G. Lalitha

Specific heat and magnetization studies of RMnO3 (R=Sm, Eu, Gd, Tb and Dy) multiferroics

A series of multiferroic materials with the compositional formula RMnO3 (where R=Sm, Eu, Gd, Tb and Dy) were prepared by the well-known citrate gel technique. After characterizing the samples structurally, a systematic investigation of specific heat and magnetization studies were performed over the temperature range 4?300?K with different magnetic fields. Based on these studies, it was found that all the samples exhibit a transition in the temperature region 39?54?K and the transition is attributed to ordering of Mn3+ ions (TNMn3+). Further, the samples GdMnO3, TbMnO3 and DyMnO3 were found to exhibit another transition (Tlock) in the temperature range 20?29?K. Finally, yet another transition was exhibited by all the samples and it is attributed to the antiferromagnetic (AF) ordering of rare-earth ion moments (TNR3+). The entropy at the AF transition TNMn3+ was computed. Finally, using the specific heat data, the Debye temperature values were also calculated.

Low-temperature resistivity anomalies and magnetic field-induced transitions in neodymium-based manganites

To investigate the magnetotransport behaviour of Nd0.67A0.33MnO3 (where A=Ca, Sr, Pb and Ba) materials, samples were prepared by the sol–gel route. The materials were characterized by x-ray diffraction (XRD) and it was found that all the samples have orthorhombic structure with pnma space group. All the samples except Nd0.67Ca0.33MnO3 (NCMO) were found to exhibit a clear metal-to-insulator (M–I) transition. Nd0.67Ca0.33MnO3, however, was found to exhibit an M–I transition on applying a magnetic field of 5 T and this transition was strengthened when the field was enhanced to 8 T. The field-induced transition in NCMO may be attributed to the presence of ferromagnetic clusters in an antiferromagnetic matrix. Further, all the samples were found to show an upturn in resistivity at about 30 K, and the observed behaviour can be explained based on theory that it is a combined effect of weak localization, the Kondo effect and electron–electron and electron–phonon scattering. Finally, the magnetoresistance (MR) of NCMO is found to be as high as 99%, indicating that it may be exploited for some devices.

Ultrasonic velocity studies in the vicinity of TC of bismuth doped manganites

Materials with the compositional formula La(0.67-x)Bi(x)Sr(0.33)MnO(3) (where x = 0, 0.1, 0.2, 0.3) prepared by a citrate gel route were used with a view to investigating the elastic behaviour of manganites in the vicinity of their T(C). The structural characterization of the materials clearly indicates that all the samples have a rhombohedral structure with [Formula: see text] space group. The magnetic (T(C)) as well as the metal-insulator (T(P)) transition temperatures determined by AC susceptibility and resistivity measurements, respectively, are found to decrease continuously with increasing bismuth concentration. Finally, the ultrasonic longitudinal velocities of all the samples are found to exhibit considerable softening in the vicinity of their magnetic transition temperatures, and an effort has been made to explain the observed behaviour by mean field theory and the Jahn-Teller phenomenon.

Elastic behavior of neodymium based manganites.

With a view to investigate the elastic behavior of Nd(0.67)A(0.33)MnO(3) (where A=Ca, Sr, Ba, Pb) manganite system, the samples were prepared by the sol gel method. After characterizing the samples structurally, a systematic investigation of ultrasonic longitudinal and transverse sound velocities of all the samples was undertaken by pulse transmission technique in the temperature range, 100-300K. It has been found that all the elasticity parameters, including Debye temperature, are found to increase continuously with increasing ionic radii of the dopant ion. All the samples are also found to exhibit anomalies in both the longitudinal and transverse velocities near their ferro to para magnetic transition (T(C)) temperatures. Apart from this, Nd(0.67)Ca(0.33)MnO(3) sample is also found to exhibit, a transition at its charge ordering temperature (T(co)). An explanation for the observed elastic anomalies based on a mean field theory has been given.