Definition and Basic Behavior
A permanent magnet maintains a magnetic field after the magnetizing process ends, without a continuous electrical supply. Its behavior depends on how strongly its internal magnetic regions remain aligned and on the conditions around it. The distinction from an energized coil is persistence: a coil’s field depends on current, while a permanent magnet’s field comes from retained magnetization. Magnet Guide is an independent English information guide, not a shop, business, service provider, school, or organisation.
Retained magnetization
A permanent magnet is defined by its ability to retain magnetization without continuous electrical current. Once magnetized, it produces a magnetic field in the space around it even when it is disconnected from a power source. This differs from an energized coil, whose field is generated by electric current and falls when that current is switched off, except for small residual effects in its core.
At the microscopic level, magnetic moments in a ferromagnetic material form regions called domains. The magnetizing process aligns many of these domains in a preferred direction. In a permanent magnet, the material’s internal structure resists their return to a random arrangement, allowing a useful field to persist. The degree of retained alignment is described by magnetic remanence: the magnetization remaining after the external magnetizing field has been removed.
Retained magnetization does not mean that the field is identical at every point or that the magnet is immune to change. Its external magnetic field strength varies with position and the magnet’s geometry, while its remanence describes a material property. A magnet can therefore have a persistent field without drawing power, yet still respond to a sufficiently strong opposing influence. This stored, stable alignment is the basis of how permanent magnets work in devices that need a magnetic field without a continuously powered coil.
Factors that change magnetization
Magnetization can change when a material is heated, exposed to an opposing magnetic field, or subjected to physical damage. The size of the effect depends on the material’s internal structure and grade; permanent magnets do not share a single temperature or field limit. A magnet’s specified operating range is therefore more useful than assuming that all materials tolerate the same conditions.
Heat increases atomic motion and can weaken the alignment that supports the magnetic field. A moderate temperature rise may cause a reversible reduction in output, with much of the field returning after cooling. If the magnet exceeds its grade-specific operating limit, some loss can be permanent; near the Curie temperature, spontaneous ferromagnetic order disappears. Curie temperatures vary substantially, from roughly 310°C for some neodymium-iron-boron compositions to around 800°C for some samarium-cobalt compositions, so one value cannot describe every magnet.
An opposing field can also reduce retained magnetization when its strength approaches or exceeds the material’s coercivity. This resistance to demagnetization differs among materials and grades. Impact or bending may crack a brittle magnet or alter its magnetic circuit, changing the field even if the material has not been heated. A crack does not automatically erase magnetization, but substantial damage can interrupt the useful field pattern. The practical limits are set by the particular magnet’s composition, construction, and operating conditions.
Common Permanent Magnet Materials
Permanent magnet materials differ in the magnetic output they can provide, their response to heat, and their resistance to corrosion. Neodymium-iron-boron, ferrite, and alnico are established material families, each suited to different design priorities. Grade and manufacturing choices can shift performance within a family, so broad comparisons are starting points rather than guarantees for a specific magnet. The contrasts below focus on typical material behavior and common compact-magnet uses.
Neodymium-iron-boron
Neodymium-iron-boron (NdFeB) is a rare-earth magnet alloy used when a strong magnetic field is needed from a compact part. Its high magnetic energy product makes neodymium magnets common in small motors, actuators, headphones, and other devices where space and mass are limited. For a given volume, an NdFeB magnet can generally provide more magnetic output than ferrite or alnico, although the actual result depends on the grade, shape, and magnetic circuit.
Temperature capability varies substantially by grade. Common grades have maximum operating temperatures broadly around 80–200 °C; specialized grades can extend beyond that range, but a magnet’s rating should be checked for the intended operating conditions. Heat can reduce useful performance, and exposure near or above a grade’s limit can cause lasting loss. The Curie temperature is not the same as a recommended operating limit: it marks the point at which a material loses its ferromagnetic order, not a safe design temperature.
Corrosion is a key consideration because the iron-rich alloy can oxidize in humid or wet environments. Many NdFeB parts therefore use protective finishes such as nickel-copper-nickel plating, zinc, or epoxy, with the appropriate finish depending on exposure and the part’s design. A coating can be damaged at edges or during handling, so the base alloy should not be treated as inherently corrosion resistant.
These properties make NdFeB useful where strong output and small size outweigh higher material cost and the need to account for heat and surface protection. In compact mechanisms, it can enable a smaller magnet or a more powerful field than lower-strength alternatives, while grade selection determines how much temperature capability is available.
