Antiferromagnetism
Antiferromagnetism is a magnetic phenomenon where the magnetic moments of electrons in a material align in opposite directions, effectively canceling each other out. Unlike ferromagnetic materials, which exhibit strong magnetism due to aligned magnetic moments, antiferromagnetic substances do not display overall magnetism under normal conditions. Common examples of antiferromagnetic materials include manganese oxide, which has a specific temperature known as the Néel temperature, above which the antiferromagnetic properties are lost due to increased electron motion. This phenomenon occurs primarily at lower temperatures and is characterized by a precise arrangement of electron spins that alternate in orientation.
Historically, the study of magnetism has deep roots, with significant early contributions from scientists like William Gilbert and Louis Néel. Contemporary research is exploring the applications of antiferromagnetic materials in technology, particularly in data storage and electronics. Antiferromagnets may offer advantages in resisting external magnetic fields, potentially leading to more stable data storage solutions. Moreover, ongoing investigations into synthetic antiferromagnetic nanoparticles and their incorporation into spintronic devices hold promise for advancements in low-power integrated circuits and neuromorphic computing, simulating brain-like functionality.
Antiferromagnetism
Antiferromagnetism occurs when the electrons in a substance form a chain of oppositely charged particles. The material itself does not exhibit magnetism, but the ions act like tiny magnets. It is the opposite of ferromagnetism. Antiferromagnetic materials include metals and alloys such as manganese oxide.
Magnetism is caused by electrons, including their interactions. All matter is magnetic at this level, but the qualities of magnetism are only evident when the magnetism is strong. Antiferromagnetism occurs at lower temperatures. As the temperature of the substance increases, the electrons become agitated and scatter and no longer form chains. The Néel temperature, or Néel point, is the antiferromagnetic Curie point at which this electron movement takes place.
Background
Humans have been interested in magnetism for thousands of years. Ancient Greeks and Chinese scholars knew about the magnetic properties of lodestone. These rare stones were most likely formed when lightning struck iron ore, magnetizing it. For centuries, sailors used lodestones to magnetize needles, which pointed north-south when freely suspended, to allow explorers to navigate the oceans. These early compasses permitted sailors to locate the earth's magnetic pole, which was more reliable as a navigation tool than relying on the stars because a compass could be used in all weather.
The scientific method was applied to studying magnetism beginning in the sixteenth century. English physician William Gilbert published De Magnete ("on the magnet") in 1600. He hypothesized that the planet was itself a huge magnet with magnetic north and south poles. Gilbert's thorough explanation of the body of scientific knowledge about magnetism was widely studied. Gilbert's experiments helped him uncover parallels between magnetic materials and the earth's polarity. His understanding of the universe, however, colored his perceptions. For example, he believed magnetism was earth's soul.
During the nineteenth century, scientists were beginning to understand the physics of electromagnetism. Danish physicist Hans Christian Oersted was teaching a class when he passed an electric current through a wire hanging above a magnetic compass. The compass needle moved, proving the relationship between magnetism and electricity.
A changing magnetic field generates an electric field, and a changing electric field results in a magnetic field. The discovery of electromagnetic waves eventually led to the development of practical applications of these waves, including radio and television waves, x rays, and microwaves. Magnetism that results from the application of the magnetic field is called induced magnetism. Materials such as iron, nickel, and cobalt that can be permanently magnetized are called magnetically hard, while those that can be temporarily magnetized are magnetically soft.
French physicist Louis Néel began researching magnetism about 1928. At that time, only three forms of magnetism were known: diamagnetism, paramagnetism, and ferromagnetism. He discovered ferrimagnetism and antiferromagnetism in 1932. He also discovered the temperature at which the greatest level of magnetism in an antiferromagnetic substance can be produced. The Néel temperature was named for him. Some materials have Néel temperatures at about room temperature, while others have extreme Néel temperatures. Manganese oxide, for example, has a Néel temperature of -240º F (151º C). Above this temperature, electrons are too disorderly to form magnetic groups.
Overview
Magnetism originates at an atomic level. Electrons circle the nuclei of atoms, usually in pairs; one goes up as the other goes down. Their spin generates a small magnetic field, but their opposite motions cancel out each other's magnetic field. This is diamagnetism.
Some metals, such as iron, are magnetic because unpaired electrons have no counter for their magnetic fields. This is paramagnetism. A magnetic field can partially align the magnetic dipole moment, but this magnetization disappears when the field is removed.
With ferromagnetism, the opposite of antiferromagnetism, particles in a substance align in a specific domain, and within that domain, it is magnetic. Ferromagnetism gets its name from iron, a ferromagnetic material. The Latin word for iron is ferrum. Ferromagnetic materials can gain magnetism from an external magnetic field, making these substances excellent for use in electromagnets. As an example, through continued strokes with a lodestone, an iron needle can become magnetic and be used as a compass needle. Antiferromagnetic materials have electrons that do not line up with the same magnetic polarity. The chains of electrons counteract one another.
In antiferromagnetic materials, the electrons that form the sublattice of the molecules line up, but they alternate in opposite orientation. The magnetism in one line cancels out the magnetism in the next opposing line. Some materials exhibit both ferromagnetic and antiferromagnetic qualities. Crystal hematite, for example, is both ferromagnetically coupled within planes and antiferromagnetically coupled between planes.
Modern digital data storage often relies on ferromagnetic materials. Research has been ongoing into using antiferromagnetics for this purpose. Ferromagnetic materials can be manipulated using an external magnetic field. This allows data to be stored on a magnetic strip, such as a credit card, hard disks, and other devices. The information can be lost, however, if a strong magnet affects the material, essentially erasing the data. Antiferromagnetic materials could be developed for data storage use. Because the magnetic points are antiparallel, they are less easily influenced by external magnetic fields. This resistance also makes it more difficult to record data on antiferromagnetic material, which has slowed development of this application.
Researchers have been working to develop synthetic antiferromagnetic nanoparticles (SAF) for biomedical use. They have also been exploring the potential of antiferromagnets in developing ultralow-power integrated circuits. Such devices could store data using magnetization without relying on outside power supplies. Such applications, described as spintronics, take advantage of the flow of electron spin, or spin current. According to Science Daily, researchers at Tohoku University in Japan layered an antiferromagnet and a ferromagnet, then applied an electric current to the bilayer system. The current affects the electron spin and redirects the magnetization in the ferromagnet layer. The research team noted that the process shows similarities to brain synapses, which could lead to further research in neuromorphic computing.
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