IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

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. ©2002 IEEE.

Department Of Physics

INTERMAG Europe 2002 - IEEE International Magnetics Conference

Electrical and magnetotransport properties of canted antiferromagnet Dy5Si2Ge2

Magnetization (M) and neutron diffraction measurements have been carried out on a polycrystalline Sc2Fe3Si5-type Tb2Mn3Si5 compound (space group P4/mnc) in the temperature range of 1.8-300 K. The compound undergoes a transition from a room temperature paramagnetic phase to a ferromagnetic phase at 89 K (TC1). A Curie-Weiss type behaviour is observed in the paramagnetic region at temperatures above 150 K. In this compound, both the rare earth (Tb) and Mn ions carry magnetic moments and there are two Mn magnetic sublattices, one coupling ferromagnetically and the other coupling antiferromagnetically with that of rare earth. © 2002 Elsevier Science B.V. All rights reserved.

Journal of Alloys and Compounds

Magnetic structure of Tb2Mn3Si5

New R2Ti3Ge4 (R = Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm5Ge4-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferro-magnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.

Pramana - Journal of Physics

Electrical transport and magnetic ordering in R2Ti3Ge4 (R = Dy, Ho and Er) compounds

A new rare-earth intermetallic compound, namely DyGd4Si2Ge2 having the monoclinic Gd5Si2Ge2-type crystal structure at room temperature has been synthesized and its magnetic susceptibility, electrical resistivity and thermoelectric power are measured in the temperature range 20-300 K. Magnetic susceptibility results reveal a transition from a room temperature paramagnetic phase to a ferromagnetic phase around 65 K (TC). Electrical resistivity of DyGd4Si2Ge2 is metal-like and has a T2 dependence at temperatures less than TC. Thermopower displays a change of slope followed by a peak feature just below TC. © 2001 Elsevier Science B.V. All rights reserved.

Magnetism in DyGd4Si2Ge2

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.

Magnetism and electronic transport in R2Mn3Si5 (R = Dy, Ho and Er) compounds

The results of ac magnetic susceptibility, dc electrical resistivity and thermoelectric power measurements on Tb2Mn3Si5 compound of Sc2Fe3Si5-type structure are reported in the temperature range 20-300 K. The compound is a paramagnet at room temperature and it undergoes a para- to ferrimagnetic transition at around 172±0.5 K (TC1) and enters the ferromagnetic region around 30±0.5 K (TC2). The resistivity shows an insulator to metal-type transition as the material goes from the disordered paramagnetic phase to the ordered magnetic phase. The thermopower is negative at room temperature and it crosses zero at around 132 K displaying a broad maximum centered at 65 K. These successive magnetic transitions are attributed to competitive Mn-Mn inter- and intralayer interactions, characteristic of the layered structure of the compound.

Multi-magnetic transitions in Tb2Mn3Si5

A compact sample holder to perform thermoelectric power measurements on small, bulk samples in the temperature range of 300-15 K is designed and attached to a closed cycle refrigerator (CCR). Differential method is employed and copper is taken as the reference material. The temperature control as well as the data acquisition are automated.

Cryogenics

Automated thermoelectric power measurement from 300 to 15 K with a closed cycle refrigerator

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.

Physica B: Condensed Matter

Reentrant ferromagnetism in Gd2Mn3Ge5

GENERAL

Solid State Phenomena

Magnetoresistance measurements on single element photoconductive Hg1-xCdxTe detectors

We report here the results of magnetic susceptibility of both n- and p-type superconductors, Nd2-xCexCuO4-y (x=0.14, 0.15, 0.16, and 0.18) and NdBa2Cu3-zZnzO7- (z=0 and 0.02) in the temperature range 2 to 300 K and specific heat measurements on p-type superconductors in the temperature range 5T300 K. Crystal-field (CF) analysis of these results has been performed considering J mixing of the lowest eleven multiplets with intermediate-coupled wave functions. The CF analysis of the magnetic susceptibility data is consistent also with the reported inelastic neutron-scattering spectra. The specific heat data have been accounted for by considering Debye, Einstein, and Schottky contributions. © 1995 The American Physical Society.

Physical Review B

Magnetic susceptibility, specific heat, and crystal-field effects in neodymium cuprate superconductors

We report here the measurements of magnetic susceptibility of Pr2-xCexCuO4-y (x=0, 0.05 and 0.15) from 2K to 300K and the specific heat of Pr1.85Ce0.15CuO4 between 6K and 60K. Fitting of the magnetic susceptibility data to a Curie-Weiss law gave effective magnetic moment (peff) values close to the free ion value of 3.58μB for Pr3+. peff was found to decrease with increase in cerium concentration. The site symmetry of Pr3+ in Pr2-xCexCuO4-y (PCCO) host is D4h and a crystal field (CF) Hamiltonian appropriate to tetragonal symmetry was used to analyze the magnetic susceptibility data. J-mixing of all the thirteen multiplets of Pr3+ and intermediate coupled wave functions have been included. A best set of CF parameters obtained not only explains the magnetic susceptibility but also the inelastic neutron scattering spectra (INS) reasonably well. The Schottky component together with Debye and Einstein contributions successfully accounts for the measured specific heat. © 1995 Plenum Publishing Corporation.

