The difference between magnetic and non-magnetic objects lies in the organization of matter at the atomic level. While some materials, such as iron, nickel, and cobalt, exhibit a powerful attraction to a magnet, many others, including wood, glass, and copper, appear entirely unaffected. This disparity stems from how a material’s internal structure dictates its response to a magnetic field. Understanding what makes an object magnetic begins with examining the microscopic forces at play within every atom.
The Atomic Source of Magnetic Force
Magnetism is an intrinsic property that originates from the movement of electrons within an atom. Every electron possesses “spin,” which generates a tiny magnetic moment, effectively turning each electron into a miniature magnet.
In many atoms, electrons exist in pairs, with one spinning in an opposite direction to its partner. When electrons are paired, their magnetic moments cancel each other out, resulting in an atom with zero net magnetic field.
Strongly magnetic materials contain unpaired electrons. Since these electrons lack a partner to neutralize their magnetic moment, the atom retains a net magnetic field. These unpaired electrons are the reason elements like iron and cobalt have the potential to interact strongly with external magnets.
The Importance of Internal Magnetic Domains
The presence of a net magnetic moment in an atom is necessary but not sufficient for an object to be strongly magnetic; the moments must be organized. In strongly magnetic materials, known as ferromagnets, the atomic magnetic moments spontaneously align over large, microscopic regions called magnetic domains. Within each domain, all the magnetic moments point in the same direction, creating a concentrated local magnetic field.
In an unmagnetized piece of iron, these magnetic domains are oriented randomly throughout the material. This random arrangement means the collective magnetic fields of all the domains cancel each other out, so the object has no overall external magnetism. The material remains neutral despite the powerful magnetic forces confined within its structure.
When this material is brought near an external magnet, the domain structure shifts in a process called magnetization. Domains aligned with the external field grow larger, while domains opposing the field shrink. The boundaries between domains move, and the magnetic moments within the domains rotate to align with the external field.
For a temporary magnet, such as soft iron, the domain alignment is easily achieved but is not permanent. Once the external magnetic field is removed, the domains quickly return to a random, unaligned state, and the material loses its magnetism. Conversely, permanent magnets are made from “hard” ferromagnetic materials where the domain alignment is locked into place, maintaining a strong external magnetic field.
Non-magnetic materials do not form these large, spontaneously aligned domains. Their atomic moments are either nonexistent, due to all paired electrons, or too weak to overcome thermal agitation and form cohesive magnetic regions. This lack of cooperative alignment prevents them from exhibiting the powerful magnetic attraction.
Classifying Material Responses to Magnetic Fields
Materials are classified based on their specific reaction when placed into an external magnetic field. Ferromagnetic materials are recognized for their intense attraction to magnets and their ability to retain magnetization, as their domains align readily and strongly. Iron, nickel, and cobalt are the most common examples of this group.
Paramagnetic materials are weakly attracted to a magnetic field, but they do not retain any magnetization once the field is removed. These materials, such as aluminum and platinum, possess unpaired electrons, but the atomic magnetic moments only align temporarily in the presence of an external field, lacking the cooperative domain structure of ferromagnets.
Diamagnetic materials are weakly repelled by a magnetic field. This slight repulsion occurs in all materials, including water, copper, and carbon, and is the only magnetic response in substances where all electrons are paired. This effect arises from a temporary shift in the electron orbits that generates a magnetic field opposing the external one.