Magnetism

MAGNETISM (from the Greek. magnetis - magnet), manifests itself in macroscales as an interaction between electricity. currents, between currents and magnets (i.e. bodies with a magnetic moment) and between magnets. In its most general form, M. can be defined as a special form of material interactions arising between moving electrically charged particles. The transmission of magnetic interaction, realizing the connection between spatially separated bodies, is carried out by a special material carrier - a magnetic field. It is, along with the electric. the field is one of the manifestations of the electromagnetic form of motion of matter (see Electromagnetic field). Between magnetic and electric. there is no complete symmetry between the fields. Sources of electricity. the fields are electric. charges possessed by elementary particles - electrons, protons, mesons, etc. Similar magnetic charges have not yet been observed in nature, although hypotheses about their existence have been expressed (see Magnetic Monopole).

The source of the magnetic field is a moving electrician. charge, i.e. electric current. At atomic scales, there are two types of microscopes for electrons and nucleons (protons, neutrons). currents are orbital, associated with the portable motion of the center of gravity of these particles, and spin (see Spin), associated with the internal. degrees of freedom of their movement.

The characteristic of M particles are their orbital and spin magnetic moments (denoted by M). Since all microstructural elements of substances - electrons, protons and neutrons - have magnetic moments, then any of their combinations - atomic nuclei and electron shells - and combinations of their combinations, i.e. atoms, molecules and macroscopic bodies, can in principle be sources of magnetism. T. O., M. substances have a universal character.

Two main effects of the external magnetic field on substances are known. Firstly, the diamagnetic effect, which is a consequence of Faraday's law of induction (see Electromagnetic induction): the external magnetic field always creates such an induction current in the substance, the magnetic field of which is directed against the initial field (Lenz rule). Therefore, the diamagnetic moment of a substance created by an external field is always negative with respect to this field.

Secondly, if an atom has a magnetic moment other than zero (spin, orbital, or both), then the external field will tend to orient it along its direction. As a result, there is a positive moment parallel to the field, which is called paramagnetic.

Of course, the influence on the magnetic properties of a substance can also be exerted by internal. interactions (electrical and magnetic nature) between atomic magnetic moments. In some cases, due to these interactions, it turns out to be energetically advantageous for a spontaneous (independent of the external field) atomic magnetic order to exist in the substance. Substances in which atomic magnetic moments are located parallel to each other, naz. ferromagnets; respectively antiferromagnets naz. substances in which neighboring atomic moments are located antiparallel. The complexity of the atomic structure of substances constructed from a huge number of atoms leads to an almost inexhaustible variety of their magnetic properties. When considering the magnetic properties of substances , a general term is used for the latter - m a g e t and k I. The interrelation of the magnetic properties of substances with their non-magnetic properties (electrical, mechanical, optical, etc.) allows very often to use studies of magnetic properties as a source of information about the interior. the structure of microparticles and bodies is macroscopic. sizes. A wide range of phenomena of M., extending from M. elementary particles to M. kosmich. bodies (Earth, Sun, stars, etc.), determines the great role of M. in natural phenomena, in science and technology.

Macroscopic description of magnetic properties of substances is usually carried out within the framework of electromagnetic field theory (see Maxwell equations), thermodynamics and statistical physics. One of the main macroscopic. characteristics of a magnet that determine its thermodynamic properties. the state, is the magnetization vector J (the total magnetic moment of the unit volume of the magnet). Experience shows that the vector J is a function of the magnetic field strength N. Graphically, the dependence of J (H) is represented by a magnetization curve having a different appearance for different magnets. In a number of substances, there is a linear relationship between J and H, J = pN, where x is the magnetic susceptibility (in diamagnets n <0, in paramagnets n>0). In ferromagnets, and is related to H non-linearly; in them, the susceptibility depends not only on the temperature T and the properties of the substance, but also on the field N.

Thermodynamically, the magnetization of a magnet J is determined through the thermodynamic potential F (H, T, p) according to the formula J - - (dF/dN)t,p (here p is pressure). In turn, the calculation of F (H, T, p) is based on the Gibbs-Boguslavsky ratio F = -kTlnZ(H,T), where k is the Boltzmann constant, Z (H,T) is the statistical sum.

From the general provisions of the classic. statistical. It follows from physics that electronic systems (without taking into account their quantum properties) cannot have a thermodynamically stable magnetic moment (the Bohr-Van-Leven-Terletsky theorem), but this contradicts experience. Quantum mechanics, which explained the stability of the atom, gave an explanation of both the motion of atoms and macroscopic bodies.

