Sundaresan A.

Temperature dependent magnetic, dielectric and Raman studies of partially disordered La2NiMnO6

2014-01-01T00:00:00Z

Temperature dependent magnetic, dielectric and Raman studies of partially disordered La2NiMnO6 Kumar, Pradeep; Ghara, Somnath; Rajeswaran, B.; Muthu, D. V. S.; Sundaresan, A.; Sood, A. K. We report a detailed magnetic, dielectric and Raman studies on partially disordered and biphasic double perovskite La2NiMnO6. DC and AC magnetic susceptibility measurements show two magnetic anomalies at T-C1 similar to 270 K and T-C2 similar to 240 K, which may indicate the ferromagnetic ordering of the monoclinic and rhombohedral phases, respectively. A broad peak at a lower temperature (T-sg similar to 70 K) is also observed indicating a spin-glass transition due to partial anti-site disorder of Ni2+ and Mn4+ ions. Unlike the pure monoclinic phase, the biphasic compound exhibits a broad but a clear dielectric anomaly around 270 K which is a signature of magneto-dielectric effect. Temperature-dependent Raman studies between the temperature range 12-300 K in a wide spectral range from 220 cm(-1) to 1530 cm(-1) reveal a strong renormalization of the first as well as second-order Raman modes associated with the (Ni/Mn)O-6 octahedra near T-C1 implying a strong spin-phonon coupling. In addition, an anomaly is seen in the vicinity of spin-glass transition temperature in the temperature dependence of the frequency of the anti-symmetric stretching vibration of the octahedra. (C) 2014 Elsevier Ltd. All rights reserved. Restricted Access

Reentrant spin-glass state and magnetodielectric effect in the spiral magnet BiMnFe2O6

2014-01-01T00:00:00Z

Reentrant spin-glass state and magnetodielectric effect in the spiral magnet BiMnFe2O6 Ghara, Somnath; Jeon, Byung-Gu; Yoo, Kyongjun; Kim, Kee Hoon; Sundaresan, A. The complex oxide BiMnFe2O6 crystallizing in a centrosymmetric orthorhombic structure (Pbcm) is known to exhibit a spiral antiferromagnetic ordering at T-N similar to 212 K and an anomaly in magnetization at low temperature (T-P similar to 34 K). To understand the origin of the low temperature anomaly and to verify experimentally whether or not the spiral magnetism induces ferroelectricity, we have carried out detailed magnetization and electrical measurements on a polycrystalline sample. Frequency-dependent ac susceptibility, dc magnetic memory effect, and dc magnetization relaxation studies show that the low temperature anomaly at T-P is associated with a reentrant spin-glass transition. From dc magnetization, we found that there exists another distinct temperature (T-irr), which corresponds to a strong thermomagnetic irreversibility. The field dependence of these two temperatures, T-P(H) and T-irr(H), follow Gabay-Toulouse and de Almeida-Thouless lines, respectively, which are indicative of the Heisenberg spin-glass state. We also observe another magnetic anomaly at T* similar to 170 K, where a dielectric anomaly with a significant magnetocapacitance effect is observed. However, pyroelectric current and positive-up-negative-down measurements reveal the absence of ferroelectricity in this compound. Restricted Access

Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and Co9S8

2014-01-01T00:00:00Z

Synthesis and Properties of Cobalt Sulfide Phases: CoS2 and Co9S8 Kumar, Nitesh; Raman, Natarajan; Sundaresan, A. Single phase cobalt disulfide (CoS2) nanoparticles were prepared by thermal decomposition of cobalt-thiourea complex at a low temperature (400 degrees C). CoS2 nanoparticles exhibit ferromagnetic ordering at 122 K below which the temperature dependent resistivity of cold pressed nanoparticles deviates from metallic behavior and shows a broad maximum. Just below T-C, it also exhibits a large magnetoresistance effect (6.5%). A mixture of CoS and Co9S8 phases were obtained between the temperature interval 400 degrees C < T < 1000 degrees C. At 1000 degrees C, a pure bulk Co9S8 phase was obtained. It exhibits magnetic hysteresis typical of a ferromagnet at room temperature and a peak in magnetization at low temperature with a strong relaxation indicating possible spin glass state. Restricted Access

Ti-rich double perovskites LnCu(3-y)Ti(2-x)Mn(2+x+y)O(12): ferrimagnetism and magnetoresistance up to room temperature

2014-01-01T00:00:00Z

Ti-rich double perovskites LnCu(3-y)Ti(2-x)Mn(2+x+y)O(12): ferrimagnetism and magnetoresistance up to room temperature Seikh, Md. Motin; Caignaert, V.; Sundaresan, A.; De, Chandan; Raveau, B. Ti-rich double perovskites LnCu(3-y)Ti(2-x)Mn(2+x+y)O(12) have been synthesized at normal pressure, with a wide homogeneity range, 0 < x < 0.5 and 0 < y <= 1. ALL these oxides are ferrimagnetic, with T-C ranging from 160 K to 293 K, in spite of the high 3d(0)-Ti content in the octahedral sites. The evolution of T-C can be monitored on the basis of three factors-antiferromagnetic superexchange interactions Cu2+up arrow-O-Mn3+/4+down arrow-O-Cu2+up arrow, double exchange Mn4+-O-Mn3+ interactions in the octahedral sites and chemical pressure due to the size effect of Ln(3+). Differently from high pressure synthesized manganites, these oxides exhibit maximum magnetoresistance at T-C, similar to single perovskite manganites. Intrinsic magnetoresistance can thus be obtained at room temperature, but with a smaller amplitude compared to single manganites, due to the dilution effect of titanium. Restricted Access