Veeturi Srinivas

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.

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.

We have investigated the magnetic behavior of Fe2VAl1−xBx (x=0, 0.03, 0.06 and 0.1) alloys under high temperature and high magnetic field conditions separately. Although, the low temperature DC magnetization data for the alloys above x>0 show clear magnetic transitions, the zero field cooled (ZFC) and field cooled (FC) curves indicate the presence of spin cluster like features. Further, critical exponent (γ) deduced from the initial susceptibility above the Tc, does not agree with standard models derived for 3 dimensional long range magnetic systems. The deviation in γ values are consistent with the short range magnetic nature of these alloys. We further extend the analysis of magnetic behavior by carrying the magnetization measurements at high temperatures and high magnetic fields distinctly. We mainly emphasize the following observations; (i) The magnetic hysteresis loops show sharp upturns at lower fields even at 900 K for all the alloys. (ii) High temperature inverse susceptibility do not overlap until T=900 K, indicating the persistent short range magnetic correlations even at high temperatures. (iii) The Arrott's plot of magnetization data shows spontaneous moment (MS) for the x=0 alloy at higher magnetic fields which is absent at lower fields (<50 kOe), while the Boron doped samples show feeble MS at lower fields. The origin of this short range correlation is due to presence of dilute magnetic heterogeneous phases which are not detected from the X-ray diffraction method.

In the present study magnetic and electrical transport properties of transition metal substituted Co-Ga alloys (near critical cobalt concentration) have been investigated. Analysis of temperature and field dependence of dc magnetization and ac susceptibility (ACS) data suggests an evidence of reentrant spin glass (RSG) phase in Co55.5TM3Ga41.5 (TM = Co, Cr, Fe, Cu). The magnetic transition temperatures (TC and Tf) are found to depend on the nature of TM element substitution with the exchange coupling strength Co-Fe > Co-Co > Co-Cu > Co-Cr. From magnetization dynamics precise transition temperatures for the glassy phases are estimated. It is found that characteristic relaxation times are higher than that of spin glasses with minimal spin-cluster formation. The RSG behavior has been further supported by the temperature dependence of magnetotransport studies. From the magnetic field and substitution effects it has been established that the magnetic and electrical transport properties are correlated in this system.

Ab initio calculations on ordered L21 structure of Fe 2VAl alloy have been carried out by introducing B2, DO3, A2' and XY-XZ type disorders in order to understand the role of anti-site disorder on magnetic and transport properties. These studies show an enhancement of individual spin moments of anti-site Fe atoms in DO3, A2' and XY-XZ type anti-site disorder, making the Fe2VAl alloy magnetically active. These calculations also show that hybridization due to covalent distribution of valance states among the atoms is important in Fe 2VAl, defining its unusual physical properties. From the density of states spectrum obtained near the Fermi level, we have noticed formation of intermediate defect-like states that couple the edges of the pseudo gap on both sides of the Fermi level, driving the material from semimetallic to metallic type in electrical transport. We also present experimental results on structural, magnetic and electrical properties of Fe2VAl Heusler alloy. A comparison of present experimental data with calculations shows an existence of DO3 type anti-site disorder due to the Al-deficiency in Fe2VAl alloy which causes deviations in theoretical results on the magnetic and transport behaviour of pure Fe2VAl. The temperature dependence of electrical transport and magnetic data analysed on the basis of impurity band model which provides convincing evidence for itinerant character of this alloy system with an antisite disorder. © 2013 Elsevier B.V. All rights reserved.