A permanent magnet retains a magnetic field after the external magnetizing field is removed. Its aligned magnetic domains resist being rearranged, so it can provide a field without continuous electrical power. An electromagnet instead relies primarily on current in a coil and can usually be switched, adjusted, or reversed. Neither type creates energy: magnets supply a field, while a motor or generator converts energy from another source.
What is a permanent magnet?
A permanent magnet is made from a hard magnetic material that retains useful magnetization under specified conditions. It has a north and south pole and produces a magnetic field around it. “Permanent” does not mean indestructible or eternal: heat, opposing fields, corrosion, and physical damage can reduce its performance.
Magnetism is part of electromagnetism, the physical interaction associated with electric charge and magnetic fields. Atoms contain electrons with intrinsic magnetic moments related to their spin. In materials such as iron, interactions between neighboring atoms can favor alignment of those moments. The resulting behavior is more useful to describe through domains than as a collection of literal tiny bar magnets (U.S. Department of Energy: The Electromagnetic Force; OpenStax: Ferromagnets and Electromagnets).
How does permanent magnetism work?
Domains, alignment, and hard materials
A magnetic domain is a region in which many atomic magnetic moments are aligned. In an unmagnetized piece of magnetic material, domains point in different directions, so their effects largely cancel at a distance. Applying a strong magnetic field can make favorably oriented domains grow and rotate into alignment.
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In a hard magnetic material, crystal structure and microstructural features make it difficult for domains and domain walls—the boundaries between domains—to move back when the external field is removed. Magnetic anisotropy makes some magnetization directions energetically more favorable than others; pinning at defects and boundaries can resist reversal. The material therefore retains a net field.
Hysteresis and coercivity
Magnetization depends partly on a material’s magnetic history, a behavior called hysteresis. A hard material has enough resistance to reversal to serve as a permanent magnet. A soft magnetic material magnetizes and demagnetizes readily, which makes it useful in transformer and relay cores and in electromagnets, where the field needs to respond to changing current.
Permanent magnets need no continuous electrical input to maintain their field in normal use. That does not make them a source of free energy: manufacturing and magnetizing them require energy, and a machine still needs an energy input to do work.
Permanent magnets versus electromagnets
| Feature | Permanent magnet | Electromagnet |
|---|---|---|
| Field maintenance | No continuous electrical input in normal operation | Usually requires current in a coil |
| On/off control | Not directly switched electrically | Can usually be switched by controlling current |
| Field adjustment and reversal | Limited by design and magnetic circuit | Controlled through current, coil turns, core, and polarity |
| Heat and power considerations | No coil resistance in the magnet itself | Coil resistance generates heat; requires suitable power and thermal design |
| Typical failure modes | Demagnetization, corrosion, fracture, or heat damage | Power loss, coil burnout, insulation failure, or core saturation |
| Typical uses | Motors, speakers, sensors, generators, and latches | Relays, solenoids, cranes, and controllable actuators |
Use an electromagnet when the field must be switched, modulated, or reversed; use a permanent magnet when a persistent field, compactness, and low electrical consumption matter. The distinction is not absolute: permanent-magnet motors use energized windings to create controlled fields that interact with the rotor magnets (The Physics Classroom: The Electromagnet; U.S. Department of Energy: Electric Motors Research and Development).
Main permanent-magnet materials
| Material | Advantages | Limitations | Common uses |
|---|---|---|---|
| Neodymium-iron-boron (NdFeB) | Highest energy density among widely commercialized permanent-magnet families; enables compact, powerful designs | Brittle, corrosion-sensitive without suitable protection, and vulnerable to heat-related performance loss; uses rare-earth elements | Motors, generators, speakers, sensors, and compact magnetic assemblies |
| Samarium-cobalt (SmCo) | Good temperature stability, high coercivity, and better corrosion resistance than NdFeB | Brittle and costly; strongest NdFeB grades have higher energy product | Aerospace, sensors, motors, generators, and high-temperature equipment |
| Ferrite or ceramic | Low cost, corrosion resistance, and no rare-earth elements | Lower energy density than rare-earth magnets; brittle and may need more volume for comparable performance | Speakers, simple motors, magnetic separators, and general-purpose holding |
| Alnico | High remanence and strong temperature capability and stability | Relatively low coercivity makes it more vulnerable to demagnetization in an unfavorable magnetic circuit | Instruments, sensors, pickups, and some motors |
NdFeB: maximum compact strength
Neodymium-iron-boron is widely used when high magnetic performance in a small volume is important. Its grade label, such as N35 or N52, is generally associated with maximum energy product; it is not a universal pull-force rating. Size, shape, air gap, pole geometry, backing steel, magnetization direction, and target surface determine how a particular magnet performs. NdFeB parts often need a protective coating, and their temperature limits vary by grade and assembly (K&J Magnetics FAQ; K&J Magnetics technical downloads and demagnetization curves).
