- 1. Magnets Amagnet(from Greekmagntislthos, "Magnesianstone") is a material or object that produces amagnetic field. It attracts ferrous objects like pieces ofiron, steel, nickel and cobalt. This magnetic field isinvisible but is responsible for the most notable propertyof a magnet: a force that pulls on otherferromagneticmaterials, such asiron, and attracts or repels othermagnets. A magnet can be made to stick to objects whichcontain magnetic material such as iron, even if theyare not magnets. But a magnet cannot be made tostick to materials which are plastic, or cotton, or anyother material, such as silicate rock, which is notmagnetic.
2. TYPES OF MAGNET Permanent magnets ELECTROMAGNETS 3. Permanent magnets Cnuie ao itws d cvr ta c t tps oetr s g,a isoe d ht e a ye f e r inm e l okps s du uul rpr s f t at nt inr rc os s n sa poe ie o a rca eett io otemt irn O epr ua m e l cld o e to e h e l o. n at l inr , ae ds n,aic ra l lo antite ( e 4 , is f n mnioe in vr or gemF3 ) Oo d et nd ue ly dh t icl e rs( bu 2 0 yasaoinE rp, adis r a r od aot 5 0 er g o c uoe nmc er rinteFrE s a asb c o cr sy uh al ieh a at s uj t f uioit.) eLt , it a e p ydinte ido nv a n a it aa r ws ml eeo h a f aigt , s wsiof n ta a ie o tis n sarc wud ed o r n o d ht p c fh u uulok ol t t oiet u enit l in a nr - ot d e io ifltf e t rt e sfeot suh ir t n hce r o otf ea( upne o a t g r n f a in a rssedd n s in o o a l t wt ) roe . Apermanent magnetis an object made from a material thatismagnetizedand creates its own persistent magnetic field. Alloys of iron, nickel, cobalt, gadolinium and certainceramic materials can become "permanent" magnets,such that they retain their magnetism for a long time. 4. Strongest Type of Permanent MagnetAneodymium magnet(alsoknownasNdFeB,NIB,orNeomagnet),themostwidely-usedtypeofrare-earthmagnet,isapermanentmagnetmadefromanalloyofneodymium,iron,andborontoformtheNd2Fe14Btetragonalcrystallinestructure.Developedin1982byGeneralMotorsandSumitomoSpecialMetals,neodymiummagnetsarethestrongesttypeofpermanentmagnetmade.Theyhavereplacedothertypesofmagnetinthemanyapplicationsinmodernproductsthatrequirestrongpermanentmagnets,suchasmotorsincordlesstools,harddiskdrives,andmagneticfasteners. NickelplatedneodymiummagnetNickel-platedneodymiummagnetcubes onabracketfromaharddrive. 5. Permanent magnets 6. Permanent Ferrite Magnet Magnets that are commonly used in speakers 7. Applicationofmagnets: G a us s C a nnon There are 3 ball bearings stuck to a magnet in a track. A fourth ball bearing is released on the opposite side of the magnet, and is attracted to it. The ball at the other end shoots off at a much higher velocity. A second version of the gauss cannon is below and uses onespherical magnet which looks identical to the ball bearings. Thedevice can first be shown without the magnet, when it acts likeNewtons cradle and conserves energy. With the magnet, the end ballshoots off the end of the ramp. 8. Applicationofmagnets:Barkhausen effect IntheBarkhauseneffect,alargecoiloffinewireis connectedthroughanamplifiertoaspeaker.When anironrodisplacedwithinthecoilandstrokedwith amagnet,anaudibleroaringsoundwillbeproduced fromthesuddenrealignmentsofthemagnetic domainswithintherod.Acopperrod,ontheother hand,producesnoeffect. 9. Agoodpermanentmagnetshouldproduceahighmagneticfieldwithalowmass,andshouldbestableagainsttheinfluenceswhichwoulddemagnetizeit.Thedesirablepropertiesofsuchmagnetsaretypicallystatedintermsoftheremanenceandcoercivityofthemagnetmaterials. 10. R e ma ne nc e Remanence orremanent ma gnetization is themagnetizationleft behind in aferromagneticmaterial (suchasiron) after an externalmagnetic fieldis removed. It isalso the measure of that magnetization.Colloquially, whena magnet is "magnetized" it has remanence.Theremanence of magnetic materials provides the magneticmemory inmagnetic storagedevices, and is used as asource of information on the past Earths magnetic field inpaleomagnetism. The equivalent termresidual ma gnetization is generallyused in engineering applications. Intransformers,electric motorsandgeneratorsa large residualmagnetization is desirable . In many other applications it isan unwanted contamination, for example a magnetizationremaining in anelectromagnetafter the current in the coilis turned off. Where it is unwanted, it can be removed bydegaussing. Sometimes the termretentivity is used for remanencemeasured in units ofmagnetic flux density. 11. Types of remanenceSaturation remanence The default definition for remanence is the magnetization remaining in zerofield after a large magnetic field is applied (enough to achievesaturation).Amagnetichysteresis loopis measured using instruments such as avibrating sample magnetometerand the zero-field intercept is a measure ofthe remanence. Inphysicsthis measure is converted to an averagemagnetization(the totalmagnetic momentdivided by the volume of the sample)and denoted in equations asMr. If it must be distinguished from other kinds ofremanence it is called thesaturation remanence orsaturationisothermal remanence (SIRM) and denoted byMrs. In engineering applications the residual magnetization is often measured usingaB-H Analyzer, which measures the response to an AC magnetic field (as inFig. 