Magnetism in Matter
Materials respond to magnetic fields by becoming magnetized, with diamagnetic, paramagnetic, and ferromagnetic behaviour.
Definition
The study of how materials acquire a magnetic moment per unit volume (magnetization) in response to applied fields, characterized macroscopically by susceptibility and permeability and classified by the sign and magnitude of the response as diamagnetic, paramagnetic, or ferromagnetic.
Scope
This topic covers the magnetization of matter, bound currents, the auxiliary field H, magnetic susceptibility and permeability, and the three principal classes of magnetic response — diamagnetism, paramagnetism, and ferromagnetism — including hysteresis and magnetic domains. It treats linear media and the qualitative microscopic origin of magnetic order, while detailed quantum theory belongs to condensed-matter physics.
Core questions
- How does magnetization give rise to bound currents and the H field?
- What distinguishes diamagnetic, paramagnetic, and ferromagnetic materials?
- Why do ferromagnets show hysteresis and retain magnetization?
Key concepts
- magnetization
- bound current
- auxiliary field H
- magnetic susceptibility
- permeability
- diamagnetism
- paramagnetism
- ferromagnetism
- hysteresis
- magnetic domains
Key theories
- Magnetization and bound currents
- Aligned atomic dipoles produce a magnetization whose spatial variation is equivalent to bound volume and surface currents, leading to the auxiliary field H sourced only by free currents.
- Classes of magnetic response
- Diamagnets weakly oppose the field, paramagnets weakly align with it, and ferromagnets show strong, often permanent magnetization with domain structure and hysteresis below the Curie temperature.
Clinical relevance
Magnetic materials are central to permanent magnets, transformer and motor cores, magnetic recording media, and the contrast and shielding considerations in magnetic resonance imaging.
History
Faraday classified substances as paramagnetic or diamagnetic in the 1840s. Pierre Curie established temperature laws for magnetic susceptibility around 1895, and Weiss introduced the molecular-field theory of ferromagnetism and domains in the early twentieth century, later grounded in quantum exchange interactions.
Key figures
- Pierre Curie
- Pierre Weiss
- Michael Faraday
Related topics
Seminal works
- landau1984
- kittel2005
Frequently asked questions
- What is the difference between B and H?
- B is the total magnetic field including the contribution of magnetized matter, while H (the auxiliary field) is constructed so that its sources are only the free currents; in linear media they are proportional through the permeability.
- Why do ferromagnets stay magnetized?
- Strong exchange coupling aligns neighbouring atomic moments into domains; once an external field aligns the domains, hysteresis lets much of the magnetization persist after the field is removed, giving a permanent magnet.