Quantum Magnetism, Spin Waves, and Light

TL;DR

This paper reviews the quantum mechanical basis of magnetism, spin waves, and their interaction with light, emphasizing phenomena like magnons, magneto-optical effects, and the emerging field of Cavity Optomagnonics.

Contribution

It provides an educational overview connecting quantum magnetism, spin excitations, and light-matter interactions, including recent research topics like Cavity Optomagnonics.

Findings

Explanation of quantum origins of magnetic ordering

Derivation of classical Faraday effect and its quantum basis

Introduction to Cavity Optomagnonics as a research frontier

Abstract

Both magnetic materials and light have always played a predominant role in information technologies, and continue to do so as we move into the realm of quantum technologies. In this course we review the basics of magnetism and quantum mechanics, before going into more advanced subjects. Magnetism is intrinsically quantum mechanical in nature, and magnetic ordering can only be explained by use of quantum theory. We will go over the interactions and the resulting Hamiltonian that governs magnetic phenomena, and discuss its elementary excitations, denominated magnons. After that we will study magneto-optical effects and derive the classical Faraday effect. We will then move on to the quantization of the electric field and the basics of optical cavities. This will allow us to understand a topic of current research denominated Cavity Optomagnonics. These notes were written as the accompanying material to the course I taught in the Summer Semester 2018 at the Friedrich-Alexander University in Erlangen. The course is intended for Master or advanced Bachelor students. Basic knowledge of quantum mechanics, electromagnetism, and solid state at the Bachelor level is assumed. Each section is followed by a couple of simple exercises which should serve as to "fill in the blanks" of what has been derived, plus specific references to bibliography, and a couple of check-points for the main concepts developed. The figures are pictures of the blackboard taken during the lecture.