20.11.201927.05.2017

The magnetic field is created not only by electric currents, but also by permanent magnets.

Magnetization of matter. Permanent magnets can be made from only relatively few substances, but all substances placed in a magnetic field become magnetized, that is, they themselves become sources magnetic field. As a result, the vector of magnetic induction in the presence of matter differs from the vector of magnetic induction in vacuum.

Ampère's hypothesis. The reason why bodies have magnetic properties was established by the French scientist Ampère. First, under the direct impression of observing a magnetic needle turning near a current-carrying conductor in Oersted's experiments, Lmier suggested that the Earth's magnetism is caused by currents passing inside the globe. The main step was taken: the magnetic properties of the body can be explained by the currents circulating inside it. Further, Ampère came to a general conclusion: the magnetic properties of any body are determined by closed electric currents inside it. This decisive step from the possibility of explaining the magnetic properties of a body by currents to the categorical statement that magnetic interactions are interactions of currents is evidence of Ampère's great scientific courage.

According to Ampère's hypothesis, elementary electric currents circulate inside molecules and atoms. (Now we know well that these currents are formed due to the movement of electrons in atoms.) If the planes in which these currents circulate are located randomly with respect to each other due to the thermal movement of molecules (Fig. 1.28, a), then their actions are mutually compensated, and the body does not show any magnetic properties. In the magnetized state, the elementary currents in the body are oriented so that their actions add up (Fig. 1.28, b). Ampere's hypothesis explains why a magnetic needle and a frame (circuit) with current in a magnetic field behave in the same way (see § 2). The arrow can be viewed as a collection of small current-carrying circuits oriented in the same way. The strongest magnetic fields are created by substances called ferromagnets. Magnetic fields are created by ferromagnets not only due to the circulation of electrons around nuclei, but also due to their own rotation.

The intrinsic torque (momentum) of an electron is called spin. Electrons always seem to rotate around their axis and, having a charge, create a magnetic field along with the field that appears due to their orbital motion around the nuclei. In ferromagnets, there are regions with parallel spin orientations called domains; the domain sizes are about 0.5 μm. The parallel orientation of the spins ensures the minimum potential energy. If the ferromagnet is not magnetized, then the orientation of the domains is chaotic, and the total magnetic field created by the domains is zero. When an external magnetic field is turned on, the domains are oriented along the magnetic induction lines of this field, and the magnetic field induction in ferromagnets increases, becoming thousands and even millions of times greater than the external field induction.

Curie temperature. At temperatures greater than some specific for a given ferromagnet, its ferromagnetic properties disappear. This temperature is called the Curie temperature after the French scientist who discovered this phenomenon. If you heat a magnetized nail strongly enough, it will lose its ability to attract iron objects to itself. The Curie temperature for iron is 753°C, for nickel 365°C, and for cobalt 1000°C. There are ferromagnetic alloys whose Curie temperature is less than 100 °C. The first detailed studies of the magnetic properties of ferromagnets were carried out by the outstanding Russian physicist A. G. Stoletov (1839-1896).

Ferromagnets and their applications. Although there are not so many ferromagnetic bodies in nature, it is their magnetic properties that have received the greatest practical use. The iron or steel core in the coil amplifies the magnetic field created by it many times over without increasing the current in the coil. This saves electricity. The cores of transformers, generators, electric motors, etc. are made from ferromagnets. When the external magnetic field is turned off, the ferromagnet remains magnetized, that is, it creates a magnetic field in the surrounding space. This is due to the fact that the domains do not return to their previous position and their orientation is partially preserved. Because of this, there are permanent magnets. Permanent magnets are widely used in electrical measuring instruments, loudspeakers and telephones, sound recorders, magnetic compasses, etc. Ferrites are widely used ferromagnetic materials that do not conduct electric current. They are chemical compounds of iron oxides with oxides of other substances. One of the well-known ferromagnetic materials - magnetic iron ore - is ferrite.

Magnetic recording of information. Ferromagnets are used to make magnetic tapes and thin magnetic films. Magnetic tapes are widely used for sound recording in tape recorders and for video recording in VCRs.

Magnetic tape is a flexible base made of PVC or other materials. A working layer is applied to it in the form of a magnetic varnish, consisting of very small needle-shaped particles of iron or other ferromagnet and binders. Sound is recorded on tape using an electromagnet, the magnetic field of which changes in time with sound vibrations. When the tape moves near the magnetic head, various sections of the film are magnetized. The diagram of the magnetic induction head is shown in Figure 1.29, a, where 1 is the core of the electromagnet; 2 - magnetic tape; 3 - working clearance; 4 - electromagnet winding.

When playing sound, the reverse process is observed: the magnetized tape excites electrical signals in the magnetic head, which, after amplification, are fed to the speaker of the tape recorder. Thin magnetic films consist of a layer of ferromagnetic material with a thickness of 0.03 to 10 µm.

They are used in the storage devices of electronic computers (computers). Magnetic tapes are designed to record, store and reproduce information. They are applied to a thin aluminum disc or drum. Information is recorded and played back in much the same way as in a conventional tape recorder. Recording information in a computer can also be done on magnetic tapes. The development of magnetic recording technology has led to the emergence of magnetic microheads, which are used in computers, making it possible to create previously unthinkable magnetic recording density. A ferromagnetic hard disk less than 8 cm in diameter stores up to several terabytes (10 12 bytes) of information. Reading and writing information on such a disk is carried out using a microhead located on a rotary lever (Fig. 1.29, b). The disk itself rotates at a tremendous speed, and the head floats above it in a stream of air, which prevents the possibility of mechanical damage to the disk. All substances placed in a magnetic field create their own field. The strongest fields are produced by ferromagnets. Permanent magnets are made of them, since the field of a ferromagnet does not disappear after the magnetizing field is turned off. Ferromagnets are widely used in practice.

