Magnet ABC

A/m: amperes per meter: Unit of magnetic field strength. 1 A/m = 0,01 A/cm (= 0.01256 Oersted).

AlNiCo: Magnetic alloy composed of Aluminum, Nickel, Cobalt, Iron and other additives – produced by casting or sintering – can only be processed by grinding and cutting.

AlNiCo p: DIN 17410 designation for polymer bonded AlNiCo materials.

Ageing: Change in magnetic properties with time, especially in the apparent remanence of a permanent magnet; can be reduced or anticipated by artificial ageing (magnetic, thermal, mechanical).

AMR-Effekt:  Anisotropic magneto resistive effect, (see also XMR-Effect).

Anisotropy: Change in magnetic properties with time, especially in the apparent remanence of a permanent magnet; can be reduced or anticipated by artificial ageing (magnetic, thermal, mechanical).

Axial magnetization: Magnetization along the axis of symmetry of a bar magnet or along one edge of a block magnet.

B: Character used to represent flux density: (also known as induction) 1 Tesla = 1 VS/m2 = 104 VS/cm2 (=104 Gauß) (see also SI units).

B * H: Product of the respective flux density B and field strength H within a magnet. Unit: 1 J/m³ =10^-3 kJ/m³ =125.6 Gauß*Oersted =125.6.6*10^-6 MGOe (see also)

B(H) curve: A curve representing the dependency of the flux density B on the field strength H (see also hysteresis loop).

Calibration: The tolerance of a magnet’s  flux is usually approximately ±7%. However, it can be set to lower values for specific applications.

cgs units: Physical units which are based on the three fundamental units cm, gram and second (see also SI units).

CMR-Effect: Colossal magneto resistive effect (see also XMR effect).

Coercive field strength: HC =Strength of the demagnetizing field where B = 0 (H_cB) or J = 0 (H_cJ).

Coercive force: Old term for coercive field strength.

Columnar crystalline materials: Especially AlNiCo alloys where an orientation of the crystals is formed by a controlled solidification of the molten material. The material AlNiCo 700 shows a very distinct anisotropy, in contrast to types where an anisotropy is produced only by applying a magnetic field during heat treatment.

Curie temperature: The characteristic temperature T_c above which the remanence of polarization in a ferro-magnetic material becomes Jr = 0. At temperatures above the Curie temperature all ferromagnetic materials are paramagnetic. However, the maximum temperature for the use of magnetic materials is usually much lower than the Curie temperature and is determined by changes in composition, self-demagnetization, the decay of bonding materials and other physical effects (see also Temperature, operating).

Demagnetization: Reduction of magnetization to J = 0; this is obtained practically by the application of an alternating field of decreasing amplitude (see also Residual magnetization).

Demagnetization curve: The second quadrant of the hysteresis loop which is of great importance for permanent magnets.

Demagnetization factor N: Shape-dependent factor which determines the pitch J/H of the Shearing lines (see also Shearing angle).

Diamagnetism: Magnetic property of materials whose permeability is smaller than 1, e.g. bismuth

Dimensional relationship: Relationship L/D = length / diameter of a bar magnet. For each magnet material there is a fixed L/D relationship at which the optimum working point is reached (intersection of the Shearing lines with the Demagnetization curve).

Dipole field: First approximation of the field of a magnet at a large distance. The dipole field is defined only by orientation and amount of the magnetic moment and decreases according to 1/r³ with increasing distance r.

Dipole moment: see moment (magnetic)

Earth field: The magnetic field strength of the earth is approximately 0.3 – 0.5 Oersted = 23.9 – 39.8 A/m.

Eddy current: A current induced in a conductor by a changing magnetic field. It is exploited for example in electricity meters for retarding without any contact. However, it causes losses and undesirable heating in motors, transformers etc.

Effective flux: Part of the magnetic flux which passes through the air gap.

Field: Space having physical properties (see also Magnetic field).

Field constant, magnetic: μ₀ = Permeability B/H of the vacuum μ₀ = 1.256 10^-6VS/Am (=1 Gauss / Oersted).

Field line: Means of visual representation of fields. In force fields (e.g. magnetic fields) the tangents to the field lines represent the directions of the effective forces; the density of field lines is a measure of the strength of the effective forces.

Field strength (magnetic) H: A quantitative representation of the strength and the direction (vector) of a magnetic field. Unit: 1 A/m (=0.01256 Oersted).

Flux density B: No. of field lines per unit of surface area. Unit: 1 Tesla = 1 VS/m² = 10-4 VS/cm² (= 10^-4Gauß) (see also SI units).