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SmCo: stability in demanding conditions
Samarium-cobalt can suit applications where temperature stability and corrosion resistance matter more than minimum cost. It is still brittle, and its limits depend on the grade and finished assembly. McMaster-Carr lists particular designs rated for temperatures up to 570°F; that product-specific rating is not a general limit for all SmCo magnets (McMaster-Carr: Samarium-Cobalt Magnets).
Ferrite and alnico: different trade-offs
Ferrite is a practical choice when low cost and corrosion resistance outweigh compact size. Alnico is useful where high remanence and temperature stability are valued, provided the magnetic circuit limits demagnetizing fields. Material behavior varies with grade and design (Oak Ridge National Laboratory technical publication; Arnold Magnetic Technologies).
How to read magnet specifications
“Strength” can refer to several different properties. Check what a specification actually measures before comparing products (IEEE Technology Navigator: Permanent Magnets).
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- Coercivity (Hc or Hcj): resistance to demagnetization by an opposing field; it is not the same as attraction strength.
- Maximum energy product ((BH)max): a material-performance measure related to magnetic energy available per unit volume.
- Maximum operating temperature: the grade- and design-specific temperature limit for acceptable performance. It is not the same as the Curie temperature.
- Curie temperature: the temperature above which long-range ferromagnetic or ferrimagnetic order disappears. A magnet can suffer irreversible losses below this temperature.
- Surface field: a field measurement, usually in gauss or tesla, that is meaningful only with its measurement location and conditions.
- Pull force: a force rating measured against a stated target under stated conditions; it does not automatically describe shear, peel, or overhead lifting capacity.
- Magnetization direction: the direction in which poles are arranged—such as through thickness, axial, diametric, or multipole—which changes how the magnet interacts with an assembly.
A larger, lower-grade magnet can outperform a smaller, higher-grade magnet in holding force. A steel-backed pot magnet can also provide more useful force than an exposed magnet of similar material by concentrating magnetic flux in its circuit.
How permanent magnets are made
- Prepare the material: Manufacturers produce an alloy or ceramic compound with a composition suited to the intended magnetic properties.
- Form the part: Depending on material and design, it may be pressed, sintered, cast, or bonded.
- Control its microstructure: Heat treatment, composition, and—in some processes—grain alignment help establish the desired behavior.
- Magnetize it: The finished or near-finished part is exposed to a strong field, often from an industrial pulsed-field or capacitor-discharge magnetizer.
- Finish and inspect: The part may be coated, machined where appropriate, and tested against its specifications.
Many strong sintered magnets are brittle and difficult to machine after magnetization. Specialized processes may be needed; grinding or cutting can damage protective coatings and create hazardous dust. Manufacturing routes vary by material and product (Oak Ridge National Laboratory technical publication; IEEE Technology Navigator).
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How magnets interact with objects
- Like poles repel; unlike poles attract.
- Magnets attract ferromagnetic materials such as iron, nickel, and cobalt. “Magnetic” does not mean “attracted to every metal”: copper, aluminum, brass, and most stainless steels are not attracted in the ordinary permanent-magnet sense, although other electromagnetic effects are possible.
- A steel object can become temporarily magnetized by induction when placed in a magnetic field.
- Magnetic force drops rapidly as the air gap increases. Paint, rust, uneven surfaces, and thin target metal can reduce holding force.
- The advertised pull force commonly assumes direct contact with a clean, flat, sufficiently thick steel target; it is not a universal rating for other setups.
These effects follow from the field and the magnetic circuit, not simply from a magnet “pulling on metal” (Georgia State University HyperPhysics: Magnets and Electromagnets; McMaster-Carr: Magnets).
Where permanent magnets are used
Motors
In many permanent-magnet motors, magnets are mounted on or within the rotor. Current in stator windings creates a controlled magnetic field that interacts with the rotor field to produce torque. Designs include permanent-magnet synchronous motors, interior and surface-mounted permanent-magnet motors, and brushless DC motors. They appear in vehicles, drones, robots, appliances, fans, and pumps. Power density and efficiency depend on the full motor, controller, cooling, and operating point; material cost, thermal limits, and design complexity are important trade-offs (U.S. Department of Energy: Electric Motors Research and Development).
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Moving a magnet relative to a coil changes the magnetic flux through the coil and induces voltage. Permanent-magnet generators can avoid the electrical input needed to energize a field winding. Examples include bicycle dynamos, some small wind turbines, portable generators, and energy-harvesting devices. Mechanical work supplies the energy converted to electricity; the magnet supplies the field, not free power (ARPA-E technical background on permanent magnets).
Speakers and microphones
A speaker’s permanent magnet supplies a static field. An electrical signal in its voice coil interacts with that field, moving the coil and diaphragm to produce sound. A dynamic microphone can use the reverse effect: motion of a coil in a magnetic field produces an electrical signal.
