Today it is almost impossible to find a technical industry where hard magnetic materials are not used, and permanent magnets are not used. This is acoustics, and electronics, and computer, and measuring equipment, and automation, and heat and power, and electric power, and construction, and metallurgy, and any kind of transport, and agriculture, and medicine, and ore preparation, and even everyone’s kitchen microwave oven stands, warms up pizza. Not to list everything, magnetotwured materials accompany us at every step of our lives. And all products with their help work on completely different principles: engines and generators have their own functions, and braking devices have their own, the separator does one thing, and the flaw detector does another. There is probably no complete list of technical devices that use magnetically hard materials, there are so many of them.
What are magnetic systems
Our planet itself is an exceptionally well-functioning magnetic system. All the others are also built on the same principle. Hard magnetic materials have very diverse functional properties. It is not in vain that suppliers' catalogs give not only their parameters, but also physical properties. In addition, it can be magnetically hard and magnetically soft materials. For example, take resonance tomographs, where systems with a highly homogeneous magnetic field are used, and compare with separators, where the field is sharply inhomogeneous. A completely different principle! Magnetic systems have been mastered, where the field can turn on and off. That’s how captures work. And some systems even change the magnetic field in space. These are well-known klystrons and traveling wave lamps. The properties of soft and hard magnetic materials are truly magical. They are like catalysts, almost always act as intermediaries, but without the slightest loss of their own energy they are able to transform someone else's, turning one species into another.
For example, a magnetic pulse is converted into mechanical energy in the operation of couplings, separators, and the like. Mechanical energy is converted by magnets into electrical energy if we are dealing with microphones and generators. And vice versa it happens! In speakers and motors, magnets turn electricity into mechanical energy, for example. And that's not all. Mechanical can even be converted into thermal energy, as the magnetic system does in the operation of a microwave oven or in a braking device. Hard magnetic and soft magnetic materials are also capable of special effects - in Hall sensors, in magnetic resonance tomographs, in the operation of microwave communications. A separate article can be written on the catalytic effect on chemical processes, how gradient magnetic fields in water affect the structures of ions, protein molecules, and dissolved gases.
Ancient magic
The natural material - magnetite - was known to mankind several millennia ago. Then they did not yet know all the properties of hard magnetic materials, and therefore they were not used in technical devices. And there were no technical devices yet. Nobody knew how to make calculations for the operation of magnetic systems. But the impact on biological objects has already been noticed. The use of hard magnetic materials at first went purely for medical purposes, until in the third century BC, the Chinese came up with a compass. However, treatment with a magnet has not stopped until today, even despite the fact that there is constant discussion about the dangers of such methods. Particularly active is the use of hard magnetic materials in medicine in the USA, China, and Japan. And in Russia there are adherents of alternative methods, although it is impossible to measure the magnitude of the effect on the body or plant with any device.
But back to the story. In Asia Minor many centuries ago, the ancient city of Magnesia already existed on the banks of the deep Meander. And today you can visit its picturesque ruins in Turkey. It was there that magnetic iron ore was discovered for the first time, which was named after the city. Quite quickly, it spread throughout the world, and the Chinese five thousand years ago with its help invented until now a non-dying navigation device. Now mankind has learned to produce magnets artificially on an industrial scale. The basis for them are a variety of ferromagnets. The largest natural magnet is stored at the University of Tartu , capable of lifting about forty kilograms, while it weighs only thirteen. Today's powder - from cobalt, iron and various other additives, they hold loads five thousand times more than they weigh.
Hysteresis loop
There are two types of artificial magnets. The first type is constants, which are made of hard magnetic materials, their properties are in no way associated with external sources or currents. The second type is electromagnets. They have a core made of iron, a magnetically soft material, and current flows through the winding of this core, which creates a magnetic field. Now you need to consider the principles of its work. The hysteresis loop for magnetically hard materials characterizes the magnetic properties . There are quite sophisticated technologies for the manufacture of magnetic systems, and therefore information is needed on magnetization, magnetic permeability, and energy losses when magnetization reversal occurs. If the change in tension is cyclic, the magnetization reversal curve (induction change) will always look like a closed curve. This is the hysteresis loop. If the field is weak, then the loop is more like an ellipse.
