Ferromagnetic ordering

Ferromagnetism is the "normal" form of magnetism with which most people are familiar, as exhibited in horseshoe magnets and refrigerator magnets. In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. A magnet (from Greek grc μαγνήτης λίθος " Magnesian stone" is a material or object that produces a Magnetic field. A refrigerator magnet is an ornament attached to a Magnet that is used to post items such as shopping lists or report cards on a Refrigerator, or simply It is responsible for most of the magnetic behavior encountered in everyday life. A magnet (from Greek grc μαγνήτης λίθος " Magnesian stone" is a material or object that produces a Magnetic field. The attraction between a magnet and ferromagnetic material is "the quality of magnetism first apparent to the ancient world, and to us today," according to a classic text on ferromagnetism. ^[1]

Ferromagnetism is defined as the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. A phenomenon (from Greek φαινόμενoν, pl φαινόμενα - phenomena) is any observable occurrence Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges

All permanent magnets are either ferromagnetic or ferrimagnetic, as are the metals that are noticeably attracted to them. A magnet (from Greek grc μαγνήτης λίθος " Magnesian stone" is a material or object that produces a Magnetic field. In Physics, a ferrimagnetic material is one in which the Magnetic moment of the atoms on different sublattices are opposed as in Antiferromagnetism; however The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across

Historically, the term ferromagnet was used for any material that could exhibit spontaneous magnetization: a net magnetic moment in the absence of an external magnetic field. In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current This general definition is still in common use. More recently, however, different classes of spontaneous magnetization have been identified when there is more than one magnetic ion per primitive cell of the material, leading to a stricter definition of "ferromagnetism" that is often used to distinguish it from ferrimagnetism. In Geometry, Solid state physics and Mineralogy, particularly in describing Crystal structure, a primitive cell, is a minimum cell corresponding In Physics, a ferrimagnetic material is one in which the Magnetic moment of the atoms on different sublattices are opposed as in Antiferromagnetism; however In particular, a material is "ferromagnetic" in this narrower sense only if all of its magnetic ions add a positive contribution to the net magnetization. If some of the magnetic ions subtract from the net magnetization (if they are partially anti-aligned), then the material is "ferrimagnetic". If the ions anti-align completely so as to have zero net magnetization, despite the magnetic ordering, then it is an antiferromagnet. Order in a crystal lattice is the arrangement of some property with respect to atomic positions In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules usuallyrelated to the spins of Electrons align in a regular pattern with neighboring All of these alignment effects only occur at temperatures below a certain critical temperature, called the Curie temperature (for ferromagnets and ferrimagnets) or the Néel temperature (for antiferromagnets). Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The Curie point ( Tc) or Curie temperature, is a term in Physics and Materials science, named after Pierre Curie (1859-1906 The Néel temperature, TN is the Temperature at which an Antiferromagnetic material becomes Paramagnetic &mdash that is the thermal energy

Ferromagnetic materials

Main article: :Category:Ferromagnetic materials

There are a number of crystalline materials that exhibit ferromagnetism (or ferrimagnetism). Cobalt (ˈkoʊbɒlt is a hard lustrous silver-grey Metal, a Chemical element with symbol Co. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical Ferrites are a class of Chemical compounds with the formula AB2O4 where A and B represent various metal Cations usually including Ferrites are a class of Chemical compounds with the formula AB2O4 where A and B represent various metal Cations usually including Ferrites are a class of Chemical compounds with the formula AB2O4 where A and B represent various metal Cations usually including Manganese (ˈmæŋgəniːz is a Chemical element, designated by the symbol Mn. Bismuth (ˈbɪzməθ is a Chemical element that has the symbol Bi and Atomic number 83 Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 Manganese (ˈmæŋgəniːz is a Chemical element, designated by the symbol Mn. Antimony (IPA (Received Pronunciation, /ˈæntɪmoʊni/ (US is a Chemical element with the symbol Sb (stibium meaning "mark" and Ferrites are a class of Chemical compounds with the formula AB2O4 where A and B represent various metal Cations usually including Yttrium iron garnet ( YIG) is a kind of synthetic Garnet, with chemical composition 32(Fe43 or Y3Fe5O12 Chromium dioxide or chromium(IV oxide is a synthetic magnetic substance once widely used in Magnetic tape Emulsion. Manganese (ˈmæŋgəniːz is a Chemical element, designated by the symbol Mn. Arsenic (ˈɑrsənɪk is a Chemical element that has the symbol As and Atomic number of 33 Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64 Europium (jʊˈroʊpiəm is a Chemical element with the symbol Eu and Atomic number 63 The table on the right lists a representative selection of them here, along with their Curie temperatures, the temperature above which they cease to exhibit spontaneous magnetization (see below).

