K Sethupathi

Magnetic susceptibility, electrical resistivity and thermoelectric power measurements on new rare earth ternary intermetallic R2Mn3Si5 (R = Dy, Ho and Er) compounds crystallizing in the Sc2Fe3Si5-type tetragonal crystal structure were carried out in the temperature range 15-300 K. Dy- and Ho-based alloys show two successive magnetic transitions (Dy2Mn3Si5 at 76 and 32 K; Ho2Mn3Si5 at 67 and 19 K) and the Er-based compound undergoes a magnetic transition around 55 K. The electrical resistivity is ferromagnetic metal-like, displaying typical low temperature T2 dependence and a high temperature spin-disorder contribution. Thermoelectric power is negative at room temperature, crosses zero and has a broadened peak feature centered around 50 K indicating a phonon drag effect at low temperatures and it does not have the signature of magnetic ordering. The successive magnetic transitions are suggestive of the presence of competing magnetic interactions in these systems. © 2001 Elsevier Science B.V.

Effects of Dy- substitution at rare earth site of recently identified giant magnetocaloric effect material Gd5Si2Ge2 on its crystal structure, magnetic and electronic transport properties have been studied. At room temperature, (DyxGd1-x)5Si2Ge2 (x = 0 - 0.8) compounds have monoclinic Gd5Si2Ge2 - type structure, but (DyxGd1-x)5Si2Ge2 (x = 0.9 - 1) have orthorhombic Sm5Ge4-type structure. The magnetic transition temperature (TC) decreases with increasing Dy- concentration. While the electrical resistivity (ρ) shows a marked decline at TC with metallic nature in the ordered state and becomes more resistive in Dy- rich side. Thermoelectric power (S) undergoes a slope change at TC and the signature of phonon drag effects at low temperature is seen for alloys with lesser Dy- concentration and end members.

We report here the low-temperature resistivity of the chemical solution deposited La1-xCaxMnO3 (x = 0.2, 0.3 and 0.33) thin films on LaAlO3 substrates. The films were post-annealed in atmosphere at 850°C. The low-temperature resistivity data has been studied in order to understand the nature of low-temperature conduction processes. The data showed T2 dependence from 60 K to 120 K consistent with the single magnon scattering process. The deviation from this quadratic temperature dependence at low temperatures is attributed to the collapse of the minority spin band. The two-magnon and electron-phonon processes contribute to scattering of carriers in the temperature range above 120 K.

Magnetization, electrical resistivity and Seebeck coefficient of ACr2Se4 (A = Cd and Zn) spinels with 10% Cu-substitution at A-site have been studied. While Cd0.9Cu0.1Cr2Se4 orders ferromagnetically at 129 K (TC), Zn0.9Cu0.1Cr2Se4 displays complex antiferromagnetic order at TN = 20 K. Magnetocaloric effect (MCE) in the vicinity of the magnetic transition has been estimated in terms of isothermal magnetic entropy change (ΔSm). The maximum values of ΔSm for Cd0.9Cu0.1Cr2Se4 and Zn0.9Cu0.1Cr2Se4 are about −4.2 J/kg K and −3.4 J/kg K respectively at 133 K and 48 K for a field change from 0 to 70 kOe. Electrical resistivity shows Mott variable range hopping behaviour below 50 K. Large positive thermoelectric power values are observed below 300 K in both Cd0.9Cu0.1Cr2Se4 and Zn0.9Cu0.1Cr2Se4.

Polycrystalline, Dy0.5Gd4.5Si2Ge 2 compound (monoclinic, space group P21/a) has been synthesized and characterized. This compound orders ferromagnetically at ∼210 K (T C) followed by an antiferromagnetic-like transition at ∼21 K (TN). The electrical resistivity, ρ, follows T2 law in the ferromagnetically ordered state indicating the presence of dominant single magnon scattering. There is a pronounced increase of resistivity in the vicinity of antiferromagnetic transition. Thermoelectric power, S, indicates a slope change near TC and has T3 dependence at low temperatures. © 2005 Elsevier B.V. All rights reserved.

Effect of Dy substitution at the Gd-site on the magnetic and electrical transport properties of the giant magnetocaloric effect material, Gd5Si2Ge2, has been explored. At room temperature, DyxGd5-xSi2Ge2 (x≤3.5) compounds have the monoclinic Gd5Si2Ge2-type crystal structure (Space group P21/a), while those with x>3.5, have orthorhombic Sm5Ge4-type structure (Space group Pnma). The Dy substitution contracts the unit cell and also reduces the monoclinic distortion. The compound Gd5Si2Ge2 orders ferromagnetically at 276 K; with increasing Dy substitution, the magnetic transition temperature decreases (TN=56 K for Dy5Si2Ge2) and the nature of magnetic coupling becomes antiferromagnetic in Dy-rich end. The electrical resistivity shows a marked drop near TC/TN. The thermoelectric power undergoes a slope change in the vicinity of magnetic transition and the signature of phonon and magnon drag effects is observed at low temperatures. Thus progressive Dy substitution in Gd5Si2Ge2 leads to a class of materials with interesting magnetic ground states, with magnetic ordering temperatures spanning from 56 to 276 K. © 2006 Elsevier B.V. All rights reserved.