Magnetic fields are created either by permanent magnets or by currents. In 1820, A. Amper put forward a bold hypothesis, according to which the magnetic properties of a substance (including permanent magnets) arise due to molecular currents circulating in the molecules of the substance. Further development Science has confirmed this idea of Ampère. However, the theory of the magnetic properties of matter was constructed only after the structure of the atom was studied. In most substances inside atoms, the magnetic fields of individual electrons, as well as the magnetic fields of individual atoms and molecules, are completely or almost completely compensated. Therefore, their magnetic properties are very weak; they are called non-magnetic. However, there are a number of substances, such as iron, cobalt, nickel and some rare earth elements (lanthanides), as well as some alloys, which have strong magnetic properties. These substances are called ferromagnets. (The word "ferromagnet" is derived from the Latin word ferrum- iron). Ferromagnets have a very strong effect on the magnetic field. If a ferromagnetic core is introduced into a coil with current, then the magnetic field is amplified hundreds and even thousands of times. This is widely used in technology: the cores of electromagnets, relays and many other devices are made from ferromagnets, and most often from special grades of steel. Ferromagnets are divided into two classes: soft and hard magnetic materials. The modern theory of ferromagnetism was created about 50 years ago. A great contribution to the creation of this theory was made by domestic scientists Ya. I. Frenkel, L. D. Landau, E. M. Livshits. Each ferromagnet is characterized by a certain temperature, above which it loses the ability to strong magnetization and its magnetic properties are the same as those of non-magnetic substances. This temperature is called the Curie point after Pierre Curie, who discovered this phenomenon in 1895. The Curie point for iron is 770 °C, for nickel 358 °C, for the rare earth element gadolinium 16 °C, for the permalloy alloy about 400 °C, for the permendur alloy about 900 °C, etc. Ferromagnetic properties are not observed in liquids , nor for gases. They are characteristic only for some crystals at temperatures below the Curie point.

A magnetic field- a force field acting on moving electric charges and on bodies with a magnetic moment, regardless of the state of their movement, the magnetic component of the electromagnetic field. The magnetic field can be created by the current of charged particles and/or by the magnetic moments of electrons in atoms (and by the magnetic moments of other particles, although to a much lesser extent) (permanent magnets). In addition, it appears in the presence of a time-varying electric field. The main power characteristic of the magnetic field is magnetic induction vector (magnetic field induction vector) . From a mathematical point of view, it is a vector field that defines and specifies the physical concept of a magnetic field. Often the vector of magnetic induction is called simply a magnetic field for brevity (although this is probably not the most strict use of the term). Another fundamental characteristic of the magnetic field (alternative magnetic induction and closely related to it, practically equal to it in physical meaning) is vector potential . Often in the literature, as the main characteristic of the magnetic field in vacuum (that is, in the absence of a magnetic medium), it is not the magnetic induction vector that is chosen, but the magnetic field strength vector, which formally can be done, since these two vectors coincide in vacuum; however, in a magnetic medium, the vector does not already have the same physical meaning, being an important, but still auxiliary quantity. Therefore, with the formal equivalence of both approaches for vacuum, from a systematic point of view, it should be considered the main characteristic of the magnetic field, namely the Magnetic field can be called a special type of matter, through which the interaction between moving charged particles or bodies with a magnetic moment is carried out. Magnetic fields are a necessary (in the context of special relativity) consequence of the existence of electric fields. Together, the magnetic and electric fields form an electromagnetic field, the manifestations of which are, in particular, light and all other electromagnetic waves. From the point of view of quantum field theory, magnetic interaction - as a special case of electromagnetic interaction is transferred by a fundamental massless boson - a photon (a particle that can be represented as a quantum excitation of an electromagnetic field), often (for example, in all cases of static fields) - virtual.

Any substance in the world has certain magnetic properties. They are measured by magnetic permeability. In this article, we will consider the magnetic properties of matter.

Ampère hypothesis

Magnetic permeability shows how many times less or more magnetic field induction in a given medium of magnetic field induction in vacuum.

A substance that creates its own magnetic field is called magnetized. Magnetization occurs when a substance is placed in an external magnetic field.

The French scientist Ampère established the cause, the consequence of which is the possession of magnetic properties by bodies. Ampere's hypothesis states that there are microscopic electric currents inside the substance (an electron has its own magnetic moment, which is of a quantum nature, orbital motion in electron atoms). It is they who determine the magnetic properties of matter. If the currents have disordered directions, then the magnetic fields they generate cancel each other out. The body is not magnetized. An external magnetic field regulates these currents. As a result, the substance has its own magnetic field. This is the magnetization of matter.

It is by the reaction of substances to an external magnetic field and by the orderliness of their internal structure that the magnetic properties of a substance are determined. In accordance with these parameters, they are divided into the following groups:

Paramagnets
Diamagnets
ferromagnets
Antiferromagnets

Diamagnets and paramagnets

Substances that have a negative magnetic susceptibility, independent of the strength of the magnetic field, are called diamagnets. Let's see what magnetic properties of a substance are called negative magnetic susceptibility. This is when a magnet is brought to the body, and at the same time it is repelled, not attracted. Diamagnets include, for example, inert gases, hydrogen, phosphorus, zinc, gold, nitrogen, silicon, bismuth, copper, silver. That is, these are substances that are in a superconducting state or have covalent bonds.
Paramagnets. For these substances, the magnetic susceptibility also does not depend on what field strength exists. She is positive though. That is, when a paramagnet approaches a permanent magnet, an attractive force arises. These include aluminum, platinum, oxygen, manganese, iron.

ferromagnets

Substances with a high positive magnetic susceptibility are called ferromagnets. In these substances, in contrast to diamagnets and paramagnets, the magnetic susceptibility depends on temperature and magnetic field strength, and to a large extent. These include crystals of nickel and cobalt.