Flux, magnetic: When a magnetic field is represented by field lines, the total number of lines through a given surface is known as the magnetic flux: measured as an electrical impulse in a coil surrounding this surface on a change to this flux. Unit: 1 Weber (Wb) = 1 Vs (= 10⁸ Maxwell) (see also SI units).

Force Line: Visual representation of a force field, especially a magnetic field.

Gauß: Old (cgs) unit of magnetic induction, 1 Gauß = 10^-4 Tesla (see also SI units).

Gaußmeter: Instrument for measuring magnetic flux density B. Instruments for measuring magnetic field strength H (Oerstedmeters) are often referred to as Gaußmeters.

Gilbert: Old unit of the magnetic potential; 1 Gilbert = 1 Oe * cm = 0.796 A. of the electric currents enclosed by the curve: .

GMR-Effekt: Gigantic magneto resistive effect (see also XMR effect).

H: Character used for magnetic field strength. Unit: 1 A/m (=0.01256 Oersted). (see also SI units).

Halbach system: An arrangement of magnets named after the American physicist K. Halbach which produces precise and very homogeneous multipolar fields(for example a dipole field) without iron components.

Hall probe: Semiconductor probe for measuring magnetic flux density in air (e.g. in an air gap of a magnet system). Hall probes always are used connected in combination with an instrument for measuring flux density (Gaußmeter).

Hard ferrite: Term used in DIN 17410 for oxide magnet materials.

Hard ferrite p: Term used in DIN 17410 for polymer bonded oxide magnet materials.

Helmholtz coil: Classically, a double coil to produce extremely homogeneous fields. The distance between the two coils is equivalent to their radius. The coil is used for measuring magnetic moments.

Hysteresis loop: Representation of flux density B as a function of the magnetizing field strength H.

Induction: The property exhibited by the magnetic field of surrounding itself with an electric field when modified. The term induction was also used earlier to mean flux density.

Isotropy: Equality of physical properties in all directions.

J: Character used to represent magnetic polarization: Unit: 1 T = 1 Vs/m² (see also SI units).

Losses, reversible, irreversible: The changes in the magnetic values which occur in magnets, for example due to the influence of external temperatures, may be reversible or irreversible. In the case of reversible changes, unlike irreversible changes, the magnetic properties revert to their original values when the initial temperature is restored.

Magnet system: Product in which a magnet is integrally combined with other materials (conductive elements, mechanical recorders etc.).

Magnetic: Commonly used to denote all materials with noticeably high permeability (especially iron, nickel, cobalt and their alloys); all other materials (brass, copper, wood, stone, etc.) are considered to be non-magnetic.

Magnetic circuit: Total of parts and gaps through which a magnetic flux passes; in the case of a permanent magnet this consists of the magnet itself, the pole shoes, the air gap and the stray field.

Magnetic field: Space in which mechanical forces have an effect on magnetic charges or in which induction phenomena occur.

Magnetic field strength H: See Field strength (magnetic).

Magnetic flux: See Flux, magnetic.

Magnetic pole: Part of the surface of a magnet where the magnetization is rectangular to the surface. This part corresponds to the regions where the magnetic flux leaves the magnet. (see also: Pole tester).

Magnetic potential Q: See Potential, magnetic.

Magnetism: Sum of magnetic phenomena arising from the electromagnetic interactions (force) and one of the fundamental physical forces. It is described in terms of the magnetic field H and the magnetic flux density B. All magnetic phenomena are associated with moving electrical charges (i.e. with electrical currents) while the term electrostatics describes the forces which occur between non-moving electrical charges. Finally, the term electrodynamics relates to the association between varying electrical and magnetic fields. When a material is magnetized, the polarization J defines an alignment of magnetic moments. These moments consist of the moments of trajectory of the electrons circling the atomic nucleus and the so-called electron spin which is caused by the rotation of the electron around its own axis. If these moments are self-compensating for all the atoms then the material is termed diamagnetic. In the case of para-, ferro-, antiferro- and ferrimagnetic materials, the sum of these moments is different from zero. They differ from one another in the way the moments of neighboring atoms are coupled: in the case of paramagnetism there is no coupling; in ferromagetism, the neighbouring atomic moments are parallel, in antiferromagnetism they have an antiparallel alignment. The term ferrimagnetism is used when the atomic moments in antiparallel alignment do not fully compensate for one another and the resulting magnetism persists.

