Experiment 3l:Giant Magnetoresistance Effect andItsApplicationsIntroductionThe2007Nobel Prize in Physics was awarded toAlbert Fert and Peter Grunbergfor the discovery of Giant magnetoresistance (GMR) which is a quantum mechanicalmagnetoresistance effect observed in multilayers composed of alternatingferromagnetic and non-magnetic conductive layers.The effect is observed as asignificant change in the electrical resistancedepending on whetherthe magnetizationof adjacent ferromagnetic layers are in a parallel or an antiparallel alignment.Theoverall resistance is relativelylowfor parallel alignment and relatively high forantiparallel alignment. The magnetization direction can be controlled, for example, byapplying an external magnetic field.The effect is based on the dependence of electronscattering on the spin orientation.The main application of GMR is magnetic fieldsensors,which are used to read data in hard disk drives, biosensors,microelectromechanical systems (MEMS) and other devices. GMR multilayerstructures are also used in magnetoresistive random-access memory (MRAM) as cellsthat storeonebitof information.In1986,German physicist Peter Grunberg used amolecular beam epitaxy (MBE)method to prepare a three-layer single crystal structure film of iron-chromium-iron. Itwas found that at a certain thickness of chromium layer without external magnetic field,themagnetic moments of theferromagnetic layers on both sides areanti-parallel.Thephenomenon becomes the premise of the GMR effect.Further research found that thestructure has high resistance when the two magnetic moments are anti-parallel and lowresistance when they are parallel. The difference between could be up to 10%.In 1988, French physicist Albert Fert's research group alternated iron andchromium films in dozens of cycles of iron-chromium superlattice films. They foundthat when changing the magnetic field strength, the resistance drops by nearly 50%This unprecedented phenomenon of large resistance change is called giantmagnetoresistanceeffect
Experiment 31: Giant Magnetoresistance Effect and Its Applications Introduction The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg for the discovery of Giant magnetoresistance (GMR) which is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. The magnetization direction can be controlled, for example, by applying an external magnetic field. The effect is based on the dependence of electron scattering on the spin orientation. The main application of GMR is magnetic field sensors, which are used to read data in hard disk drives, biosensors, microelectromechanical systems (MEMS) and other devices. GMR multilayer structures are also used in magnetoresistive random-access memory (MRAM) as cells that store one bit of information. In 1986, German physicist Peter Grunberg used a molecular beam epitaxy (MBE) method to prepare a three-layer single crystal structure film of iron-chromium-iron. It was found that at a certain thickness of chromium layer without external magnetic field, the magnetic moments of the ferromagnetic layers on both sides are anti-parallel. The phenomenon becomes the premise of the GMR effect. Further research found that the structure has high resistance when the two magnetic moments are anti-parallel and low resistance when they are parallel. The difference between could be up to 10%. In 1988, French physicist Albert Fert's research group alternated iron and chromium films in dozens of cycles of iron-chromium superlattice films. They found that when changing the magnetic field strength, the resistance drops by nearly 50%. This unprecedented phenomenon of large resistance change is called giant magnetoresistance effect
ExperimentalObjective(1)UnderstandtheprincipleofGMReffectwithmultilayerfilms(2)MasterthecharacteristicsofGMRmagnetoresistance(3) Understand the structure and characteristics of GMR sensors and know how to useGMR sensors.Experimental Content and Procedure(1) Measure the characteristics of GMR magnetoresistance.