Antiferromagnets and ferrimagnets

Substances in which, during heating, a phase transition of a given substance occurs, accompanied by the appearance of paramagnetic properties, are called antiferromagnets. If the temperature becomes below a certain one, these properties of the substance will not be observed. Examples of these substances would be manganese and chromium.
Ferrimagnets are characterized by the presence of uncompensated antiferromagnetism in them. Their magnetic susceptibility also depends on temperature and magnetic field strength. But they still have differences. These substances include various oxides.

All of the above magnets can be further divided into 2 categories:

hard magnetic materials. These are materials with a high value of coercive force. For their magnetization reversal, it is necessary to create a powerful magnetic field. These materials are used in the manufacture of permanent magnets.
Soft magnetic materials, on the contrary, have a small coercive force. In weak magnetic fields, they are able to enter saturation. They have low losses for magnetization reversal. Because of this, these materials are used to make cores for electrical machines that run on alternating current. This, for example, is a current and voltage transformer, or a generator, or an asynchronous motor.

We examined all the basic magnetic properties of matter and figured out what types of magnets exist.

MAGNETIC PROPERTIES OF SUBSTANCES

Magnetism is a fundamental property of matter. Since ancient times, the property of permanent magnets to attract iron objects has been known. For many centuries, there was a legend among navigators about a magnetic rock, which is supposedly capable of attracting iron nails from a ship sailing too close to it and destroying it. Fortunately, such a strong magnetic field can only exist in the vicinity of neutron stars. The development of electromagnetism made it possible to create electromagnets stronger than the constants existing in nature. In general, various instruments and devices based on the use of electromagnetic phenomena are so widely distributed that now it is impossible to imagine life without them.

However, not only permanent magnets interact with a magnetic field, but also all other substances. The magnetic field, interacting with matter, changes its magnitude compared to vacuum (here and below, all formulas are written in the SI system):

where m0 is the magnetic constant equal to 4p " 10-7 H/m, m is the magnetic permeability of the substance, B is the magnetic induction (in T), H is the magnetic field strength (in A/m). For most substances, m is very close to unit, therefore, in magnetochemistry, where the main object is a molecule, it is more convenient to use the value c defined by the equation, which is called magnetic susceptibility.c can be attributed to a unit of volume, mass or amount of substance, then it is called, respectively, volumetric (dimensionless) cv, specific cd ( in cm3 / g) or molar cm (in cm3 / mol) magnetic susceptibility. It is clear that, following formula (2), vacuum c is zero. Substances can be divided into two categories: those that weaken the magnetic field (c 0), - paramagnets (Fig. 1). One can imagine that in an inhomogeneous magnetic field, a force acts on a diamagnet, pushing it out of the field, on a paramagnet, on the contrary, it pulls in. The methods considered below for measuring the magnetic properties of substances are based on this. Diamagnets (and this is the vast majority of organic and high-molecular compounds) and mainly paramagnets are the objects of study of magnetochemistry.

Diamagnetism is the most important property of matter, due to the fact that under the influence of a magnetic field, electrons in filled electron shells (which can be represented as small conductors) begin to precess, and, as you know, any movement of an electric charge causes a magnetic field, which, according to the Lenz rule, will be directed as to reduce the impact from the external field. In this case, the electronic precession can be considered as circular currents. Diamagnetism is characteristic of all substances, except for atomic hydrogen, because all substances have paired electrons and filled electron shells.

Paramagnetism is caused by unpaired electrons, which are called so because their own magnetic moment (spin) is not balanced by anything (respectively, the spins of paired electrons are directed in opposite directions and compensate each other). In a magnetic field, the spins tend to line up in the direction of the field, strengthening it, although this order is disturbed by chaotic thermal motion. Therefore, it is clear that the paramagnetic susceptibility depends on temperature - the lower the temperature, the higher the value of cm. In the simplest case, this is expressed by a relationship called the Curie law: where C is the Curie constant, or the Curie-Weiss law, where q is the Weiss correction. This type of magnetic susceptibility is also called orientational paramagnetism, since its cause is the orientation of elementary magnetic moments in an external magnetic field.

The magnetic properties of electrons in an atom can be described in two ways. In the first method, it is considered that the own (spin) magnetic moment of the electron does not affect the orbital moment (due to the movement of electrons around the nucleus) or vice versa. More precisely, such a mutual influence always exists (spin-orbit interaction), but for 3d ions it is small, and the magnetic properties can be described with sufficient accuracy by two quantum numbers L (orbital) and S (spin). For heavier atoms, such an approximation becomes unacceptable and one more quantum number of the total magnetic moment J is introduced, which can take values from | L+S | before | L-S | . Van Vleck considered the energy contributions of the orbitals depending on the influence of the magnetic field (according to the quantum mechanical perturbation theory, they can be expanded into a series and summarized): where H is the magnetic field strength and, accordingly, E (0) is the contribution independent of the external field, E (1 ) is the contribution directly proportional to the field, and so on. It turned out that the zero-order energy is determined by the spin-orbit interaction, which is important in the description of chemical bonds:

where l is the spin-orbit interaction constant. The first-order energy (of the interaction of the magnetic moment of an unpaired electron (m = gbS) with the magnetic field H) is equal to

where g is the Lande factor, usually equal to two for most compounds, b is the Bohr magneton, equal to 9.27 " 10-19 erg / Oe (recall that the energy of magnetic interactions is the scalar product of the vectors of magnetic moments m and H). E ( 2) - the energy contribution, which will have to be taken for granted, since it depends on the subtle features of the electronic structure and is difficult to explain from the point of view of classical physics.You should pay attention to the smallness of the magnetic interaction energy (for room temperatures and magnetic fields, common in laboratories, the energy of magnetic interactions is three to four orders of magnitude less than the energy of thermal motion of molecules).

