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
11、remained in this state even after turning off the power.Random access memory(RAM):The charge can be refreshed frequently.information is lost when the power removed from the device.(DRAM,SRAM)Read only memory(ROM):Information is not lost when the power is switched off,but the charge stored in chip ca
12、nt be refreshed.Flash:The charge can be refreshed frequently,and information is not lost when the power is switched off.DRAMThe presence of a charge represents the logical value“1”and its absence the logical value“0”Parasitic capacitanceDRAM write and read operationwritereadROM Mask ROMPROMEPROMEEPR
13、OM(Flash)Flash DielectricTunnel oxideMOSFET+Floating GateThreshold shift due to the electric chargeMOSFET GDSDSGMetal-oxide-semiconductor field effect transistorFlash write/erase/read operation Apply voltage to control gate(CG)e-tunneling occurs from channel to FG Apply voltage to source e-transfer
14、occurs from FG to source Apply voltage to CG.If e-present in FG,no conduction between S and D.If e-is absent,conduction happens.NAND&NOR FlashNAND Flash:erased and programmed block-wise.NOR Flash:erased and programmed byte-wise.Performance and requirements Fast accessNon-volatilityUnlimited R/W cycl
15、esLow powerWide temperature rangeLow costEmerging memory FeRAM Organic Memory Nano-Crystal Floating-Gate Flash Memory Phase Change Memory NRAMFerroelectric unitHysteresis curveTwo states of polarization under applied field can correspond to a stored“0”or“1”Remnant polarizationCoercive fieldFerroelec
16、tric memory(FeRAM)FeRAM(capacitor)Plateline(PL)has a variable voltage level to enable the switching of the polarization of the ferroelectric capacitor.1T-1CFeRAM operationTo write“1”in the cell,BL is set to VDD and PL is grounded,then a pulse is applied to activate the cell transistor.To write“o”,ac
17、complished in the same manner but PL and BL are exchanged to reverse the polarization of Ferroelectric capacitor.Read:first BL is grounded,then it is made floating.After the cell is selected by WL,the PL voltage is raised from GND to VDD,raised voltage of BL is dependent of the polarization(data)sto
18、red in FeCAP.FeFET(polarization)FeFET is in principle a MOSFET transistor whose gate dielectric is ferroelectric.Advantage:reading operation is nondestructive.Disadvantage:retention time is very short to nonvolatile memory.Electrical bistability:Organic electric bistable devices A phenomenon exhibit
19、 two kinds of different stable conductive state by applying appropriate voltage.Typical I-V characteristicsSilicon memory:encode“0”and“1”as the amount of charge stored in device cell Organic memory:store date based on high&low conductivity response to applied voltageDevice structuresCross-BarsShadow
20、 maskDevice configurationsPolyaniline nanofiberGold nanoparticlesOrganic/nanoparticles systemMetal complex DonorAcceptorDonor-Acceptor systemPerformance and Characterization ON/OFF current ratioWrite-read-erase cyclesSwitching timeRetention abilityNano-crystal floating-gate memoryOxide gate too thin
21、Leakage pathCause electron stored to leak outHow to alleviate the scaling limitation?use thinner tunnel oxides without sacrificing nonvolatility oxide thickness operating voltage operating speeds DielectricTunnel oxidePhase change memoryChange the phase to crystalline(set or conductive)and amorphous(reset or resistive)by passing a programming current of different magnitudes(higher current,pulse current,lower current)through memory element.l Material:Ge2Sb2Te5(GST)l Switching:10-30 nsl Cycling time:1012Carbon nanotube memoryThanks for your attention!