(2) Measuring the current of GMR analog sensor.Experimental InstrumentExperimental instruments based on GMR effect and application, and componentsforbasiccharacteristicandcurrentmeasurementExperimentalPrinciple1.PrincipleofGMReffectAccording to the microscopic mechanism of conduction,the electrons in the metaldo not travel straight along the electric field when conducting electricity, butcontinuously collide with the atoms in the lattice position (also called scattering), andthe electrons change the direction of motion after each scattering. The total motion isthe superposition of the directional acceleration along the electric field and the randomscattering motion of the electrons. The average distance of the electrons movingbetween the two scattering centres is called the mean free path. The smaller the electronscattering probability, the longer the mean free path and the lower the resistivity.Equation R-pl/S can be used for the macroscopic materials. Usually the boundary effectcan be ignored when the film is thick enough. However, when the film is as thin as ananometer level, for example, there is only a few atoms, the scattering probability ofthe electrons on the boundary is greatly increased. Therefore, the phenomenon that theresistivityis increasedwiththereducingthicknesscanbeclearlyobserved.Theelectroncarries the charge and it also has a spin characteristic.Under the external magnetic field
Experimental Objective (1) Understand the principle of GMR effect with multilayer films. (2) Master the characteristics of GMR magnetoresistance (3) Understand the structure and characteristics of GMR sensors and know how to use GMR sensors. Experimental Content and Procedure (1) Measure the characteristics of GMR magnetoresistance. (2) Measuring the current of GMR analog sensor. Experimental Instrument Experimental instruments based on GMR effect and application, and components for basic characteristic and current measurement. Experimental Principle 1. Principle of GMR effect According to the microscopic mechanism of conduction, the electrons in the metal do not travel straight along the electric field when conducting electricity, but continuously collide with the atoms in the lattice position (also called scattering), and the electrons change the direction of motion after each scattering. The total motion is the superposition of the directional acceleration along the electric field and the random scattering motion of the electrons. The average distance of the electrons moving between the two scattering centres is called the mean free path. The smaller the electron scattering probability, the longer the mean free path and the lower the resistivity. Equation R=l/S can be used for the macroscopic materials. Usually the boundary effect can be ignored when the film is thick enough. However, when the film is as thin as a nanometer level, for example, there is only a few atoms, the scattering probability of the electrons on the boundary is greatly increased. Therefore, the phenomenon that the resistivity is increased with the reducing thickness can be clearly observed. The electron carries the charge and it also has a spin characteristic. Under the external magnetic field
therearetwopossibleorientationsofthemagneticmomentfortheelectrons,namelyparallel or anti-parallel orientation to the magnetic field. As early as 1936, physicistN.F. Mott pointed out that in the transition metals, the electrons whose spin magneticmoment is parallel to the magnetic field are much less likely to be scattered comparedwith those whose spin magnetic moment isTopmagneticfield direction withoutantiparallel to the magnetic field of the material.externalmagneticfieldThe total current through the materials is the sumTopferromagnetic filmof two types of spintronic currents in eachindependentchannel.ThetotalresistanceoftheIntermediateconductivelayerBottom ferromagnetic filmmaterials is the parallel resistance of twoindependent resistances from each independentBottommagneticfield directionchannel. This is called “"two current model"withoutexternal magneticfieldAs shown in Fig. 31-1, in the multi-layerFig. 31-1. GMR structure based onGMR structure, when there is no externalMultilaverfilmsmagneticfield, themagneticmomentsoftheupperandlowerferromagneticfilmsareanti-parallel coupling since the energy is the smallest. Under the external magnetic field,themagneticmomentdirectionof theferromagneticfilmwill beturnedtobeconsistentwith the direction of themagneticfield.Theexternal magnetic field changes thetwolayers of ferromagnetic film from anti-parallel coupling into parallel coupling.Thereare two types of spin-dependent scattering that contribute to the GMR effect(1) Interface scatteringWhenthereisnoexternalmagneticfieldthemagneticfieldsoftheupperand lowerferromagnetic films are opposite in direction.Regardless of the initial spin state of theelectrons,theelectron statechangeswhengoingfromaferromagneticfilm intoanotherferromagnetic film (parallel→ anti-parallel or anti-parallel→ parallel).The probabilityof electron scattering on the interface is very large which is corresponding to a highresistance state.However,when thereis an external magnetic field,the magneticfieldsof the upper and lower ferromagnetic films are in the same direction, and the scatteringprobability of electrons on the interface is relatively small which is corresponding to alow resistance state