After mathematical transformations, the expression for the macroscopic magnetic susceptibility, taking into account the Boltzmann distribution of the ensemble of magnetic moments over energy levels, takes the form (its derivation is presented, for example, in )

This is the Van Vleck equation - the main one in magnetochemistry, which relates magnetic properties to the structure of molecules. Here NA is Avogadro's number, k is Boltzmann's constant. We have already met with some extreme cases of it above. If = 0, and can be neglected, then we obtain as a result the Curie law (cf. equation (3)), but in a more rigorous form.

It can be seen that the Curie law reflects the so-called pure spin magnetism, which is characteristic of most paramagnetic compounds, such as salts of copper, iron, nickel, and other transition metals. If = 0 and @ kT, then the Van Vleck equation is greatly simplified: where Na is the temperature independent (Van Vleck) paramagnetism. As can be seen from the above, Van Vleck paramagnetism is a purely quantum phenomenon and is inexplicable from the standpoint of classical physics. It can be represented as an admixture of excited energy levels to the ground state of the molecule.

There are quite a few substances that, when the temperature is lowered, first behave like paramagnets, and then, when a certain temperature is reached, their magnetic properties change dramatically. The most famous example is ferromagnets and the substance from which they got their name, iron, whose atomic magnetic moments below the Curie temperature (in this case equal to TC = 770 ° C) line up in the same direction, causing spontaneous magnetization. However, macroscopic magnetization does not occur in the absence of a field, since the sample is spontaneously divided into regions about 1 μm in size, called domains, within which the elementary magnetic moments are directed in the same way, but the magnetizations of different domains are randomly oriented and, on average, compensate each other. The forces that cause a ferromagnetic transition can only be explained using the laws of quantum mechanics.

Antiferromagnets are characterized by the fact that the spin magnetic moments at the antiferromagnetic transition temperature (Néel temperature TN) are ordered in such a way that they cancel each other out. The maximum value of the magnetic susceptibility is reached at TN, above which c decreases according to the Curie-Weiss law, below - due to the so-called exchange interactions. Antiferromagnets are, for example, MnO and KNiF3.

If the compensation of magnetic moments is incomplete, then such substances are called ferrimagnets, for example Fe2O3 and FeCr2O4. The last three classes of compounds (Table 1) are solids and are studied mainly by physicists. Over the past decades, physicists and chemists have created new magnetic materials, more details about the properties of which can be found in.

In a molecule containing an unpaired electron, the remaining (paired) electrons weaken the magnetic field, but the contribution of each of them is two to three orders of magnitude smaller. However, if we want to measure the magnetic properties of unpaired electrons very accurately, then we must introduce the so-called diamagnetic corrections, especially for large organic molecules, where they can reach tens of percent. The diamagnetic susceptibilities of the atoms in a molecule add to each other according to the Pascal-Langevin additivity rule. To do this, the diamagnetic susceptibilities of atoms of each type are multiplied by the number of such atoms in the molecule, and then constitutive corrections are introduced for structural features (double and triple bonds, aromatic rings, etc.). Let us turn to the consideration of how the magnetic properties of substances are experimentally studied.

EXPERIMENTAL MEASUREMENT OF THE MAGNETIC SUSCEPTIBILITY

The main experimental methods for determining the magnetic susceptibility were created in the last century. According to the Gouy method (Fig. 2, a), the change in the sample weight in a magnetic field is measured compared to its absence, which is equal to where Dmg = F is the force acting on the substance in the magnetic field gradient, c is the measured magnetic susceptibility of the substance, c0 - magnetic susceptibility of the medium (air), S is the sample cross-sectional area, Hmax and Hmin are the maximum and minimum strengths of the external magnetic field.

According to the Faraday method (Fig. 2, b), the force acting on the sample in an inhomogeneous magnetic field is measured:

The sample is chosen small so that H0dH / dz remains constant within it, and the maximum value of the parameter is achieved by choosing a special profile of the magnet tips. The main difference between the Gouy method and the Faraday method is that in the first case, inhomogeneity is maintained along the (extended) sample, and in the second, along the magnetic field.

The Quincke method (Fig. 2, c) is used only for liquids and solutions. It measures the change in the height of a liquid column in a capillary under the influence of a magnetic field.

In this case, for diamagnetic liquids, the height of the column decreases, for paramagnetic liquids it increases.

The viscometer method measures the time of fluid flow through a small hole with the magnetic field on (tH) and off (t0). The outflow time of paramagnetic liquids in a magnetic field is noticeably shorter than in the absence of a field, and vice versa for diamagnetic liquids. The difference between the two flow times is determined by the magnetic susceptibility, and the value of the calibration constant k is determined by measuring a liquid with a known magnetic susceptibility. The bulk magnetic susceptibilities of some common solvents are given below.

