Magnetic excitations

Abstract

Modern physics provides a framework in order to describe the behavior of many interacting particles as a whole system characterized by a ground state and a spectrum of excited states. The corresponding elementary excitations are a basic ingredient of the description of many physical properties. Their knowledge constitutes the best microscopic understanding of a system and from the initial bricks constituted by the energy spectrum E n and the associated wave-functions | n >, one can calculate expectation value of an observable A: = n p n < n |A| n > where p n is the probability of occupation of level n at the temperature T , p n = 1 Z exp(− E n k B T). Inelastic Neutron Scattering (INS) is a very powerfull probe to measure elementary excitations being either related to the lattice (phonons) or to the magnetism (spin waves, spin fluctuations.. .). In this lecture, we focus on magnetic excitations starting by recalling basic notions of INS and introducing basic instrumentation. Different kind of excitations are then reviewed: local excitations, dispersive excitations and diffuse modes. These excitations are related to different terms of the Hamiltonian describing the magnetic system: spin-orbit coupling, crystal field potential, exchange interaction. The bulk of the topic of magnetic excitations is covered by the 1987 review article of Stirling and McEwen [1]. In order to study thoroughly the subject, the reader must refer to classical textbooks by order of difficulty: A. Furrer, J. Mesot and T. Strässle [2], G.L. Squires [3], S.W. Lovesey [4], Y.A. Izyumov and N.A. Chernoplekov [5]. An overview of the different inelastic neutron scattering techniques (three axis, time of flight, spin echo and backscattering) can be found in the book "Neutron and X-ray Spectroscopy" [6]. A book by G. Shirane, S.M. Shapiro and J. Tranquada is devoted to the three axis spectrometer [7]. One can also find practical details (in French) on three axis and time of flight instruments in the JDN16 lecture book [8, 9].