there are two possible orientations of the magnetic moment for the electrons, namely parallel or anti-parallel orientation to the magnetic field. As early as 1936, physicist N.F. Mott pointed out that in the transition metals, the electrons whose spin magnetic moment is parallel to the magnetic field are much less likely to be scattered compared with those whose spin magnetic moment is antiparallel to the magnetic field of the material. The total current through the materials is the sum of two types of spintronic currents in each independent channel. The total resistance of the materials is the parallel resistance of two independent resistances from each independent channel. This is called “two current model”. As shown in Fig. 31-1, in the multi-layer GMR structure, when there is no external magnetic field, the magnetic moments of the upper and lower ferromagnetic films are anti-parallel coupling since the energy is the smallest. Under the external magnetic field, the magnetic moment direction of the ferromagnetic film will be turned to be consistent with the direction of the magnetic field. The external magnetic field changes the two layers of ferromagnetic film from anti-parallel coupling into parallel coupling. There are two types of spin-dependent scattering that contribute to the GMR effect. (1) Interface scattering When there is no external magnetic field, the magnetic fields of the upper and lower ferromagnetic films are opposite in direction. Regardless of the initial spin state of the electrons, the electron state changes when going from a ferromagnetic film into another ferromagnetic film (parallel → anti-parallel or anti-parallel → parallel). The probability of electron scattering on the interface is very large which is corresponding to a high resistance state. However, when there is an external magnetic field, the magnetic fields of the upper and lower ferromagnetic films are in the same direction, and the scattering probability of electrons on the interface is relatively small which is corresponding to a low resistance state. Bottom magnetic field direction without external magnetic field Top ferromagnetic film Intermediate conductive layer Bottom ferromagnetic film Top magnetic field direction without external magnetic field Fig. 31-1. GMR structure based on Multilayer films
(2) Bulk scatteringR(Q)Considering the random scattering,5000electrons also have a certain probability49004800to pass between the upper and lower4700ferromagnetic films. When there is no4600external magneticfield,themagnetic4500fieldsof theupperandlower44004300ferromagnetic films are opposite inB(Gs)4200direction,Regardless of the initial spin500-2502500500state of the electrons, theywill undergoFig.31-2. Magnetoresistance characteristic curves.small scattering (parallel) and largescattering (anti-parallel)processes.Theparallel resistance from the two types ofspintronic currents is similar to the parallel connection of two medium-resistanceresistors which is corresponding to a high resistance state. When the upper and lowerferromagnetic films have the same magnetic field direction, the scattering probabilityof the spin parallel electrons is small and the antiparallel one is large. The parallelresistanceofthetwotypesofspintroniccurrent issimilartotheparallel connectionofa small resistance and a large resistance which is corresponding to a low resistance state.Fig. 31-2 shows the magnetoresistance characteristic curves of typical GMRstructure. During the forward magnetic field, as the external magnetic field increases,the resistance gradually decreases (solid line in the figure). There is a linear regionWhen the magnetic moments of the two ferromagnetic films are completely parallel-coupled with the external magnetic field, the resistance is no longer reduced and themagnetic saturation state is reached even though the magnetic field continues toincrease. Then when the magnetic field is reduced from the magnetic saturation state,the resistance will gradually increase (dashed line in the figure).The two lines do notoverlap because of the hysteresis characteristic of the ferromagnetic material. Themagnetoresistance characteristics under the reverse magnetic field are symmetricalwith that under the forward magnetic field, as shown in Fig. 31-2.2. GMR analog sensor structure
