A magnet is any material or object that produces its own magnetic field, an invisible force that attracts certain substances. Every magnet possesses North and South poles, where the magnetic force is strongest. Opposite poles attract each other, while similar poles repel. Magnets are categorized by how their field is generated, their material composition, and how they interact with an external field.
Permanent and Electromagnets
The most practical distinction separates magnets into permanent magnets and electromagnets. Permanent magnets, often called “hard” magnets, maintain their magnetic field without any external influence or power source once they are initially magnetized. The internal structure of these materials keeps their microscopic magnetic regions, called domains, aligned indefinitely, allowing them to retain their magnetic properties.
Electromagnets, conversely, are considered “soft” magnets because their field is temporary and requires a continuous flow of electricity. They are typically constructed by wrapping a conductive wire into a coil around a core made of a ferromagnetic material, such as soft iron. The magnetic field is generated only when the electric current passes through the wire, temporarily aligning the core’s magnetic domains.
Changing the amount of electric current or the number of turns in the wire coil controls the strength of the magnetic attraction. This ability to turn the field on and off and adjust its strength makes electromagnets invaluable for industrial applications like scrap metal lifting and medical devices. In contrast, the fixed nature of a permanent magnet makes it suitable for devices that require a consistent, reliable magnetic pull.
Major Permanent Magnet Compositions
Permanent magnets fall into three material categories defined by their chemical makeup. Neodymium magnets (NdFeB) are alloys of Neodymium, Iron, and Boron, making them the strongest magnets available today. Their high magnetic strength makes them preferred components in high-performance applications like computer hard drives, wind turbine generators, and audio equipment, though they are prone to corrosion.
Ferrite magnets, also known as ceramic magnets, are composed primarily of iron oxide and strontium carbonate. These magnets offer a significantly lower magnetic field strength compared to Neodymium but are more cost-effective to produce. Ferrite magnets are utilized in mass-market items such as refrigerator seals, small electric motors, and loudspeakers due to their resistance to demagnetization and corrosion.
Alnico magnets are alloys that include Aluminum, Nickel, and Cobalt. While not as powerful as Neodymium at standard room temperature, Alnico alloys are valued for their ability to maintain magnetic stability at high operational temperatures. This superior heat resistance makes them suitable for use in measuring instruments, sensors, and aerospace applications.
How Materials Respond to Magnetism
Materials are classified based on how their atomic structure responds to an external magnetic field. Ferromagnetic materials, which include substances like iron, nickel, and cobalt, show a strong attraction to magnetic fields because their electron spins align easily. These materials are the only ones capable of being permanently magnetized and retain their field after the external source is removed.
Paramagnetic materials, such as aluminum and platinum, are only weakly attracted to an external field, and their magnetic properties disappear when the field is removed. Their electron spins align only partially and randomly in the presence of the field. Finally, diamagnetic materials, which include copper, gold, and even water, are characterized by a weak repulsion from a magnetic field. This slight opposing force occurs because the external field causes a minor misalignment in the material’s orbiting electrons.