A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, attracts or repels other magnets. A permanent magnet is an object made from a material, magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are the ones that are attracted to a magnet, are called ferromagnetic; these include the elements iron and cobalt, some alloys of rare-earth metals, some occurring minerals such as lodestone. Although ferromagnetic materials are the only ones attracted to a magnet enough to be considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism. Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, magnetically "hard" materials, which do.
Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, the total magnetic flux it produces; the local strength of magnetism in a material is measured by its magnetization. An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops; the coil is wrapped around a core of "soft" ferromagnetic material such as mild steel, which enhances the magnetic field produced by the coil. Ancient people learned about magnetism from lodestones which are magnetized pieces of iron ore.
The word magnet was adopted in Middle English from Latin magnetum "lodestone" from Greek μαγνῆτις meaning " from Magnesia", a part of ancient Greece where lodestones were found. Lodestones, suspended so they could turn, were the first magnetic compasses; the earliest known surviving descriptions of magnets and their properties are from Greece and China around 2500 years ago. The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia Naturalis Historia. By the 12th to 13th centuries AD, magnetic compasses were used in navigation in China, the Arabian Peninsula and elsewhere; the magnetic flux density is a vector field. The magnetic B field vector at a given point in space is specified by two properties: Its direction, along the orientation of a compass needle, its magnitude, proportional to how the compass needle orients along that direction. In SI units, the strength of the magnetic B field is given in teslas. A magnet's magnetic moment is a vector.
For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole, the magnitude relates to how strong and how far apart these poles are. In SI units, the magnetic moment is specified in terms of A·m2. A magnet both responds to magnetic fields; the strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. In addition, when the magnet is put into an external magnetic field, produced by a different source, it is subject to a torque tending to orient the magnetic moment parallel to the field; the amount of this torque is proportional both to the external field. A magnet may be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. If the field is uniform in space, the magnet is subject to no net force, although it is subject to a torque. A wire in the shape of a circle with area A and carrying current I has a magnetic moment of magnitude equal to IA.
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume denoted M, with units A/m. It is a vector field, rather than just a vector, because different areas in a magnet can be magnetized with different directions and strengths. A good bar magnet may have a magnetic moment of magnitude 0.1 A•m2 and a volume of 1 cm3, or 1×10−6 m3, therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter; such a large value explains. Two different models exist for magnets: atomic currents. Although for many purposes it is convenient to think of a magnet as having distinct north and south magnetic poles, the concept of poles should not be taken literally: it is a way of referring to the two different ends of a magnet; the magnet does not have distinct south particles on opposing sides. If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two b
Carl Friedrich Gauss
Johann Carl Friedrich Gauss (. Sometimes referred to as the Princeps mathematicorum and "the greatest mathematician since antiquity", Gauss had an exceptional influence in many fields of mathematics and science, is ranked among history's most influential mathematicians. Johann Carl Friedrich Gauss was born on 30 April 1777 in Brunswick, in the Duchy of Brunswick-Wolfenbüttel, to poor, working-class parents, his mother was illiterate and never recorded the date of his birth, remembering only that he had been born on a Wednesday, eight days before the Feast of the Ascension. Gauss solved this puzzle about his birthdate in the context of finding the date of Easter, deriving methods to compute the date in both past and future years, he was christened and confirmed in a church near the school he attended as a child. Gauss was a child prodigy. In his memorial on Gauss, Wolfgang Sartorius von Waltershausen says that when Gauss was three years old he corrected a math error his father made. Many versions of this story have been retold since that time with various details regarding what the series was – the most frequent being the classical problem of adding all the integers from 1 to 100.
There are many other anecdotes about his precocity while a toddler, he made his first groundbreaking mathematical discoveries while still a teenager. He completed his magnum opus, Disquisitiones Arithmeticae, in 1798, at the age of 21—though it was not published until 1801; this work was fundamental in consolidating number theory as a discipline and has shaped the field to the present day. Gauss's intellectual abilities attracted the attention of the Duke of Brunswick, who sent him to the Collegium Carolinum, which he attended from 1792 to 1795, to the University of Göttingen from 1795 to 1798. While at university, Gauss independently rediscovered several important theorems, his breakthrough occurred in 1796 when he showed that a regular polygon can be constructed by compass and straightedge if the number of its sides is the product of distinct Fermat primes and a power of 2. This was a major discovery in an important field of mathematics. Gauss was so pleased with this result that he requested that a regular heptadecagon be inscribed on his tombstone.