1). This is represented by aflux densityBR. This value of remanence is oneof the most important parameters characterizingpermanent magnets; itmeasures the strongest magnetic field they can produce.Neodymium magnets, for example, have a remanence approximately equal to 1.3teslas. 12. Is o t h e r m a l r e m a n e n c e Often a single measure of remanence does not provide adequateinformation on a magnet. For example, magnetic tapes contain a largenumber of small magnetic particles, and these particles are not identical.Magnetic minerals in rocks may have a wide range of magneticproperties. One way to look inside these materials is to add or subtractsmall increments of remanence. One way of doing this is firstdemagnetizingthe magnet in an AC field, and then applying a fieldHandremoving it. This remanence, denoted byMr(H), depends on the field. It iscalled theinitial remanence or theisothermal remanentmagnetization (IRM) . Another kind of IRM can be obtained by first giving the magnet asaturation remanence in one direction and then applying and removing amagnetic field in the opposite direction. This is calleddemagnetizationremanence ordc demagnetization remanence and is denoted bysymbols likeMd(H), whereHis themagnitudeof the field. Yet anotherkind of remanence can be obtained by demagnetizing the saturationremanence in an ac field. This is calledac demagnetizationremanence oralternating field demagnetization remanence andis denoted by symbols likeMaf(H). If the particles are noninteracting single-domain particles with uniaxialanisotropy, there are simple linear relations between the remanences. 13. Anhysteretic remanence Another kind of laboratory remanence is anhystereticremanenceoranhysteretic remanent magnetization (ARM). Thisis induced by exposing a magnet to a large alternatingfield plus a small dc bias field. The amplitude of thealternating field is gradually reduced to zero to getananhysteretic magnetization, and then the bias field isremoved to get the remanence. The anhystereticmagnetization curve is often close to an average of thetwo branches of thehysteresis loop,and is assumed insome models to represent the lowest-energy state for agiven field.ARM has also been studied because of itssimilarity to the write process in some magnetic recordingtechnologyand to the acquisition ofnatural remanent magnetizationin rocks. 14. Coercivity Inmaterials science, thecoercivity, also called thecoercive fieldorcoerciveforce, of aferromagnetic materialis the intensity of the appliedmagnetic fieldrequired to reduce the magnetizationof that material to zeroafterthemagnetization of the sample has been driven tosaturation. Thus coercivitymeasures the resistance of a ferromagnetic material to becomingdemagnetized. Coercivity is usually measured inoerstedorampere/meter unitsand is denoted HC. It can be measured using aB-H Analyzerormagnetometer. Ferromagnetic materials with high coercivity are calledmagneticallyhardmaterials, and are used to makepermanent magnets.Permanent magnets find application inelectric motors, magnetic recordingmedia (e.g.hard drives,floppy disks, ormagnetic tape) andmagnetic separation. Materials with low coercivity are said to be magneticallysoft. They are used intransformerandinductorcores,recording heads.microwavedevices, andmagnetic shielding. 15. Experimental determination Typically the coercivity of a magnetic material is determined by measurement ofthehysteresis loop, also called themagnetization curve, as illustrated in the figure.The apparatus used to acquire the data is typically avibrating-sampleoralternating-gradientmagnetometer. The applied field where the data line crosseszero is the coercivity. If anantiferromagnetis present in the sample, the coercivitiesmeasured in increasing and decreasing fields may be unequal as a result of theexchange biaseffect. The coercivity of a material depends on the time scale over which a magnetizationcurve is measured. The magnetization of a material measured at an appliedreversed field which is nominally smaller than the coercivity may, over a long timescale, slowlyrelaxto zero. Relaxation occurs when reversal of magnetization bydomain wall motion isthermally activatedand is dominated bymagnetic viscosity.The increasing value of coercivity at high frequencies is a serious obstacle to theincrease ofdata ratesin high-bandwidthmagnetic recording, compounded by thefact that increased storage density typically requires a higher coercivity in themedia. The coercivity of a material depends on the time scale over which a magnetizationcurve is measured. The magnetization of a material measured at an appliedreversed field which is nominally smaller than the c