Exploring one and two-channel Kondo effect and investigating dielectric properties in ferrimagentic nanocomposites of LaNiO3 and CoFe2O4

Abstract

The Kondo effect is a well-known and intensely studied phenomenon in condensed matter physics. After the theoretical prediction of Zawadowski and Noizères about the two-channel Kondo (2CK) effect, several successful attempts have been made to experimentally realize the 2CK effect. Usually the 2CK fixed point is not stable in the presence of magnetism as it can break the channel symmetry. But Zhu et al. have recently reported the coexistence of ferromagnetism and 2CK effect in thin films of L10- MnGa and L10- MnAl. The small spin polarization due to the disorder-induced antiparallel aligned between Mn–Mn atoms results in weak channel asymmetry and hence coexistence of 2CK with ferromagnetism. In order to get new insights about the simultaneous presence of magnetism and the 2CK effect, the low temperature magneto-transport properties of the composites containing LaNiO3 (LNO) and CoFe2O4 (CFO) [(1−x)LNO + xCFO; x = 0, 0.10, 0.15, 0.20, 0.25] are studied extensively. For composite with lower percentage of CFO (x = 0.10), spin one channel Kondo effect dominates at low temperature. However, in case of x ≥ 0.15 resistivity below the upturn is governed by orbital 2CK effect which originates from the scattering of conduction electrons with the structural disorders created at the interfaces between the two phases (LNO and CFO). The magnetoresistance and anomalous Hall effect (AHE) are studied in the two composites with x = 0.15 and 0.20 to understand the origin of AHE in systems with structural defects and to unravel the correlation between AHE and 2CK physics. The AHE shows two important phenomena, one is the AHE of the composites with 15 % and 20 % CFO follow scaling behaviour with longitudinal resistivity at high temperature, but shows a deviation around Kondo temperature (TK) and a sudden jump in the temperature variation of anomalous Hall coefficient near TK. These two observations suggest the AHE is strongly influenced by the presence of orbital 2CK effect. The complex impedance spectroscopy (CIS) is a useful tool to correlate the electrical properties with the microstructure and to determine various polarization process present in grain, grain boundaries and electrode-interfaces of polycrystalline materials. In the second part of my thesis, the same compounds (LNO and CFO) are used with changing the composition in such a way that CFO acts as insulating matrix and LNO plays the role of conductive filler [xLNO + (1-x)CFO; x = 0, 0.05, 0.10, 0.15]. The physics of transport and dielectric properties are investigated in detail applying the CIS technique. Adding LNO affects the grain boundary transport in the composites enabling short range hopping of the ions across it