The stonemason declined, stating that the difficult construction would look like a circle. The year 1796 was productive for both Gauss and number theory, he discovered a construction of the heptadecagon on 30 March. He further advanced modular arithmetic simplifying manipulations in number theory. On 8 April he became the first to prove the quadratic reciprocity law; this remarkably general law allows mathematicians to determine the solvability of any quadratic equation in modular arithmetic. The prime number theorem, conjectured on 31 May, gives a good understanding of how the prime numbers are distributed among the integers. Gauss discovered that every positive integer is representable as a sum of at most three triangular numbers on 10 July and jotted down in his diary the note: "ΕΥΡΗΚΑ! num = Δ + Δ' + Δ". On 1 October he published a result on the number of solutions of polynomials with coefficients in finite fields, which 150 years led to the Weil conjectures. Gauss remained mentally active into his old age while suffering from gout and general unhappiness.
For example, at the age of 62, he taught himself Russian. In 1840, Gauss published his influential Dioptrische Untersuchungen, in which he gave the first systematic analysis on the formation of images under a paraxial approximation. Among his results, Gauss showed that under a paraxial approximation an optical system can be characterized by its cardinal points and he derived the Gaussian lens formula. In 1845, he became an associated member of the Royal Institute of the Netherlands. In 1854, Gauss selected the topic for Bernhard Riemann's inaugural lecture "Über die Hypothesen, welche der Geometrie zu Grunde liegen". On the way home from Riemann's lecture, Weber reported that Gauss was full of excitement. On 23 February 1855, Gauss died of a heart attack in Göttingen. Two people gave eulogies at his funeral: Gauss's son-in-law Heinrich Ewald, Wolfgang Sartorius von Waltershausen, Gauss's close friend and biographer. Gauss's brain was preserved and was studied by Rudolf Wagner, who found its mass to be above average, at 1,492 grams, the cerebral area equal to 219,588 square millimeters.
Developed convolutions were found, which in the early 20th century were suggested as the explanation of his genius. Gauss was a Lutheran Protestant, a member of the St. Albans Evangelical Lutheran church in Göttingen. Potential evidence that Gauss believed in God comes from his response after solving a problem that had defeated him: "Finally, two days ago, I succeeded—not on account of my hard efforts, but by th
A magnetar is a type of neutron star believed to have an powerful magnetic field. The magnetic field decay powers the emission of high-energy electromagnetic radiation X-rays and gamma rays; the theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar had been detected on March 5, 1979. During the following decade, the magnetar hypothesis became accepted as a explanation for soft gamma repeaters and anomalous X-ray pulsars. Like other neutron stars, magnetars are around 20 kilometres in diameter and have a mass 2–3 times that of the Sun; the density of the interior of a magnetar is such that a tablespoon of its substance would have a mass of over 100 million tons. Magnetars are differentiated from other neutron stars by having stronger magnetic fields, by rotating comparatively quicker. Most neutron stars rotate once every one to ten seconds, whereas magnetars rotate once in less than one second.
A magnetar's magnetic field gives rise to strong and characteristic bursts of X-rays and gamma rays. The active life of a magnetar is short, their strong magnetic fields decay after about 10,000 years, after which activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more. Starquakes triggered on the surface of the magnetar disturb the magnetic field which encompasses it leading to powerful gamma ray flare emissions which have been recorded on Earth in 1979, 1998, 2004. Magnetars are characterized by their powerful magnetic fields of 108 to 1011 tesla; these magnetic fields are hundreds of millions of times stronger than any man-made magnet, quadrillions of times more powerful than the field surrounding Earth. Earth has a geomagnetic field of 30–60 microteslas, a neodymium-based, rare-earth magnet has a field of about 1.25 tesla, with a magnetic energy density of 4.0×105 J/m3.
