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.

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.

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.

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.

Coupling between nearest neighbor metal nanoparticles can lead to large shifts in plasmon resonance wavelength. Here we investigate the interaction between identical silver nanoparticles numerically using finite element analysis method. The results show that for all particle shapes and sizes, the surface plasmon resonance shifts to longer wavelengths as the interparticle distance is decreased, and the shift decays exponentially with increasing interparticle distance. The shifts become vanishingly small when the distance is more than about 1.25 times the length of the particle along the direction of polarization.

Equi-atomic intermetallic Co-Ga compound exhibits unusual properties, such as, non-metallic electrical transport and weakly magnetic behavior. Electronic band structure calculations suggest the presence of deep psuedogap in the density of states near Fermi level. As the cobalt concentration varies over 50-60 at% it presents an interesting magnetic phase diagram along with drastic variation in electronic properties across a critical concentration (x∼58) in CoxGa100-x alloy system. The magnetic characteristics observed in these alloy were attributed to antisite defects [1, 2]. Despite several works on this alloy no clear consensus emerged on the nature of magnetic phase transitions observed and the magnetic ground state. A detailed analysis of thermal and magnetic field (H) dependence of magnetization (M) can throw light on magnetic ground state and the origin of anomalous behavior at critical composition. In this work we present dc magnetization, magnetoresistance (MR) and specific heat study of Co58.5Ga41.5 composition prepared under different process conditions to elucidate magnetic ground state.