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PMD Glossary of Terms - Page 1

PMD Glossary of Terms 

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Area of the Air Gap (Ag) – The cross sectional area of the air gap perpendicular to the flux path, or the average cross sectional area of that portion of the air gap within which the application interaction occurs. This area is measured in the plane perpendicular to the central flux line of the air gap.

Area of a Magnet (Am) – The cross sectional area of the magnet perpendicular to the central flux line, usually measured in sq. cm. at any point along its length. In magnet design, Am is usually considered the area at the neutral section of the magnet.

Air Gap Volume (Vg) – The useful volume of air or nonmagnetic material between magnetic poles; often measured in cubic centimeters.

Absolute Permeability – Absolute Permeability is a ratio of the magnetic induction caused by an applied magnetic field. Permeability of magnetic materials is rarely expressed as an absolute permeability. Typically it is reported in terms of relative permeability

Air Gap – A region in the flux path of a magnetic circuit where the flux is available for use, as in a motor. Often the air gap is just air, but may also include other materials such as paint, aluminum, epoxy etc.

Alnico Magnets – Developed in the 1930s. Alnico magnets are an iron-based alloy system and named from their main alloying elements – aluminum, nickel and cobalt. Alnico magnets have the widest range of temperature stability of any standard permanent magnet material. Alnico must be carefully stored since it is prone to demagnetization. Manufacture is by sintering or casting.

Alloy – A metallic compound containing two or more elemental metals.

Amorphous – Any material that is metallurgically non-crystalline in nature, i.e. the atoms are not arranged in a regular crystalline pattern.

Anisotropic Magnet – A magnet having a preferred direction of magnetic orientation, the magnetic characteristics are optimum in that direction.

Anisotropic Materials – Materials having different characteristics in different directions are called anisotropic materials.

Anisotropy – Means having different properties depending on the measured direction. Magnets which are anisotropic, or have an easy axis of magnetization, have their anisotropy developed by either shape or magnetocrystalline differences.

Anneal – The high-temperature conditioning treatment of a material to relieve the stresses introduced when the material was originally formed. To prevent oxidation, annealing is often performed in a vacuum or inert gas atmosphere.

Axial Pressing – In axial pressing, pressure is applied parallel to the direction of alignment. Most rings are axially pressed.

Axis of Easy Magnetization – For an anisotropic magnetic body, this is the crystal direction in which magnetization is easiest to achieve.

Axis of Hard Magnetization – For an anisotropic magnetic body, this is the crystal direction in which magnetization is difficult.

Bd/Hd – Slope of the operating line determined by the magnetic circuit, or the ratio of the remnant induction, Bd, to a demagnetizing force, Hd. It is also referred to as the permeance coefficient, shear line, load line and unit permeance.

Bastnasite – A yellowish to reddish-brown fluorocarbonate mineral that is a common source of rare earth elements.

Bending Strength – A characteristic value showing the resistance to bending and is also called bend strength. Bending strength shows the maximum strength at which a test piece is broken by the bend load. This value is often used to characterize the mechanical strength of a magnet.

Bonded Magnets – Magnetic powder is blended in a rubberized or plastic compound material. The magnetic compound is then either compression or injection molded, extruded or calendered. A bonded magnet is always less powerful than a traditional sintered magnet of any formulation. (See Flexible Magnets, Injection Molded or Compression Bonded Magnets)

Calendering – A calender is a series of hard pressure rollers used to form or smooth a sheet of material. Although calendaring is most commonly used to manufacture paper products, it is also utilized in the manufacture of flexible bonded ferrite magnets.

Capture – The act of magnetically extracting ferrous tramp from a product stream.

CGS System – A system of units based on the Centimeter-Gram-Second and the one commonly used for presenting magnetic data.

Center Flow Magnets – Center-Flow Magnets are in-line magnet separators designed to remove ferrous fines as well as larger pieces of tramp iron from dry particulates as they travel through pneumatic or gravity-flow conveying lines. To achieve optimum contact with the product flow, a bullet-shaped magnet is suspended in the centerline of the housing. The tapered, exposed-pole cartridge has a stainless steel “nose cone” to direct the flow of materials around the magnet. This design allows ferrous fines to collect out of the direct air stream, and tramp metal will collect there.

Ceramic Magnets (see Ferrite Magnets)

Closed Magnetic Circuit – Exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit. No magnetic flux is available outside the flux path.

CMMDA – The China Magnetic Materials Device Association. CMMDA is associated with the Southwest institute of Applied Magnetics of China. Membership represents a relatively small percentage of the total Chinese magnet industry.

Coating – A thin layer of material applied to the outside of a magnet to prevent corrosion. Neodymium magnets are the most prone to corrosion and the most likely to require a coating. Various coatings applied to permanent magnets, including Zn, Ni, Epoxy, CR3+Zn or Ni+Ag.

