Magnets from A to Z!

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Permanent magnet: A ferromagnetic material that creates a persistent magnetic field in its surroundings without a continuous supply of electrical energy. This magnetic state can be eliminated only by a sufficiently strong opposing external field.

Reversal process (during magnetisation): The reversible part of the remagnetisation process which, at high field strengths, merges into the irreversible process. During this process, Weiss domains of equal potential energy are oriented and arranged in a particular direction, requiring a considerable amount of energy. At low field strengths, a pure reversal process can be observed only in the so-called single-domain case. Here, the entire volume reverses simultaneously rather than successively in time, so the process occurs two to three orders of magnitude faster. The intermediate state between such “coherent” rotation and domain-wall displacement is called an “incoherent” reversal process.

Curie point / Curie temperature: Ferromagnetic and ferrimagnetic properties depend on the distance between atoms with uncompensated electron spins and therefore on temperature. When the thermal energy in a Weiss domain becomes comparable with the potential energy of the atomic moments, ferromagnetism disappears and the material becomes paramagnetic. In that state, the kinetic energy of the atoms dominates over exchange energy and there is no longer a common spin orientation observable externally. For pure iron, the Curie temperature Tc is approximately 778 °C. In iron alloys this temperature is generally lower, although for most alloys it remains above 100 °C and therefore above the normal operating range of many practical components and circuits.

Absolute permeability: Absolute permeability is the ratio of magnetic flux density to magnetic field strength. The permeability of vacuum is the ratio of the magnetic flux density B0 in vacuum to the applied magnetic field strength H.

Shape anisotropy: For energetic reasons, shape anisotropy tends to make the magnetisation align along the longitudinal direction of a specimen.

AlNiCo alloy: An alloy of aluminium, nickel and cobalt used to manufacture permanent magnets and capable of providing high magnetic energy density.

Anisotropy energy: The energy required to rotate the magnetisation vector of an anisotropic ferromagnetic specimen from an “easy” direction into a “hard” direction.

Anisotropy: A body is anisotropic if it does not exhibit the same properties in every spatial direction; for example, its deformation or magnetisation behaviour depends on direction.

Antiferromagnetism: A generally non-magnetic state occurring in certain oxide magnetic materials when the saturation magnetisations of two sublattices are equal in magnitude but opposite in polarity.

Barium ferrite: An oxide magnetic material with the approximate composition BaO·6Fe2O3. It has very good hard-magnetic properties due to its strong hexagonal anisotropy.

Diamagnetism: Materials with a relative permeability lower than 1, such as copper, silver, glass, water, zinc, bismuth and almost all gases and liquids, are called diamagnetic materials after Faraday. In an external magnetic field their energy increases, so they experience a force toward regions of lower field strength. Their electron shells are completely filled.

Surface anisotropy: Surface anisotropy occurs when some constituents that would otherwise be symmetrically arranged in the crystal structure within the bulk material are absent at the material surface.

Ferrimagnetism: The type of magnetism found in ferrites, produced by the antiparallel alignment of the free spin moments of two sublattices. The resulting external magnetisation is generally lower than that exhibited by metallic ferromagnetic materials.

Ferrites: Oxide magnetic materials, usually with a spinel crystal structure. Their general formula is MeO·Fe2O3, where Me is a divalent ion such as Mn, Mg, Co, Ni, Zn or Fe(II). They are technically important in high-frequency telecommunications and data processing and are used, for example, in transmission transformers and memory cores.

Ferromagnetism: A magnetic solid-state effect associated with the crystal lattice, observed especially in nickel, cobalt, iron and their alloys. The interaction between atomic magnetic moments is so strong that, despite thermal motion, they spontaneously align parallel to one another within certain regions (Weiss domains) even without an external field. The ease with which the magnetic moments of uncompensated electron spins can rotate results in high relative permeability.

Hysteresis: A phenomenon in which an effect remains even after the cause that produced it has been removed. In magnetism, this means that magnetic flux density does not follow increasing and decreasing magnetic field strength along the same path. The elementary magnetic domains do not return completely to their original state after excitation is removed. This behaviour is related to imperfections in the crystal structure. The relationship between B and H is represented geometrically by the hysteresis loop.

Hysteresis heat: The heat transferred from the remagnetising field into a specimen and then to its surroundings during one isothermal cycle around the hysteresis loop. The amount of heat transferred per magnetisation cycle and per unit volume increases with the Br and Hc values of the material and is therefore relatively high for permanent-magnet materials.

Hysteresis loop: The relationship between magnetic field strength and magnetic flux density over a complete remagnetisation cycle of a ferromagnetic material from positive saturation to negative saturation. This point-symmetric curve does not coincide with the initial magnetisation curve because, due to hysteresis, one value of H corresponds to two values of magnetisation M or flux density B depending on whether the field is changing from positive or negative saturation. The B=f(H) and M=f(H) curves differ by the quantity μ0H; for the latter, the branches run exactly parallel to the H-axis once saturation has been reached. The form of the loop depends to some extent on the condition of the material and on the measurement method used.

