材料物理 材料磁性性质ppt课件

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= /

2、0 II, Magnetic momentThe concept 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 beha

3、ves the same. This torque increases 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).

4、 Here the torque, , increases with the current, 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

5、materialsParamagnetic materials are those whose atioms 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 permane

6、nt moments are continually knocked out of alignment with 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 a

7、nd manganese, or their compounds (and a few more exotic ones as 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 Ma

8、gnetic Memory Mechanism: Main applicationsTape Diskette Magnetic drum 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 acces

9、s, large boxMain application:3 Semiconductor memoryBased on semiconductor devices；Advantage: fast access, high data storage, low power；Cache memoryStacked memoryFlash memoryComparison of memory technologiesOptical MemoriesMagnetic DisksMagnetic TapesMagnetic Bubble MemoriesSemiconductor RAMsSemicond

10、uctor ROMs10010-110-210-310-410-510-610-710-810-910-1010-910-810-710-610-510-410-310-2Access 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: R

11、AM, ROM and FlashNonvolatile: after transition from OFF state to ON state, device 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

12、): Information is not lost when the power is switched off, but the charge stored in chip cant 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 logi

13、cal value “0”Parasitic capacitanceDRAM write and read operationwritereadROM Mask ROMPROMEPROMEEPROM (Flash)Flash DielectricTunnel oxide+Floating GateThreshold shift due to the electric chargeMOSFET GDSDSGMetal-oxide-semiconductor field effect transistorFlash write/erase/read operation Apply voltage

14、to control gate (CG) e- tunneling occurs from channel to FG Apply voltage to source e- transfer 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 F

15、lash: erased and programmed byte-wise. Performance and requirements Fast accessNon-volatilityUnlimited R/W cyclesLow powerWide temperature rangeLow costEmerging memory FeRAM Organic Memory Nano-Crystal Floating-Gate Flash Memory Phase Change Memory NRAMFerroelectric unitHysteresis curveTwo states of

16、 polarization under applied field can correspond to a stored “0” or “1”Remnant polarizationCoercive fieldFerroelectric 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

17、” 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” , accomplished 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 th

18、e cell is selected by WL, the PL voltage is raised from GND to VDD, raised voltage of BL is dependent of the polarization (data) stored in FeCAP. FeFET (polarization)FeFET is in principle a MOSFET transistor whose gate dielectric is ferroelectric.Advantage: reading operation is nondestructive.Disadv

19、antage: retention time is very short to nonvolatile memory.Electrical bistability:Organic electric bistable devices A phenomenon exhibit two kinds of different stable conductive state by applying appropriate voltage. Typical I-V characteristicsSilicon memory: encode “0” and “1” as the amount of char

20、ge stored in device cell Organic memory: store date based on high & low conductivity response to applied voltageDevice structuresCross-BarsShadow maskDevice configurationsPolyaniline nanofiberGold nanoparticlesOrganic/nanoparticles systemMetal complex DonorAcceptorDonor-Acceptor systemPerformance and Characterization ON/OFF current ratioWri