Ferromagnetic Cluster Spin Wave Theory

This book begins by introducing magnetism and discusses magnetic properties of materials, magnetic moments of atoms and ions, and the elements important to magnetism. It covers magnetic susceptibilities and electromagnetic waves in anisotropic dispersive media among other topics. There are problems at the end of each chapter, many of which serve to expand or explain the material in the text. The bibliographies for each chapter give an entry to the research literature.

Modern Problems in Condensed Matter Sciences, Volume 22.2: Spin Waves and Magnetic Excitations focuses on the processes, methodologies, reactions, principles, and approaches involved in spin waves and magnetic excitations, including magnetic systems, fluctuations, resonance, and spin dynamics. The selection first elaborates on spin-wave resonance in metals, excitations in low-dimensional magnetic systems, and the theory of magnetic excitations in disordered systems. Topics include spin waves in ferromagnets with weak fluctuations of the exchange interaction; dynamics of propagating excitations; models of two-dimensional magnetic systems; spin-wave resonance in bulk metals; and standing spin-wave resonance in thin films. The manuscript then ponders on spin dynamics of amorphous magnets and magnetic excitations in spin glasses, including dynamics in reentrant spin glasses, dynamics of classical spin glasses, spin dynamical theory, spin dynamics of locally isotropic materials, and effects of dilution. The book takes a look at nuclear spin and magnetoelastic excitations and magnetic impuritons in antiferromagnetic dielectric crystals. Discussions focus on coherent and incoherent impurity excitations, equations of motion and the energy of a magnetoelastic medium, magnetoelastic excitations near magnetic orientational phase transitions, and the effect of frequency pulling on the behavior of nuclear spin echo signals. The selection is a vital source of data for researchers interested in spin waves and magnetic excitations.

This book presents recent scientific achievements in the investigation of magnetization dynamics in confined magnetic systems. Introduced by Bloch as plane waves of magnetization in unconfined ferromagnets, spin waves currently play an important role for description of very small systems.Spin wave confinement effect was experimentally discovered in the 1990s in permalloy microstripes. The diversity of systems where this effect is observed has been steadily growing since then, most of which will be addressed in this book. The book includes six chapters which originate from different groups of experimentalists and theoreticians dominating the field since the discovery of the effect. Different chapters of the book reflect different facets of spin wave confinement, providing a comprehensive description of the effect and its place in modern magnetism. It will be of value for scientists and engineers working on magnetic storage elements and magnetic logic, and is also suitable as an advanced textbook for graduate students.

This book is intended for postgraduate students as well as researchers in various areas of physics such as statistical physics, magnetism and materials sciences. The content of the book covers mainly frustrated spin systems with possible applications in domains where physical systems can be mapped into the spin language. Pedagogical effort has been made to make each chapter to be self-contained, comprehensible for researchers who are not really involved in the field. Basic methods are given in detail.

The topic of lattice quantum spin systems is a fascinating and by now well established branch of theoretical physics. Based on a set of lectures, this book has a level of detail missing from others, and guides the reader through the fundamentals of the field.

Two of the most powerful tools used to study magnetic materials are inelastic neutron scattering and THz spectroscopy. Because the measured spectra provide a dynamical fingerprint of a magnetic material, those tools enable scientists to unravel the structure of complex magnetic states and to determine the microscopic interactions that produce them. This book discusses the experimental techniques of inelastic neutron scattering and THz spectroscopy and provides the theoretical tools required to analyze their measurements using spin-wave theory. For most materials, this analysis can resolve the microscopic magnetic interactions such as exchange, anisotropy, and Dzyalloshinskii-Moriya interactions. Assuming a background in elementary statistical mechanics and a familiarity with the quantized harmonic oscillator, this book presents a comprehensive review of spin-wave theory and its applications to both inelastic neutron scattering and THz spectroscopy. Spin-wave theory is used to study several model magnetic systems, including non-collinear magnets such as spirals and cycloids that are produced by geometric frustration, competing exchange interactions, or Dzyalloshinskii-Moirya interactions. Several case studies utilizing spin-wave theory to analyze inelastic neutron-scattering and THz spectroscopy measurements are presented. These include both single crystals and powders and both oxides and molecule-based magnets. In addition to sketching the numerical techniques used to fit dynamical spectra based on microscopic models, this book also contains over 70 exercises that can be performed by beginning graduate students.

This thesis deals with the propagation of spin waves in thin Ni80Fe20 films. Its specific aims are to experimentally determine a dispersion relation for propagating spin waves in thin ferromagnetic films, and to determine phase velocities, group velocities and damping. Additonally, an analytic dispersion relation based upon the linearized Landau-Lifshitz-Gilbert (LLG) equation is derived, from which the experimental findings can be reproduced using a theoretical model. Further on, this thesis reports on the direct observation of the interference of spin waves. This thesis is structured as follows: After an introduction in chapter 1, the basic principles of magnetostatics are sketched in chapter 2, followed by the principles of magnetodynamics in chapter 3. Based upon these principles, in chapter 4 a theory for the propagation of spin waves in ferromagnetic thin films is developed. The experimental part of this thesis begins with an overview of the experimental setup for Time Resolved Scanning Kerr Microscopy (TRSKEM) in chapter 5, and continues with the results of spin wave propagation and interference in chapter 6. The experimental results are compared to the theoretical predictions of the model developed in chapter 4. Chapter 7 gives a summary of the results and an outlook towards possible future developments. The appendix gives additional information on experimental and theoretical details.

Spin wave theory of magnetism and BCS theory of superconductivity are typical theories of the time before renormalization group (RG) theory. The two theories consider atomistic interactions only and ignore the energy degrees of freedom of the continuous (infinite) solid. Since the pioneering work of Kenneth G. Wilson (Nobel Prize of physics in 1982) we know that the continuous solid is characterized by a particular symmetry: invariance with respect to transformations of the length scale. Associated with this symmetry are particular field particles with characteristic excitation spectra. In diamagnetic solids these are the well known Debye bosons. This book reviews experimental work on solid state physics of the last five decades and shows in a phenomenological way that the dynamics of ordered magnets and conventional superconductors is controlled by the field particles of the infinite solid and not by magnons and Cooper pairs, respectively. In the case of ordered magnets the relevant field particles are called GSW bosons after Goldstone, Salam and Weinberg and in the case of superconductors the relevant field particles are called SC bosons. One can imagine these bosons as magnetic density waves or charge density waves, respectively. Crossover from atomistic exchange interactions to the excitations of the infinite solid occurs because the GSW bosons have generally lower excitation energies than the atomistic magnons. According to the principle of relevance the dynamics is governed by the excitations with the lowest energy. The non relevant atomistic interactions with higher energy are practically unimportant for the dynamics.