Veeturi Srinivas

We report magnetic and magnetoimpedance (MI) properties of cobalt-rich amorphous alloys to investigate the role of small quantities of 3d or 4d transition-metal elements in Co-rich alloys. The microstructure, thermal stability, AC soft magnetic properties and magnetoimpedance of the ribbons are investigated by substituting Nb in place of Ni and Mn. Present study shows an improvement in glass forming ability and reduction in electrical conductivity of the ribbon with Nb substitution. Higher Nb containing alloy shows minimum core loss compared to other compositions over a range of frequencies. Ribbon with higher core loss exhibited higher MI ratio (∼100% at 3 MHz), but decreased rapidly at higher frequencies due to its larger electrical conductivity. The ribbon with lower electrical conductivity showed a steady MI ratio and field sensitivity with frequency. These results indicate that MI and core losses at medium range of frequencies can be tuned by adding Nb that provides most favourable electrical conductivity and magnetic permeability values, which result in optimum skin depth and MI. The effect of electrical conductivity on MI is also validated through numerical simulations using standard electrodynamics models.

We present a detailed magnetic critical behavior study of Fe 2CrAl Heusler alloy, for the first time, with rigorous analysis of high precision magnetization data obtained over the critical temperature region. Our studies confirm that B2 type site-disordered in Fe 2CrAl alloy exhibits long-range ferromagnetic order below a well defined Curie temperature (T=208 K). Though the nature of this transition is found to be of second order, the estimated critical exponents β=0.42, γ=1.356 and δ=4.25, are in between the theoretically predicted values for three-dimensional Heisenberg and mean-field interaction models. However, it is noteworthy that the scaling relations are obeyed indicating renormalization of interactions around the Curie temperature (T C), where magnetization data collapse into two separate branches, above and below T C. This conclusively shows that calculated critical exponents as well as critical temperature are unambiguous and intrinsic to the system. However, magnetization vs temperature data shows another magnetic transition (at T=313 K) above the Curie temperature. It is shown that the short range magnetic correlation exist even beyond T C with cluster moment ∼10 2 μ B. This is attributed to a site disorder which results in formation of Cr clusters with short range ferromagnetic order. © 2011 Elsevier B.V. All rights reserved.

The electrical transport and magnetic properties of Co 58.5 Ga 41.5 binary alloy have been investigated in the temperature range 2-300 K. Analysis of field and temperatures dependence magnetization data suggests a reentrant magnetic behavior with Curie temperature (T C ) ∼90 K. Temperature dependence of resistivity shows large residual resistivity values which is sensitive to the applied magnetic field. The temperature and field dependent resistivity behavior indicates the presence of spin fluctuations at low temperature. Magnetoresistance (MR) of 17 % has been observed at 2 K in 90 kOe field, which follows a power law with exponent varying form 0.5 at low temperature to linear behavior at higher temperature. The temperature variation of magnetization and MR can be understood on the basis of ferromagnetic matrix with finite spin clusters.

Low temperature electrical transport properties of rare earth (RE) element doped Fe2VAl Heusler alloys have been investigated. Significant improvement of thermal power S (peak values -125 to -160 μV/K) is observed compared to the pure Fe2VAl (peak value 40 μV/K). It is observed that the thermal conductivity reduced by 50%in RE doped samples. As the carrier concentration of these samples is high compared conventional semiconductors, single parabolic band model (SPB) has been used to analyze the thermoelectric data. We have estimated the values of Lorentz number, β, and maximum possible figure of merit (zTmax) by using SPB model. The experimental data corroborate well with estimated values from the model.

Polymer-metal composites with different fillers, such as nanocrystalline nickel (n-Ni), core shell n-Ni and nickel oxide (NiO) [n-Ni@NiO] were prepared under the same processing conditions with polyvinyledene fluoride matrix. The larger value of critical exponents (s and s') and percolation threshold (f c ∼ 0.30) for n-Ni@NiO composites as compared to n-Ni composites (fc ∼ 0.07) and a comparable effective dielectric constant (εeff ∼ 300) with low loss tangent (tan δ ∼ 0.1) at 100 Hz in case of percolative n-Ni@NiO composite was observed. The core shell structure [n-Ni@NiO] also shows a very high value of εeff ∼ 6000 with tan δ ∼ 8 at 40 Hz. The results have been explained by using boundary layer capacitor effect and the percolation theory. The difference in fc and critical exponents is attributed to NiO insulating layer that gives rise to different extent of continuumness at fc and have been explained with the help of Swiss cheese model. © 2014 World Scientific Publishing Company.