Ferrite and alnico
Ferrite and alnico offer different trade-offs from NdFeB. Ferrite is a ceramic material valued for low cost and good resistance to corrosion, while alnico is a metal alloy noted for temperature stability and useful magnetic output. The comparison is relative: specific grades and magnetic circuits affect performance, and a material’s temperature rating is not a substitute for checking the component’s own specification.
| Material | General strength | Relative cost | Typical maximum operating temperature | Corrosion resistance |
|---|---|---|---|---|
| Neodymium-iron-boron | Very high for its size | Generally higher | About 80–200 °C for many grades; grade dependent | Low without a protective coating |
| Ferrite | Lower than NdFeB in comparable compact designs | Generally low | Often about 200–300 °C; grade dependent | High; ceramic material resists rust |
| Alnico | Moderate to high, depending on grade and design | Generally moderate to high | Often about 450–550 °C; grade dependent | Good in many environments |
Ferrite magnets are a practical choice when low material cost, corrosion resistance, or operation at elevated temperatures matters more than achieving the strongest field in a small volume. Their lower output for a given compact size can mean that a design needs a larger magnet or a different magnetic circuit. Ferrite is also electrically resistive, a useful characteristic in some alternating-field applications.
Alnico is selected where high-temperature stability and reliable magnetic performance over a broad temperature range are priorities. Its relatively low coercivity compared with NdFeB means the surrounding magnetic circuit and operating conditions matter; alnico is not simply a drop-in replacement based on temperature rating alone. Its alloying elements, including aluminum, nickel, and cobalt, also affect its relative cost and availability.
For compact, high-output designs, NdFeB is often favored; ferrite suits cost-sensitive or corrosion-prone settings, while alnico is compelling when temperature stability is central. Comparing the required field, operating temperature, surface exposure, and total part size identifies which trade-off best fits the application.
Permanent Magnets and Electromagnets
Permanent magnets and electromagnets can both create a magnetic field, but they produce it in different ways. One relies on magnetization within a material; the other relies on current flowing through a coil. That difference affects how a field can be adjusted, switched off, and maintained. It also helps explain why each design suits particular tasks, from holding parts to lifting loads.
Sources of the magnetic field
In a permanent magnet, the field comes from the material’s net magnetization: many microscopic magnetic moments, mainly associated with electron spin, are aligned so their effects add together. The magnetized material produces a field outside itself without a continuous supply of electrical current. Its field is therefore not generated by current passing through an external coil, even though the underlying magnetic interactions are electromagnetic.
An electromagnet produces its working field when electric current flows through a coil. Each turn contributes to the field, and the contributions reinforce one another along the coil’s axis. For a long ideal solenoid, the field is approximately proportional to the number of turns per unit length and the current; a ferromagnetic core can concentrate and strengthen it. In practical devices, core saturation limits how much the field increases as current rises.
The distinction is the immediate physical source: material magnetization in the first case, and moving electric charge in a wound conductor in the second. An iron core inside an electromagnet may itself become magnetized while current flows, but the coil current supplies the controllable field that drives that magnetization. With the current removed, the coil’s field disappears; a soft-iron core generally retains much less magnetization than a permanent magnet, although a small residual field can remain.
Control and typical uses
Current gives an electromagnet a direct control input: changing the current changes the coil’s field, while interrupting the supply removes its active field. A permanent magnet needs no continuous electrical input, but its field is not adjusted by turning a current knob. The table compares these operating differences and common uses.
| Comparison | Permanent magnet | Electromagnet |
|---|---|---|
| Current control | No coil current is required; continuous supply current is 0 A. | Field strength responds to coil current; below core saturation, it is approximately proportional to ampere-turns, N × I. |
| Switching and variation | No electrical on/off switch controls the field during ordinary operation; field adjustment requires a change to the magnetic circuit or magnetization. | Reducing current to 0 A removes the coil-generated field; a current controller can vary its strength. |
| Electrical power | Maintains its field without continuous electrical power. | Requires electrical power while the coil is energized; coil resistance also produces heat. |
| Typical uses | Used for holding fixtures and in permanent-magnet motors and generators. | Used in lifting systems that need release on command, wound-field motors and generators, and electrically operated switches. |
For a fixed holding force without a powered coil, a permanent magnet avoids continuous power use. Where a machine must vary the field or release a load electrically, an electromagnet provides more direct control; in either case, the required behavior determines the suitable arrangement.
Shapes and Applications
A magnet’s shape affects how it can be mounted and where its field is concentrated. Bar, disc, ring, and block forms are available with different dimensions and magnetization directions, so similar-looking parts may produce different field patterns. The intended air gap, nearby components, and mounting method all influence how a form works in a device. The examples below connect common geometries with practical uses without treating one shape as universally suitable.
Common magnet forms
Bar magnets have a long rectangular profile, with poles commonly located at opposite ends when magnetized along their length. This orientation creates a field that extends from one end to the other, while transverse magnetization places the poles on opposing broad faces. A bar can be held in a slot, clamped along a face, or fixed at an end; the chosen method should leave the required pole surfaces accessible. Long bars suit layouts that need an extended magnetic path, whereas shorter bars fit compact assemblies.
