A permanent magnet is a material that generates and retains its own persistent magnetic field after the external magnetizing force is removed. This stable force distinguishes them from temporary magnets, like electromagnets, which only exhibit magnetism when an electric current is flowing. Permanent magnets are made from magnetically hard materials. These materials are difficult to magnetize but maintain their internal alignment once the process is complete. This ability to sustain a magnetic field makes them indispensable components in electric motors, speakers, and various electronic devices.
Choosing the Right Materials
Creating a permanent magnet begins with selecting a magnetically hard material, which possesses a high coercivity, meaning it strongly resists demagnetization. The most powerful magnets today are the rare-earth types, specifically Neodymium-Iron-Boron (NdFeB) and Samarium Cobalt (SmCo), which offer an exceptional strength-to-weight ratio. Neodymium magnets are the strongest commercially available, but they are prone to corrosion and have a lower maximum operating temperature. Samarium cobalt magnets are slightly less powerful but offer superior resistance to demagnetization from heat and corrosion.
Another common type is the Ceramic or Ferrite magnet, composed of iron oxide and strontium carbonate. These magnets are much cheaper and highly resistant to corrosion. While not as strong as the rare-earth types, their low cost and high coercivity make them popular for many general applications. Alnico magnets, an alloy primarily of aluminum, nickel, and cobalt, offer excellent temperature stability, maintaining a constant field over a wide temperature range. The specific application determines the material choice, balancing strength, cost, and temperature requirements.
The Inner Workings of Magnetism
The ability of a material to become a permanent magnet is determined at the atomic level, specifically by internal regions called magnetic domains. Within ferromagnetic materials, the magnetic moments of individual atoms spontaneously align themselves in the same direction. This creates small, uniformly magnetized regions, or domains, each acting like a tiny individual magnet. In an unmagnetized state, these domains are oriented randomly throughout the material, meaning their magnetic fields cancel one another out, resulting in no net external magnetism.
The magnetization process involves forcing these randomly oriented domains to align along a common direction. When a strong external magnetic field is applied, the domain walls shift, increasing the size of domains aligned with the external field. If the external field is strong enough, all the domains will rotate and become fully aligned, reaching a state known as magnetic saturation. In a magnetically hard material, this alignment is locked in by the material’s crystalline structure, allowing the bulk material to retain a strong residual magnetic field after the external force is removed.
Practical Magnetization Methods
The most common method for manufacturing permanent magnets involves placing the material into a coil and applying a powerful external magnetic field, a process known as electromagnetic magnetization. This is typically achieved using a magnetizer, which discharges a short, high-energy pulse of current through the coil to create a momentary, intense magnetic field. For materials with very high coercivity, like Neodymium, a pulse magnetizer is necessary to generate the required strong field to achieve full domain alignment. Less coercitive materials, such as ferrite magnets, can be magnetized using a constant direct current to generate the field.
Another technique, often used during the manufacturing process, is induction. For certain alloys, like some Alnico grades, a strong external magnetic field is applied while the material is cooled from a high temperature. The material is heated above its Curie temperature, the point where it loses all magnetic properties, and then cooled in the presence of the external field. This cooling process forces the magnetic domains to solidify in alignment with the applied field, permanently setting the preferred direction of magnetization.
Maintaining and Demagnetizing Permanent Magnets
Once a permanent magnet is created, it will maintain its strength indefinitely under normal conditions, but certain environmental factors can cause a loss of magnetism. The primary threat to a magnet’s strength is exposure to excessive heat, which increases the atomic movement within the material. When a magnet’s temperature approaches its Curie point, the thermal energy can overcome the forces holding the magnetic domains in alignment, leading to a significant and often irreversible loss of magnetic performance.
Another major cause of demagnetization is exposure to a strong opposing magnetic field, which can force the domains to realign in the reverse direction. This demagnetizing force can come from nearby magnets or from external electrical equipment. Severe mechanical shock or impact can also physically jostle the domains out of their locked alignment, resulting in a reduction of the magnet’s overall strength. To intentionally demagnetize a magnet, one can heat it above its Curie temperature or expose it to a continuously reversing, alternating magnetic field that is slowly reduced to zero, which scrambles the domain alignment.