Sensors and switches
A magnet can provide a reference field or trigger for Hall-effect sensors, reed switches, position and speed sensors, magnetic encoders, and door or lid detectors. The sensing element may be electronic or mechanical; the magnet is only one part of the system.
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Latches, fixtures, and magnetic separation
Magnets provide closures for cabinets and doors, hold tools or workpieces, and help position jigs and fixtures. In recycling, mining, food processing, and manufacturing, magnetic separators remove ferrous material from other streams (Bunting Magnetics).
Medical and scientific equipment
Permanent magnets are used in laboratory instruments, magnetic separation, and some medical and scientific systems. MRI systems are not all alike: depending on the design, they may use permanent, resistive, or superconducting magnet technologies.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why magnets lose strength or stop working well
Excessive heat
Heat increases thermal motion and weakens magnetic order. Above the Curie temperature, long-range ferromagnetic or ferrimagnetic order disappears. Irreversible demagnetization can also occur at lower temperatures if the magnet’s operating point exceeds the stability of its grade and magnetic circuit. A device’s safe operating temperature is therefore not determined by its Curie temperature alone.
Opposing magnetic fields
A sufficiently strong field in the opposite direction can move a magnet down its demagnetization curve and cause partial or complete irreversible loss. The risk depends on the material grade and operating point.
Impact, corrosion, and assembly design
Hard magnets may chip or fracture under impact; repeated shock can alter their performance, though one drop does not automatically demagnetize a magnet. Corrosion is particularly important for NdFeB: the part may remain magnetized while swelling, flaking, or losing structural integrity. An excessive air gap, unsuitable pole piece, or unfavorable orientation can also make an undamaged magnet perform poorly. That is a design problem, not necessarily a loss of magnetization.
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How to choose a permanent magnet
Start with the job the magnet must do. For holding applications, distinguish direct pull from shear (sliding), peel, and torque; then evaluate the complete assembly and working conditions rather than selecting by grade label alone.
Choose a material for the conditions
| Consider | Likely starting point | Trade-off to check |
|---|---|---|
| Small size or high field in a controlled-temperature environment | NdFeB | Corrosion protection, heat tolerance, brittleness, and material sourcing |
| High temperature or demanding corrosion conditions | SmCo | Cost, brittleness, and the specific grade’s limits |
| Low cost, corrosion resistance, and room for a larger magnet | Ferrite | Lower energy density and brittleness |
| High remanence and temperature stability in a suitable circuit | Alnico | Lower coercivity and susceptibility to opposing fields |
Check geometry and environment
- Determine the required field, holding force, dimensions, shape, and magnetization direction.
- Measure the air gap and consider the target’s material, thickness, flatness, coating, and pole geometry.
- Allow for the full temperature range and duty cycle, not only room-temperature operation.
- Account for water, salt, chemicals, humidity, vibration, and mechanical shock; specify a suitable coating or encapsulation.
- Decide whether a raw magnet or a steel-backed magnetic assembly better suits the task.
- For a product or engineered system, check grade documentation, drawings, testing, certification, and traceability where needed.
Safety and practical limits
Pinching, fracture, and ingestion
Strong magnets can snap together with enough force to pinch skin or shatter brittle material, creating fragments that can injure eyes. Keep loose powerful magnets away from children. If multiple small magnets are swallowed, they can attract across intestinal tissue and cause severe internal injury; seek urgent medical care if ingestion is suspected (U.S. Consumer Product Safety Commission: Briefing Package on Magnet Sets).
Holding loads safely
A pull rating is conditional, not a safe lifting capacity. McMaster-Carr warns that pull ratings depend strongly on target material and conditions and should not be used for lifting over people (McMaster-Carr: Samarium-Cobalt Magnets). A magnet mounted vertically may slide under a load even if its nominal pull force appears much greater than the load’s weight. Surface friction, coatings, vibration, load direction, and a suitable safety factor all matter. Thin or undersized steel pole pieces can saturate, so adding more steel does not always improve performance.
Devices, coatings, and transport
Strong fields may interfere with magnetic storage, compasses, watches, speakers, sensors, and some implanted or wearable medical devices; follow the device maker’s guidance. Coatings such as nickel, zinc, epoxy, plastic, or rubber affect corrosion protection, thickness, friction, durability, and electrical insulation. Strong magnets may also be subject to air-shipping packaging and transport rules that vary by carrier, route, and jurisdiction; check the applicable requirements rather than assuming one vendor’s guidance is universal (K&J Magnetics FAQ and shipping guidance).
Quick Recap
Sources and further reading
- OpenStax: Ferromagnets and Electromagnets
- IEEE Technology Navigator: Permanent Magnets
- U.S. Department of Energy: Critical Materials Assessment
- U.S. Department of Energy: Neodymium Magnets Supply Chain Report
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