Substitution Induced Structural, Magnetic and Electrical Properties in LaFeO3 Nanoparticle

Abstract

This thesis work preferentially aspires to tune the structural, magnetic and electrical properties of undoped and various doped LaFeO3 nanoparticles synthesized by ethylene glycol assisted sol-gel technique. The enhanced properties of the as-synthesized nanoparticles have been explained in terms of cation distribution, structural modifications and exchange interactions. A detail description of the synthesis techniques used to synthesize LaFeO3 nanoparticles and its various substitutions by several magnetic and nonmagnetic metals with different valence. The experimental techniques used for the structural, morphological, dielectric and magnetic characterization of synthesized nanoparticles are systematically documented. Orthorhombic distorted perovskite LaFeO3 undergoes an antiferromagnetic (G-type) transition at ~740 K due to a strong superexchange interaction. This compound also exhibits complex electrical and magnetic behaviour with strong correlation among spin, charge and orbital degrees of freedom. Moreover, in this class of materials, many of the physical properties changes drastically under influence of internal disorder and/or external stimuli. Furthermore, a very few reports are available regarding the spin induced weak ferromagnetism and spin current induced polarization especially in the nanocrystalline form but the understanding of the magnetization, polarization and conduction mechanism in nanocrystalline LaFeO3 is still debatable. Additionally, though this wide-bandgap charge transfer type insulator and its substitutions show a very good dielectric, electric transport properties but the mechanism remains poorly understood. In this work, an effort is made to address such issues in detail and explore few new mechanisms to understand the drastic behavior shown by bare and substituted LaFeO3 nanoparticle. Detail structural, dielectric and magnetic properties of low-dimensional LaFeO3 nanoparticles are studied. For completeness, the related data of ceramic LaFeO3 is also presented. The effect of Na, a monovalent metal, on the magnetic phase transitions and electrical conductions in LaFeO3 nanoparticles is studied extensively. The influence of Zn(ll) on the structural, magnetic and dielectric dynamics of nano-LaFeO3 is documented. The explicit zero field cooled exchange bias effect is studied in Ni substituted LaFeO3. Finally, the dielectric and magnetic properties of co-doping of Na and Mn in their respective sites of LaFeO3 is studied. It is found that, LaFeO3 nanoparticle is having a better functionality than its bulk counterpart, where the former exhibits a weak ferromagnetic behavior. The Na substituted LaFeO3 nanoparticles show a coexistence of superparamagnetic and weak ferromagnetic phase and the ratio of two distinct phases vary with Na. From the dielectric measurement, a p-type polaronic conduction mechanism is found in 25%Na incorporation, which is mainly due to hole hopping between Fe4+ and Fe3+ states. Temperature-dependent magnetization of Zn doped systems show a non-ergodic state at low-T and the incompleteness of the phase transition even at very high-T. Impedance spectra reveals a non-Debye type of relaxation and grain boundary dominates over grain effect with Zn. A substantial exchange bias (EB) field is acquired below spin reorientation transitions which further reduces with higher chemical pressure of Ni. EB field is found to be ~5.71 kOe at 5 K, revealing a large value compared to similar zero field cooled EB systems. Finally, the magnetization data reveals a drastic magnetic phase change of the modified co-doping (Na, Mn) system than LaFeO3. This phase change is attributed to the mixed valence state of Mn ions present in the system due to substitutions. The above features induced due to the alkali metal in orthoferrites enhances further its applications towards spintronics and dielectrics in a wide field and temperature windows. The higher exchange bias effect in this material will certainly help in searching new materials for practical applications related to similar EB effect. Additionally, an order of change in dielectric response along with improved magnetic property makes this doped system a potential candidate for various electromagnetic devices.