K Sethupathi

The magnetic and electrical transport properties of Ce doped LaMnO3 bulk samples have already been reported for various Ce concentrations. However there are few reports on the magnetotransport properties of these compounds. In this paper, we report the magnetic and magnetotransport properties of nanocrystalline La0.8Ce0.2MnO3 and La0.7Ce0.3MnO3 samples with different particle size. The nanocrystalline samples with different particle size were prepared through citrate-complex method by calcining the precursor at different temperatures. The transmission electron microscopy analysis of both the compounds reveal that the particle size increases with calcination temperature. Temperature dependence of magnetization of all the samples shows ferro- to paramagnetic transition. The zero field cooled and field cooled magnetization curves show irreversibility just below the magnetic transition temperature. The temperature dependence of resistivity of all the samples exhibits a metal to insulator transition without an impurity peak around 250 K, which is generally observed in multiphase samples. The results indicate that magnetoresistance (MR) increases with decreasing particle size. The observed MR will be discussed by spin dependent tunneling and enhancement of double exchange mechanism upon application of magnetic field. The physical properties suggest that the samples are single phase in nature. © 2006 Elsevier B.V. All rights reserved.

Sol-gel prepared nanocrystalline La0.7Te0.3MnO3 has rhombohedral crystal structure (space group R3-C) at room temperature and orders ferromagnetically at ∼280 K (TC). A large magnetic entropy change of ∼12.5 J kg-1 K-1 is obtained near TC for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at TC and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La0.7Te0.3MnO3 a promising material for magnetic refrigeration and spintronic device applications. © 2010 Elsevier B.V. All rights reserved.

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.

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.