What Are Magnets?

A magnet is any object or device that produces a magnetic field capable of attracting ferromagnetic materials and exerting forces on moving electric charges. Magnets range from sub-micron particles used in biomedical research to kilometer-scale superconducting coil systems used in particle accelerators. Their design and analysis draw on materials science, electromagnetic theory, and advanced manufacturing, making magnets a central topic across many engineering disciplines.

Permanent Magnets

Permanent magnets retain their magnetization without an external power source. Their performance is characterized by three key parameters: remanence (B_r), the flux density retained after the magnetizing field is removed; coercivity (H_c), the field required to demagnetize the material; and the maximum energy product (BH)_max, which measures the energy stored per unit volume.

Modern high-performance permanent magnets are rare-earth compounds. Neodymium-iron-boron (Nd-Fe-B) holds the highest room-temperature energy product of any commercial material, reaching about 400 kJ/m³ in sintered grades. Samarium-cobalt (Sm-Co) magnets sacrifice some energy product for superior temperature stability and corrosion resistance. These materials enabled the compact, lightweight motors found in electric vehicles, wind turbine generators, and consumer electronics.

Rare-earth permanent magnet research continues to focus on reducing dependence on heavy rare earths such as dysprosium, which are geopolitically concentrated, while maintaining coercivity at elevated operating temperatures.

Micromagnetics is the computational discipline that models magnetization behavior at the nanometer to micrometer scale by solving the Landau-Lifshitz-Gilbert equation over a discretized geometry. Micromagnetic simulations reveal how domain walls nucleate at defects, how reversal propagates through a grain, and how demagnetizing fields from neighboring grains interact, guiding the design of permanent magnets with optimized coercivity and remanence.

Electromagnets

Electromagnets generate magnetic fields only when electric current flows through a coil wound around a ferromagnetic core. The field strength is controlled by adjusting the current, and the field drops to zero when power is removed. This switchability makes electromagnets indispensable in applications requiring rapid field changes or precise control.

Industrial electromagnets lift steel scrap in recycling plants and hold workpieces on precision machining tables. Medical electromagnets deflect particle beams in proton therapy systems and position magnetic capsule endoscopes inside the body. The field of a conventional resistive electromagnet is ultimately limited by the thermal load of the current-carrying windings; increasing current increases both field strength and resistive losses.

Superconducting Magnets

Superconducting magnets replace resistive windings with coils of superconducting wire that carry current with zero resistance below a critical temperature. Eliminating resistive loss allows much higher current densities and, consequently, much larger magnetic fields. Niobium-titanium (NbTi) and niobium-tin (Nb₃Sn) are the dominant low-temperature superconductor materials, operated in liquid helium at 4.2 K.

Superconducting magnets in MRI systems routinely produce fields of 1.5 to 3 T in clinical scanners, with research systems reaching 10.5 T for human imaging. The Large Hadron Collider at CERN uses thousands of NbTi dipole magnets to bend 7 TeV proton beams around an 27-km ring. High-temperature superconductors (HTS) based on rare-earth barium copper oxide (REBCO) are enabling a new generation of compact fusion reactor designs and high-field research magnets that operate above 20 T.

Quench protection is a critical engineering challenge in superconducting magnets: if a small section loses superconductivity, the stored energy (sometimes hundreds of megajoules) must be redistributed rapidly to prevent localized damage. Quench detection and protection systems use voltage sensing, heaters, and crowbar circuits to spread the energy safely across the entire winding.

Applications

• Electric vehicles: High-energy-product Nd-Fe-B magnets power the traction motors in most battery electric cars, providing high torque density in compact packages.
• Medical MRI: Superconducting solenoids create the uniform high fields needed for clinical magnetic resonance imaging.
• Wind energy: Permanent magnet generators in direct-drive wind turbines eliminate the gearbox, improving reliability in offshore installations.
• Industrial automation: Electromagnets and permanent magnet arrays drive linear motors, magnetic couplings, and conveyance systems in manufacturing lines.
• Particle accelerators: Superconducting dipole and quadrupole magnets steer and focus high-energy particle beams in physics research facilities.
• Consumer electronics: Thin rare-earth magnets enable compact speakers, vibration motors, and sensors in smartphones and wearables.