Magnetic susceptibility can also be measured using an NMR spectrometer. You can read about the physical foundations of the NMR method in. We confine ourselves to what we note: the value of the chemical shift of the NMR signal in the general case is determined not only by the screening constant, which is a measure of the electron density on the nucleus under study, but also by the magnetic susceptibility of the sample. For a sample in the form of a rectangular parallelepiped, the chemical shift is also determined by the orientation of the sample in a magnetic field, where the calibration constants A and B are determined by measuring two liquids with a known magnetic susceptibility (most often water and acetone). This method was developed at the Department of Inorganic Chemistry of Kazan University and is the only one that allows the instrument to be calibrated according to diamagnetic standards, and then measurements can also be made with paramagnetic samples. The magnetic susceptibilities of many substances have been measured in this way. What did they allow to learn about their structure?

The obtained value of the magnetic susceptibility for paramagnets is determined by the number of unpaired electrons (compare with (9) for one unpaired electron)

In this way, the spin quantum number S can be determined, and hence the number of unpaired electrons. It should be noted that in real compounds the g factor somewhat changes from the "purely spin" value, which, as noted above, is equal to two.

The cm values of paramagnetic substances are small and not very convenient in explaining the structure of compounds. Therefore, the paramagnetic susceptibility is more often characterized by the effective magnetic moment meff, which is determined by the equation.

Then, at a temperature of 298 K, the "purely spin" value for one unpaired electron is ms = 1.73 Bohr magnetons (mB), for two - 3.46 mB, and so on. (Table 2). The contribution of other factors, primarily the spin-orbit interaction, is reflected in the value of the g factor and leads to the fact that meff differs from ms.

Knowing the number of unpaired electrons helps to understand some of the features of the placement of elements in the Periodic Table of D.I. Mendeleev. So, electron shells, filled completely or exactly half, have increased stability. With increasing relative atomic mass, we first encounter this with chromium. Compare the electronic configurations in the ground state: Sc 3d 14s 2, Ti 3d 24s 2, V 3d 34s 2, the next chromium is not 3d 44s 2, but 3d 54s 1, the more stable half-filled shell is underlined:

And this was established precisely by measuring the magnetic susceptibility, when it was found that the chromium atom contains six unpaired electrons, and not four. True, for this it was necessary to perform rather subtle measurements on isolated atoms in the gas phase, since the magnetic properties of conductors are not related to the number of unpaired electrons (because valence electrons in metals are not attached to certain atoms, but move randomly throughout the crystal), but are determined quantum laws (the so-called Fermi diamagnetism and Landau paramagnetism). At the same time, for example, the order in which the 5d and 4f orbitals are filled in the lanthanide series does not change the number of unpaired electrons; therefore, the correct electronic configurations were established only in the 1960s by quantum mechanical calculations (the 5d1 and 4f configurations cannot be distinguished from magnetic measurements). 1). Nevertheless, magnetochemical studies make it possible to establish the electronic configuration, as the attentive reader has probably already noticed, of transition metal compounds, which form the basis of the chemistry of coordination (complex) compounds.

Coordination compounds are formed, as a rule, due to a donor-acceptor bond, that is, lone pairs of ligand electrons occupy vacant places in the orbitals of the central atom. In this case, the number of unpaired electrons and the magnetic moment of complexing ions remain the same as for a free ion in the gas phase. This is true for transition metal aqua complexes, for example, iron(II) (Fig. 3). However, there are also magnetically anomalous complexes whose magnetic moment is lower than that of a gaseous ion. Their electronic structure can be explained within the framework of the valence bond method as follows. Very many complex compounds have a coordination number of six. Six ligands are symmetrically located at the vertices of the octahedron. In order to get six hybrid orbitals, six valence orbitals of the central atom must take part in their formation: this redistribution of electron density is called sp3d 2 hybridization (compare with sp3 hybridization of the carbon atom in alkanes, where four bonds are directed to the vertices of the tetrahedron). Please note that d-orbitals with the same serial number as s, p-orbitals take part in the formation of hybrid orbitals. This is explained by the fact that the inner d-orbitals located lower in energy are occupied by the intrinsic electrons of the metal ion. In order to occupy the lower energy orbitals, the ligands must force the metal ion's own electrons to pair and release the inner d-orbitals for the so-called d 2sp 3 hybridization. This can be done only by strong field ligands that form strong bonds with the metal ion, for example, cyanide ions in complex hexacyanoferrate(II) (see Fig. 3).

Accordingly, the first type of complexes, which has a high magnetic moment, is called an outer-orbital complex, and the second type, with a reduced magnetic moment, is called an intra-orbital complex. This difference, which leads to a change in the number of unpaired electrons in the complex, leads to a change in the magnetic moments of the outer- and inner-orbital complexes, respectively, and is caused by the energy inequality of the corresponding d-orbitals (usually called the splitting energy in the ligand field and denoted by D or 10Dq).

According to the ability to form intraorbital complexes (in terms of D value), all ligands can be arranged in a series, which is called the spectrochemical series of ligands:

CN->NO2->SO32->NH3>NCS->H3O>

>OH->F->Cl->Br->I-

It got its name because the color of the complex depends on the position of the ligand in this series, and this shows the relationship between the optical and magnetic properties of coordination compounds.

Thus, by measuring the magnetic susceptibility, one can easily judge the degree of oxidation and the geometry of the first coordination sphere in the complex. Data on the magnetic susceptibility of a number of transition metal and lanthanide ions are given in Table. 2. It can be seen that the magnetic properties of 3d ions in most cases are in good agreement with purely spin values ms, and to explain the magnetic properties of lanthanides, a more complex model is required with the use of the quantum number J mentioned above.

It is known that most important in practice chemical reactions occur in solutions, they also include complex formation reactions; therefore, in the next section we will consider the magnetic properties of solutions in which transition metal compounds are realized in the form of complexes.

