Study of Multi-functional Materials Using First-principles Method

Abstract

The realization that the electric current through a sequence of thin alternating magnetic and metallic non-magnetic layers is strongly affected depending on the relative orientation of the magnetization gave birth to the new paradigm of electronics, now commonly known as spintronics, based on the manipulation of the spin degree of freedom of the electrons. Spintronics devices consist of a spin injector and a detector, and the efficiency of such a device crucially depends on the effectiveness of the injection and detection of polarized spins. Typically, a ferromagnetic metal is used as a spin injector or detector. However, a ferromagnetic half-metal, which has a band gap for one spin channel while the other spin channel is conducting, is the most suitable material for the spin injection or detection process. There are several theoretical predictions of bulk half-metallic ferromagnets, some of which have also been experimentally verified. However, a common problem seems to be that the half-metallic ferromagnetic states do not always survive at the surfaces, especially on the (001) surface, which is desirable from the point of view of experimental fabrication of the same system or at the interface with other materials. Recently, density functional theory-based first-principles calculations have predicted that the multi-functional perovskite BiFeO3 having spacegroup-R3c becomes a magnetic semiconductor in the presence of ferromagnetic ordering. Motivated by these findings, in this thesis, we have investigated the tetragonal BiFeO3 structure having P4mm space-group symmetry in the presence of ferromagnetic ordering using first-principles methods based on density functional theory. Since the tetragonal phase can exist with a wide range of possible structural parameters, it is expected that it will host much richer electronic properties compared to the R3c phase. In this thesis, we report, for the first time, that this is indeed true. Out of four different possible structural parameters of tetragonal BiFeO3 studied in this thesis, we find that one of the bulk structures is a half-metallic ferromagnet. Interestingly, we find that the (001) surface of the same structure is also a half-metallic ferromagnet, and the half-metallic states are further retained at the interface in certain heterostructure geometry as well.