(2) Bulk scattering Considering the random scattering, electrons also have a certain probability to pass between the upper and lower ferromagnetic films. When there is no external magnetic field, the magnetic fields of the upper and lower ferromagnetic films are opposite in direction. Regardless of the initial spin state of the electrons, they will undergo small scattering (parallel) and large scattering (anti-parallel) processes. The parallel resistance from the two types of spintronic currents is similar to the parallel connection of two medium-resistance resistors which is corresponding to a high resistance state. When the upper and lower ferromagnetic films have the same magnetic field direction, the scattering probability of the spin parallel electrons is small and the antiparallel one is large. The parallel resistance of the two types of spintronic current is similar to the parallel connection of a small resistance and a large resistance which is corresponding to a low resistance state. Fig. 31-2 shows the magnetoresistance characteristic curves of typical GMR structure. During the forward magnetic field, as the external magnetic field increases, the resistance gradually decreases (solid line in the figure). There is a linear region. When the magnetic moments of the two ferromagnetic films are completely parallelcoupled with the external magnetic field, the resistance is no longer reduced and the magnetic saturation state is reached even though the magnetic field continues to increase. Then when the magnetic field is reduced from the magnetic saturation state, the resistance will gradually increase (dashed line in the figure). The two lines do not overlap because of the hysteresis characteristic of the ferromagnetic material. The magnetoresistance characteristics under the reverse magnetic field are symmetrical with that under the forward magnetic field, as shown in Fig. 31-2. 2. GMR analog sensor structure －500 －250 0 250 500 B (Gs) 50 0 0 49 0 0 48 0 0 47 0 0 46 0 0 45 0 0 44 0 0 43 0 0 42 0 0 R (Ω) Fig. 31-2. Magnetoresistance characteristic curves
RRoutputoutpuRRFlux collectorInput-(a)Geometric structure(b) Circuit diagramFig. 31-3. Structure schematic of GMR analog sensor.As shown in Fig.31-3, a bridge structure is generally employed to eliminate theinfluenceofenvironmentalfactors suchastemperaturefluctuationonthemeasurementresulteffect whentheGMRisusedas asensor.There is no signal outputwhenthefourGMRs in the bridge structure are fully synchronized with the response of the magneticfield. Therefore, the two resistors R3, R4 at the diagonal position of the bridge arecovered with a layer of high magnetic permeability material to shield the externalmagnetic field while the resistors Ri and R2 are directly facing the external magneticfield. Thus the resistance value will change with the change of the external magneticfield. The geometric structure is shown in Fig. 31-3(a). The input voltage of thecorresponding end of the bridge circuit is Uin and the output voltage of the othercorresponding end is Uou. When there is no external magnetic field, the resistance ofeachGMRis R.Underthe external magneticfield,theresistance oftheresistorsRiandR2decreasesbyR,andtheoutput voltageisUINAR(31-1)Uour=(2R-△R)From the above formula, the magnetoelectric conversion characteristics of theGMR analog sensors can be obtained and the related physical parameters can becalculated.FromEq.31-l,onecan seethatthereisalinearrelationshipbetweentheoutput voltage and the magnetic induction intensity within a certain range, and thesensitivityis relativelyhighTherefore,theGMR canbeconvenientlymade into amagnetic field meter to measure magnetic induction or other related parameters.Forexample, it can be used to measure the current without the requirement of connectingthe instrument to the circuit, and it will not interfere with the circuit operation. Inpractical applications, in order to improve the measurement accuracy, the GMR analogsensor must be operated within the linear region.And a fixed magnetic field, calledmagnetic bias, is often applied to the sensor in advance. This is similar to the DC bias
