THERMAL STRUCTURAL INSTABILITY AND MAGNETORESISTANCE IN EUTIO3

Abstract

Due to the potential of being multiferroics, the interest in $EuTiO_3$ (ETO) has aroused in recent years. Rich phenomena due to coupling between magnetism, electron, and phonon, make ETO so fascinating. Among all these properties, magnetoresistance and soft mode phonon frequency are studied in this thesis. Firstly, the influence of second-order Jahn-Teller effect on anharmonic phonon is investigated, and I find second-order coupling can enhance the softness of soft mode frequency. The anharmonic oscillation is also stabilized by such coupling under proper condition. Secondly, a combinational method of NEGF and Boltzmann equation is used to inspect several possible causes for large colossal magnetoresistance, and it confirms the important contribution of the molecular field. Thirdly, an algorithm to obtain spin Green's functions of the arbitrary nano-size spin system is proposed, which suggests a possible way to investigate the quantum transport of nano-size spintronic devices.