Magnetic Properties And M(?)ssbauer Spectroscopy Of Charge-ordering Manganite Y_0.5Ca_0.5MnO₃

Perovskite manganites of the form R1-xAxMnO3 where R is a trivalent rareearth ion such as La3+, Pr3+, and Y3+, and A is a divalent alkaline earth ion such asCa2+, Sr2+, and Ba2+ are mixed valent systems containing Mn3+ and Mn4+. Theyexhibit a multitude of magnetic, electronic and structural phase transitions asfunctions of doping level x (which controls the ratio Mn3+/Mn4+), temperature,magnetic, and electric fields. The colossal magnetoresistance (CMR) behaviors inperovskite manganites for 0.2 < x < 0.5 have attracted great attention due to bothfundamental theory of condensed matter physics and potential applications of CMRmaterials in magnetic devices, e.g., read and write heads for magnetic disk drivers,magnetic random access memories as well as magnetic field sensors. Therefore, thestudy on manganites has become a focus of recent condensed physics and materialscience. The interplay of charge, spin, lattice and orbital degrees of freedom inthese electronic strong collection systems takes a key role to determine theproperties what they display. Understanding the behavior of these Mn-basedmaterials is a challenge for both experimentalists and theorists. CMR and relatedproperties essentially arise from the double-exchange(DE) mechanism of electronhopping between the Mn3+ (t2 e1 ) and Mn4+ (t2 e0 ) ions, but this mechanism aloneis insufficient to quantitatively explain the phenomenon. Other effects such aselectron-lattice coupling due to Jahn-Teller (JT) distortion, magnetic polarons, andphase separation (PS) of carriers are advanced as an additional physics. Charge-ordering (CO) is a phenomenon observed in solids wherein electronsbecome localized due to the ordering of cations of differing charges on specificlattice sites. The charge-ordered phases are novel manifestations arising from theinteraction between the charge carriers and the phonons where in the JT distortionsplay a significant role. This thesis is based on the CO manganite Y0.5Ca0.5MnO3 (YCMO) andFe-doped YCMO, performed a detailed studies by electron paramagnetic resonance(EPR) and M?ssbauer spectroscopy (MS). In this study, YCMO and Fe-dopedYCMO were prepared by conventional solid-state reaction method. The structureand phase purity was tested by x-ray diffraction patterns. Magnetization andmagnetic susceptibility were measured with superconducting quantum interferencedevice magnetometer (Quantum Design SQUID). The EPR measurements wereperformed at 9.51 GHz employing a Bruker X-band spectrometer (model200D/SRC). The Fe MS in transmission geometry was collected with a Co γ-ray 57 57source, while the absorber was kept fixed in a variable-temperature cryostat. Theconclusions are summarized as follows:(1) For YCMO, as the temperature decreases, the system first undergoes atransition to a CO state at TCO = 275 K and then to a CE type antiferromagneticinsulating state at TN = 125 K.(2) The magnetization of the sample shows an onset of irreversibility (there is adifference between the field-cooling (FC) and zero-field-cooling (ZFC)magnetization) when the sample is cooled down to low temperature. We believethat the onset of irreversibility is due to that the FC and ZFC measurements areperformed under different experimental conditions on polycrystalline samples andnot the reason of spin glass state.(3) Below TCO, the EPR linewidth increases as temperature decreasing, thisconfirms that the temperature-dependent linewidth in the temperature range TN -TCO is the 'fingerprint'of the CO state in manganites. The observed change in thelinewidth in the temperature range below TCO can be explained by the anisotropiccrystal-field effects and the DM exchange interactions. The large decrease in thelinewidth from TN to TCO is caused by the hopping of the Jahn-Teller polarons.(4) Below TCO g factor decreases almost indistinguishably as the temperature ,decreases. We discussed there are two possible reasons in this g behavior: (i) Onepossibility may be due to the weakening of orbital order as the temperaturedecreasing. (ii) Another possible origin of the behavior of g could be that YCMObelongs to the different categories of CO manganites and have a very strong COstate.(5) The dynamic isomer shift (δDY) has been observed in MS for Fe doped YCMO.