Fe73.5Si13.5Cu1Nb3B9 and Fe77.2Si11.2Cu0.8Nb3.3B 7.5 nanocomposite materials consisting of nanocrystalline phase in an amorphous matrix were obtained by heat-treatment of their precursor amorphous ribbons. The influence of structural modifications induced during the heat-treatment on soft magnetic properties and magnetoimpedance (MI) effect have been studied. The structural investigations on both these ribbons revealed the presence of two phases, fine grained Fe3Si phase and a residual amorphous phase on heat-treatment. The maximum MI ratio obtained in the present study is 95 at f 4 MHz, for the optimized heat-treated Fe77.2Si 11.2Cu0.8Nb3.3B7.5 ribbon. This is ascribed to the increase in magnetic permeability and decrease in coercive force and intrinsic resistivity. Moreover, a maximum magnetic field sensitivity (ζ) of 8.3/Oe at f 2.5 MHz is obtained, for the optimized nanocrystalline Fe73.5Si13.5Cu1Nb3B9 ribbon. This suggests that tailoring of the nanocrystalline microstructures induced by optimum heat-treatment conditions can result in obtaining excellent combinations of the magnetic permeability and resistivity. Our results indicate that these Fe-based nanocrystalline materials can be ideally used for low magnetic field and high frequency sensor applications. © 2011 American Institute of Physics.

Dielectric behavior of polymer (polar/nonpolar)-metal nanocomposites (PMCs) prepared under identical processing conditions have been compared. A high effective dielectric constant (εeff > 2500) with a moderate loss and a lower εeff (74) with low loss was observed, respectively, for polar and nonpolar PMC at their respective percolation thresholds (fc). The results have been explained with the help of percolation theory and dipolar polarization. Similar value of fc observed in both the PMC is attributed to the same order of conductivity of polymer matrices. The dipolar polarization present in the polymer plays a major role in the enhancement of εeff. © 2011 American Institute of Physics.

AC magnetic properties and core loss behavior of differently prepared Fe-P sheets have been analyzed in this paper. Furthermore, the results have been compared with non-oriented silicon (Si) steel. The coercivity, magnetic flux density, and permeability measurements in dc magnetic field show Fe-P has better soft magnetic properties than Si steel. The ac magnetic behavior of both the samples has been studied in the frequency range from dc to 1 kHz for a range of flux density values. The above studies indicate that crystalline Fe-P alloys can be the replacement of so-called Si steel for low-power device applications.

Fine particles of Mn 0.6 Zn 0.4 Fe 2 O 4 (MZF) are prepared by sol-gel method and investigated the structural and magnetic properties by coating the particles with passive shell. Among different samples coated on MZF we have observed in ZnO coated sample the fraction of spinel phase is increased by diminishing Fe 2 O 3 phase compared to the all other samples. The saturation Magnetization of the MZF sample is 2.67 emu/g decreased to 1.87emu/g for the sample coated with ZnO and annealed at 600 °C. From the present investigation we conclude that the reduction of impurity phase is due surface layer reaction that results in formation of new impurity phase.

Abstract We report on magnetoresistance, Hall and magnetization measurements of Fe2VAl1-xSix Heusler compounds for x = 0.005, 0.015, 0.02. There is a systematic change in the temperature coefficient of resistance (TCR) from negative to positive as the Si composition is increased. The Hall co-efficient shows that the carriers are electron like and the carrier density increases with Si concentration. Resistance measurements under magnetic field indicate a decreasing behavior under the application of magnetic field at low temperature region (T < 60 K), suggesting the suppression of scattering by magnetic field. Temperature and field dependent magnetization measurements did not show any significant change apart from the fact that the presence of super paramagnetic (SPM) cluster and its ordering at low temperatures. Arrott plot analysis of magnetization versus field also indicates the magnetic ordering with applied field below 60 K.