The motion of atoms and molecules is caused by the spin magnetic moments of their electrons, the motion of electrons in the shells of atoms and molecules (the so-called p-bitalnym), the spin and orbital motion of nucleons of nuclei. In multielectronic atoms, the addition of orbital and spin magnetic moments is performed according to the laws of spatial quantization: the resulting magnetic moment is determined by the total angular quantum number j and is equal to .nj = gj is the root of j(j+1) pv, where gj is the Lande multiplier, pv is the Boron magneton (see Magnetic moment).

The magnetic properties of substances are determined by the nature of atomic carriers and the nature of their interactions. The significant influence of these interactions on the magnetic properties is indicated, in particular, by the comparison of the magnetic properties of isolated atoms of various elements. Thus, in inert gas atoms (Ne, Ag, Ne, etc.), the electron shells are magnetically neutral (their total magnetic moment is zero). In an external magnetic field, inert gases exhibit diamagnetic properties (see Diamagnetism). The electron shell of alkali metal atoms (Li, Na, K, etc.)it has only the spin magnetic moment of the valence electron, the orbital moment of these atoms is zero. Alkali metal vapors are paramagnetic (see Paramagnetism). Transition metal atoms (Fe, Co, Ni, etc.) have, as a rule, large spin and orbital magnetic moments due to unfinished d- and f-layers of their electron shell (see Atom).

The strong dependence of the chemical substances on the nature of the bond between the microparticles (carriers of the magnetic moment) leads to the fact that the substance of the unchanged chemical. composition depending on external conditions, as well as crystallinity. or the phase structure (e.g., the degree of ordering of atoms in alloys, etc.) may have different magnetic properties. For example, Fe, Co, Ni in crystal. states below a certain temperature (Curie points) have ferromagnetic properties; above the Curie point they lose these properties (see Ferromagnetism). Quantitatively , the interaction between atomic carriers M in a substance, it can be characterized by the value of the EVZ energy of this interaction, calculated for a separate pair of magnetic moment carrier particles. The energy of the bases caused by electricity. and the magnetic interaction of microparticles and depending on their magnetic moments, can be compared with the values of the energies of others. atomic interactions: with the energy of the magnetic moment pv in a certain effective magnetic field Neff, i.e. with En = pvNeff, and with the average energy of thermal motion of particles at a certain effective critical. temp-re Tk, i.e. with ET = kTx. At the values of the external field strength H <Neff or at temp-pax T <Tc, the magnetic properties of the substance due to the EVZ - internal interactions of the atomic carriers of M. (the so-called "strong" M. substances) will be strongly manifested. On the contrary, in the areas of H > Neff or T >Tc, external factors will dominate - temp-pa or field, suppressing the effects of internal interaction ("weak" M. substances). This classification is formal, because it does not reveal the physical. nature Neff and Tk. To fully clarify the phys . of the nature of the magnetic properties of a substance, it is necessary to know not only the magnitude of the evz energy in comparison with Et or EN, but also its physical. the origin and nature of the magnetic moment of the carriers (orbital or spin). If we exclude the case of nuclear M., in which the effect of nuclear interactions manifests itself, then 2 types of forces act in the electronic shells of atoms and molecules, as well as in the electronic system of condensed substances (liquids, crystals)- electric and magnetic. Measure electric. the interaction can serve as electro-static. the EEL energy of two electrons located at an atomic distance (a = = 10-8CM): eel ~ e2/a ~ 10-12 erg (here e is the electron charge). The measure of magnetic interaction is the binding energy of two microparticles with magnetic moments (is and located at a distance of a, i.e. emagn ~ p2b/a3 ~ 10-16 erg. Thus, EEL exceeds the energy of e"agn by at least three orders of magnitude.

In this regard, the preservation of magnetization by ferromagnets (Fe, Co, Ni) to a temperature of ~ 1000 K can only be due to electricity. interaction, because at an energy of EMagn ~ 10-16erg, thermal motion would destroy the orienting action of magnetic forces already at 1 K. Based on quantum mechanics, it was shown that along with Coulomb electro-static. by the interaction of charged particles, there is also a purely quantum electrostatic. exchange interaction depending on the mutual orientation of the magnetic moments of the electrons. t. A., this part of the electrical interaction by its nature has a significant effect on the magnetic state of electronic systems. In particular, this interaction favors the ordered orientation of the magnetic moments of atomic carriers M. The upper limit of the energy of the exchange interaction is Eob~10-13erg.

