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R Nirmala
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R Nirmala
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R Nirmala
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Nirmala, R.
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29 results
Now showing 1 - 10 of 29
- PublicationMagnetic and magnetocaloric properties of nanocrystalline Pr1 xA xMn 1 yCo yO 3 (A = Ca, Sr) (x = 0.3; y = 0.5) manganites(01-03-2011)
;Mahato, Rabindra Nath; ; ; Malik, S. K.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. - PublicationMagnetic and magnetotransport properties of Ce doped nanocrystalline LaMnO3(12-07-2007)
;Krishnamoorthy, C.; ; ; Malik, S. K.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. - PublicationLarge magnetic entropy change in nanocrystalline [formula omitted](01-05-2010)
;Mahato, Rabindra Nath; ; Nanocrystalline [formula omitted] sample has been prepared by sol-gel method. The room temperature powder x-ray diffraction data show single phase nature of the sample and confirm the cubic crystal structure with [formula omitted] space group. The average crystallite size is calculated using Scherrer formula, and it is found to be [formula omitted]. Transmission electron microscopy image shows that the particles are spherical in shape and the average particle size is [formula omitted]. The sample undergoes ferromagnetic ordering at 235 K [formula omitted] and obeys the Curie–Weiss law in the paramagnetic region. The maximum value of the magnetic entropy change [formula omitted] is [formula omitted], and the relative cooling power is [formula omitted] for a field change of 50 kOe. The Arrott plot confirms that the magnetic ordering is of second order nature. The experimentally observed magnetic entropy change of the sample obeys Landau theory of phase transition well. © 2010, American Institute of Physics. All rights reserved. - PublicationParticle size effect on magnetotransport properties of nanocrystalline Nd0.7Sr0.3MnO3(01-10-2007)
;Krishnamoorthy, C.; ; ; Malik, S. K.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. - PublicationReentrant ferromagnetism in Gd2Mn3Ge5(01-01-2000)
; ; 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. - PublicationElectrical and magnetotransport properties of canted antiferromagnet Dy5Si2Ge2(01-01-2002)
; ; ; ;Morozkin, A. V. ;Chu, Z. ;Yelon, W. B. ;Malik, S. K. ;Prasad, V.Subramanyam, S. V.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. - PublicationMagnetism and electronic transport in R2Mn3Si5 (R = Dy, Ho and Er) compounds(26-07-2001)
; ; ; Morozkin, A. V.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. - PublicationMagnetization and neutron diffraction studies on Dy 5Si 2Ge 2(15-05-2005)
; ; ; ;Morozkin, A. V. ;Cai, Q. ;Chu, Z. ;Yang, J. B. ;Yelon, W. B.Malik, S. K.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. - PublicationMagnetic, transport, and magnetocaloric properties of double perovskite oxide LaCaMnCo O6(27-04-2009)
;Mahato, Rabindra Nath ;Bharathi, K. Kamala; ; ; ;Nigam, A. K.Lamsal, JagatMagnetic, 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. - PublicationGiant magnetoresistance and table-like magnetocaloric effect in double perovskite oxide PrSrMnCoO6(01-04-2011)
;Nath Mahato, Rabindra; ; ; ;Nigam, A. K.Malik, S. K.Magnetoresistance and magnetocaloric effect of a double perovskite oxide PrSrMnCoO6 (cubic, Fm 3 m) has been studied in fields up to 7 T. This compound is semiconductor-like and its electrical resistivity increases by 5 orders while going from 300 to 50 K. Giant magnetoresistance of ∼40 is observed at 200 K in 7 T field. PrSrMnCoO6 orders ferromagnetically at ∼150 K and shows a maximum magnetic entropy change of ∼4.6 J/kg/K for 5 T field change in the temperature range of 110-190 K. This nearly constant magnetocaloric effect over a broad temperature span is highly suitable for Ericsson-cycle magnetic refrigeration. © 2011 American Institute of Physics.
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