When the magnetic field strength increases, a whole series of such loops, enclosed in each other, is obtained. In the process of magnetization, all vectors are oriented along, and at the end there will be a state of technical saturation, the material will be completely magnetized. The loop obtained during saturation is called the limit loop; it shows the maximum achieved value of the induction Bs (saturation induction). When the tension decreases, residual induction is maintained. The area of the hysteresis loops in the limiting and intermediate state indicates energy dissipation, i.e., hysteresis losses. It depends most of all on the magnetization reversal frequency, material properties, and geometric dimensions. The following characteristics of magnetically hard materials can be determined by the limiting hysteresis loop: saturation induction Bs, residual induction Bc, and coercive force Hc.
Magnetization curve
This curve is the most important characteristic, since it shows the dependence of the magnetization and intensity of the external field. Magnetic induction is measured in tesla and is associated with magnetization. The switching curve is the main one, this is the place of the vertices on the hysteresis loops that were obtained during cyclic magnetization reversal. This reflects a change in magnetic induction, which depends on the field strength. When the magnetic circuit is closed, the field strength reflected in the form of a toroid is equal to the strength of the external field. If the magnetic circuit is open, poles appear at the ends of the magnet, which create demagnetization. The difference between these stresses determines the internal tension of the material.
The main curve has characteristic regions that stand out when a single crystal of a ferromagnet is magnetized. The first section shows the process of shifting the boundaries of unfavorably tuned domains, and in the second, the magnetization vectors unfold to an external magnetic field. The third section is the para process, the final stage of magnetization, here the magnetic field is strong and directed. The use of soft magnetic and hard magnetic materials to a large extent depends on the characteristics obtained using the magnetization curve.
Permeability and energy loss
To characterize the behavior of the material in the field of tension, it is necessary to use such a concept as absolute magnetic permeability. There are definitions of impulse, differential, maximum, initial, normal magnetic permeability. Relative is traced along the main curve, therefore, this definition is not used - for simplicity. Magnetic permeability under conditions when H = 0 is called initial, and it can be determined only in weak fields, up to about 0.1 units. The maximum, on the contrary, characterizes the greatest magnetic permeability. The normal and maximum values provide an opportunity to observe the normal course of the process in each particular case. In the saturation region in strong fields, the magnetic permeability always tends to unity. All these values are necessary for the use of hard magnetic materials; they are always used.
Loss of energy during magnetization reversal. Electricity is released in the material as heat, and its losses are composed of dynamic losses and hysteresis losses. The latter are obtained by shifting the walls of the domains, when the magnetization process is just beginning. Since the magnetic material has a heterogeneous structure, energy is necessarily expended in aligning the walls of the domains. And dynamic losses are obtained in connection with eddy currents that occur at the moment of changing the intensity and direction of the magnetic field. Energy is dissipated in the same way. And losses due to eddy currents exceed even hysteresis losses at high frequencies. Also, dynamic losses are obtained due to residual changes in the state of the magnetic field after the intensity has changed. The amount of aftereffect losses depends on the composition, on the heat treatment of the material, they appear purely at high frequencies. The consequence is magnetic viscosity, and these losses are always taken into account if ferromagnets are used in pulsed mode.
Classification of hard magnetic materials
The terms referring to softness and hardness are not strictly related to mechanical properties. Many solid materials are actually magnetically soft, and also, from a mechanical point of view, soft materials are quite magnetically hard. The process of magnetization in both groups of materials is the same. First, the domain boundaries are shifted, then the rotation begins in the direction of an increasingly magnetizing field, and finally the paraprocess occurs. And here the difference appears. The magnetization curve shows that it is easier to shift the boundaries, less energy is spent, but the rotation process and the para process are more energy-intensive. Soft magnetic materials are magnetized by shifting the boundaries. Hard magnetic - due to rotation and para process.
The shape of the hysteresis loop is approximately the same for those and other groups of materials, saturation induction and residual are also close to equal, but the difference exists in the coercive force, and it is very large. For magnetically hard materials, Hs = 800 kA-m, and for magnetically soft materials, only 0.4 A-m. Total, the difference is huge: 2 * 106 times. That is why, based on these characteristics, such a separation was adopted. Although, we must admit that it is quite arbitrary. Soft magnetic materials can saturate even in a weak magnetic field. Apply them in low-frequency fields. For example, in magnetic memory devices. Hard magnetic materials are difficult to magnetize, but they remain magnetized for a very long time. It is from them that good permanent magnets are obtained. The fields of application of hard magnetic materials are numerous and extensive, some are listed at the beginning of the article. There is another group - magnetic materials for special purposes, their scope is very narrow.