As shown in Fig. 31-3, a bridge structure is generally employed to eliminate the influence of environmental factors such as temperature fluctuation on the measurement result effect when the GMR is used as a sensor. There is no signal output when the four GMRs in the bridge structure are fully synchronized with the response of the magnetic field. Therefore, the two resistors R3, R4 at the diagonal position of the bridge are covered with a layer of high magnetic permeability material to shield the external magnetic field while the resistors R1 and R2 are directly facing the external magnetic field. Thus the resistance value will change with the change of the external magnetic field. The geometric structure is shown in Fig. 31-3(a). The input voltage of the corresponding end of the bridge circuit is UIN and the output voltage of the other corresponding end is UOUT. When there is no external magnetic field, the resistance of each GMR is R. Under the external magnetic field, the resistance of the resistors R1 and R2 decreases by ΔR, and the output voltage is: (2 ) IN OUT R R U R U − = (31-1) From the above formula, the magnetoelectric conversion characteristics of the GMR analog sensors can be obtained and the related physical parameters can be calculated. From Eq. 31-1, one can see that there is a linear relationship between the output voltage and the magnetic induction intensity within a certain range, and the sensitivity is relatively high. Therefore, the GMR can be conveniently made into a magnetic field meter to measure magnetic induction or other related parameters. For example, it can be used to measure the current without the requirement of connecting the instrument to the circuit, and it will not interfere with the circuit operation. In practical applications, in order to improve the measurement accuracy, the GMR analog sensor must be operated within the linear region. And a fixed magnetic field, called magnetic bias, is often applied to the sensor in advance. This is similar to the DC bias Flux collector R3 R4 R2 R1 (a) Geometric structure (b) Circuit diagram Fig. 31-3. Structure schematic of GMR analog sensor. R2 R3 R4 R1 Input + output − + output Input −
caseinintegrated circuitExperimental RequirementsR1.Measurement on magnetoresistanceSpiral coil(1) Measurement principleAdjustable current sourceAs shown in Fig.31-4, GMR is placedin the solenoid and then the constantFig. 31-4. Schematic diagram ofvoltage is applied. Adjust the coil currentmeasurement process onto change the magnetic field value, recordmagnetoresistivecharacteristicsthe current data and calculate themagnetoresistance.(2)Measurementrequirementsandsteps@Place the GMR analog sensor into the center of the solenoid and switch thefunctionswitchto"GMRmeasurement"（“巨磁阻测量").②Connectthe“circuitsupply"（“电路供电")oftheexperimentinstrumenttothe“circuit supply"(“电路供电"）ofthe basicmeasurementcomponent.Then“GMRpower supply”（巨磁电阻供电"）isconnectedinserieswiththeamperemeterandGMRpower supply”（"巨磁电阻供电"）of thebasicmeasurementcomponent. The constant current output is connected to the "solen current input"（螺线管电流输入"）ofthebasicmeasurementcomponent.③Turn on thepower and adjustthe“constant current adjustment"(“恒流调节")knob to make the solenoid current gradually changed from 100mA-0-.100mA-0-100mA(negative current can be obtained by exchanging thepolarity of current source output). Record a series of data from A1 and A2Adjust the“constant current adjustment"（“恒流调节"）knob to make theconstant current output be zero and turn off the power of the machine
case in integrated circuit. Experimental Requirements 1. Measurement on magnetoresistance (1) Measurement principle As shown in Fig. 31-4, GMR is placed in the solenoid and then the constant voltage is applied. Adjust the coil current to change the magnetic field value, record the current data and calculate the magnetoresistance. (2) Measurement requirements and steps ① Place the GMR analog sensor into the center of the solenoid and switch the function switch to "GMR measurement" (“巨磁阻测量”). ② Connect the “circuit supply” (“电路供电”) of the experiment instrument to the “circuit supply” (“电路供电”) of the basic measurement component. Then “GMR power supply” (“巨磁电阻供电”) is connected in series with the amperemeter and “GMR power supply” (“ 巨磁电阻供电 ”) of the basic measurement component. The constant current output is connected to the "solen current input" (“螺线管电流输入”) of the basic measurement component. ③ Turn on the power and adjust the “constant current adjustment” (“恒流调节”) knob to make the solenoid current gradually changed from 100mA0- 100mA0100mA (negative current can be obtained by exchanging the polarity of current source output). Record a series of data from A1 and A2. ④ Adjust the “constant current adjustment” (“恒流调节”) knob to make the constant current output be zero and turn off the power of the machine. E A1 A2 Adjustable current source Spiral coil R Fig. 31-4. Schematic diagram of measurement process on magnetoresistive characteristics