Ferromagnetic metal alloys whose constituents are not themselves ferromagnetic in their pure forms are called Heusler alloys, named after Fritz Heusler. A Heusler alloy is a Ferromagnetic Metal Alloy based on a Heusler phase Friedrich Heusler (1866 &ndash 1947 was a German mining engineer and Chemist.

One can also make amorphous (non-crystalline) ferromagnetic metallic alloys by very rapid quenching (cooling) of a liquid alloy. A quench refers to a rapid Cooling. In Polymer chemistry and Materials science, quenching is used to prevent low-temperature processes such as phase These have the advantage that their properties are nearly isotropic (not aligned along a crystal axis); this results in low coercivity, low hysteresis loss, high permeability, and high electrical resistivity. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system A typical such material is a transition metal-metalloid alloy, made from about 80% transition metal (usually Fe, Co, or Ni) and a metalloid component (B, C, Si, P, or Al) that lowers the melting point. Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Phosphorus, (ˈfɒsfərəs is the Chemical element that has the symbol P and Atomic number 15 WikipediaNaming

A relatively new class of exceptionally strong ferromagnetic materials are the rare-earth magnets. Rare-earth magnets are strong permanent magnets made from Alloys of Rare earth elements Rare-earth magnets are substantially stronger than They contain lanthanide elements that are known for their ability to carry large magnetic moments in well-localized f-orbitals.

Physical origin

The property of ferromagnetism is due to the direct influence of two effects from quantum mechanics: spin and the Pauli exclusion principle. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925

The spin of an electron, combined with its orbital angular momentum, results in a magnetic dipole moment and creates a magnetic field. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position In physics there are two kinds of dipoles ( Hellènic: di(s- = two- and pòla = pivot hinge An electric dipole is a In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current (The classical analogue of quantum-mechanical spin is a spinning ball of charge, but the quantum version has distinct differences, such as the fact that it has discrete up/down states that are not described by a vector; similarly for "orbital" motion, whose classical analogue is a current loop. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. ) In many materials (specifically, those with a filled electron shell), however, the total dipole moment of all the electrons is zero (i. An electron shell may be crudely thought of as an Orbit followed by Electrons around an Atom nucleus. e. , the spins are in up/down pairs). Only atoms with partially filled shells (i. e. , unpaired spins) can experience a net magnetic moment in the absence of an external field. A ferromagnetic material has many such electrons, and if they are aligned they create a measurable macroscopic field.

These permanent dipoles (often called simply "spins" even though they also generally include orbital angular momentum) tend to align in parallel to an external magnetic field, an effect called paramagnetism. Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field (A related but much weaker effect is diamagnetism, due to the orbital motion induced by an external field, resulting in a dipole moment opposite to the applied field. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied Magnetic field, thus causing a repulsive effect ) Ferromagnetism involves an additional phenomenon, however: the dipoles tend to align spontaneously, without any applied field. This is a purely quantum-mechanical effect.

According to classical electromagnetism, two nearby magnetic dipoles will tend to align in opposite directions (which would create an antiferromagnetic material). Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules usuallyrelated to the spins of Electrons align in a regular pattern with neighboring In a ferromagnet, however, they tend to align in the same direction because of the Pauli principle: two electrons with the same spin cannot also have the same "position", which effectively reduces the energy of their electrostatic interaction compared to electrons with opposite spin. The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925 In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin (Mathematically, this is expressed more precisely in terms of the spin-statistics theorem: because electrons are fermions with half-integer spin, their wave functions are antisymmetric under interchange of particle positions. The spin-statistics theorem in Quantum mechanics relates the spin of a particle to the statistics obeyed by that particle In Particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi. A wave function or wavefunction is a mathematical tool used in Quantum mechanics to describe any physical system This can be seen in, for example, the Hartree-Fock approximation to lead to a reduction in the electrostatic potential energy. In Computational physics and Computational chemistry, the Hartree-Fock ( HF) method is an approximate method for the determination of the ground-state ) This difference in energy is called the exchange energy. In Physics, the exchange interaction is a Quantum mechanical effect which increases or decreases the expectation value of the Energy or Distance