In detail about magnetic hardness
As already mentioned, magnetically hard materials have a wide hysteresis loop and a large coercive force, low magnetic permeability. They are characterized by the maximum specific magnetic energy given into space. And the "harder" the magnetic material, the higher its strength, the lower the permeability. Specific magnetic energy is given the most important role in assessing the quality of the material. A permanent magnet practically does not give off energy to the outer space when the magnetic circuit is closed, because all the lines of force are inside the core, but not outside its magnetic field. In order to maximize the use of the energy of permanent magnets, an air gap of a strictly defined size and configuration is created inside the closed magnetic circuit.
Over time, the magnet “ages”, its magnetic flux decreases. However, such aging can be both irreversible and reversible. In the latter case, the causes of its aging are shocks, shocks, temperature fluctuations, constant external fields. Magnetic induction is reduced. But it can be re-magnetized, thus restoring its excellent properties. But if the permanent magnet has undergone any structural changes, re-magnetization will not help, aging will not be eliminated. But they serve for a long time, and the purpose of magnetically hard materials is great. Examples are literally at every turn. These are not only permanent magnets. This material is for storing information, for recording it - and audio, and digital, and video. But the above is only a small part of the application of hard magnetic materials.
Hard magnetic cast materials
By the method of preparation and composition, hard magnetic materials can be cast, powder, and others. They are based on alloys of iron, nickel, aluminum and iron, nickel, cobalt. These compositions are the most basic in order to obtain a permanent magnet. They relate to precision, because their number is determined by the strictest technological factors. Cast magnetically hard materials are obtained during dispersion hardening of the alloy, where cooling occurs at a calculated rate from melting to the onset of decomposition, which occurs in two phases.
The first is when the composition is close to pure iron with pronounced magnetic properties. As it were, plates of single-domain thickness appear. And the second phase is closer to the intermetallic compound in composition, where nickel and aluminum have low magnetic properties. The result is a system where the non-magnetic phase is combined with strongly magnetic inclusions with a large coercive force. But this alloy is not good enough in magnetic properties. The most common is another alloyed composition: iron, nickel, aluminum and copper with cobalt for alloying. Cobalt-free alloys have lower magnetic properties, but they are much cheaper.
Hard magnetic powder materials
Powder materials are used for miniature, but complex shapes of permanent magnets. They are cermet, metalloplastic, oxide and micropowder. Cermets are especially good. In terms of magnetic properties, inferior to cast ones quite a bit, but somewhat more expensive than them. Cermet magnets are made by pressing metal powders without any binding material and sintering them at very high temperatures. Powders are used with the alloys described above, as well as based on platinum and rare earth metals.
Powder metallurgy is superior to casting in mechanical strength, but the magnetic properties of cermet magnets nevertheless turn out to be slightly lower than in cast ones. On a platinum basis, magnets have very high coercive forces, and the parameters are highly stable. Alloys with uranium and rare-earth metals record maximum values of maximum magnetic energy: the limit value is 112 kJ per square meter. Such alloys are obtained by cold pressing of the powder to the highest degree of density, then the briquettes are sintered with the presence of a liquid phase and molding of a multicomponent composition. By simple casting, it is not possible to mix the components so well.
Other hard magnetic materials
Hard magnetic materials include those with a highly specialized purpose. These are elastic magnets, plastically deformable alloys, materials for information carriers and liquid magnets. Deformable magnets have remarkable plastic properties, lend themselves perfectly to any type of mechanical processing - stamping, cutting, machining. But such magnets are expensive.Kunife magnets made of copper, nickel and iron are anisotropic, that is, they are magnetized towards rolling, they are used in the form of stamping and wire. Vicall magnets made of cobalt and vanadium are made in the form of a magnetic tape of high strength, as well as wire. This composition is good for very small magnets with the most complex configuration.
Elastic magnets - on a rubber basis, in which the filler is a fine powder of hard magnetic material. Most often it is barium ferrite. This method allows to obtain products of any shape with high manufacturability. They are also perfectly cut with scissors, bent, stamped, twisted. They cost much cheaper. Magnetic rubber is used as sheets of magnetic memory for computers, in television, for corrective systems. As storage media, magnetic materials meet many requirements. This is a high-level residual induction, a small effect of self-demagnetization (otherwise information will be lost), a high value of coercive force. And to facilitate the process of erasing records, you need just a small amount of this force, but this contradiction is removed with the help of technology.