Disc magnets are round and relatively flat. Many are magnetized axially, with north and south poles on the two circular faces, making them straightforward to seat in a shallow counterbore or bond to a flat surface. A diametrically magnetized disc has poles on opposite sides of its curved edge instead, which can suit rotating or position-sensing arrangements. A disc’s diameter and thickness affect its field distribution, so its outline alone does not specify how the field will emerge.
Ring magnets have a central opening for a shaft, fastener, or other component. Axial magnetization places opposite poles on the two annular faces; radial or diametric arrangements can place poles around the circumference, depending on how the ring is manufactured. Blocks provide broad, flat faces that can be bonded, clamped, or located in a machined pocket. They are often magnetized through their thickness or across a face. For all four forms, orientation marks or a field check help confirm pole placement during assembly.
Applications by shape and field
Shape and pole orientation help define the field available to a component, but the working gap, motion, and mounting space determine whether a particular form is suitable. Examples of permanent magnet applications include:
- A small axially magnetized disc can face a Hall-effect sensor across a narrow air gap, providing a changing field as a shaft or moving part passes.
- A ring magnet mounted around a motor rotor can provide a circumferential pole pattern; the required pole count and air gap depend on the motor design.
- In a loudspeaker, a ring-shaped magnetic assembly can surround the voice-coil region, leaving central space for the pole piece and moving coil.
- A bar magnet can operate a magnetic reed switch when a door or cover closes, with orientation and spacing set to bring the switch into its operating field.
- Blocks placed in a steel-backed fixture can hold small ferrous parts during assembly, with contact area and pole arrangement affecting the holding pattern.
These examples are starting points, not fixed pairings: a different form may work when it provides the necessary field at the working location and fits the assembly. For instance, sensor response depends on the field direction at the sensing element, while a closure depends on the magnetic path across its mating surfaces.
Choosing a Permanent Magnet
Choosing a permanent magnet means matching its properties to the assembly, not selecting a material by name alone. The required field depends on the magnet’s dimensions, placement, and the magnetic circuit around it. Temperature, moisture, mounting space, and nearby components can also change which grade works reliably. Compare the complete design conditions with the manufacturer’s ratings before specifying a part.
Selection factors
Start with the field and mechanical requirements at the location where the magnet will operate. These selection factors interact: a change in dimensions or mounting can alter the field delivered by the same grade.
- Material: Compare grade-level remanence, coercivity, temperature stability, and corrosion resistance; a compact rare-earth magnet may meet a tight field target, while another composition suits heat better.
- Shape: Choose a form that fits the magnetic circuit and assembly; a ring can surround a shaft, while a block offers a broad pole face.
- Dimensions: Specify length, width, thickness, and tolerances together; increasing magnet thickness can change the operating point, while added width may not improve field at every location.
- Operating environment: Check the actual peak and continuous temperatures, humidity, chemicals, and vibration; grade limits and coating durability differ, so the environment can rule out otherwise suitable materials.
- Mounting: Account for adhesive thickness, fasteners, retainers, and contact surfaces; even a thin nonmagnetic gap increases separation, and a steel holder can redirect flux.
- Required field: Define the target at a precise position and distance, in tesla or gauss, and state whether the assembly needs a uniform field or a steep gradient.
For a compact sensor, field strength at the sensing element may matter more than the magnet’s surface value; for a latch, force across the installed gap is the relevant output. The permanent magnet materials should therefore be compared in the intended assembly, with the required field and mechanical fit assessed together.
Checking ratings for the design
A catalog rating describes a magnet under defined conditions, not every assembly in which that part might be used. Confirm whether the listed value is remanence, maximum energy product, pull force, or field measured at a stated distance; these quantities are not interchangeable. For example, a pull-force figure measured against a thick, flat steel plate does not predict the force across a coated surface or a working air gap.
Geometry changes the magnetic circuit and the magnet’s operating point. A short, wide block and a long, slender bar made from the same grade can produce different field distributions. Check the stated dimensions and tolerances, the direction of magnetization, and the orientation of the poles relative to the target. If the design uses several magnets, their spacing and polarity also affect the combined field.
Compare the design’s continuous and peak temperatures with the rating for the exact grade; permanent magnet properties and temperature limits vary by material and grade. Include the air gap, coatings, adhesive, steel returns, and nearby magnets or ferromagnetic parts in a calculation or prototype measurement. A field probe should be positioned at the working point, with its sensing axis aligned to the expected field direction.
For a reliable choice, use the manufacturer’s grade-specific data and validate the assembled geometry at its worst operating conditions. Select the smallest grade and size that meet the measured field or force requirement with the design’s actual gap and temperature margin.