At long distances (after many thousands of ions), the exchange energy advantage is overtaken by the classical tendency of dipoles to anti-align. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge This is why, in an equilibriated (non-magnetized) ferromagnetic material, the dipoles in the whole material are not aligned. Rather, they organize into magnetic domains (also known as Weiss domains) that are aligned (magnetized) at short range, but at long range adjacent domains are anti-aligned. A magnetic domain describes a region within a material which has uniform Magnetization. The transition between two domains, where the magnetization flips, is called a domain wall (i. A domain wall is a term used in Physics which can have one of two distinct but similar meanings in either Magnetism or String theory. e. , a Bloch/Néel wall, depending upon whether the magnetization rotates parallel/perpendicular to the domain interface) and is a gradual transition on the atomic scale (covering a distance of about 300 ions for iron). A Bloch wall is a narrow transition region at the boundary between Magnetic domains over which the Magnetisation changes from its value in one domain to that in the

Thus, an ordinary piece of iron generally has little or no net magnetic moment. However, if it is placed in a strong enough external magnetic field, the domains will re-orient in parallel with that field, and will remain re-oriented when the field is turned off, thus creating a "permanent" magnet. This magnetization as a function of the external field is described by a hysteresis curve. Although this state of aligned domains is not a minimal-energy configuration, it is extremely stable and has been observed to persist for millions of years in seafloor magnetite aligned by the Earth's magnetic field (whose poles can thereby be seen to flip at long intervals). Paleomagnetism is the study of the record of the Earth's magnetic field preserved in various Magnetic Minerals through time Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see The net magnetization can be destroyed by heating and then cooling (annealing) the material without an external field, however. Annealing, in Metallurgy and Materials science, is a Heat treatment wherein a material is altered causing changes in its properties such as strength

As the temperature increases, thermal oscillation, or entropy, competes with the ferromagnetic tendency for dipoles to align. In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy When the temperature rises beyond a certain point, called the Curie temperature, there is a second-order phase transition and the system can no longer maintain a spontaneous magnetization, although it still responds paramagnetically to an external field. In Thermodynamics, phase transition or phase change is the transformation of a thermodynamic system from one phase to another Below that temperature, there is a spontaneous symmetry breaking and random domains form (in the absence of an external field). The Curie temperature itself is a critical point, where the magnetic susceptibility is theoretically infinite and, although there is no net magnetization, domain-like spin correlations fluctuate at all lengthscales. In Physical chemistry, Thermodynamics, Chemistry and Condensed matter physics, a critical point, also called a critical state In Electromagnetism the magnetic susceptibility ( Latin: susceptibilis “receptiveness” is the degree of Magnetization of a material in response

The study of ferromagnetic phase transitions, especially via the simplified Ising spin model, had an important impact on the development of statistical physics. The Ising model, named after the physicist Ernst Ising, is a mathematical model in Statistical mechanics. There, it was first clearly shown that mean field theory approaches failed to predict the correct behavior at the critical point (which was found to fall under a universality class that includes many other systems, such as liquid-gas transitions), and had to be replaced by renormalization group theory. A Many-body system with interactions is generally very difficult to solve exactly except for extremely simple cases ( Gaussian field theory, 1D Ising model. In Theoretical physics, renormalization group (RG refers to a mathematical apparatus that allows one to investigate the changes of a physical system as one views

Unusual ferromagnetism

It is thought that other similarly-formed materials, such as isoelectronic compounds of boron and nitrogen, may also be ferromagnetic. Two or more molecular entities ( Atoms Molecules Ions are described as being isoelectronic with each other if they have the same number of Boron nitride ( BN) is a binary chemical compound, consisting of equal numbers of Boron and Nitrogen atoms The alloy ZnZr₂ is also ferromagnetic below 28. Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 Zirconium (zɚˈkoʊniəm /ˌzɝˈkoʊniəm/ is a Chemical element with the symbol Zr and Atomic number 40 5 K.

Sources

• Charles Kittel, Introduction to Solid State Physics (Wiley: New York, 1996).
• Neil W. Ashcroft and N. David Mermin, Solid State Physics (Harcourt: Orlando, 1976).
• John David Jackson, Classical Electrodynamics (Wiley: New York, 1999).
• E. P. Wohlfarth, ed. , Ferromagnetic Materials (North-Holland, 1980).
• "Heusler alloy," Encyclopedia Britannica Online, retrieved Jan. 23, 2005.
• F. Heusler, W. Stark, and E. Haupt, Verh. der Phys. Ges. 5, 219 (1903).
• S. Vonsovsky Magnetism of elementary particles (Mir Publishers, Moscow, 1975). Sergei Vasilyevich Vonsovsky (also spelled as Vonsovskii or Vonsovskiy, Russian: Сергей Васильевич Вонсовский 1910-1998 was

External links

• Electromagnetism - a chapter from an online textbook

References

1. ^ Richard M. Bozorth, Ferromagnetism, first published 1951, reprinted 1993 by IEEE Press, New York as a "Classic Reissue. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization " ISBN 0-7803-1032-2.