A magnetar's 1010 tesla field, by contrast, has an energy density of 4.0×1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead. The magnetic field of a magnetar would be lethal at a distance of 1000 km due to the strong magnetic field distorting the electron clouds of the subject's constituent atoms, rendering the chemistry of life impossible. At a distance of halfway from Earth to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth; as of 2010, they are the most powerful magnetic objects detected throughout the universe. As described in the February 2003 Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength. "X-ray photons split in two or merge. The vacuum itself is polarized, becoming birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic de Broglie wavelength of an electron." In a field of about 105 teslas atomic orbitals deform into rod shapes.
At 1010 teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter. The strong fields of magnetars are understood as resulting from a magnetohydrodynamic dynamo process in the turbulent dense conducting fluid that exists before the neutron star settles into its equilibrium configuration; these fields persist due to persistent currents in a proton-superconductor phase of matter that exists at an intermediate depth within the neutron star. A similar magnetohydrodynamic dynamo process produces more intense transient fields during coalescence of pairs of neutron stars; when in a supernova, a star collapses to a neutron star, its magnetic field increases in strength. Halving a linear dimension increases the magnetic field fourfold. Duncan and Thompson calculated that when the spin and magnetic field of a newly formed neutron star falls into the right ranges, a dynamo mechanism could act, converting heat and rotational energy into magnetic energy and increasing the magnetic field an enormous 108 teslas, to more than 1011 teslas.
The result is a magnetar. It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar. On March 5, 1979, a few months after the successful dropping of satellites into the atmosphere of Venus, the two unmanned Soviet spaceprobes, Venera 11 and 12, that were drifting through the Solar System were hit by a blast of gamma radiation at 10:51 EST; this contact raised the radiation readings on both the probes from a normal 100 counts per second to over 200,000 counts a second, in only a fraction of a millisecond. This burst of gamma rays continued to spread. Eleven seconds Helios 2, a NASA probe, in orbit around the Sun, was saturated by the blast of radiation, it soon hit Venus, the Pioneer Venus Orbiter's detectors were overcome by the wave. Seconds Earth received the wave of radiation, where the powerful output of gamma rays inundated the detectors of three U. S. Department of Defense Vela satellites, the Soviet Prognoz 7 satellite, the Einstein Observatory.
Just before the wave exited the Solar System, the blast hit the International Sun–Earth Explorer. This powerful blast of gamma radiation constituted the strongest wave of extra-solar gamma rays detected; because gamma rays travel at the speed of light and the time of the pulse
A molecular cloud, sometimes called a stellar nursery, is a type of interstellar cloud, the density and size of which permit the formation of molecules, most molecular hydrogen. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most used to determine the presence of H2 is carbon monoxide; the ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps; these clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse. Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium, yet it is the densest part of the medium, comprising half of the total gas mass interior to the Sun's galactic orbit.
The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs from the center of the Milky Way. Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy; that molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of 50 to 75 parsecs, much thinner than the warm atomic and warm ionized gaseous components of the ISM; the exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances.
A vast assemblage of molecular gas with a mass of 103 to 107 times the mass of the Sun is called a giant molecular cloud. GMCs are around 15 to 600 light-years in diameter. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun; the substructure of a GMC is a complex pattern of filaments, sheets and irregular clumps. The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia; the concentration of dust within molecular cores is sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large. These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt; the most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs. The Sagittarius region is chemically rich and is used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules; the densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are included in the same studies. In 1984 IRAS identified a new type of diffuse molecular cloud; these were diffuse filamentary clouds. These clouds have a typical density of 30 particles per cubic centimeter; the formation of stars occurs within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse.
