Crystal Structure, Magnetic Structure and Magnetic Properties of (Nd, K)-Mn-O and (Nd, Na)-Mn-O based Manganites

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Colossal magnetoresistive oxides, one of the amazing classes of materials have been the subject of research for more than a decade, due to their fascinating electrical and magnetic properties and, their potential applications in spintronics devices and magnetic storage. The alkaline earth doped rare earth manganites based materials are mostly known to exhibit colossal magneto-resistivity (CMR), i.e. large negative magneto-resistivity in the vicinity of ferromagnetic (FM) transition temperature (TC) due to the double exchange ferromagnetic interaction. There are several reports on Nd based divalent alkaline earth doped manganites. The magnetic and transport properties of such materials are found to be quite interesting. In Ca doped Nd-Mn-O series, FM transition with TC around 110 K, along with the presence of charge ordering and antiferromagnetism (AFM) has been observed without any metalinsulator transition. The application of magnetic field was found to induce semiconductorD metal and AFM-FM transitions in the composition range of 0.30 ः x ः 0.45. The charge ordering and its destruction by the application of large magnetic field were reported in half doped, Nd0.5Ca0.5MnO3 series. In Nd0.7Sr0.3MnO3, FM and metal-insulator (M-I) transitions have been observed with TC in the range of 230 K to 250 K because of its relatively large A site ionic size ( rA > =1.212) compared to that of (Nd, Ca)-Mn-O series. However, in halfdoped Nd0.5Sr0.5MnO3 material, charge ordering transition has been reported at around 150 K from the detailed neutron diffraction, magnetization and electrical resistivity studies. In (Nd, Ba)-Mn-O series, even though the rA > value is quite large, FM was observed only in a narrow composition range of x = 0.2 to 0.4 with relatively lower TC 120 K. In this research work, mainly monovalent (K and Na) doped Nd-based manganites have been taken up for study. The monovalent doping has the advantage of creation of optimum concentration of Mn3+/Mn4+ ions with relatively small level of doping. Thus, the lattice distortion can be kept minimum in the monovalent doped materials. Crystal structure, magnetic structure and lattice distortion have been investigated using neutron and X-ray diffraction measurements. DC magnetization and ac susceptibility measurements have been carried out to investigate the nature of magnetic properties for different doping concentrations. Moreover, FM, spin glass like behavior, critical exponent behavior in the vicinity of FM transition, charge ordering, etc. were studied in detail in these materials. Suppression of charge-ordering in Nd0.8Na0.2MnO3 compound by varying two parameters, namely particle size down to nano-metric scale and magnetic field up to 10 T has been investigated. The following three series of samples were prepared. 1. Nd1-xKxMnO3 (x = 0.10 - 0.30) 2. Nd1-xNaxMnO3 (x = 0 - 0.20) 3. Nanoparticle samples of Nd0.8Na0.2MnO3 The above samples were characterized by using powder X-ray diffractometer (XRD), scanning electron microscope (SEM), field emission SEM, transmission electron microscope (TEM), energy-dispersive spectrometer (EDS), and chemical titration. To determine the crystal structure and magnetic structure of the above materials, neutron powder diffraction (NPD) patterns were recorded at selected temperatures down to 5 K by using multi-position sensitive ....
Supervisor: S. Ravi