PERMANENT MAGNET

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Introduction to permanent magnet theory:

polarization or intrinsic magnetization.

The characteristic property of a permanent magnet to create a magnetic field around itself without the presence of electrical windings is due its atomic structure. At this level the magnetic moments of each atom, with mutual interaction in the microscopic regions of the material, create an uniform and spontanous magnetization or polarization. These regions, called Weiss's domains, or single domains are separated from each other by so called Bloch walls.

Dimensions, shape and orientation of the polarization are depending on material structure at crystalline level (e.g. lattice, defects) and on external condictions (e.g. temperature, electromagnetic field). The polarization of Weiss domains in permanent magnets has a strong preference to orient itself and to remain oriented in one privilegded direction of the crystalline lattice. These domain act, inside the material, as small permanent magnets: in the demagnetized state of the magnet due, for instance to thermal agitation at high temperature the polarizations of the domains are random i.e. oriented in a disorderly way. Under the effect an external magnetic field, polarizaton become oriented and at saturation they are arranged parallel to the direction of the applied external magnetic field. Switching of the external magnetic field, polarizations of the Weiss domains return to the prefered direction of the formerly applied magnetic field at saturation, originating a frequency distribution with a maximum in the direction of the saturation field and with a minimum close to zero in the opposite direction. In this way the magnet creates and external magnetic field around itself which continues to be effective also after the elimination of the external saturation field.

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Glossary of magnetic terms

 

Ampere turn. A unit of magnetomotive force it is a product of the number of turns in the coil and amperes passing through the coil. 

Anisotropic magnet. A material havin preferred  orientation so that the magnetic characteristics are better along one axis  than along any other axis CGS System. A system of measurement in which the centimetrer gram and second are fundamental units. The cgs electromagnetic system is used in this manual.

Coercitive force - Hc. The demagnetizing force in oersteds corresponding to zero magnetic induction in a magnetic material after saturation.

Coercitive force intrinsic - Hci. The demagnetizing force in oersteds corresponding to zero iintrinsic induction in a megnetic materal after saturation. It is a measure of resistance to demagnetizing.

Demagnetization. The partial or complete reduction of induction.

Demagnetization curve. That portion of the normal hysteresis loop in the second quadrant ahowing the induction in a magnetic material as related to the magnetizing force.

Dimension ratio - L/D. The ratio of the length of a magnet in the direction of magnetization to its diameter or the ratio of the length of the magnet to the diameter of a circle which has an area equal to the cross sectional area of the magnet.

Energy product curve. It is the graphical representation of the external energy produced by a magnet and is the product of the flux density and demagnetizing force as shown on the normal demagnetization curve. The maximum of this product as shown on such a curve is known as (Bd Hd) max.  This value divided by 8 π gives the theoretical optimum magnetic energy in ergs per cubic centimeter of material which can be set ou in any external magnetic circuit associated with it.

Ferromagnetic. A material which in general exhibits hysteresis phenomena and whose permeability is dependent upon the magnetizing force.

Flux, magnetic Ø . The physicfal manifestation of a condition existing in a medium or material subjected to a magnetizing by the fact that an electromotive force is induced in a conductor surrounding the flux changes in magnitude. The unit in the Cgs sysytem is the Maxwell.

Flux, density - B. The number of lines or maxwells per unit area in a section normal to the direction of the flux.

Gap. That portion of the magnetic circuit that does not contain ferromagnetic material e.g. an ait gap.

Gauss. Unit of flux density.  Gauss = total flux in maxwell : area in cm².

Gilbert. A Cgs unit of magnetomotive force. The magnetomotive force required to produce one maxwell magnetic flux in a magnetic circuit of unit reluctance. Magnetomotive force in Gilbert = 0,4 π ampereturns.

High energy materials. This therm refers to magnetic material having a comparatively high energy products. Permanent magnet materials are of this class and have been known also as  hard magnetic materials.

Hysteresis, magnetic. A property of a magnetic material by virtue of which the magneticinduction for a given magnetizin force depends upon the previous conditions of magnetization.

Hysteresis, loop. A normal hysteresis loop is the graphical representation of the relationship  between the magnetizing force and the resultant induced magnetization of a ferromagnetic material when the magnetizing force is carried through a complete cycle of equal and opposite values under cyclic conditions.

Incremental permeability - µ ∆. The ratio of a change in magnetic induction to the corresponding change in magnetizin force when the mean induction differs from zero. It equals the slope of a straight line joining the excursion limits of an incremental hysteresis loop.

Incremental permeability - µ ∆. The ratio of cyclic change in induction to the cyclic change in magnetizing force from any position on the magnetization curve on hysteresis loop.

Induction, intrinsic - Br. The excess in the induction in a megnatic material over the induction in vacuum. for a given value of magnetizing force. The equation for intrinsic induction is  Bi = B - µH.

Induction, magnetic B. The magnetic flux for unit area of a section normal to the direction of flux.  The unit of measurement for flux density in the Cgs system is the Gauss.

Isotropic, magnetic. A material having the same magnetic characteristics along any axis or direction.

Kilogauss. One Kilogauss equals 1000 Gauss.

Leakage flux. That portion of the magnetic field that is not useful.