Magnetization: 1.) M = J/μ₀ Polarization divided by vacuum permeability. Therefore corresponds to the density of the aligned magnetic moments. 2.) Noun from “to magnetize”, the type of magnetization (axial M., radial M., pole orientation etc.) defines the polarization direction and the position of the Magnetic pole.

Magnetizing: Process of aligning the magnetic moments by an external magnetic field.

Magneto resistive sensor: (MR) sensor using the change of electric resistance in a magnet field to measure it. Due of recent developments in thin layer technology which offer extremely high magneto resistive effects, we are currently witnessing a renaissance in the use of MR sensors. (see also XMR effect).

Magnetomotive force Θ: electrical: Term for the line-integral of field strength H along a closed curve. This line-integral is the sum of the electrical currents enclosed within the curve: .

Melt spin process: Method involving the extremely fast cooling of molten metal which is sprayed onto a rotating cylinder. This results in materials with physical properties different from those exhibited by molten metals cooled under normal conditions. This method is used to produce the basic powder of Neofer p ®.

Moment, magnetic: (also dipole moment) Product of polarization J and volume V of a homogeneously magnetized magnet. The moment, expressed in terms of Vsm, corresponds to the mechanical torque in Nm experienced by the magnet in a magnetic field H of 1 A/m perpendicular to the magnetization (Coulomb’s magnetic moment m_Coul). The magnetic moment is measured directly in a Helmholtz coil in combination with a fluxmeter. (see also SI units) Formerly the Ampere magnetic moment m_Amp was commonly used as the product of the magnetization M and volume V of a solid, where m_Coul=μ₀ m_Amp.

Multicomponent injection molding: Injection molding process in which several different materials are injected one after another, for example a non-magnetic material injected on top of a magnetic compound.

Neofer ®: Permanent magnet material based on neodymium, iron and boron.

Oersted: Former (cgs) unit of magnetic field strength (see also SI units). 1 Oersted = 79.6 A/m = 0.796 A/cm = 0.0796 kA/m.

Oxide magnet: Hard ferrite, ceramic magnet material, e.g. one composed of iron oxide and barium oxide (BaO + 6 Fe₂O₃).

Paramagnetism: Magnetic property of materials with permeability μ > 1. All ferromagnetic materials exhibit paramagnetism above the Curie temperature.

Permagraph: Measuring instrument for plotting the entire hysteresis loop of a magnet. It consists of an electromagnetic yoke to apply an external field, measuring instruments for measuring field strength H and flux density B and a computer or an X/Y plotter to record the curve.

Permeability: μ = B/H; ratio of the flux density B to the magnetic field H. In permanent magnet technology, permanent permeability is important as this gives the change in B when small changes in H occur ( u_p = dB/dH), in particular close to the optimum working point. Unit: 1 T m/A. The permeability of a vacuum is μ₀ = 1.256-10^-6 T/m = 1.256 * 10^-6 Vs/Am.

Permeance: Magnetic conductance ( in the case of an air gap, surface/length)

Polarization J: Magnetic orientation of a magnetic material. It is J=B-μ₀H.

Pole: See Magnetic pole.

Pole orientation: Expression of the preferred orientation in a magnet in accordance with multipolar magnetization, i.e. the preferred orientation runs in a curve between neighboring poles.

Pole tester: Tool supplied by the Magnetfabrik Bonn to distinguish between North and South poles and to visualize multipolar magnetization.

Polymer bonded magnet material: If a magnet powder is blended with plastic material it is possible to apply methods used in the plastic industry (injection molding, rolling, extrusion) to produce magnets of very complex shapes. The advantages: cheap manufacturing processes, small tolerances and many kinds of shapes must be compared with the disadvantages: expensive tools and lower magnetic properties.

Potential, magnetic: Physical quantity whose the gradient gives the magnetic field H. Only a potential difference can be measured (magnetic potential between two points) as an integral of the field strength over any path between these two points, provided this path does not enclose an current-carrying conductor (see also SI units).

Prac ®: Pressed magnet consisting of AlNiCo powder and bonding agent.

Preferred orientation: Orientation, resulting from the production process, in a magnet in which the magnetic properties are at their highest level. Magnets with a preferred orientation (anisotropic materials) can only be magnetized in this orientation, whereas isotropic materials can be magnetized in any direction using appropriate equipment/coils.

Pressed magnet: A magnet by means of a compression process followed by thermal hardening (see Prac, Prox, Neofer p).

Prox: Pressed magnet consisting of oxide powder and a bonding agent.

