This chapter presents the applications of magnetic materials for nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), including the history and latest developments. Magnetic materials are applied for MRI/NMR mainly in two ways. One is the use of permanent magnets for generating the main magnetic fields B 0 and the other is magnetic nanoparticles (MNPs) injected in the object under scan for contrast enhancement. The physics and techniques of these two main applications will be detailed. Discussions and future perspectives will be provided at the end of the chapter.

In this chapter, the physics of magnetic materials, especially those related to MRI/NMR, are introduced in detail in Section 3.1. It is followed by Section 3.2 which details the working principles of MRI and NMR. In Section 3.3, magnetic materials used to provide the main magnetic field for imaging are presented. Examples of MRI systems using permanent magnets for imaging are demonstrated. In Section 3.4, the other main application of magnetic materials for MRI, contrast enhancement using MNPs, is presented. The history, physics, design criteria, and classifications of MNPs are introduced in detail. Different types of contrast agents (CAs) are systematically introduced and compared.