve for each alloy. The anisotropy energy is dominantly magnetostrictive and is found to be proportional to Hc. An equation of state M/MS vs. H/Hk; Hk is the anisotropy field) is derived using phenomenology, and can be fitted well to the experimental data.

Thermoelectric power of undoped and doped Y-Ba-Cu-O superconductor between 77 and 300K

Temperature dependence of electrical resistivity and thermoelectric power of undoped Y-Ba-Cu-O sintered pellet and that doped with small amount of sulphur and silver are presented. While the nature of the temperature dependence of electrical resistivity and thermoelectric power remains similar, their absolute values are strongly influenced by the presence of the dopant. Thermoelectric power data for all the samples can be fitted to an expression of the type, S = AT +B/T. It is argued from the data that the precursor peak in thermoelectric power near T > Tc is of electronic origin due to superconducting fluctuations.

Superconductivity in sulphur doped YBaCuO system

Sulphur substituted YBa2Cu3SO6-x superconducting powder was prepared by the ceramic route. The material, in bulk, showed zero resistivity at 88 K and the onset of diamagnetism at 90 K. Magnetisation behaviour confirms type II behaviour of the superconducting state. The ESCA and IR spectrum on the superconducting compound at room temperature, indicate the location of the sulphur at the apex of the octahedral coordination sphere of the Cu(2)-ion in the crystal lattice.

A simple fluxgate magnetometer using amorphous alloys

Abstract A simple fluxgate magnetometer is developed using low magnetostrictive ferromagnetic amorphous alloy acting as a sensing element. It uses the fact that the magnetization of sensing element symmetrically magnetized by a sinusoidal field contains even harmonic components in presence of dc signal field H and the amplitude of the second harmonic component of magnetization is proportional to H. The sensitivity and linearity of the magnetometer with signal field are studied for parallel configuration and the field ranging from 10 nT to 10 μT can be measured. The functioning of the magnetometer is demonstrated by studying the shielding and flux-trapping phenomena in high-Tc superconductor.

Magnetic properties of amorphous Fe-Ni-B alloy under heat-pulse annealing and stress

Abstract Amorphous Fe 40 Ni 40 B 20 alloy is treated with infrared heat pulses in the presence of a dc field to facilitate the relaxation of the internal stress. Magnetic properties like initial susceptibility, magnetization, remanence, magnetostriction and effective anisotropy are measured in as-received and heat treated samples. Magnetic softness increases when treated with short duration heat pulses. The stress dependence of the above properties have been studied. It is found that the normal curves of as-received and heat treated samples under different stresses reduce to the single curve when expressed in terms of reduced variables M / M D vs. H / H c , where M D and H c are the domain saturation magnetization and coercive field, respectively. The results are consistent with the model due to Vazquez et al.

Influence of stress and stress‐flash annealing on the magnetic properties of amorphous Fe77.5Si7.5B15 wire

The influence of the tensile stress on the flux reversal phenomena in as‐quenched state of Fe77.5Si7.5B15 is investigated. A large and stable Barkhausen jump is observed at a field H*, called the switching field. The switching field H* passes through a minimum as tensile stress increases and the jump in magnetization M* at H* grows with stress. It is observed that H* of the sample can be modified by flash annealing under stress. The results on magnetic properties like coercive field, squareness ratio, power loss of as‐quenched and stress‐annealed samples are presented. The stress annealing induces elastic and plastic components in anisotropy.

Critical current density and magnetic behaviour of superconducting YBa2Cu3+yO7- delta : Agx composite

The effects of silver additive on the superconducting properties of YBa2Cu3+yO7- delta :Agx for x=0.1, 0.3, 0.6 and 1.2 are studied by measuring the real and imaginary components of the magnetic response in an AC field. The data are fitted to Beans critical state model and are used to obtain the macroscopic critical current density. The magnetic susceptibility and critical current density are found to depend on the silver content whereas Tc remains essentially unchanged. For x=1.2 the critical current density sharply increases below 77 K. The results suggest that the silver improves the properties of grain boundaries and enhances the intergranular coupling. The real part of the magnetic response is found to be a function of Ha/H*, where H* is the value of the field when the critical-state profile reaches the centre of the specimen.

Influence of hydrogen on the magnetic properties of amorphous FeSiB fibres

The saturation magnetization and switching characteristics of amorphous Fe77.5Si7.5B15 fibres were investigated after hydrogen incorporating by the electrochemical method. The switching field (H*) increased from its as-received value (5.57 Am−1 to 18.62 Am−1 after charging with a current density (Ich) of 30 mA cm−2 for 2 h. The saturation magnetization (Ms) decreased with hydrogen content up to a critical concentration corresponding to Ich = 60 mA cm−2 after 2 h of charging. At this critical current density, Ms decreased from 1.57 T, its as-received value, to 0.89 T. The results are explained by considering various kinds of interaction of hydrogen with metals, such as electron exchange, volume dilation, etc.