There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity; the physics of molecular clouds is poorly much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are supersonic but comparable to the speeds of magnetic disturbances; this state is thought to lose energy requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most the effects of massive stars—before a significant fraction of their mass has become stars. Molecular clouds, GMCs, are
A neutron star is the collapsed core of a giant star which before collapse had a total of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, not counting hypothetical quark stars and strange stars. Neutron stars have a radius of the order of a mass lower than a 2.16 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei. Once formed, they no longer generate heat, cool over time. Most of the basic models for these objects imply that neutron stars are composed entirely of neutrons. Neutron stars are supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure; however neutron degeneracy pressure is not sufficient to hold up an object beyond 0.7M☉ and repulsive nuclear forces play a larger role in supporting more massive neutron stars.
If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit, it continues collapsing to form a black hole. Neutron stars that can be observed are hot and have a surface temperature of around 600000 K, they are so dense that a normal-sized matchbox containing neutron-star material would have a weight of 3 billion metric tons, the same weight as a 0.5 cubic kilometre chunk of the Earth. Their magnetic fields are between 1015 times stronger than Earth's magnetic field; the gravitational field at the neutron star's surface is about 2×1011 times that of Earth's gravitational field. As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence newly formed neutron stars rotate at up to several hundred times per second; some neutron stars emit beams of electromagnetic radiation. Indeed, the discovery of pulsars by Jocelyn Bell Burnell in 1967 was the first observational suggestion that neutron stars exist; the radiation from pulsars is thought to be emitted from regions near their magnetic poles.
If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space. The fastest-spinning neutron star known is PSR J1748-2446ad, rotating at a rate of 716 times a second or 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.24 c. There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, most are old and cold, neutron stars can only be detected in certain instances, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are undetectable. Soft gamma repeaters are conjectured to be a type of neutron star with strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them.
Neutron stars in binary systems can undergo accretion which makes the system bright in X-rays while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified X-ray pulsar systems. Additionally, such accretion can "recycle" old pulsars and cause them to gain mass and spin-up to fast rotation rates, forming the so-called millisecond pulsars; these binary systems will continue to evolve, the companions can become compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or merger. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are strong sources of gravitational waves. In 2017, a direct detection of the gravitational waves from such an event was made, gravitational waves have been indirectly detected in a system where two neutron stars orbit each other. In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars.
The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, both may be a kilonova, which may be more common in the universe than understood, according to the researchers. Any main-sequence star with an initial mass of above 8 times the mass of the sun has the potential to produce a neutron star; as the star evolves away from the main sequence, subsequent nuclear burning produces an iron-rich core. When all nuclear fuel in the core has been exhausted, the core must be supported by degeneracy pressure alone. Further deposits of mass from shell burning cause the core to exceed the Chandrasekhar limit. Electron-deg
A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed from subatomic particles to galaxies. In everyday life, the effects of magnetic fields are seen in permanent magnets, which pull on magnetic materials and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location; as such, it is an example of a vector field. The term'magnetic field' is used for two distinct but related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla, equivalent to newton per meter per ampere.
H and B differ in. In a vacuum, B and H are the same aside from units. Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, are both components of the electromagnetic force, one of the four fundamental forces of nature. Magnetic fields are used throughout modern technology in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric generators; the interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect; the Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass. Although magnets and magnetism were studied much earlier, the research of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles.
Noting that the resulting field lines crossed at two points he named those points'poles' in analogy to Earth's poles. He clearly articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them. Three centuries William Gilbert of Colchester replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet. Published in 1600, Gilbert's work, De Magnete, helped to establish magnetism as a science. In 1750, John Michell stated that magnetic poles attract and repel in accordance with an inverse square law. Charles-Augustin de Coulomb experimentally verified this in 1785 and stated explicitly that the north and south poles cannot be separated. Building on this force between poles, Siméon Denis Poisson created the first successful model of the magnetic field, which he presented in 1824. In this model, a magnetic H-field is produced by'magnetic poles' and magnetism is due to small pairs of north/south magnetic poles. Three discoveries in 1820 challenged this foundation of magnetism, though.