Journal of Superconductivity

Magnetic susceptibility and specific heat of Pr2-xCexCuO4-y

Thermal conductivity of a series of well characterized samples of YBa2Cu3-xZnxO7-y has been measured. The phonon part of the thermal conductivity has been fitted to a modified BRT type expression. The fit parameters, among other things, indicate that the gap parameter 2/kBTc and the electron-phonon coupling strength decrease with increasing zinc concentration in the sample. © 1994.

Physica C: Superconductivity and its applications

Thermal conductivity of YBa2Cu3-xZnxO7-y

Significant reduction in the magnitude of thermal conductivity of a sample of YBa2Cu3O7-y (YBCO) has been observed when the sample was stored in dry air for a period of one year, though it was reoxygenated to restore superconductivity. A modified BRT type analysis points to an increase in point defects and planar defects. © 1994.

Ageing effects on thermal conductivity of YBa2Cu3O7-y

A new compound, NdBa2Ca3Sr4Cu5Ox, has been synthesised using the conventional solid state reaction technique. The material was characterised by powder XRD, electrical resistivity, ac susceptibility and dc magnetisation studies. The results of powder X-ray diffraction show that the structure is pseudo orthorhombic, having unit cell dimensions a = 5.47 Å, b = 5.46 Å and c = 14.58 Å. Magnetisation studies on a SQUID magnetometer showed a superconducting transition at 52 K. This was confirmed by the measurements of electrical resistance and ac susceptibility of this sample, which also showed superconductivity at 52 K. Details of preparation and characterisation are discussed. © 1993, Taylor &amp; Francis Group, LLC. All rights reserved.

Phase Transitions

Superconductivity in a new NdBa2Ca3Sr4Cu5Ox system

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean's critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (-Hmax to +Hmax) and reverse (+Hmax to -Hmax) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified as Hpeak, HI, HII etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (Mrem(H)-MFC(H)+MZFC(H)) being equal to, less than or greater than zero, where Mrem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate. © 1992 Indian Academy of Sciences.

Pramana

Magnetization studies in niobium and YBa2Cu3O7 samples

Fluctuations in phase across the weak links in a granular superconductor contribute to the excess conductivity below Tm, the temperature at which the plot of temperature derivative of electrical resistivity against temperature shows a maximum. In a narrow range of temperature above T∞ (the temperature at which the resistivity at which the resistivity becomes zero), the excess conductivity, Δσ = [σ(T) - σ(Tm)], is expected to vary as (T-T∞)-γ. In this work, resistivity measurements on CaREBaCu3O7-y (RE=La, Sm), Ag coated YBa2Cu3O7-y, Tl2CaBa2Cu2O8 and YBa2Cu3-xZnxCu3O7-y (x=0.05, 0.075, 0.1 and 0.15) in the paracoherence region are reported. Analysis of the data indicates the excess conductivity in this region is better described by the critical exponent y = 1.33. From an analogy between the tunneling Hamiltonian in the granular material and the Hamiltonian for an XY ferromagnet one expects the critical exponent, γ, to be the same as the critical exponent for susceptibility in the ferromangetic case. © 1992.

Excess conductivity studies in the paracoherence region in high- temperature superconductors

Thermal conductivity of YBa2Cu3O7 has been measured on samples of a few mm thickness. Above Tc thermal conductivity is found to decrease with increase in temperature, pointing towards a contribution to thermal resistivity from three-phonon Umklapp processes. Below Tc thermal conductivity increases rapidly before reaching a maximum at about 50 K and then falls towards zero at lower temperatures. The experimental set up is described and results discussed. © 1992, the Indian Academy of Sciences. All rights reserved.

Pramana: Journal of Physics

Thermal conductivity of YBa2Cu3O7 from 7 K to 260 K

Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data for fields applied parallel and perpendicular to the plane of flat superconducting Pb discs (Tc=7.2 K) are presented. A quasi-irreversibility temperature T*, which differs for the two orientations, can be identified in these data. T* is found to vary with the field approximately as (1-T*/Tc)H1.085. © 1991 The American Physical Society.

Quasi-irreversibility temperature in a type-I superconductor

The compounds CaREBaCu3O7-y (RE=La and Sm) are tetragonal at room temperature with Tc between 60 and 70 K. The single-phase compounds were prepared by solid-state reaction. The resistivity was measured by a four-probe technique in a continuous flow cryostat with the temperature being controlled to an accuracy of 10 mK. The resistivity vs temperature showed a break in slope around 180 K in CaLaBaCu3O7-y and around 220 K in CaSmBaCu3O7-y . The results were analysed for fluctuation conductivity from 180 K downwards. A plot of d ρ/d T vs T showed a sharp peak at Tm =69·69 K for La compound and 66·00 K for the Sm compound. Detailed analysis of the resistivity in the region Ton to 180 K was carried out using the procedure due to Veira and Vidal. The results are discussed in this paper. © 1991 Indian Academy of Sciences.

Fulltext

Bulletin of Materials Science

Fluctuation-induced excess conductivity in the compounds CaREBaCu3O7-y (RE=La and Sm)

The magnetic susceptibility measurements were performed on CuO before and after quenching from high temperature. A large increase in the magnetic susceptibility is observed by quenching or consequent decrease in the oxygen content. It is argued that the large rise in the magnetization at temperatures much below the Néel temperature observed in certain samples may not be of impurity origin but results from the random freezing of isolated Cu2+ ions near the oxygen defects. © 1990.