The magnetic properties of a material are a class of physical phenomena mediated by fields. Electric currents and magnetic moments of elementary particles generate a field that acts on other currents. The most familiar effects occur in ferromagnetic materials, which are strongly attracted by magnetic fields and can be magnetized, turning into constant ones, creating the charged fields themselves.
Only a few substances are ferromagnetic. To determine the level of development of this phenomenon in a particular substance, there is a classification of materials by magnetic properties. The most common are iron, nickel and cobalt and their alloys. The prefix ferro refers to iron, because permanent magnetism was first observed in empty form of natural iron ore, called the magnetic properties of the material, Fe3O4.
Paramagnetic materials
Although ferromagnetism is responsible for most of the effects of magnetism found in everyday life, all other materials are somewhat affected by the field, as well as some other types of magnetism. Paramagnetic substances, such as aluminum and oxygen, are weakly attracted to the applied magnetic field. Diamagnetic substances, such as copper and carbon, are weakly repelled.
While antiferromagnetic materials, such as chromium and spin glasses, have a more complex relationship with the magnetic field. The strength of a magnet on paramagnetic, diamagnetic and antiferromagnetic materials is usually too weak to be felt, and it can only be detected by laboratory devices, so these substances are not included in the list of materials with magnetic properties.
Conditions
The magnetic state (or phase) of a material depends on temperature and other variables, such as pressure and applied magnetic field. A material can exhibit more than one form of magnetism when these variables change.
History
The magnetic properties of the material were first discovered in the ancient world when people noticed that magnets, naturally magnetized pieces of minerals, can attract iron. The word "magnet" comes from the Greek term μαγνῆτις λίθος magnētis lithos, "magnesian stone, foot stone."
In ancient Greece, Aristotle attributed the first of what can be called a scientific discussion about the magnetic properties of materials to the philosopher Thales of Miletus, who lived from 625 BC. e. until 545 BC e. The ancient Indian medical text “Sushruta Samhita” describes the use of magnetite to remove arrows embedded in the human body.
Ancient China
In ancient China, the earliest literary reference to the electrical and magnetic properties of materials is contained in a book of the 4th century BC, named after its author, “The Sage of the Ghost Valley”. The earliest mention of needle pulling is in the 1st century Lunheng (Balanced Requests): “The magnet pulls the needle.”
An 11th-century Chinese scientist, Shen Kuo, was the first person to describe - in The Dream Pool Essay - a magnetic compass with a needle and that it improved navigation accuracy using astronomical methods. The concept of true north. By the 12th century, the Chinese were known to use a compass magnet for navigation. They fashioned a guide spoon out of stone so that the handle of the spoon always pointed south.
Middle Ages
Alexander Neckam, by 1187, was the first in Europe to describe the compass and its use for navigation. This researcher for the first time in Europe has thoroughly established what properties magnetic materials possess. In 1269, Peter Peregrine de Maricourt wrote Epistola de magnete, the first surviving treatise describing the properties of magnets. In 1282, the properties of compasses and materials with special magnetic properties were described by al-Ashraf, a Yemeni physicist, astronomer and geographer.
Renaissance
In 1600, William Gilbert published his Magnetic Corps and Magnetic Tellurium (On Magnet and Magnetic Bodies, and also on the Great Magnet of the Earth). In this work, he describes many of his experiments with his model earth called terrella, with which he conducted a study of the properties of magnetic materials.
From his experiments, he came to the conclusion that the Earth itself is magnetic and that this is why compasses pointed to the north (previously some believed that it was a polar star (Polaris) or a large magnetic island at the North Pole that attracted the compass).
New time
An understanding of the relationship between electricity and materials with special magnetic properties appeared in 1819 in the work of Hans Christian Oersted, a professor at the University of Copenhagen, who discovered as a result of accidental twitching of the compass needle near the wire that an electric current could create a magnetic field. This landmark experiment is known as the Oersted Experiment. Several other experiments followed with Andre-Marie Ampère, who in 1820 discovered that a magnetic field circulating in a closed path was associated with a current flowing along the perimeter of the path.