Leakage factor - σ. The ratio of the total flux produced in the neutral section of the magnet to the useful flux.

Line. A term commonly used inter chamgeably for a Maxwell.

Magnetizing force - H. The magnetomotive force for unit length at any given point in a magnetic circuit in the Cgs system the unit is the Oersted and defined by equation = magnetomotive force in Gilbert ÷ magnetizing force in Oersted.

Maxwell. The Cgs unit of magnetic flux.

Oersted. The Cgs unit of magnetizing force.

Permeability - µ.  The ratio of the magnetic induction in a given medium to the induction which would be produced in a vacuum with the same magnetizing force. In the Cgs system permeability is given by the equation: Permeability = magnetic induction in gauss ÷ magnetizin force in Oersted.

Permeance - P. The ratio of the flux through any cross section of a tubular portion of a magnetic circuit bounded by lines of force and by two equipotential surfaces to the magnetic potential difference between the surfaces taken within the portion under consideration. The defining equation in the Cgs sistem is: Permeance = magnetic flux in Maxwell ÷ magnetomotive force in Gilbert.

Permeance coefficent - Pc. Ratio of the magnetic induction Bd to its self demagnetizing force Hd Pc = Bd/Hd.

Reluctance - R. The reciprocal of permeance.

Reluctance factor - rf. Ratio of nnf produced by the magnet to te mmf in the gap.

Remanence - Bd. The magnetic induction which remains in a magnetic circuit after the removal of an applied magnetomotive force. If there is an air gap in the magnetic circuit, the remanence will be less than the residual induction.

Residual induction - Br. The magnetic induction corrisponding to zero magnetizing force in a magnetic material after saturation in a closed circuit (no air gap).

Saturation. The condition under which all elementary moments have become oriented in one direction. A magnetic material is satured when an increase in the aplied magnetizing force produces no increase in intrinsic induction.

Stabilization. The process of subjecting magnet to various condition such as heat, or demagnetizing conditions so that magnet will produce a constant magnetic field.

Tractive force. The force which a permanent magnet exerts on a ferromagnetic object.

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ALNI - ALNICO - MAXALCO C.O MAGNETS

Alni is an alloy of Fe-Al-Ni and Alnico an alloy of Fe-Al-Ni-Co to which other components in small quantities may be added. They are obtained by melting in medium frequency induction furnaces and cast in sand (shell) molds. Final magnetic properties are obtained by special heat treatment. In the case of the anisotropic Alnico Types the thermal treatment occurs the presence of strong magnetic fields. The purpose of these fields is to orintate particles of a highly magnetic phase, precipitated during heat treatment, thus increasing magnetic properties in the direction of the magnetic field during treatment. Final magnetization and application must occur in the same direction in order to have best result. Therefore these magnet are called oriented and anisotropic.

Alni and especially Alnico magnets have high remanence (Br) and high value of the energy product combined with the lowest temperature coefficent of all known permanent magnetic material. Thanks to these characteristics they are particulary fit for usage in instruments for electric measurements and in other equipment where high precision and thermal stability is required.

Alnico V is the most popular alloy for almost every application.

Maxalco C.O. as above but with oriented crystals. Oriented crystal structure is obtained by directing heat flow durin solification of the alloy after casting, which supports even better orientation during heat treatment and permits to obtain highest value of remanence and energy product.

Alnico VIII is an alloy with highest contents of Cobalt and Titanium. It is characterized by the highest coescitive force and stability and is the typical material for electrical instruments.

 

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FERROXDURE MAGNETS

This sintered ceramic ferrite material, based on the oxydes of Ba or Sr and Fe is of similar nature as the ancient mineral called "load stones" or "magnets", being "magnetite" the Fe3 O4. Ferroxdure was invented in the fifties: It is used for volume production of permanent magnets at low cost. It is produced by calcination of the raw materials in the order to obtain a megnetic compound of the right chemical composition. The compound is first ball milled and then pressed in multiple die tools, without or with magnetic fields for orientation, then the green pressed pieces are sintered in long sinter kilns to obtain maximal strength and density. Finally they are ground on the pole faces to tight tollerances.

Ferroxdure present very high coercive force, i.e. high resistance to demagnetizing field and has very high specific electric resistance. Typical application are: Loudspeakers, separators, magnetic choucks motors, toys.

More specifically:

Ferroxdure I - Isotropic (not oriented) magnets, for rotor of small motors, timers and bicycle dinamos, magneti panels and toys.

Ferroxdure II - High remanence and energy magnets, fit for loudspeaker rings.

Ferroxdure III - High coercive force magnet suitable for segment shaped motor magnets.

 

FERROXDURE I  (ISOTROPIC)

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FERROXDURE II  (ANISOTROPIC)

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FERROXDURE III  (ANISOTROPIC)

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SAMARIUM - COBALT  MAGNETS

This type of magnet is obtained by a sinterization process, in controlled atmosphere, of Rare Eart powder. The use of Samarium, grants a specific high energy product (BHmax), with a good residual magnetic flux (Br) and a very good coercitive force (JHc). The Cobalt also grants the stability of the magnetic properties. Although the temperature resistance is not very high, the high magnetic properties of SM.CO magnets, are expecially qualified for applications with limited dimensions.

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Simbols and units of measure

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