Cobalt – A metallic element that is usually obtained as a by-product of nickel mining. Cobalt pricing can occasionally be volatile. It is a key ingredient in Samarium Cobalt and Alnico magnets. It is also a critical additive to NdFeB magnets, to improve the high-temperature performance of the magnet.

Coercive Force (Hc) - The demagnetizing force necessary to reduce the induction, B, to zero, in a previously saturated magnet.

Collection Area – Locations on the magnetic face where tramp metal collects due to high concentrations of magnetic flux. These locations may be above internal magnetic poles or at external pole extensions.

Columnar Alnico – The magnetic properties of certain Alnico grades can be improved applying a magnetic field to the magnet during heat treatment to create long columnar grains parallel to the applied magnetic field.

Compression Bonded Magnets – These magnets are made by mixing magnet powder with a polymer such as epoxy or plastic to form a matrix of magnetic and bonding materials. The magnets are then compression molded to final shape and final solidification occurs by curing instead of sintering.

Cow Magnets – Alnico or ceramic magnets that are fed to cows to collect the various ferrous contaminants and tramp iron from their stomach which they consume during grazing. These magnets remain in the cow for life.

Corrosion Resistance – The ability to resist oxidation. Corrosion resistance can sometimes be an issue with NdFeB magnets, particularly in high-humidity applications. Optimization of the magnet’s chemistry and carefully controlled processing and coating of the finished part can significantly reduce a magnet’s tendency to corrode in most applications.

Cunife – An alloy primarily composed of copper, nickel and iron. The alloy has a linear coefficient of expansion similar to certain compositions of glass, and thus makes an ideal material for the lead wires in light bulbs. Cunife is ferromagnetic and can occasionally be utilized as a weak permanent magnet.

Curie Temperature (Tc) - This is the temperature where thermal vibrations initially overtake the parallel alignment of magnetic moments, and the material has no magnetization.

Demagnetization – A process through which induction is reduced to or approaches zero. This can be done by applying an adverse field or by heating a magnet above its Curie temperature.

Demagnetization Curve – The second quadrant of the hysteresis loop, generally describing the behavior of the magnet in actual use. Also known as the B-H Curve.

Demagnetization Force – A magnetizing force, typically applied in the direction opposite to the force used to magnetize a magnet in the first place. Temperature and radiation can also be demagnetizing forces.

Demagnetized – A magnetic material with no magnetization. A magnet can be either thermally or field demagnetized. Thermal demagnetization can be achieved by heating the magnets above the Curie temperature. Field demagnetization can be achieved by exposure to a continually decreasing AC field.

Diamagnetism – A magnetic characteristic in which the individual magnetic moments in a material are oriented randomly in the absence of an external magnetic field, such that the body as a whole is not magnetized. When an external magnetic field is applied, the body becomes weakly magnetized in the opposite direction to the applied field, with intensity proportional to the magnetic field. Diamagnetism is normally seen in inert gases, metals such as copper, gold, silver, and zinc, and many organic compounds

Dimension Ratio (lm/D) – Is the ratio of the length of a magnet to its diameter, or alternately, the diameter of a circle of equivalent cross-sectional area. For simple geometries, such as bars and rods, the dimension ratio is related to the slope of the operating line of the magnet, Bd/Ha.

Dimensional Tolerance – An allowance, given as a permissible range, for the nominal dimensions of a finished magnet. The purpose of a tolerance is to specify the allowed leeway for dimensional variations in manufacturing.

Dimensions – The physical size of a magnet, occasionally including any plating or coating.

Dipole Ring Magnets – A magnetic circuit that generates a uniform magnetic field in a specific direction. This is an extraordinarily efficient magnetic circuit, considering the magnetic field generated for the weight of magnets. Dipole ring magnets are cylindrically shaped, with magnets placed along the circumference of the cylinder cross sections. The segment magnets are pasted into place in such a way that the magnetic orientation direction is continuous.

Direction of Magnetization – This refers to the "easy axis" or the axis of choice for the direction of alignment, which exists for anisotropic magnets. For example, most rings are aligned axially so the direction of magnetization is through the axis (or thickness). Other possibilities for rings include "across the diameter" or "radial" magnetization.

Domains – Regions in a magnetic alloy which have the same orientation. Domains may be rotated and manipulated by an external magnetizing field to create a useful magnet with a net magnetic moment. In the case of unmagnetized material, the domains cancel each other out. Thus, in this condition there is no net external field.

Dysprosium – A widely used rare earth element, which is often added to NdFeB magnets to increase resistance to demagnetization at elevated temperatures.

Drum Magnets – Magnetic Drums and Drum Separators are self-cleaning devices which provide continuous removal of ferrous contaminants from a wide range of free-flowing bulk and granular materials in high-volume applications.

