Veeturi Srinivas

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 magnetic properties of binary CoxGa100-x alloy with Co concentration in the range 54 ≤ x ≤; 61.5 at% have been investigated. From the temperature and magnetic field dependent magnetization measurements magnetic phase diagram has been identified. Cluster spin glass like features are noticed in x = 54, 55 compositions, while the compositions x > 57 exhibit double magnetic transition i.e., at higher temperatures paramagnetic (PM) - ferromagnetic (FM) and at lower temperatures FM-SG like transition. The critical concentration is identified to be near x = 57 composition where discernible spontaneous magnetization emerges and the long range ferromagnetic order develops above this composition in addition to the spin glass transition (or mixed magnetic phase). Analysis of temperature dependence magnetization data in the different temperature ranges for the compositions x = 60 and 61.5 indicate that the mean field models are not suitable to understand the phase transition. Magnetic isotherms in the critical region were analyzed using non-mean-field approach and the critical exponents (γ = 1.31 and β = 0.337) found to be close to 3D Heisenberg model suggesting the importance of short range magnetic order. The data satisfies magnetic equation of state characteristic of a second order phase transition. The results obtained from the present study corroborate well with the phenomenological interacting spin cluster model.

We report effect of V substitution in Cr-sites of Fe2CrAl on electronic properties in the present article. The electronic structure calculations of Fe2Cr1-xVxAl alloys clearly exhibit a significant effect of decreasing number of valence electron on density of state (DOS) for majority-spin state as well as magnetic properties of half-metallic ferromagnetic Fe2CrAl Heusler alloy, but retains high degree of spin polarization across this series (100% for x= 0.75). © 2012 American Institute of Physics.

Low temperature magnetic properties of binary CoxGa100−x (x=54–57) alloy have been investigated. Analysis of frequency dependence of ac susceptibility provided a conclusive evidence for the existence of cluster spin glass like behavior with the freezing temperature ~8, 14 K for x=54, 55.5 respectively. The parameters for conventional ‘slowing down’ of the spin dynamics have been extracted from the acs data, which confirm the presence of glassy phase. The magnitude of Mydosh parameter obtained from the fits is larger than that reported for typical canonical spin glasses and smaller than those for non-interacting ideal superparamagnetic systems but comparable to those of known cluster-glass systems. Memory phenomena using specific cooling protocols also support the spin-glass features in Co55.5Ga44.5 composition. Further the development of ferromagnetic clusters from the cluster spin glass state has been observed in x=57 composition.

A detailed analysis of the thermal and the magnetic field dependence of the magnetization and the electrical transport properties of Co58.5Ga41.5 prepared under different process conditions has been reported. All the samples show double magnetic transitions. The sample annealed at lower temperature (S1) shows heterogeneous behavior (structural and magnetic) with high anisotropy and exhibits maximum magnetoresistance at low temperature (-12.4% at 5 K on the application of 50 kOe field), whereas the quenched sample shows stronger magnetic interaction with lower anisotropy. The analysis of magnetization data suggests a shortrange ferromagnetic order. The itinerant behavior has been observed through the analysis of magnetic and resistivity data, and the characteristic spin-fluctuation temperature (TSF) for high temperature annealed sample (S2) has been calculated from a specific heat measurement.

A detailed study of low temperature electrical transport properties of CoxGa100-x (x = 54, 55.5, 57) alloy has been carried out. The origin of the resistivity anomalies and correlation between magnetic and electrical transport properties are identified through an elaborate analysis. The weak localization and enhanced electron-electron interaction effects partially support the electrical transport properties of the system. Further, the observed magnetoresistance can be well represented by localized model along with quantum corrections. The low temperature magnetoresistance value near critical composition is comparable to that reported in giant magnetoresistance materials.