Karl Friedrich Gauss studied magnetism. Jean-Baptiste Bio and Felix Savard in 1820 came up with the Bio-Savart law, which gives the necessary equation. Michael Faraday, who in 1831 discovered that a time-varying magnetic flux through a loop of wire caused voltage. And other scientists found further connections between magnetism and electricity.
XX century and our time
James Clerk Maxwell synthesized and expanded this understanding of Maxwell's equations by combining electricity, magnetism and optics in the field of electromagnetism. In 1905, Einstein used these laws, motivating his theory of the special theory of relativity, requiring that laws be preserved in all inertial reference frames.
Electromagnetism continued to develop in the 21st century, being included in more fundamental theories of gauge theory, quantum electrodynamics, electroweak theory, and, finally, in the standard model. Nowadays, scientists are already in full swing studying the magnetic properties of nanostructured materials. But the greatest and most amazing discoveries in this area are probably still ahead of us.
Essence
The magnetic properties of materials are mainly due to the magnetic moments of the orbital electrons of their atoms. The magnetic moments of atomic nuclei are usually thousands of times smaller than that of electrons, and therefore they are insignificant in the context of magnetization of materials. Nuclear magnetic moments are nevertheless very important in other contexts, especially in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).
Usually a huge number of electrons in a material are arranged so that their magnetic moments (both orbital and internal) are nullified. To some extent, this is due to the fact that the electrons are combined in pairs with opposite intrinsic magnetic moments as a result of the Pauli principle (see. Electron configuration) and are combined into filled subshells with zero total orbital motion.
In both cases, the electrons mainly use schemes in which the magnetic moment of each electron is neutralized by the opposite moment of the other electron. Moreover, even when the electron configuration is such that there are unpaired electrons and / or unfilled subshells, it is often the case that various electrons in a solid will introduce magnetic moments that point in different, random directions, so that the material will not be magnetic.
Sometimes, either spontaneously, or because of an applied external magnetic field, each of the magnetic moments of the electrons will be, on average, lined up. Suitable material can then create a strong, pure magnetic field.
The magnetic behavior of the material depends on its structure, in particular on the electronic configuration, for the reasons mentioned above, as well as on temperature. At high temperatures, random thermal motion makes alignment of electrons difficult.
Diamagnetism
Diamagnetism manifests itself in all materials and represents the tendency of the material to withstand the applied magnetic field and, therefore, repel from the magnetic field. However, paramagnetic behavior dominates in a material with paramagnetic properties (i.e., with a tendency to amplify an external magnetic field). Thus, despite the universal appearance, diamagnetic behavior is observed only in a purely diamagnetic material. There are no unpaired electrons in the diamagnetic material; therefore, the intrinsic magnetic moments of the electrons cannot create any volume effect.
Please note that this description is only intended as a heuristic. The Bohr-Van Leuven theorem shows that diamagnetism is impossible according to classical physics, and that a correct understanding requires a quantum-mechanical description.
Please note that all materials pass this orbital response. However, in paramagnetic and ferromagnetic materials, the diamagnetic effect is suppressed by much stronger effects caused by unpaired electrons.
Paramagnetic material has unpaired electrons; that is, atomic or molecular orbitals with exactly one electron in them. While the Pauli exclusion principle requires paired electrons to have their own (“spin”) magnetic moments pointing in opposite directions, as a result of which their magnetic fields are compensated, an unpaired electron can align its magnetic moment in any direction. When an external field is applied, these moments will tend to coincide in the same direction as the applied field, strengthening it.
Ferromagnets
A ferromagnet, as a paramagnetic substance, has unpaired electrons. However, in addition to the tendency of the intrinsic magnetic moment of the electrons to be parallel to the applied field, there is also a tendency for these magnetic moments to orient themselves parallel to each other in order to maintain a state of reduced energy. Thus, even in the absence of an applied field, the magnetic moments of the electrons in the material spontaneously line up parallel to each other.