MAGNETIC SUSCEPTIBILITY OF SOLUTIONS

When moving from solid body the magnetic susceptibility of the solvent and all solutes should be taken into account. In this case, the simplest way to take this into account will be the summation of the contributions of all components of the solution according to the additivity rule. The principle of additivity is one of fundamental principles in the processing of experimental data. At times, it even fails experimenters, because it is difficult for the human mind to imagine any other mechanism for the interaction of various factors, other than their simple addition. Any deviations from it are more often associated with the fact that the principle of additivity itself is fulfilled, and the components of the solution change their properties. Therefore, it is assumed that the magnetic susceptibility of the solution is equal to the sum of the magnetic susceptibilities of the individual components, taking into account the concentration where ci is the concentration (in mol/l), cmi is the molar magnetic susceptibility of the i-th component of the solution, the coefficient 1/1000 is used to convert to molar concentration. In this case, the summation is carried out over all solutes and the solvent. It can be seen that the contributions of paramagnetic and diamagnetic substances to the measured magnetic susceptibility are opposite in sign and can be separated

cv(meas) = cv(pair) - cv(dia).

When studying the magnetic properties of the same substance in different solvents (Table 3), it can be seen that they can significantly depend on the nature of the solvent. This can be explained by the entry of solvent molecules into the first coordination sphere and the corresponding change in the electronic structure of the complex, the energies of the d-orbitals (D), and other properties of the solvate complex. Thus, magnetochemistry also makes it possible to study solvation, that is, the interaction of a solute with a solvent.

In solutions, the determination of cm and meff of coordination compounds makes it possible, as can be seen from the above theoretical material, to determine a number of structural parameters (l, S, D), which makes magnetochemical studies very valuable. Various complexes of the same metal ion can differ markedly in the magnitude of the effective magnetic moment. Using copper(II) as an example, it can be seen that the effective magnetic moment increases during complex formation, and when a dimeric complex is formed, it decreases due to the antiferromagnetic interaction of unpaired electrons of copper(II) ions. The magnetic properties of copper(II) complex compounds are given below. (When writing the formulas, the abbreviations for ligands used in coordination chemistry are used: acac - acetylacetone CH3COCH3COCH3, H4Tart - tartaric acid HOOC(CHOH)2COOH.)

A few words about "magnetic" water, more precisely, about aqueous solutions (because even distilled water contains impurities, such as dissolved oxygen, and it is paramagnetic). This topic, of course, requires a separate consideration; we will touch on it only in connection with magnetochemistry. If the magnetic field affects the properties of the solution, and numerous experimental facts (measurements of density, viscosity, electrical conductivity, proton concentration, magnetic susceptibility) indicate that this is so, then it should be recognized that the interaction energy of the individual components of the solution and an ensemble of water molecules is quite high, then is comparable to or exceeds the energy of the thermal motion of particles in the solution, which averages out any effect on the solution. Recall that the energy of the magnetic interaction of one particle (molecule) is small compared to the energy of thermal motion. Such an interaction is possible if we accept that in water and aqueous solutions, due to the cooperative nature of hydrogen bonds, large ice-like structural ensembles of water molecules are realized, which can be strengthened or destroyed under the influence of dissolved substances. The energy of formation of such "ensembles" is apparently comparable to the energy of thermal motion, and under magnetic influence, the solution can remember it and acquire new properties, but Brownian motion or an increase in temperature eliminates this "memory" for some time.

Note that by precisely adjusting the concentrations of paramagnetic substances in a diamagnetic solvent, it is possible to create a non-magnetic liquid, that is, one in which the average magnetic susceptibility is zero or in which magnetic fields propagate in exactly the same way as in vacuum. This interesting property has not yet found application in technology.

Magnetic properties of matter

In all bodies placed in a magnetic field, a magnetic moment arises. This phenomenon is called magnetization.

A magnetized body (magnet) creates an additional magnetic field with induction B′, which interacts with the induction B 0 = μ a H due to macroscopic currents. Both fields give the resulting field with induction B, which is obtained as a result of vector addition B' And B 0 .

Closed currents circulate in the molecules of a substance; each such current has a magnetic moment; in the absence of an external magnetic field, the molecular currents are randomly oriented and the average field created by them will be equal to zero. Under the action of a magnetic field, the magnetic moments of molecules are oriented predominantly along the field, as a result of which the substance is magnetized. The measure of the magnetization of a substance (magnet) is the magnetization vector. Vector magnetization I is equal to the vector sum of all magnetic moments pm molecules enclosed in a unit volume of a substance:

The quantity χ is called magnetic susceptibility is a dimensionless quantity.

In the SI system:	In the SGSM system:
B′ = μ I	B′ = 4χ I	2)
B = μ 0 H + μ I	B = H+ 4χ I	3)
μ = 1 + χ	μ = 1 + 4π χ	4)

Curve expressing the relationship between H And B or H And I, is called magnetization curve.

Substances for which χ > 0 (but only slightly) are called paramagnetic ( paramagnets); substances for which χ< 0, называются диамагнитными (diamagnets). Substances with χ much greater than one are called ferromagnets.

Ferromagnets differ from paramagnets and diamagnets in a number of properties.

A) The magnetization curve of ferromagnets has a complex character (Fig. 1), for paramagnets it is a straight line with a positive angular
coefficient, for diamagnets - a straight line with a negative slope. The magnetic susceptibility and permeability of ferromagnets depends on the field strength; paramagnets and diamagnets do not have this dependence.

For ferromagnets, the initial magnetic permeability (μ initial) is usually indicated - the limiting value of the magnetic permeability when the field strength and induction are close to zero, i.e.

