Iron can be magnetized due to the fundamental physical property known as magnetism. Iron is the most well-known example of a substance that can exhibit this strong force. Understanding this process requires examining the unique atomic structure of iron and the internal changes that occur when it is exposed to an external magnetic field.
Defining Ferromagnetism: Why Iron is Unique
Iron is classified as a ferromagnetic material, a category shared by only a few elements at room temperature, including nickel and cobalt. This classification is rooted in the atomic structure of iron, specifically the intrinsic magnetic moments of its electrons. Electrons act as tiny magnets due to their spin, but in most elements, these magnetic effects are canceled out because electrons pair up with opposite spins.
Iron atoms possess unpaired electrons in their outer shells, which gives each atom a net magnetic moment. The internal quantum mechanics of iron cause these individual atomic moments to align parallel to each other, creating a powerful internal magnetic field. This spontaneous internal alignment defines ferromagnetism and is why iron, nickel, and cobalt are highly magnetic metals.
The Internal Mechanism of Magnetization
Before magnetization, iron is composed of microscopic regions called magnetic domains. Within each domain, the magnetic moments of the atoms are uniformly aligned in a single direction, but the domains themselves are oriented randomly throughout the material. This random orientation causes their magnetic fields to cancel out, leaving the iron magnetically neutral.
When an external magnetic field is applied, the boundaries of these domains, known as Bloch walls, begin to shift. Domains that are already aligned with the external field grow larger by consuming their neighbors.
As the strength of the external field increases, the remaining magnetic domains rotate until they are all fully aligned in the direction of the applied field. Once all atomic moments are aligned parallel to the external field, the iron is said to be magnetically saturated, achieving its maximum magnetic strength.
Temporary Versus Permanent Magnetism
Whether iron retains its magnetic state depends on its composition and physical structure, leading to the distinction between soft and hard magnetic materials. Soft magnetic materials, such as pure iron, are easily magnetized but possess low coercivity, meaning they quickly lose their magnetic field when the external source is removed. They are commonly used in applications like electromagnets, where the magnetic field needs to be rapidly switched on and off.
Hard magnetic materials, conversely, require a stronger field to become magnetized but exhibit high coercivity, allowing them to retain a strong residual magnetic field, or remanence, after the external field is gone. These materials are typically iron alloys, such as certain types of steel, where other elements introduce defects that “pin” the magnetic domains in place. This pinning effect prevents the domains from easily returning to their random orientation, resulting in a permanent magnet.
A magnetized piece of iron can be demagnetized using methods that disrupt the domain alignment and restore the neutral state. Heating the material above its Curie temperature will cause the intense thermal motion of the atoms to overcome the magnetic forces, making the material lose its ferromagnetism. For pure iron, this temperature is approximately 770°C, above which it transitions to a non-magnetic state. Demagnetization can also be achieved by subjecting the material to a strong, reversing, and gradually decreasing alternating magnetic field.