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Ferromagnetic materials, such as iron, exhibit a natural magnetic field and maintain a strong, long-lasting, uniform magnetic field when exposed to an external magnetic source. Remanence and the Curie temperature limit their use. Advanced research includes liquid mirrors and ferromagnetism in superconductivity and semiconductors.
Ferromagnetic materials are usually based on the element iron and represent one of three naturally occurring types of magnetism, distinct from diamagnetism and paramagnetism. The primary characteristics of ferromagnets are that they exhibit a natural magnetic field in the absence of this first imposed on the substance by an external magnetic field source, and the field is, for all intents and purposes, permanent. Diamagnetic materials, in contrast, exhibit a weak induced magnetic field that is directly opposite to that present in iron. Paramagnetic materials include the metals aluminum and platinum, which can be induced to have even 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 can also 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 the spins of the electrons are aligned in parallel with each other, known as domains. These domains are strongly magnetic, but scattered randomly throughout the bulk of a material itself, giving it an overall weak or neutral natural magnetism. By taking these natural magnetic fields and exposing them to an external magnetic source, the domains themselves will align and the material will maintain a strong, long-lasting, uniform magnetic field. This increase in the general magnetism of a substance is known as its relative permeability. The ability of iron and rare earths to maintain 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 held at a fraction of the original strength over time. This is known as remainder. Remanence is important in calculating the strength of ferromagnetism-based permanent magnets, 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 change to those of a paramagnet. Curie’s law of paramagnetic susceptibility uses the Langevin function to calculate the change in ferromagnetic to paramagnetic properties in known material compositions. The transition from one state to another follows a predictable, increasing curve that has a parabolic shape as the temperature increases. This tendency of ferromagnetism to weaken and eventually disappear with increasing temperature is known as thermal agitation.
The electrical hum heard in a transformer with no moving parts is due to its use of a ferromagnet and is known as magnetostriction. This is a response of the ferromagnet to the induced magnetic field created by the electric 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 usually alternates in 60 hertz cycles, or 60 times per second.
Advanced research using ferromagnet properties has several interesting potential applications. In astronomy, a ferromagnetic liquid is engineered 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 shape of the mirror could also be changed by activating the magnetic field actuators in one-kilohertz cycles.
Ferromagnetism was also discovered in concert with superconductivity in ongoing research conducted in 2011. A compound of nickel and bismuth, Bi3Ni, engineered to the nanoscale, or one-billionth of a meter, exhibits different properties than the same compound in larger samples. big . Material properties on this scale are unique, as ferromagnetism usually nullifies superconductivity and its potential uses are still being explored.
German research on semiconductors built on a ferromagnet involves the gallium manganese arsenic compound, GaMnAs. This compound is known to have the highest Curie temperature of any ferromagnet semiconductor, at 212° Fahrenheit (100° Celsius). Such compounds are being researched as a means of dynamically tuning the electrical conductivity of superconductors.