Quality: See also (B * H)_max value

Radial magnetization: Magnetizing a ring magnet between two coils carrying currents of opposite directions leads to a radial magnetization. One pole is then located on the inner circumference of the magnet and the other pole on the outer circumference.

Rare earth magnet materials: The rare earth metals Nd and Sm are used in different alloys for manufacturing permanent magnets with very high magnetic properties. The Seco and Neofer materials which are in use today are based on the compositions Sm Co₅, Sm₂ (Fe/Co)₁₇ und Nd₂Fe₁₄B.

Remanence B_r: Residual flux density in a solid which has been subjected to a magnetizing field (true remanence in the case of a closed magnetic circuit, apparent remanence in the case of an open magnetic circuit).

Residual magnetism: Because of the manufacturing process, delivered magnets exhibit a greater or lesser degree of residual magnetism. This can only be reduced by means of a time-consuming demagnetization process.

Saturation: Better termed saturation polarization.

Saturation polarization: Highest practically achievable magnetic polarization of a material.

Seco ®: A magnet material composed of an alloy of rare earths and cobalt.

Seco p ®: A magnet material composed of a rare earth cobalt alloy and a polymer bonding agent.

Shearing: Shifting of the working point on the demagnetization curve by opening the previously closed magnetic circuit.

Shearing angle: Angle between the shearing lines and the J or B axis in the J(H) or B(H) chart. The tangent of the shearing angle corresponds to the demagnetization factor N.

Shearing line: Line in a magnet material’s B(H) or J(H) chart which specifies the shape of a magnet or the air gap in a magnet system. The intersection of the shearing lines with the demagnetization curve represents the magnets working point.

SI units: Physical units according to the System International (SI) which is based on the units kilogram, meter second and ampere. All other units are expressed as a product, a quotient or a power of these four basic units. The older cgs units (cm, gram, second) and the Gauß units in the field of magnetism are still in use but their adaptation is now a legal requirement. The following table indicates some magnetic units and their conversion.

Sintered magnet: A magnet formed user powder and metalworking technologies involving the heating below the melting point of a green product compressed from metal or powder material.

Sprac: Polymer-bonded, AlNiCo magnet manufactured using an injection molding procedure.

Sprox ®: A magnet manufactured from hard ferrite oxide powder and bonding agents. This material may be injection molded, extruded or rolled.

Stabilization: Treatment of a magnet which renders it immune to external influences and ageing.

Stray flux (leakage): Part of the magnetic flux which does not pass through the air gap.

Stray ratio (factor): Ratio C_B of the effective flux and the total flux of a magnet.

Supraconductor: Electrical conductors with no resistance to electric current flow at very low temperatures. In this way it is possible to construct closed coils which generate a durable magnetic field in the absence of a power supply in the same way as a permanent magnet.

Susceptibility (magnetic) X_r: Change of magnetization resulting from the change of an applied field H X_r=dM/dH.

Temperature coefficient: Indicates the dependence of a physical quantity on temperature. In the case of magnet materials, the temperature coefficients, for example, are remanence and coercive field strength. As relative values, these are designated as a(Br), b(Hc) and specified in %/K (percentage per degree Kelvin).

Temperature compensation: Reduction or elimination of temperature dependence due to apparent remanence; this occurs mainly as a result of a temperature-dependent magnetic shunt.

Temperature, (operational): The upper operational temperature of any magnetic material is limited, in particular due to a greater or lesser irreversible loss of magnetization above the maximum operational temperature depending on the magnetic working point.

Tolerances, magnetic: According to DIN 17410, magnetic materials are characterized by minimum values. Although no upper limits are defined, most materials typically lie in the range up to 114% of minimum remanence and up to 125% of minimum energy product. If the magnetic values are subject to more precisely defined requirements, then it is advisable to perform (very time-consuming) comparisons during the magnetization process.

Vacuum permeability: see Field constant, magnetic.

Working capacity: The maximum amount of magnetic energy per cm³ of a magnet volume which can be converted into mechanical force (work).

Working point: Point in the 2nd quadrant of the hysteresis curve representing the flux density B and inner field strength H under working conditions. In the case of permanent magnets, this is usually optimal if it lies at the end the external demagnetization curve and B * H is maximum, i.e. (B * H)_max-value.

XMR effect: Sum total of the AMR, GMR and CMR effects. These are huge changes of electric resistance within thin layers of different materials exposed to a magnetic field and are utilized in sensors which are used to capture/measure the strength and orientation of magnetic fields. As they were discovered, the increasingly strong effects were named, in succession, magneto resistive effect, AMR effect, GMR effect, CMR effect.