Electronic Structure, Magnetic Moment and Magnetocrystalline Anisotropy in Two and Three Dimensional Materials: A First Principles Study

Abstract

Magnetic properties are very important both in two and three dimensional materials as it increases one more degree of freedom which extend their range of applications. Using first principles calculations within the framework of density functional theory, we investigate the electronic structure and magnetic properties of some selected 2D and bulk materials for various applications. This thesis mainly composed of two parts. The first part of this thesis will primarily emphasis on the magnetic properties of 2D layered materials such as black phosphorene and feroxyhyte for spintronics related applications. The second part of this thesis will focus on the magnetic properties of two ferromagnetic bulk materials FeCo and α″-Fe16N2 for rare-earth free permanent magnet applications. Phosphorene is a newly discovered 2D material, which was mechanically exfoliated from layered bulk black phosphorous in 2014. Although phosphorene has some fascinating properties such as, an intrinsic band gap, anisotropic electrical, mechanical and optical properties but it mainly consist of non-magnetic phosphorous atoms. Thus pristine phosphorene turn out to be a non-magnetic semiconductor. In this thesis we will explore the different possibilities to induce magnetism in phosphorene extrinsically. In section 3.1, we investigate the manipulation of magnetic state in phosphorene by doping of light non-magnetic elements. These impurities disrupt the sp3 hybridization of pristine phosphorene, causing a change in electronic structure and thereby induces a magnetic state. In a more traditional way in section 3.2, transition metal dimers are used to create magnetism in non-magnetic phosphorene layer. Having an intrinsic ferromagnetism with semiconducting band gap, is what needs for next generation spintronic devices. Therefore, in section 4.1 we investigate the long range ferromagnetic ordering in a semiconducting 2D graphene-like structure namely feroxyhyte. The magnetism in feroxyhyte stems from layer to layer interaction. The second part of this thesis is associated with the possibility of rare earth free Fe based permanent magnet for which we need a high value of saturation magnetization and magnetocrystalline anisotropy (MCA). The binary FeCo alloy possesses a high saturation magnetization, but very weak MCA due to its cubic symmetry. Therefore, in section 5.1 we explore the possibility of introducing tetragonal lattice distortion due to small interstitial doping of 2p elements, which leads to very high value of MCA. Furthermore, in section 5.2 we investigate the strain and interface effects on MCA in the hybrid bilayer FeCo/FePt. Additionally, the metastable Fe16N2 alloy is a semi-hard permanent magnet with very high saturation magnetization. So, in section 6.1 we explore the possibility of further enhancement of MCA by interstitial doping of light element. Finally, in section 6.2 the heavy 4d and 5d elements are substitutionally doped in Fe16N2 in order to incorporate the associated spin orbit coupling effect for enhancing the MCA.