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.

Nanocrystalline samples with an average particle size of 40 and 52 nm have been synthesized by citrate-complex auto-ignition method. Magnetic properties of the samples show para- to ferromagnetic transition at around 135 K. The electron magnetic resonance (EMR) study on these samples indicates the presence of coexistence of two magnetic phases below 290 K. Electrical resistivity follows variable range hopping (VRH) mechanism in the paramagnetic regime. The magnetoresistance (MR) data has been analysed by spin dependent hopping between the localized spin clusters together with the phase-separation phenomenon. These clusters are assumed to be formed by distribution of canted spins and defects all over the nanoparticle. In addition, the hopping barrier depends on the magnetic moment orientation of the clusters. The magnetic moments of the clusters are narrowly oriented in ferro- and are randomly oriented in paramagnetic phase. The ferromagnetic phase contributes to the total MR at low applied magnetic fields whereas the paramagnetic phase contributes at relatively high fields in both the samples. The average cluster size in ferromagnetic phase is bigger than that in paramagnetic phase. It is also observed that the cluster size, in ferromagnetic phase, in 52 nm sample is bigger than that in the 40 nm sample. However, the average cluster size in paramagnetic phase is almost same in both the samples. © 2006 Springer Science+Business Media, Inc.

Preliminary results of AC magnetic susceptibility, DC electrical resistivity and thermopower measurements on the new ternary compound Gd2Mn3Ge5 in the temperature range 15-300 K are reported. It is a ferromagnet at room temperature and the onset of ferromagnetism is observed around 236 K whereas reentrant ferromagnetism occurs near 88 K. The successive magnetic transitions are attributed to the layered structure of the compound. © 2000 Elsevier Science B.V. All rights reserved.

Since the giant magnetocaloric effect is encountered in a ferromagnetic Gd5Si2Ge2 alloy near room temperature it is considered as a suitable material for magnetic refrigerator applications. Also a commensurate structural transition occurs at the magnetic transition temperature and there is a good correlation between the crystal structure and magnetic properties. Such observations have triggered numerous experimental studies on similar rare earth alloys and compounds. We have synthesized its Dy- analogue, namely, Dy5Si2Ge2 and have characterized it by means of room temperature X-ray diffraction, ac magnetic susceptibility (15 K-300 K), electrical resistivity (at zero field and at 6 T), thermoelectric power (15 K-300 K) and neutron diffraction (at 300 K and 9.2 K) experiments.

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.

The compound Dy5 Si2 Ge2 crystallizes in an orthorhombic structure (Sm5 Ge4 type, space group Pnma). Magnetization measurements performed in the temperature range of 2-300 K in applied fields up to 7 T reveal that this compound orders antiferromagnetically at 56 K (TN) but with a positive paramagnetic Curie temperature θP. Magnetization-field isotherms, obtained at 5 K and 20 K, display a field-induced antiferromagnetic to ferromagnetic transition. The magnetization approaches saturation in a field of 6 T with a moment value of ~8 μB Dy3+. Neutron diffraction measurements, carried out at 9.2 K, suggest that Dy moments arrange spirally along the a axis giving rise to a canted antiferromagnetic structure. The analysis of neutron diffraction data yields an ordered state magnetic moment of 7.63 μB per Dy3+ ion. © 2005 American Institute of Physics.

Magnetic, magnetoresistive, and magnetocaloric properties of a novel double perovskite oxide, namely, LaCaMnCo O6 have been studied. Polycrystalline sample of LaCaMnCo O6 has been synthesized by sol-gel technique. It has cubic crystal structure (space group Fm 3- m) at room temperature. The temperature variation in magnetization reveals a steep increase in magnetization around 168 K (TC). The magnetization does not even saturate at 5 K and a magnetic moment of 0.7 μB f.u. is obtained at 5 K in an applied field of 50 kOe. The electrical resistivity measurement indicates that the material is semiconducting-like in the temperature range of ∼300-50 K and below ∼50 K the sample becomes insulating. A maximum magnetoresistance (MR) of about 8% is found at 200 K in an applied field of 7 T and MR has a negative sign. The magnetocaloric effect is calculated from the magnetization versus temperature data and a maximum magnetic entropy change of 3.1 Jkg K for a field change of 11 kOe is obtained near TC. Thus a moderate magnetocaloric effect is achieved in rather low magnetic fields. © 2009 American Institute of Physics.

High quality magnetoresistive La(1-x)CaxMnO3 thin films have been prepared by the chemical solution deposition technique. A solution of propionate precursors of lanthanum, calcium, and manganese in propionic acid was used for this purpose. Films of varying compositions (x varying from 0.1 to 0.4) were spin coated on to LaAlO3(100) and SrTiO3(100) substrates at room temperature and pyrolyzed in the temperature range 600-850°C. For fixed compositions, annealing at higher temperatures shifts the insulator-metal transition temperature (T1-M) to higher values accompanied by a reduction in the resistivity values. The T1-M variation for different x values was found to be less pronounced in the compositions x = 0.2, 0.3, and 0.4. Typical T1-M values of 283 K and 290 K were obtained for La0.7Ca0.3MnO3 coated on LaAlO3 and SrTiO3 substrates, respectively, when annealed at 850°C. The substrate effect was found to be more pronounced for the x value 0.1 which showed two peaks (one at 271 K and another at 122 K) in the ρ-T curve. The roles of substrate mismatch, composition variation, and annealing temperatures are discussed. © 2001 American Institute of Physics.

It has been reported that the electron (Ce) doped LaMnO3 compounds are multi-phased when prepared through solid state reaction and precipitation method. However, the laser ablated thin films of the above show single phase nature. Here, we report the synthesis of single phase nanocrystalline La0.85Ce0.15MnO3 and La0.8Ce0.2MnO3 powders through citrate-complex method. In this method, the metal ions are homogeneously dispersed in amorphous organic (citrate) complex and are allowed to react while calcination. Thus, the reacted metal ions give single phase compounds. The measured average particle size from transmission electron micrograph is found to be 30 nm for La0.8Ce0.2MnO3 sample. Temperature dependence of magnetization of both the samples show ferro- to paramagnetic transition and zero field cooled and field cooled magnetization curves suggest cluster glass behaviour. Temperature dependence of resistance of the La0.8Ce0.2MnO3 sample exhibits a metal to insulator transition at 110 K and shows no impurity peak around 250 K, which is generally observed in multi-phase samples. Magnetoresistance decreases with increasing temperature in ferromagnetic regime, as observed in typical hole doped nanocrystalline manganites. © 2006 Elsevier B.V. All rights reserved.

The magnetic and optical properties of rare earth (Gd, Dy, and Er) ion doped SnO2 bulk and optical properties of thin films are investigated. The bulk samples were found to show an upturn in ac magnetic susceptibility at low temperatures TU and exhibited a paramagnetic moment at room temperature. The band gap of the bulk samples was found to be independent of the rare earth (RE) ions while for the thin films it varied from 3.48 eV to 3.58 eV. The observed low temperature upturn suggests an interesting magnetic property in SnO2 with RE doping. © 2005 American Institute of Physics.