Chapter 2 - Magnetic Structures

Abstract

This chapter discusses magnetic structures. Most of the information on the nature of ordered magnetic phases or magnetic structures comes from neutron diffraction experiments. If the ordered magnetic phase is ferromagnetic, then we expect magnetic reflections superimposed on the nuclear reflections. The magnetic cell is the same as the nuclear cell and the propagation vector. The magnetic structure determination is then reduced to determining the moment direction and the magnitude of the ordered magnetic moment. A modulated magnetic structure is a superstructure of the crystal structure for which the magnetic periodicity, caused by the spin modulation, is large compared to the periodicity of the crystalline lattice. The translational properties of the modulated structure are fixed by the propagation or the wave vector. Chromium is the archetypical itinerant antiferromagnet, whose incommensurate spin density wave is characterized by a wave vector, which is determined by the nesting properties of its Fermi surface. It is found that the axial next nearest-neighbor Ising model has been used extensively to explain modulated structures, which are incommensurate or high-order commensurate.