Chapter 20: Magnetic Properties

Magnetic properties emerge from electron orbital motion and spin, quantified through the Bohr magneton, which establishes the relationship between atomic structure and bulk magnetic response. The chapter defines essential quantities including magnetic field intensity, flux density, permeability, magnetization, and magnetic susceptibility, clarifying how these parameters interconnect to describe material behavior in external magnetic fields. Materials exhibit distinct magnetic responses depending on their atomic structure and electron configuration. Diamagnetic materials develop weak, opposing magnetization in response to applied fields due to Lenz law effects, while paramagnetic materials show weak positive magnetization through dipole alignment. Ferromagnetic materials like iron, cobalt, and nickel demonstrate strong, permanent magnetization arising from exchange coupling interactions that align atomic dipoles. Antiferromagnetic materials achieve zero net magnetization through antiparallel spin alignment, whereas ferrimagnetic materials like magnetite exhibit permanent magnetization despite incomplete spin cancellation. Temperature profoundly influences magnetic behavior, with saturation magnetization decreasing until reaching critical transition temperatures—the Curie temperature for ferro- and ferrimagnets or the Néel temperature for antiferromagnets—beyond which materials become paramagnetic. The formation of magnetic domains and hysteresis loops explains remanent magnetization and coercivity, determining whether materials serve as permanent magnets or soft magnetic components. Magnetic anisotropy, arising from crystallographic symmetry, creates preferred magnetization directions in single crystals. Soft magnetic materials exhibit narrow hysteresis loops and low energy losses, making them ideal for transformer cores and electromagnetic devices, while hard magnetic materials display wide hysteresis loops and large energy products, enabling their use in permanent magnets and electromagnetic applications. The chapter addresses practical magnetic storage technologies, including perpendicular magnetic recording in hard disk drives using nanoscale cobalt-chromium grains and magnetic tape systems employing ferromagnetic particles. Finally, the chapter explores superconductivity, wherein certain metals, alloys, and ceramic compounds achieve zero electrical resistance and perfect diamagnetism below critical temperatures, with Type I superconductors exhibiting complete flux exclusion through the Meissner effect and Type II superconductors permitting partial flux penetration. High-temperature ceramic superconductors enable practical applications in medical imaging, particle acceleration, and transportation technologies.