Magnetism is a fundamental physical phenomenon governing the interaction between moving electrical charges, resulting in attractive or repulsive forces. Although all matter contains charged particles, only a small fraction of materials exhibits a strong response to a magnetic field, like the common attraction to a refrigerator magnet. How a material is affected by a magnetic field depends entirely on its internal atomic structure and the collective behavior of its electrons.
The Atomic Basis of Magnetism
A material’s magnetic properties originate within the electron, a subatomic particle possessing electric charge and spin. This spin generates a small magnetic field, making each electron a microscopic magnet. In most atoms, electrons exist in pairs with opposite spins, causing their individual magnetic fields to cancel out. Only atoms containing unpaired electrons possess a net magnetic moment, meaning the atom itself acts as a tiny magnet.
In materials with these atomic magnetic moments, their behavior is further determined by how these tiny magnets interact. In materials that can be strongly magnetized, groups of millions of atoms spontaneously align their magnetic moments into regions called domains. When the material is not magnetized, these domains point randomly, canceling out their collective fields and resulting in no net external magnetism. Applying an external magnetic field causes the domains to shift and rotate, aligning their moments with the external field.
Classifying Magnetic Material Behavior
Materials are categorized into three main groups based on how they interact with an external magnetic field.
Ferromagnetism
Ferromagnetism is the most familiar category, including elements like iron, nickel, cobalt, and gadolinium. These materials exhibit a strong attraction because internal forces cause their magnetic domains to align parallel to the external field. Ferromagnetic materials can retain their magnetization after the external field is removed, making permanent magnets possible.
Paramagnetism
Paramagnetism is observed in materials such as aluminum, platinum, and oxygen. Paramagnetic materials are weakly attracted to a magnetic field, but this effect is far less pronounced than ferromagnetism. Their magnetic moments align with the external field only while it is present, and this alignment is disrupted by thermal energy. Once the external field is withdrawn, the atomic magnetic moments return to a random, non-magnetic state.
Diamagnetism
Diamagnetism is a property all substances possess, though it is usually masked by stronger magnetic effects. Diamagnetic materials, including water, copper, gold, and most organic compounds, are weakly repelled by a magnetic field. This repulsion occurs because the external field slightly alters the electrons’ orbital motion, inducing a tiny magnetic moment that opposes the applied field. Since diamagnetism is caused by paired electrons, it is always a weak effect.
Permanent and Temporary Magnets
The distinction between materials used for stable magnets and those used for temporary magnetization is determined by coercivity.
Hard Magnetic Materials (Permanent Magnets)
Materials with high coercivity are known as hard magnetic materials. They resist demagnetization and are used to create permanent magnets. These materials require a strong reverse magnetic field to destroy their domain alignment after magnetization. Modern permanent magnets are typically made from rare-earth alloys, such as Neodymium-Iron-Boron, which maintain a strong magnetic field over long periods due to their exceptionally high coercivity.
Soft Magnetic Materials (Temporary Magnets)
In contrast, soft magnetic materials have low coercivity and are easily magnetized and demagnetized. These materials, including pure iron and silicon steel, are used in devices like electromagnets, transformers, and electric motors. Since the magnetic field must be quickly and repeatedly reversed in these applications, the ease with which soft magnets align and disalign their domains minimizes energy loss. They are highly responsive to an external current, making them suitable for temporary or controlled magnetic fields.
The Curie Temperature
All ferromagnetic materials are subject to the Curie temperature, the specific temperature at which they lose their spontaneous magnetic properties. Above this point, increased thermal energy overcomes the atomic forces that keep the magnetic moments aligned in domains. When heated past its Curie temperature, a ferromagnetic material transitions into a paramagnetic material, losing its ability to retain magnetization. This temperature varies widely; for instance, the Curie temperature for iron is approximately 770°C, while for Neodymium magnets, it is significantly lower.