材料物理课件-8材料磁性性质资料.ppt
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1、Magnetic Properties of Materials I,Magnetization CurvesFig.MPA a)is the curve in the absence of any material:a vacuum.The gradient of the curve is 4.10-7 which corresponds to the fundamental physical constant 0.magnetic flux density:(A/m,Gs)VPMmB=0(H+M).M=mHB=H=0(1+m)H,r=/0 II,Magnetic momentThe con
2、cept of magnetic moment is the starting point when discussing the behaviour of magnetic materials within a field.If you place a bar magnet in a field then it will experience a torque or moment tending to align its axis in the direction of the field.A compass needle behaves the same.This torque incre
3、ases with the strength of the poles and their distance apart.So the value of magnetic moment tells you,in effect,how big a magnet you have.It is also well known that a current carrying loop in a field also experiences a torque(electric motors rely on this effect).Here the torque,increases with the c
4、urrent,i,and the area of the loop,A.is the angle made between the axis of the loop normal to its plane and the field direction.=B i A sin =B m sin Diamagnetic materials are those whose atoms have only paired electrons.III,Diamagnetic and paramagnetic materialsParamagnetic materials are those whose a
5、tioms have unpaired electrons and has permanent magnetic moments.Although paramagnetic substances like oxygen,tin,aluminium and copper sulphate are attracted to a magnet the effect is almost as feeble as diamagnetism.The reason is that the permanent moments are continually knocked out of alignment w
6、ith the field by thermal vibration,at room temperatures anyway(liquid oxygen at-183 C can be pulled about by a strong magnet).IV,Ferromagnetic materialsThe most important class of magnetic materials is the ferromagnets:iron,nickel,cobalt and manganese,or their compounds(and a few more exotic ones as
7、 well).The magnetization curve looks very different to that of a diamagnetic or paramagnetic material.V,Hysteresis loopMemory devicesOutlineu Background u Semiconductor Conventional memory technologies Emerging memory technologies1 Magnetic Memory Mechanism:Main applicationsTape Diskette Magnetic dr
8、um Magnetic Memory materials:-Fe2O3,CrO2,Fe-Co et alRead/write heads2 Optical MemoryDVD-RWDVDCDApplications:Optical storage materials:PC、PMMA、Epoxy et al.Mechanism:Advantage:low price,high storage density;disadvantagelow access,large boxMain application:3 Semiconductor memoryBased on semiconductor d
9、evices;Advantage:fast access,high data storage,low power;Cache memoryStacked memoryFlash memoryComparison of memory technologiesOptical MemoriesMagnetic DisksMagnetic TapesMagnetic Bubble MemoriesSemiconductor RAMsSemiconductor ROMs10010-110-210-310-410-510-610-710-810-910-1010-910-810-710-610-510-4
10、10-310-2Access timeCost per bitMain memoryCacheSemiconductor memories Cell arrayPeripheral circuitI/O unit circuit 2m+n+k-1Categories of Semiconductor memories Memory technologies Primary categories of electrical memory:RAM,ROM and FlashNonvolatile:after transition from OFF state to ON state,device
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