Permanentmagnet2. Current measurement using GMRTerminalTerminaanalog sensor(1) Measurement principleGMRThe GMR sensor is placed next to thewire, as shown in Fig.31-5.Amagnetic biasEis applied to the sensor by using a permanentFig.31-5.Schematic diagram of currentmagnet.Then change the wire current andmeasurement onanalog sensorrecord the voltage value.(2) Measurement requirements and steps①“GMRpowersupply"（“巨磁电阻供电"）isconnectedtothe“GMRpowersupply"(“巨磁电阻供电）ofthebasicmeasurementcomponent.Theconstantcurrentoutput is connected to the“currentinput tobetested”（待测电流输入"）ofthecurrent measurement component. And then the voltmeter is connected to the“"signaloutput"（"信号输出").② Turn on the power and adjust the position of the magnet to be far from the GMRanalog sensor, corresponding to the weak magnetic bias state③ Adjust the current step by step (0-200mA-→0) and record the reading data of aseries of current and correspondingvoltage@ Adjust the biasing magnet to be closer to the GMR analog sensor, correspondingto the strong magnetic bias state.③Graduallyadjustthecurrent(0-200mA-0)andrecord thereadingdata ofa seriesof current and corresponding voltage.? Turn off the power, arrange the instruments, and end the experimentExperimental data recording and processing(1) Taking the magnetic field B as x-axis and the resistance R as y-axis to make theGMR characteristic curves (B = μon).(2) Taking current I as x-axis and voltage U as y-axis to plot and compare the
2. Current measurement using GMR analog sensor (1) Measurement principle The GMR sensor is placed next to the wire, as shown in Fig. 31-5. A magnetic bias is applied to the sensor by using a permanent magnet. Then change the wire current and record the voltage value. (2) Measurement requirements and steps ① “GMR power supply” (“巨磁电阻供电”) is connected to the “GMR power supply” (“巨磁电阻供电”) of the basic measurement component. The constant current output is connected to the “current input to be tested” (“待测电流输入”) of the current measurement component. And then the voltmeter is connected to the “signal output” (“信号输出”). ② Turn on the power and adjust the position of the magnet to be far from the GMR analog sensor, corresponding to the weak magnetic bias state. ③ Adjust the current step by step (0200mA0) and record the reading data of a series of current and corresponding voltage. ④ Adjust the biasing magnet to be closer to the GMR analog sensor, corresponding to the strong magnetic bias state. ⑤ Gradually adjust the current (0200mA0) and record the reading data of a series of current and corresponding voltage. ⑥ Turn off the power, arrange the instruments, and end the experiment. Experimental data recording and processing (1) Taking the magnetic field B as x-axis and the resistance R as y-axis to make the GMR characteristic curves (B = μ0nI). (2) Taking current I as x-axis and voltage U as y-axis to plot and compare the Fig. 31-5. Schematic diagram of current measurement on analog sensor. E GMR V A R Terminal Terminal Permanent magnet
measurementsensitivityunderdifferentmagneticbiasesQuestions(1) What are the reasons for different magnetic biases affecting the sensitivity ofcurrent measurement?(2)Howto understand thephysical mechanism of theprocess of magnetoresistancechange?(3) Discuss the characteristics of the gradient sensor and analyze whether the gradientsensorcanbeusedforvehicleflowmeasurement.References(1）吴镐，都有为、巨磁电阻效应的原理及其应用[]自然杂志,2007,29(6):322-327.(2）邢定钰.自旋输运和巨磁电阻[]、物理，2005，34（5)：348-361.(3）吴建华，李伯减，蒲富格等.平行与垂直磁化下多层磁膜巨磁电阻与外磁场关系的唯象理论计算[J].物理学报，1994,43(1):110-116Translated by Huolin Huang2018.08.10
measurement sensitivity under different magnetic biases. Questions (1) What are the reasons for different magnetic biases affecting the sensitivity of current measurement? (2) How to understand the physical mechanism of the process of magnetoresistance change? (3) Discuss the characteristics of the gradient sensor and analyze whether the gradient sensor can be used for vehicle flow measurement. References (1) 吴镝，都有为. 巨磁电阻效应的原理及其应用[J]. 自然杂志 , 2007, 29(6): 322- 327. (2) 邢定钰. 自旋输运和巨磁电阻[J]. 物理，2005，34（5）：348-361. (3) 吴建华,李伯减, 蒲富格等.平行与垂直磁化下多层磁膜巨磁电阻与外磁场关 系的唯象理论计算[J]. 物理学报, 1994, 43(1):110–116. Translated by Huolin Huang 2018.08.10