The value E0b >0 corresponds to the parallel orientation of atomic magnetic moments, i.e. spontaneous (spontaneous) magnetization of bodies (ferromagnets). At Eob<0, there is a tendency to antiparallel orientation of neighboring magnetic moments, characteristic of the atomic magnetic structure of antiferromagnets. The above allows you to conduct the following physics . classification of chemical substances.

b) organic. compounds with a nonpolar bond, in which molecules or radicals either do not have a magnetic moment, or the paramagnetic effect in them is suppressed by a diamagnetic one; these compounds have x ~ - 10-6 and also practically does not depend on the temperature, but has a noticeable anisotropy (see Magnetic anisotropy);

c) substances in condensed phases- liquid and crystalline: some metals (Zn, Au, Hg, etc.); solutions, alloys and chemical compounds (e.g., halides) with predominance of diamagnetism of ion cores (ions similar to atoms of inert gases-Li+, Be2+, A13+, Cl-, etc.). The M. of this group of substances is similar to M. of "classical" diamagnetic gases.

B. The predominance of paramagnetism is characteristic of: a) free atoms, ions and molecules with a resultant magnetic moment. Paramagnetic gases are O2, NO, alkali and transition metal vapors. Their susceptibility x>0 is small in magnitude (~10-3-10-5) and at not very low temp-pax and not very strong magnetic fields (pvN/kT < 1) does not depend on the field, but significantly depends on the temperature, for x there is a Curie law x = S/T, where C is the Curie constant; b) for ions of transition elements in liquid solutions, as well as in crystals, provided that magnetically active ions interact weakly with each other and their immediate environment in the condensed phase has little effect on their paramagnetism. Under the condition of pvN /kT < 1, their susceptibility % does not depend on H, but depends on T - there is a Curie-Veps law x = C/(T - K), where C‘ and K are the constants of the substance; c) for ferro- and antiferromagnetic substances above the Curie point 0.

B. Diamagnetism of conduction electrons in metals (Landau diamagnetism) is inherent in all metals, but, as a rule, it is masked either by a stronger spin electron paramagnetism, or by the dia- or paramagnetism of ion cores.

B. Para- and diamagnetism of conduction electrons in semiconductors. Compared with metals, there are few conduction electrons in semiconductors, but their number increases with increasing rate; % in this case also depends on T.

G. M. superconductors are caused by electricity. currents flowing in a thin surface layer ~ 10-5cm thick. These currents shield the thickness of the superconductor from external magnetic fields, therefore, in a massive superconductor at T <Tk, the magnetic field is zero (Meissner effect).

B. Antiferromagnetism takes place in substances with negative. exchange energy (E0b<0): crystals of Sg and Mp, a number of rare earth metals (Ce, Pr, Nd, Sm, Eu), as well as in numerous compounds and alloys involving elements of transition groups.

Magnetically, the crystal lattice of these substances is divided into so-called magnetic sublattices, the vectors of spontaneous magnetization are either antiparallel (collinear anti-ferromagnetic coupling), or directed to each other at angles other than 0° and 180° (non-collinear coupling, see Magnetic structure). If the total moment of all magnetic sublattices in an antiferromagnet is zero, then compensated antiferromagnetism takes place; if there is a difference spontaneous magnetization other than zero, then uncompensated antiferromagnetism, or ferrimagnetism, is observed, which is realized mainly in crystals of metal oxides with a crystal lattice such as spinel, garnet, perovskite and other minerals (they are called ferrites). These bodies (usually semiconductors and insulators) are similar in magnetic properties to conventional ferromagnets. In case of violation of the compensation of magnetic moments in antiferromagnets due to the weak interaction between the atomic carriers of M. there is a very small spontaneous magnetization of substances (~ 0.1% of the usual values for ferro- and ferrimagnets). Such substances are aas. weak ferromagnets (e.g. hematite a-Fe2O3, carbonates of a number of metals, orthoferrites, etc.).

The magnetic state of a ferro- or antiferromagnet in an external magnetic tulle H is determined, in addition to the magnitude of the field, also by the previous states of the magnet (the magnetic background of the sample). This phenomenon is called hysteresis. Magnetic hysteresis manifests itself in the ambiguity of the dependence of J on H (in the presence of a hysteresis loop). Due to hysteresis, it is insufficient to eliminate the external field for demagnetization of the sample, at H = 0 the sample will retain the residual magnetization Jr. To demagnetize the sample, it is necessary to apply the reverse magnetic field Ns, to the swarm naz. coercive force. Depending on the value of Hc, magnetically soft materials (Hc<800 a/M, or 10 e) and magnetically hard or highly coercive materials (Hc>4 ka/m, or 50 e) are distinguished. Jg N" depend on the rate and, as a rule, decrease with its increase, tending to zero with the approach of T to 6.