Eddy Currents – These are circulating electrical currents that are induced in electrically conductive elements when exposed to a varying magnetic field. The currents flow in a direction which creates an opposing force to the change. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences in certain designs such as transformers, which should be accounted for or minimized.

Eddy Current Separator – An eddy current separator uses a powerful magnetic field to separate metals from non-metals (usually in garbage). When a magnetic field is applied to a conveyor belt carrying a thin layer of mixed waste, electrically conductive materials, such as metals are thrown off the belt, and nonmetals simply fall off the end of the belt.

EDM –Electric discharge machining (EDM) is a process called spark machining, spark eroding, burning, die sinking or wire erosion. A common process for cutting permanent magnet materials. Material is removed from the magnet by rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage.

Electric Bicycles – A bicycle with a motor and battery to propel it. This is a major application of NdFeB magnets, especially in China.

Electromagnet – A magnet, consisting of a solenoid which creates a magnetic field only while current flows through the coil.

EMERF – The Electric Motor Education and Research Foundation. A group devoted to the promotion of educational and research activites associated with small electric motors and their components, affiliated with the MCMA.

Energy Product – Is related to the energy that a permanent magnet can supply to a magnetic circuit, when operating in a device. It is calculated by Bd x Hd.

Exchange Spring Magnets – A composite magnet comprised of fine sub-micron hard and soft magnetic phases. In spite of the two components, this magnet behaves like a homogenous and uniform magnet, with a magnetic exchange coupling between the two phases. Because its magnetization behavior is as though the hard and soft magnetic phases are linked with a magnetic spring, this type of magnet is called an exchange spring magnet. In hysteresis loops, these magnets show the best qualities of the two materials used to make them, with improved magnetization and resistance to demagnetization.

Extrusion – The process pushing a material through a mold to form it. Extrusion is used to produce flexible ferrite magnets for applications such as refrigerator door seals and similar holding applications.

Ferrimagnetic Materials – These are materials in which the magnetic moments of the atoms on different sublattices are opposed. It is similar to antiferromagnetism, except the magnetic moments of the two sublattices do not cancel out completely and a spontaneous magnetization is observed.

Ferrimagnetism – Is exhibited by ferrites and magnetic garnets. The oldest-known magnetic substance, magnetite (Lodestone), is a ferrimagnet. It was originally classified as a ferromagnet before Neel's discovery of ferrimagnetism and antiferromagnetism.

Ferrite Magnets – A hard or soft magnet made from combining magnetic oxides whose primary component is Fe2O3. In addition, hard ferrite magnets use barium or strontium oxides. These magnets are relatively inexpensive, have high corrosion resistance and exhibit high electrical resistivity. However, their magnetic characteristics are normally lower than those of rare earth magnets and their maximum energy product is about 1/10 of that for a rare earth magnet.

Ferromagnetic Material – A material which is easily magnetized with a small magnetic field, i.e. its permeability is very much larger than 1 (from 60 to several thousand times), and which exhibits hysteresis phenomena. A material that either is a source of magnetic flux or a conductor of magnetic flux.

Flexible Magnets – A magnet material that is easily bent. It is normally produced with a mixture of magnetic powder (usually hard ferrite) and organic binding materials. The resulting compound is then either extruded or calendered. The resulting flexible magnetic sheet is then commonly printed and presented to the market as the traditional “Refrigerator Magnet”.

Flux Meter – An instrument that measures the change of flux linkage with a search coil. This signal can be converted to B, induction.

Fringing Fields – A leakage flux particularly associated with edge effects in a magnetic circuit.

Fully Dense Magnet – A magnet produced through a metallurgical casting or sintering method. Fully dense magnets exhibit the highest magnetic performance for their material type (as opposed to Bonded Magnets).

Gauss Meter – An instrument that measures the instantaneous value of magnetic field, H. Its principle of operation is usually based on one of the following: the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle.

GHz – l,000,000,000 Hz (giga Hertz).

Gradient – Indicates a change in magnetic strength between two points measured at different distances perpendicular to the magnetic field of a magnet.

Grate Magnets – Magnet assemblies which normally incorporate hard ferrite or rare earth tube magnets into an array, to remove ferrous contamination from free flowing liquids or solids as they pass through the grate.

Hall Effect Transducer – A semiconductor device that produces a voltage dependent on an applied DC voltage and an incident magnetic field. The magnitude of the output is proportional to the field strength and the angle of incidence with the Hall device. This type of sensor is often used to provide an output signal for use in a Gauss meter to measure the incident magnetic force, H.

Hard Magnetic Material – A magnetic material with a large coercive force, meaning that it is not easily demagnetized by external magnetic fields. This generally describes a magnet or permanent magnet but also includes magnetic recording materials. Ferrite magnets, NdFeB magnets, SmCo magnets, platinum iron, and FeCoCr alloys are common hard magnetic materials.