Heat treatment: A collective term for thermal processes, particularly those applied to ferromagnetic alloys, to establish defined internal stresses and lattice arrangements that decisively influence magnetic properties and, where relevant, anisotropy.

Ideal hysteresis loop: Also called an ideal linear loop with a saturation knee. It is a point-symmetric ideal magnetisation characteristic. The ideal rectangular loop takes hysteresis and saturation into account but does not represent the frequency dependence of the magnetic material.

Ideal magnetisation curve: A curve consisting of straight-line sections with two constant slopes. It takes saturation into account but does not include the frequency dependence of the magnetic material.

Induced anisotropy: Anisotropy produced in a material that is otherwise isotropic by treatment in a magnetic field; for example, by annealing the magnetic material below its Curie temperature while an external magnetic field is applied.

Law of electromagnetic induction: A fundamental law of magnetodynamics stating that a time-dependent change of the magnetic flux enclosed by a conductor loop induces an electrical voltage in that loop.

Isotropic material: A material having the same properties in all spatial directions. Hard magnetic materials are materials with an Hc value greater than approximately 790 A/m; examples include permanent magnets and the materials used in magnetic tapes and magnetic sheets.

Initial permeability: The permeability measured at the beginning of the magnetisation curve when a magnetic material is subjected to a very small excitation.

Exchange energy: The part of the total free energy of a ferromagnetic crystal lattice that depends only on the angle between neighbouring spins and is independent of their particular positions in the crystal lattice.

Exchange force: The interaction responsible for producing parallel alignment of atomic magnetic moments. It is quantum-mechanical in nature and disappears when the Curie temperature is reached.

Coercive field strength (Hc): The magnetic field strength, opposite to the direction of the previous saturation-producing field, required to eliminate the remanent magnetic flux density or magnetisation remaining after magnetisation. It corresponds to the intersection of the hysteresis loop with the H-axis. When B=0, the field strength is +Hc or −Hc. A distinction is made between the coercive field strength BHC in the B–H diagram and MHC in the M–H diagram. Only MHC is a true material constant. For soft magnetic materials the difference between BHC and MHC is small, whereas for hard magnetic materials it may not be negligible.

Coherent rotation: Uniform rotation of all spins during the remagnetisation of thin magnetic layers. Crystal anisotropy is anisotropy originating from the spatial lattice structure. For example, permeability can differ along a cube's body diagonal and along its edge.

Annealing: Heat treatment of a magnetic material carried out either to facilitate further mechanical processing (intermediate annealing) or to obtain a defined crystal structure (final annealing).

Air gap: In a magnetic circuit containing parts that move relative to one another, such as a motor, relay, lifting magnet or holding magnet, a working air gap is necessary. The associated magnetic “shearing” significantly changes measured values. To minimise deterioration of these values, the ratio of the mean magnetic path length to the air-gap length should be as large as possible. Air gaps may also be introduced into magnetic circuits without moving parts to improve circuit characteristics, for example in filter cores where an air gap can increase the quality factor or improve temperature stability.

Demagnetisation: A process used to eliminate remanent magnetic flux density.

Demagnetisation process: The demagnetising factor Nt is used to determine the demagnetising field H0 that develops through pole formation at the ends of an open magnetic circuit and reduces the external field strength Ha to the effective internal field strength Hi. The relationship is Hi=Ha−NtM. Nt is also the slope of the demagnetisation or shear line in the B–H plane: Nt=tan β. The angle β is the angle between the shear (demagnetisation) line and the B-axis.

Lenz's law: A general law of physics stating that an induced effect opposes the change that causes it. An induced electrical voltage is therefore directed so as to oppose changes in the magnetic field; in a coil, an “inertial” effect opposes both increases and decreases in magnetic flux. The measure of this effect is the self-inductance L of the coil.

Lorentz force: The force exerted by a magnetic field on a moving electric charge or current-carrying conductor. Pulse magnetisation refers to magnetising a permanent magnet with a short, high-current pulse generated by suitable magnetising equipment.

Magnet: A body that creates a magnetic state or magnetic field in its surroundings. By formal analogy with electrostatics, a magnet may be described as a magnetic dipole because it has two poles of equal strength and opposite polarity that exert forces on another magnet.

Magnetic anisotropy energy: The energy determined by the anisotropy in question and described by anisotropy constants. In a magnetically isotropic body, a field of equal magnitude magnetises the material equally in every direction. In a magnetically anisotropic body, a preferred “easy” direction exists in which the material can be magnetised much more readily. Anisotropy may arise from several different causes. Its magnitude is characterised by the anisotropy constant K, expressed as energy per unit volume.

Magnetic materials (soft magnetic materials): Magnetic materials whose coercive field strength Hc is generally below approximately 100 A/m are called soft magnetic materials. Their hysteresis loops may be linear or approximately rectangular. Magnetic shielding is based on the fact that magnetic field lines preferentially follow a path of lower magnetic reluctance, even if this path is geometrically longer. A high-permeability material can therefore divert or “draw” magnetic flux lines away from a protected region, such as the interior of a magnetic shield.