Electron Paramagnetic Resonance and Magnetic Studies of Certain Doped Rare Earth Manganites

Abstract

Doped rare-earth manganites with the general formula T1ð€€€xDxMnO3 where `T' is a trivalent rare earth ion, e.g. La3+;Nd3+; Pr3+ etc. and `D' is a divalent alkaline earth ion, e.g Ca2+; Sr2+;Ba2+ etc. display various interesting properties. They attracted research attention because of their unique properties and potential applications [1{5]. These materials exhibit exciting properties like CMR (colossal magneto resistance), charge ordering (CO), orbital ordering (OO), ferromagnetism (FM), antiferromagnetism (AFM), phase separation (PS), electron-hole asymmetry, phase complexities, exchange bias effects etc. [1,2,6]. These exciting properties exhibited by manganite system are explained using various interactions taking place in the system like, super exchange (SE), double exchange (DE), Dzyaloshinsky-Moriya interaction (DM interaction), eg ð€€€ eg coulombic interactions, electron-phonon interactions etc. [7{9]. In parent compounds TMnO3 and DMnO3, all Mn ions are in Mn3+ and Mn4+ states respectively. These parent compounds are governed by SE interaction resulting in AFM insulating phase. On doping with divalent alkaline earth ion in TMnO3, there is a transition from AFM insulating phase to canted AFM phase, then to FM insulating phase and FM conducting phase depending on the ratio of Mn3+ and Mn4+ ions. Manganites depict principal magnetic and electronic properties across the temperature and composition range. So the phase diagrams of manganites are complex with various phases like AFM insulating, canted AFM, FM insulating, FM metallic, PM, CO, OO etc. These phases are very sensitive to external pressure and magnetic eld. Further research in manganites shows that these are complicated systems exhibiting strong correlations between electron, spin, lattice and orbital degrees of freedom. It is also found that some of the above properties are suppressed when the size of these particles are reduced to nano-scale. For example the CO phase has been observed to suppress/completely disappear depending on the size of the particle [10{12]. Electron-hole asymmetry has also been seen to disappear in nanomanganites [13, 14]. Phase complexities, seen in the bulk [14] particles are observed to disappear. These nanomanganites also display other properties which are not seen in their bulk counterpart, like ferromagnetism, superparamagnetism, exchange-bias, spin-glass state etc. [15]. In nanomanganites, researchers have come up with various theories to explain the properties exhibited by them. Among these core-shell model is generally beleived to be most applicable [16]. Our main objective in this thesis was to perform EPR/FMR studies on certain doped rare-earth manganites, in polycrystalline bulk and nano form. In this we study di erent manganite systems exhibiting unique and interesting properties at certain doping level and try understanding them by performing temperature dependent EPR measurements. These studies are supplemented by DC magnetization studies to characterize the magnetic states of the samples. The thesis is organized into chapters as follows.