In addition to M. atomic particles and substances, the modern doctrine of magnetic phenomena includes M. celestial bodies and cosmic environments. The following articles are devoted to the consideration of related issues: Terrestrial magnetism, Solar magnetism, Magnetic stars, Interstellar magnetic field, Cosmic rays, as well as Magnetic field, Magnetic Hydrodynamics, etc.

Magnetism in science and technology. The main scientific problems of the modern teaching about M. is to clarify the nature of the exchange interaction and interactions that cause anisotropy in various types of magnetically ordered crystals; the spectra of elementary magnetic excitations (magnons) and the mechanisms of their interaction with each other, as well as with phonons (quanta of crystal lattice vibrations). An important problem remains the creation of a theory of transition from paramagnetic to ferromagnetic state. The study of chemical substances is widely used in various fields of science as a means of studying chemical chemistry. bonds and structures of molecules (magnetochemistry). The study of the dia- and paramagnetic properties of gases, liquids, solutions, and compounds in the solid phase allows us to understand the details of physics. and chemical processes occurring in these bodies and in their structure. The study of magnetic dynamical characteristics (para-, dia- and ferromagnetic, electronic and nuclear resonances and relaxations) helps to understand the kinetics of many physicists. and physical and chemical processes in various substances (see Magnetic resonance). Magnetobiology is developing intensively.

To the most important problems of M, kosmich. bodies include: elucidation of the origin of the magnetic fields of the Earth, planets, the Sun, stars (in particular, pulsars), extragalactic.radio sources (radio galaxies, quasars, etc.), as well as the role of magnetic fields in cosmic processes.

Basic technical. M. finds applications in electrical engineering, radio engineering, electronics, instrumentation, electronic computing devices, marine, aviation and space. navigation, geophysicist. methods of mineral exploration, automation and telemechanics. Magnetic flaw detection and magnetic control methods are also widely used in the technique. Magnetic materials are used for the manufacture of magnetic conductors of generators, motors, transformers, relays, magnetic amplifiers, magnetic memory elements, compass arrows, magnetic recording tapes, etc.

The history of the doctrine of magnetism. The first written evidence of M. (China) is more than two thousand years old. They mention the use of natural permanent magnets as a compass. In the works of ancient Greek. and Roman scientists mention the attraction and repulsion of natures. magnets and magnetization of iron filings in the presence of a magnet (for example, in Lucretius's poem "On the Nature of Things", 1st century BC). In the Middle Ages, a magnetic compass began to be widely used in Europe (from the 12th century), attempts were made to experimentally study the interaction of magnets of different shapes (Pierre Peregrine de Maricourt, 1269). The results of M.'s research in the Renaissance were summarized in the work of W. Hilbert "On the magnet, magnetic bodies and the large magnet - the Earth" (1600). Hilbert showed, in particular, that the Earth is a magnetic dipole, and proved the impossibility of separating two opposite poles of a magnet. Further, the doctrine of M. developed in the works of R. Descartes, F. Epinus, S. Coulomb. Descartes was the author of the first detailed metaphysical. theories of M. and geomagnetism ("The Beginnings of Philosophy", Part 4, 1644); he proceeded from the existence of a special magnetic substance that determines by its presence and movement of M. bodies.

In his treatise "The Experience of the Theory of Electricity and Magnetism" (1759), Epinus emphasized the close analogy between electricity. and magnetic phenomena. This analogy, as shown by Coulomb (1785-89), has a certain amount of expression: the interaction of point magnetic poles obeys the same law as the interaction of point electricity. charges (Coulomb law). In 1820 X. Oersted discovered the electric magnetic field. current.

In the same year, A. Ampere established the laws of magnetic interaction of currents, the equivalence of the magnetic properties of a circular current and a thin flat magnet; M. he explained the existence of molecular currents. In the 30s of the 19th century, K. Gauss and V. Weber developed mathematics. the theory of geomagnetism and developed methods of magnetic measurements.