Curve of dependence of μ on H for ferromagnets passes through a maximum. The tables usually indicate the maximum value (μ max).

b) The magnetic susceptibility of ferromagnets increases with increasing temperature. At some temperature T to a ferromagnet turns into a paramagnet; this temperature is called Curie temperature (Curie point). At temperatures above the Curie point, matter is paramagnetic. Near the Curie temperature, the magnetic susceptibility of the cutting ferromagnet increases.

The magnetic susceptibility of diamagnets and some paramagnets (for example, in alkali metals) does not depend on temperature. The magnetic susceptibility of paramagnets (with a few exceptions) varies inversely with absolute temperature.

V) A demagnetized ferromagnet is magnetized by a magnetic field; addiction B(or I) from H during magnetization will be expressed by the curve 0–1 (Fig. 1). This curve is called the initial magnetization curve. The magnetization in weak fields grows rapidly, then the growth slows down, and, finally, a saturation state sets in, in which the magnetization remains practically constant with a further increase in the field.

The maximum value of magnetization is called saturation magnetization (I s).

When decreasing H down to zero B(And I) will vary along curve 1–2; the change in induction lags behind the change in field strength. This phenomenon is called magnetic hysteresis.

The magnitude of the induction that is preserved in the ferromagnet after the removal of the field (when H= 0), is called residual induction ( B r). In Fig.1 B r is equal to the segment 0–2. To demagnetize a ferromagnet, it is necessary to remove the residual induction. To do this, you need to create a field of the opposite direction. The change in induction in the pope of the opposite direction will be represented by a curve 2-3-4.

Field strength Hc(segment 0–3 in Fig. 8), at which the induction is zero, is called the coercive intensity (force).

Addiction B(or I) from periodically changing magnetic field strength from + H before - H expressed as a closed curve 1–2–3–4–5–6–1. Such a curve is called hysteresis loop.

For one cycle of changing the field strength from + H before - H the energy consumed is proportional to the area of the hysteresis loop.

The properties of ferromagnets are explained by the presence of regions in them that, in the absence of an external magnetic field, are spontaneously magnetized to saturation. These areas are called domains. But the location and magnetization of these regions are such that even in the absence of a field, the total magnetization of the entire body is zero.

When a ferromagnet is in a magnetic field, the boundaries between the domains shift (in weak fields) and the magnetization vectors of the domains rotate in the direction of the magnetizing field (in stronger fields), as a result of which the ferromagnet becomes magnetized.

A ferromagnet placed in a magnetic field changes its linear dimensions, i.e., it is deformed. This phenomenon is called magnetostriction. The relative elongation depends on the nature of the ferromagnet and the strength of the magnetic field.

The magnitude of the magnetostrictive effect does not depend on the direction of the field; in some substances shortening (nickel) is observed, in others elongation (iron in weak fields) along zero. This phenomenon is used to produce ultrasonic vibrations with frequencies up to 100 kHz.

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FEDERAL EDUCATIONAL AGENCY STATE EDUCATIONAL INSTITUTION OF HIGHER AND PROFESSIONAL EDUCATION

"VORONEZH STATE UNIVERSITY"

(GOU VPO VSU)

Faculty of Geology

Department of Ecological Geology

Essay

on the topic: Magnetic properties of substances

Completed by: 1st year student, gr. #9

Agoshkova Ekaterina Vladimirovna

Reviewer:

Associate Professor, Candidate of Sciences Voronova T.A.

Magnetic properties of substances

Magnetic permeability of a substance

Classification of substances according to the action of an external magnetic field on them

Antiferromagnets and ferrimagnets

permanent magnets

Curie point

Literature

Magnetic properties of substances

Magnetism- a form of interaction of moving electric charges, carried out at a distance by means of a magnetic field.

The magnetic properties of matter are explained according to Ampère's hypothesis.

Ampère hypothesis- the magnetic properties of the body can be explained by currents circulating inside it.

Inside the atoms, due to the movement of electrons in orbits, there are elementary electric currents that create elementary magnetic fields.

1. if the substance does not have magnetic properties - elementary magnetic fields are unoriented (due to thermal motion);

2. if the substance has magnetic properties - the elementary magnetic fields are equally directed (oriented) and the substance's own internal magnetic field is formed.

Magnetized the substance that creates its own magnetic field is called. Magnetization occurs when a substance is placed in an external magnetic field.

magnetism ampere antiferromagnet curie

Magneticand Isubstance permeability

The effect of a substance on an external magnetic field is characterized by the quantity m , which is called the magnetic permeability of a substance.

Magnetic permeability is a physical scalar quantity showing how many times the magnetic field induction in a given substance differs from the magnetic field induction in vacuum.

where is B? -- magnetic field induction in matter; b? 0 -- magnetic field induction in vacuum.

Substance classificationby the action of an external magnetic field on them

1. D and magnetics [m<1]- слабомагнитные вещества, внутреннее магнитное поле направлено противоположно внешнему магнитному полю, но слабовыраженно. Вещества, которые имеют отрицательную магнитную восприимчивость, не зависящую от напряженности магнитного поля.

Negative magnetic susceptibility- this is when a magnet is brought to the body, and at the same time it is repelled, not attracted.

Diamagnets include, for example, inert gases, hydrogen, phosphorus, zinc, gold, nitrogen, silicon, bismuth, copper, silver. That is, these are substances that are in a superconducting state or have covalent bonds.

