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1、8.5.6POLYCRYSTALLINEMATERIALSANDTHEMVERSUSHBEHAVIORThemajorityofthemagneticmaterialsusedinengineeringispolycrystallineandthereforehaveamicrostructurethatconsistsofmanygrainsofvarioussizesandorientationsdependingonthepreparationandthermalhistoryofthecomponen
2、t.Inanunmagnetizedpolycrystallinesample,eachcrystalgrainwillpossessdomains,asdepictedinFigure8.30.Thedomainstructureineachgrainwilldependonthesizeandshapeofthegrainand,tosomeextent,onthemagnetizationsinneighboringgrains.Althoughverysmallgrainsperhapssmaller
3、than0.1µm,maybesingledomains,inmostcasesthemajorityofthegrainswillhavemanydomains.Overall,thestructurewillpossessnonetmagnetization,providedthatitwasnotpreviouslysubjectedtoanappliedmagneticfield.WecanassumethatthecomponentwasheatedtoatemperatureabovetheCur
4、iepointandthenallowedtocooltoroomtemperaturewithoutanappliedfield.Supposethatwestartapplyingaverysmallexternalmagneticfield(µ0H)alongsomedirection,whichwecanarbitrarilylabelas+x.Thedomainwallswithinvariousgrainsbegintomovesmalldistances,andfavorablyoriented
5、domains(thosewithacomponentofMalong+x)growalittlelargerattheexpenseofthosepointingswayfromthefield,asindicatedbypointaninFigure8.31.Thedomainwallsthatarepinnedbyimperfectionstendtobowout.Thereisaverysmallbutnetmagnetizationalongthefield,asindicatebytheOareg
6、ioninthemagnetizationversusmagnetizingfield(MversusH)behaviorinFigure8.31.Asweincreasethemagnetizingfield,thedomainmotionextendlargerdistance,asshownforpointbinFigure8.31,andwallsencountervariousobstaclessuchascrystalimperfections,impurities,secondphase,and
7、soonwhichtendtoattractthewallsandtherebyhindertheirmotions.Adomainwallthatisstuck(orpinned)atanimperfectionatagivenfieldcannotmoveuntilthefieldincreasessufficientlytoprovidethenecessaryforcetosnapthewallsnapsfree,whichthensuddenlysurgesforwardtothenextobsta
8、cle.Asawallsuddenlysnapsfreeandshootsforwardtothenextobstacle,essentiallytwocausesleadtoheatgeneration:Suddenchangeinthelatticedistortion,duetomagnetostriationcreatelatticewavesthatcarryoffsomeoftheene
