Ferromagnetic materials are usually based on the element iron and represent one of the three types of magnetism found in nature, distinct from diamagnetism and paramagnetism. The primary features of ferromagnets are that they exhibit a natural magnetic field in the absence of this first being imposed on the substance by an external magnetic field source, and the field is, for all intents and purposes, permanent. Diamagnetic materials, by contrast, display a weak, induced magnetic field that is directly opposite of the one present in iron. Paramagnetic materials include aluminum and platinum metals, which can be induced to also have a slight magnetic field, but quickly lose the effect when the inducing field is removed.
The most common material in nature that exhibits ferromagnetic properties is iron, and this quality has been known for over 2,000 years. Other rare earths also can exhibit ferromagnetism, such as gadolinium and dysprosium. Metals that act as ferromagnetic alloys include cobalt alloyed with samariam or neodymium.
The magnetic field in a ferromagnet is centered in atomic regions where electron spins are aligned in parallel with each other, known as domains. These domains are strongly magnetic, yet randomly scattered throughout the bulk of a material itself, which gives it an overall weak or neutral natural magnetism. By taking such naturally magnetic fields and exposing them to an external magnetic source, the domains themselves will align and the material will retain a uniform, strong and enduring magnetic field. This increase in the general magnetism of a substance is known as relative permeability. The ability of iron and rare earths to retain this alignment of domains and general magnetism is known as hysteresis.
While a ferromagnet retains its field when the inducing magnetic field is removed, it is only retained at a fraction of the original strength over time. This is known as remanence. Remanence is important in calculating the strength of permanent magnets based on ferromagnetism, where they are used in industrial and consumer devices.
Another limitation of all ferromagnet devices is that the property of magnetism is completely lost at a certain temperature range known as the Curie temperature. When the Curie temperature is exceeded for a ferromagnet, its properties switch to that of a paramagnet. The Curie law of paramagnetic susceptibility uses the Langevin function to calculate the change in ferromagnetic to paramagnetic properties in known material compositions. The change from one state to another follows a predictable, rising, parabolic-shaped curve as temperature increases. This tendency for ferromagnetism to weaken and eventually disappear as temperature increases is known as thermal agitation.
The electrical hum heard in a transformer with no moving parts is due to its utilization of a ferromagnet, and is known as magnetostriction. This is a response by the ferromagnet to the induced magnetic field created by electrical current fed to the transformer. This induced magnetic field causes the natural magnetic field of the substance to change direction slightly to align with the applied field. It is a mechanical response in the transformer to alternating current (AC), which alternates usually in 60 hertz cycles, or 60 times per second.
Advanced research using ferromagnet properties has several exciting potential applications. In astronomy, a ferromagnetic liquid is being designed as a form of liquid mirror that could be smoother than glass mirrors and created at a fraction of the cost for telescopes and space probes. The mirror shape could also be changed by cycling magnetic field actuators at one kilohertz cycles.
Ferromagnetism has also been discovered in concert with superconductivity in ongoing research conducted in 2011. A nickel and bismuth compound, Bi3Ni, engineered at the nanometer scale, or one-billionth of a meter, exhibits properties different from that of the same compound in larger samples. Material properties at this scale are unique, as ferromagnetism usually cancels out superconductivity, and its potential uses are still being explored.
German research into semiconductors built upon a ferromagnet involve the compound gallium manganese arsenic, GaMnAs. This compound is known to have the highest Curie temperature of any ferromagnet semiconductor, of 212° Fahrenheit (100° Celsius). Such compounds are being researched as a means of dynamically tuning the electrical conductivity of superconductors.