2. P aramagnets [m>1] - weakly magnetic substances, the internal magnetic field is directed in the same way as the external magnetic field. For these substances, the magnetic susceptibility also does not depend on what field strength exists. She is positive though. That is, when a paramagnet approaches a permanent magnet, an attractive force arises. These include aluminum, platinum, oxygen, manganese, iron.

3. F erromagnets [m>>1] - highly magnetic substances, the internal magnetic field is 100-1000 times greater than the external magnetic field.

In these substances, in contrast to diamagnets and paramagnets, the magnetic susceptibility depends on temperature and magnetic field strength, and to a large extent.

These include crystals of nickel and cobalt.

Antiferromagnets and ferrimagnets

Substances in which, during heating, a phase transition of a given substance occurs, accompanied by the appearance of paramagnetic properties, are called antiferromagnets. If the temperature becomes below a certain one, these properties of the substance will not be observed. Examples of these substances would be manganese and chromium.

Magnetic susceptibility ferrimagnets also depends on temperature and magnetic field strength. But they still have differences. These substances include various oxides.

All of the above magnets can be further divided into 2 categories:

hard magnetic materials. These are materials with a high value of coercive force. For their magnetization reversal, it is necessary to create a powerful magnetic field. These materials are used in the manufacture of permanent magnets.

Soft magnetic materials, on the contrary, have a small coercive force. In weak magnetic fields, they are able to enter saturation. They have low losses for magnetization reversal. Because of this, these materials are used to make cores for electrical machines that run on alternating current. This, for example, is a current and voltage transformer, or a generator, or an asynchronous motor.

permanent magnets

Permanentmagnets are the bodies long time retaining magnetization.

A permanent magnet always has 2 magnetic poles: north (N) and south (S).

The strongest magnetic field of a permanent magnet is at its poles.

Permanent magnets are usually made of iron, steel, cast iron and other iron alloys (strong magnets), as well as nickel, cobalt (weak magnets). Magnets are natural (natural) from iron ore magnetic iron ore and artificial, obtained by magnetizing iron when it is introduced into a magnetic field.

Interaction of magnets: Like poles repel, and opposite poles attract.

The interaction of magnets is explained by the fact that any magnet has a magnetic field, and these magnetic fields interact with each other.

Magnetic field of permanent magnets

What are the reasons for the magnetization of iron? According to the hypothesis of the French scientist Ampere, inside the substance there are elementary electric currents (Ampère currents), which are formed due to the movement of electrons around the nuclei of atoms and around their own axis. When electrons move, elementary magnetic fields arise. When a piece of iron is introduced into an external magnetic field, all elementary magnetic fields in this iron are oriented in the same way in the external magnetic field, forming their own magnetic field. So a piece of iron becomes a magnet.

What does a magnetic field look likepermanent magnets?

An idea of the form of the magnetic field can be obtained using iron filings. One has only to put a sheet of paper on the magnet and sprinkle it with iron filings on top.

For permanent bar magnet For permanent arc magnet

Curie point

Curie point, or Curie temperature, is the temperature of a phase transition of the second kind, associated with an abrupt change in the symmetry properties of a substance with a change in temperature, but for given values of other thermodynamic parameters(pressure, electric or magnetic field strength). The second-order phase transition at the Curie temperature is associated with a change in the symmetry properties of the substance. At T c, in all cases of phase transitions, some type of atomic order disappears, for example, the order of electron spins ( ferroelectrics), atomic magnetic moments ( ferromagnets), orderliness in the arrangement of atoms of different components of the alloy over the nodes of the crystal lattice (phase transitions in alloys). Near T c sharp anomalies of physical properties are observed, for example, piezoelectric, electro-optical, thermal.

The magnetic Curie point is the temperature of such a phase transition, at which the spontaneous magnetization of the domains of ferromagnets disappears, and the ferromagnet passes into a paramagnetic state. With comparatively low temperatures the thermal motion of atoms, which inevitably leads to some violations of the ordered arrangement of magnetic moments, is insignificant. With increasing temperature, its role increases and, finally, at a certain temperature (T c), the thermal motion of atoms is able to destroy the ordered arrangement of magnetic moments, and the ferromagnet turns into a paramagnet. Near the Curie point, a number of features are observed in the change in the non-magnetic properties of ferromagnets (specific resistance, specific heat, temperature coefficient of linear expansion).

The value of T c depends on the strength of the bond of magnetic moments with each other, in the case of a strong bond it reaches: for pure iron T c \u003d 768 ° C, for cobalt T c \u003d 1131 ° C, exceeds 1000 ° C for iron-cobalt alloys. For many substances, T c is small (for nickel, T c \u003d 358 ° C). The value of T c can be used to estimate the binding energy of magnetic moments with each other. To destroy the ordered arrangement of magnetic moments, the energy of thermal motion is needed, which is much greater than both the energy of the interaction of dipoles and the potential energy of the magnetic dipole in the field.

At the Curie temperature, the magnetic permeability of a ferromagnet becomes approximately equal to one, above the Curie point, the change in magnetic susceptibility obeys Curie-Weiss law.

For each ferromagnet there is a certain temperature - the Curie point.

1. If t substances< t Кюри, то вещество обладает ферромагнитными свойствами.

2. If t of a substance > t Curie, then the ferromagnetic properties (magnetization) disappear, and the substance becomes a paramagnet. Therefore, permanent magnets lose their magnetic properties when heated.

Literature

Zhilko, V. V. Physics: textbook. allowance for the 11th grade. general education school from Russian lang. training / V.V. Zhilko, A.V. Lavrinenko, L. G. Markovich. -- Mn.: Nar. asveta, 2002. - S. 291-297.

http://msk.edu.ua/

http://elhow.ru/

http://class-fizika.narod.ru/

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