A new stage in the study of M. begins with the works of M. Faraday, who gave a consistent interpretation of the phenomena of M. on the basis of ideas about the reality of the electromagnetic field. A number of important discoveries in the field of electromagnetism (electromagnetic induction - Faraday, 1831; Lenz's rule - E.H. Lenz, 1833, etc.), generalization of open electromagnetic phenomena in the works of J. K. Maxwell (1872), systematic study of the properties of ferromagnets and paramagnets (A. G. Stoletov, 1872; P. Curie, 1895, etc.) laid the foundations of modern physics. the macroscopic theory of M.

A microscopic approach to the study of M. became possible after the discovery of the electron-nuclear structure of atoms. On the basis of the classical electronic theory of X. A. Lorentz, P. Langevin constructed the theory of diamagnetism in 1905 (he also created the quasi-classical theory of paramagnetism). In 1892 B.L. Rosing and in 1907 P. Weiss expressed the idea of the existence of an internal molecular field that determines the properties of ferromagnets. The discovery of the electron spin and its magnetism (S. Goudsmith, J. Y. Uhlenbeck, 1925), the creation of a sequence. microscopic theories. phenomena - quantum mechanics - led to the development of the quantum theory of dia-, para- and ferromagnetism. On the basis of quantum mechanical representations (spatial quantization), L. Brillouin in 1926 found the dependence of the magnetization of paramagnets on the external magnetic field and the temp. F. Hund in 1927 compared experimental and theoretical. the values of the effective magnetic moments of ions in various paramagnetic salts, which led to the elucidation of the influence of electricity. fields of a paramagnetic crystal on the "freezing" of the orbital moments of ions - as it was found, the magnetization of the crystal is determined almost exclusively by spin moments (V.Penny and R. Schlepp; J.Van Fleck, 1932). In the 30s, a quantum-mechanical was built. tevria of magnetic properties of free electrons (paramagnetism of Pauli, 1927; Landau diamagnetism, 1930). The phenomenon of electron paramagnetic resonance (EPR) predicted by J. G. Dorfman (1923) and then discovered by E. K. Zavoysky (1944) was of significant importance for the further development of the theory of paramagnetism.

The creation of the quantum theory of ferromagnetism was preceded by the work of him. physics of E. Ising (1925, two-dimensional model of ferromagnets), Dorfman (1927, he proved the non-magnetic nature of the molecular field), V. Heisenberg (1926, quantum mechanical calculation of the helium atom), V. Geitler and F. London (1927, calculation of the hydrogen molecule). In the last two works, the effect of the exchange (electrostatic) interaction of electrons, discovered in quantum mechanics, was used (p. Dirac, 1926) in the shell of atoms and molecules and its connection with the magnetic properties of electronic systems obeying Fermi-Dirac statistics (Pauli principle) is established. The quantum theory of ferromagnetism was initiated by the works of J. I. Frenkel (1928, collectivized model) and Heisenberg (1928, localized spin model). Consideration of ferromagnetism as a quantum cooperative phenomenon (F. Bloch, J. Slater, 1930) led to the discovery of spin waves. In 1932-33 l . Neel and L. D. Landau predicted the existence of antiferromagnetism. The study of new classes of magnetic substances - antiferromagnets and ferrites - allowed a deeper understanding of the nature of M. The role of magnetoelastic energy in the origin of magnetic anisotropy energy was clarified, the theory of the domain structure was constructed and methods of its experimental study were mastered.

The development of M. has been greatly facilitated by the creation of new experimental methods for the study of substances. Neutronographic. the methods allowed us to determine the types of atomic magnetic structures. Ferromagnetic resonance, originally discovered and investigated in the works of V. K. Arkadiev (1913), and then J. Griffiths (1946), and antiferromagnetic resonance (K. Gorter et al., 1951) allowed experimental studies of magnetic relaxation processes to begin, and also provided an independent method for determining effective anisotropy fields in ferro- and antiferromagnets.

Nuclear magnetic resonance (E. Purcell et al., 1946) and the Mössbauer effect (1958) have significantly deepened our knowledge about the distribution of spin density in matter, especially in metals. ferromagnets. Observation of neutron and light scattering made it possible to determine the spectra of spin waves for a number of substances. In parallel with these experimental works, various aspects of the theory of M were also developed: the theory of magnetic symmetry of crystals, the ferromagnetism of collectivized electrons, the theory of phase transitions of the second kind and critical phenomena, as well as models of one-dimensional and two-dimensional ferro- and antiferromagnets.

The development of the physics of magnetic phenomena has led to the synthesis of new promising magnetic materials: ferrites for RF and microwave devices, highly coercive compounds of the SmCo5 type (see permanent magnet), transparent ferromagnets, etc.