K Sethupathi

Structural, magnetic and magnetocaloric properties of sol-gel prepared, nanocrystalline oxides Pr 1-xA xMn1-yCo yO 3 (A = Ca, Sr) (x = 0.3; y = 0.5) (cubic, space group Fm3̄m) have been studied. From the X-ray data, the crystallite size of Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 samples is found to be ∼24 nm and ∼15 nm respectively. High resolution transmission electron microscopy image shows average particle size of ∼34 nm and ∼20 nm. Magnetization measurements indicate a Curie temperature of ∼153 K and ∼172 K in applied magnetic field of 100 Oe for Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 compounds. The magnetization versus applied magnetic field curves obtained at temperatures below 150 K show significant hysteresis and magnetization is not saturated even in a field of 7 T. The magnetocaloric effect is calculated from M versus H data obtained at various temperatures. Magnetic entropy change shows a maximum near T C for both the samples and is of the order ∼2.5 J/kg/K. © 2011 American Scientific Publishers.

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.

Samarium iron garnet (Sm3Fe5O12) is a potential but less explored ferromagnetic rare earth iron garnet. Cobalt substitution induced structural modifications that results enhancement in room temperature as well as low temperature magnetic behavior of Sm3Fe5O12 prepared by co-precipitation has been probed using superconducting quantum interface device. At room temperature, replacement of 0.3 atomic% Fe by Co improves the saturation value from 12.63 to 25.26 emu/g and the coercivity is found to be increased from 0.023 to 0.09 kOe. Similarly, at low temperature (∼5K) the enhancement in saturation magnetization and coercivity is found to be 17.62 to 31.23 emu/g and 1.55 to 2.32 kOe respectively. Rooted from the morphological modifications, the enhanced magnetism observed here might also be from the modifications in the super-exchange interaction and formation of oxygen vacancies due to the larger ionic radius of replaced Co2+ ions in the Fe3+ sites of Sm3Fe5O12.

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.

Nanocrystalline Pr1-xAxMn1-yCo yO3 (A = Sr, Ca; x = 0.3, y = 0.5) samples are prepared by sol-gel method. The XRD pattern show single phase and cubic crystal structure with a (=b=c) = 3.862 nm at room temperature. The crystallite size of the samples are calculated using Scherrer formula and found to be ∼15 nm and ∼24 nm for Pr0.7Sr0.3Mn0.5Co 0.5O3 and Pr0.7Ca0.3Mn 0.5Co0.5O3 respectively. The SEM images show the particles are spherical in shape and average particles sizes are about ∼20 nm and ∼25 nm. The samples undergo paramagnetic to ferromagnetic transition about ∼172 K and ∼153 K. The temperature dependence of resistivity (ρ) data show insulating behaviour below 80 K and from 80 K to 300 K, the samples behave like semiconductor. Variable range hopping conduction is found to be dominating for temperatures above 80 K. The maximum magnetoresistance is found to be ∼35 % and ∼24 % at 100 K and it has negative sign. © 2010 American Institute of Physics.

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.