Holding Magnets – Constructed of a magnet embedded in a steel channel, or a magnet sandwiched between two steel plates. These magnet assemblies provide strong pull for their sizes. Most holding magnets come with mounting holes for easy installation and use.

Hysteresis and Hysteresis Loss – Hysteresis is the tendency of a magnetic material to retain its magnetization. This tendency is strong in permanent magnets and weak in soft magnetic materials. Hysteresis causes the graph of magnetic flux density versus magnetizing force to form a closed loop rather than a line. The area enclosed by the loop represents the difference between energy stored and energy released per unit volume of material per cycle.

Hysteresis Loop – A plot of magnetizing force versus resultant magnetization or induction (also called a B-H curve) of a material as it is successively magnetized to saturation, demagnetized, magnetized in the opposite direction and finally remagnetized. This plot is a closed loop which completely describes the characteristics of the magnetic material at a given temperature. The size and shape of this loop is important for both hard and soft materials. With hard materials the "fatter" the loop, the magnet has more resistance to demagnetization. The first quadrant of the loop (that is +X and +Y) is called the magnetization curve. It is of interest because it shows how much magnetizing force must be applied to saturate a magnet. The second quadrant (+X and -Y) is called the Demagnetization Curve.

Hysteresisgraph – An instrument that draws hysteresis loops. Also called a permeameter.

Hz – Frequency in units of Hertz (cycles per second).

IEEE Magnetics Society – One of 38 divisions within the IEEE. This organization works to characterize basic permanent magnet materials and work on standardization of definitions, symbols, and operating characteristics of permanent magnet materials.

Induction – The magnetic flux per unit area of a section normal to the direction of flux.

Induction Curve – A graph depicting the relationship between induction, B, and the applied field (magnetizing force), H.

Injection Molded Magnets – A mixture of either ferrite or NdFeB powder is thoroughly mixed with a plastic material and a finished magnet is molded to final shape in an injection molding machine. Although injection molded magnets sacrifice significant magnetic strength (compared to a fully dense magnet), the injection molding process offers the ability to produce magnets with complex shapes and magnetization patterns not achievable by other processes. See Bonded Magnets.

Intrinsic Coercive Force, Hci – Is a measure of the material's inherent ability to resist demagnetization. It is the demagnetization force corresponding to zero magnetization in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.

Intrinsic Demagnetization Curve – The hysteresis loop of M versus H, where M is the magnetization resulting from only the magnetic material.

Iron Nitride Magnets – A theoretically strong permanent magnet that does not require any rare earth materials. The theoretical magnetic energy product for iron nitride magnets is reported to be 130 MGOe, which is more than twice the maximum reported magnet energy product for rare-earth magnets. This material is still in research and is not yet commercially proven.

Iron Oxide – Iron oxide is used in the production of both bonded and sintered ferrite magnets. A level of high purity is necessary to achieve optimum magnetic properties. Most iron oxide used in magnet production is produced with the Ruthner process, which reclaims iron oxide from steel industry pickle liquor.

Irreversible Flux Loss – A partial demagnetization of the magnet, caused by exposure to elevated temperatures, external demagnetizing fields, radiation or other factors. These losses can be recovered by re-magnetization. Magnets can be stabilized to remove irreversible losses, either by partial demagnetization induced by temperature cycles or by external magnetic fields. This loss can be recovered by remagnetization.

Isotropic Magnet – A material whose magnetic properties are the same in any direction. Isostropic materials can be magnetized in any direction without loss of magnetic characteristics or performance.

Isostatic Pressing – A pressing method where the pressure is applied equally in all directions, it usually provides a higher degree of alignment and higher magnetic properties, compared to axial pressing.

Isotropic (Non-oriented) – A material that has no preferred direction of magnetic orientation, allowing for magnetization in any direction.

JABM – The Japan Association of Bonded Magnetic Materials. A consortium of Japanese bonded magnet producers and related companies.

Joule – SI unit for energy.

kHz – 1,000 Hz (Kilo Hertz).

Keeper – A keeper is a high permeability material, typically mild steel, which is installed on an alnico magnet or magnetic assembly to reduce the risk of demagnetization. This reduces the overall leakage fields generated by the magnet or magnetic assembly.

Kilogauss – One Kilogauss = 1,000 Gauss = Maxwells per square centimeter.

Knee of the Demagnetization Curve – The point at which the B-H curve ceases to be linear. All magnet materials, even if their second quadrant curves are straight line at room temperature, develop a knee at some temperature. If the operating point of a magnet falls below the knee, small changes in H produce large changes in B, and the magnet will not be able to recover its original flux output without remagnetization.

Your suggestions for potential improvement or expansion of this industry glossary are always welcomed. Please send your valued suggestions to: wbglossary@aol.com

Glossary created by Walter T. Benecki LLC

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