Hans Christian Ørsted demonstrated that a current-carrying wire is surrounded by a circular magnetic field. André-Marie Ampère showed that parallel wires with currents attract one another if the currents are in the same direction and repel if they are in opposite directions. Jean-Baptiste Biot and Félix Savart announced empirical results about the forces that a current-carrying long, straight wire exerted on a small magnet, determining that the forces were inversely proportional to the perpendicular distance from the wire to the magnet. Laplace deduced, but did not publish, a law of force based on the differential action of a differential section of the wire, which became known as the Biot–Savart law. Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model.
This has the additional benefit of explaining. Further, Ampère derived both Ampère's force law describing the force between two currents and Ampère's law, like the Biot–Savart law described the magnetic field generated by a steady current. In this work, Ampère introduced the term electrodynamics to describe the relationship between electricity and magnetism. In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field, he described this phenomenon in. Franz Ernst Neumann proved that, for a moving conductor in a magnetic field, induction is a consequence of Ampère's force law. In the process, he introduced the magnetic vector potential, shown to be equivalent to the underlying mechanism proposed by Faraday. In 1850, Lord Kelvin known as William Thomson, distinguished between two magnetic fields now denoted H and B; the former applied to the latter to Ampère's model and induction. Further, he derived how H and B relate to each other
Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. Colloquially, when a magnet is "magnetized" it has remanence; the remanence of magnetic materials provides the magnetic memory in magnetic storage devices, is used as a source of information on the past Earth's magnetic field in paleomagnetism. The equivalent term residual magnetization is used in engineering applications. In transformers, electric motors and generators a large residual magnetization is not desirable as it is an unwanted contamination, for example a magnetization remaining in an electromagnet after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing. Sometimes the term retentivity is used for remanence measured in units of magnetic flux density; the default definition of magnetic remanence is the magnetization remaining in zero field after a large magnetic field is applied. The effect of a magnetic hysteresis loop is measured using instruments such as a vibrating sample magnetometer.
In physics this measure is converted to an average magnetization and denoted in equations as Mr. If it must be distinguished from other kinds of remanence it is called the saturation remanence or saturation isothermal remanence and denoted by Mrs. In engineering applications the residual magnetization is measured using a B-H analyzer, which measures the response to an AC magnetic field; this is represented by a flux density Br. This value of remanence is one of the most important parameters characterizing permanent magnets. Neodymium magnets, for example, have a remanence equal to 1.3 teslas. A single measure of remanence does not provide adequate information on a magnet. For example, magnetic tapes contain a large number of small magnetic particles, these particles are not identical. Magnetic minerals in rocks may have a wide range of magnetic properties. One way to look inside these materials is to subtract small increments of remanence. One way of doing this is first demagnetizing the magnet in an AC field, applying a field H and removing it.
This remanence, denoted by Mr, depends on the field. It is called the isothermal remanent magnetization. Another kind of IRM can be obtained by first giving the magnet a saturation remanence in one direction and applying and removing a magnetic field in the opposite direction; this is called demagnetization remanence or DC demagnetization remanence and is denoted by symbols like Md, where H is the magnitude of the field. Yet another kind of remanence can be obtained by demagnetizing the saturation remanence in an ac field; this is called AC demagnetization remanence or alternating field demagnetization remanence and is denoted by symbols like Maf. If the particles are noninteracting single-domain particles with uniaxial anisotropy, there are simple linear relations between the remanences. Another kind of laboratory remanence is anhysteretic remanence or anhysteretic remanent magnetization; this is induced by exposing a magnet to a large alternating field plus a small dc bias field. The amplitude of the alternating field is reduced to zero to get an anhysteretic magnetization, the bias field is removed to get the remanence.
The anhysteretic magnetization curve is close to an average of the two branches of the hysteresis loop, is assumed in some models to represent the lowest-energy state for a given field. There are several ways for experimental measurement of Anhysteretic Magnetization Curve, based on fluxmeters and DC biased demagnetization. ARM has been studied because of its similarity to the write process in some magnetic recording technology and to the acquisition of natural remanent magnetization in rocks. Coercivity Hysteresis Rock magnetism Thermoremanent magnetization Viscous remanent magnetization Coercivity and Remanence in Permanent Magnets Magnet Man