Ultrafast magnetodynamics of self-localized spin textures driven by spin current

Abstract

Spintronics is a developing field of electronics that utilizes electron spin. This additional degree of freedom opens up new horizons for the design of non-volatile memory, logic, signal, and data processing devices. In particular, a spin-polarized charge current or a "pure" spin current can locally excite the magnetic order of the material, which is the basis for the operation of spintronic oscillators. Meeting the nanometer size requirements needed to compete with the achievements of semiconductor technology, these nano-oscillators are promising candidates for building unconventional computing schemes, such as neuromorphic computing or Ising machines. This thesis examines the dynamics of self-localized spin structures, dynamically or topologically stable configurations of magnetic order localized in space, for their application in spintronic devices.

The thesis starts with an introduction to the theoretical framework and an overview of spintronic nano-oscillators — a major focus of this work. Subsequent chapters are organized based on the type of magnetic order examined.

Chapter 2 presents an experiment on injection-locking of field-localized and self-localized spin wave modes, which are easily reconfigurable in a ferromagnetic spin Hall nano-oscillator. A theoretical model of synchronization in the presence of noise that explains this experiment is presented.

Chapters 3 and 4 focus on antiferromagnets, investigating quasi-1D and 2D spin textures, respectively. Most of the results cover domain walls, whose internal dynamics offer a pathway for constructing a nano-oscillator with high (sub-THz) frequency and overcoming challenges related to uniformly ordered antiferromagnets. A long domain wall can act as a transmission line, with information carried by another localized inhomogeneity within it: a Bloch line. The scheme for such a transmission is explored at the beginning of Chapter 4. The antiferromagnetic section concludes with a study of a dynamic droplet-like texture formed by the mutual attraction of elementary excitations, magnons, and excited by a spin current.

Chapter 5 shifts focus to the unique properties of ferrimagnets, revisiting domain wall dynamics. Although ferrimagnets combine the advantages of the two previous ordering types, we show that the forced dynamics of domain walls in ferrimagnets do not always reduce to these limiting cases and exhibit distinctive behavior - periodic "explosive" instabilities.

Chapter 6 concludes the thesis by discussing possible future research directions based on the obtained results.