Each ferromagnetic substance has its own individual temperature, called the Curie temperature, or the Curie point, above which it loses its ferromagnetic properties. This is due to the fact that the thermal tendency to disorder suppresses a decrease in energy due to the ferromagnetic order.
Ferromagnetism is found only in a few substances; common are iron, nickel, cobalt, their alloys and some rare-earth metal alloys.
The magnetic moments of atoms in a ferromagnetic material make them behave like tiny permanent magnets. They stick together and combine into small areas of more or less uniform alignment, called magnetic domains or Weiss domains. Magnetic domains can be observed with a magnetic force microscope to reveal the boundaries of magnetic domains that resemble white lines in a sketch. There are many scientific experiments that can physically show magnetic fields.
Domain role
When a domain contains too many molecules, it becomes unstable and divides into two domains aligned in opposite directions so that they stick together more stably, as shown on the right.
Under the influence of a magnetic field, the domain boundaries move, so that the domains aligned with the magnetic field grow and dominate the structure (dashed yellow region), as shown on the left. When the magnetizing field is removed, the domains may not return to the non-magnetized state. This leads to the fact that the ferromagnetic material is magnetized, forming a permanent magnet.
When magnetization is strong enough, so that the predominant domain overlaps all the others, leading to the formation of only one separate domain, the material is magnetically saturated. When the magnetized ferromagnetic material is heated to the temperature of the Curie point, the molecules are mixed to such an extent that the magnetic domains lose their organization, and the magnetic properties that they cause cease. When the material cools, this domain alignment structure spontaneously returns, in much the same way that a liquid can freeze into a crystalline solid.
Antiferromagnet
In an antiferromagnet, in contrast to a ferromagnet, the intrinsic magnetic moments of neighboring valence electrons tend to indicate in opposite directions. When all atoms are located in a substance so that each neighbor is antiparallel, the substance is antiferromagnetic. Antiferromagnets have a zero total magnetic moment, which means that they do not create a field.
Antiferromagnets are less common than other types of behavior and are most often observed at low temperatures. At various temperatures, antiferromagnets exhibit diamagnetic and ferromagnetic properties.
In some materials, neighboring electrons prefer to point in opposite directions, but there is no geometric arrangement in which each pair of neighbors is anti-aligned. This is called spin glass and is an example of geometric frustration.
Magnetic properties of ferromagnetic materials
Like ferromagnetism, ferrimagnets retain their magnetization in the absence of a field. However, like antiferromagnets, neighboring pairs of electron spins tend to point in opposite directions. These two properties do not contradict each other, because in the optimal geometric arrangement the magnetic moment from the sublattice of electrons that point in one direction is greater than from the sublattice that points in the opposite direction.
Most ferrites are ferrimagnetic. The magnetic properties of ferromagnetic materials are today considered indisputable. The first magnetic substance discovered, magnetite, is ferrite and was initially considered a ferromagnet. However, Louis Neel has denied this by discovering ferrimagnetism.
When a ferromagnet or ferrimagnet is small enough, it acts as a single magnetic spin, which is subject to Brownian motion. Its reaction to a magnetic field is qualitatively similar to the reaction of a paramagnet, but much more.
Solenoids
An electromagnet is a magnet in which a magnetic field is created by an electric current. The magnetic field disappears when the current is turned off. Electromagnets usually consist of a large number of closely spaced turns of wire that create a magnetic field. Wire turns are often wound around a magnetic core made of a ferromagnetic or ferrimagnetic material, such as iron; The magnetic core concentrates the magnetic flux and creates a more powerful magnet.
The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the magnitude of the electric current in the winding. However, unlike a permanent magnet that does not require power, an electromagnet requires a continuous supply of current to maintain a magnetic field.
Electromagnets are widely used as components of other electrical devices such as motors, generators, relays, solenoids, speakers, hard drives, MRI devices, scientific instruments and equipment for magnetic separation. Electromagnets are also used in industry to trap and move heavy iron objects such as scrap metal and steel. Electromagnetism was discovered in 1820. Then the first classification of materials by magnetic properties came out.