The Giant Magneto Resistance Effect - Essay Example

Giant Magneto-Resistance 479811) Introduction The GMR or Giant Magneto Resistance was a pioneering effort made in the field of material science. The efforts were subsequently rewarded as Albert Fert and Peter Grunberg were awarded the Nobel Prize for Physics in 2007 for their contributions. (The Royal Swedish Academy of Sciences, 2007) Working of a GMR The GMR effect can be explained as a simple phenomenon which causes the change of electrical resistance of a conductor when subjected to an external magnetic field by virtue of the alignment of its spin electrons. In materials of large thickness the electrons can travel for long distances without any change of direction since intermolecular spaces are large and restriction in movement is minimal. The path thus travelled is called the mean free path length. The electrical resistance is directly proportional to this mean free length. The inference is, more the free length less is the bombardment of the electron with other atomic particles hence less is the resistance of the material. However in materials that are very thin the chances of electrons reaching its outer boundary very quickly and getting dispersed are more than getting scattered by other electrons within the material itself. The low thickness of the material thus creates it difficult for the electrons to travel thereby increasing the resistance. The diagram indicated above show the manner in which resistivity increases with decrease of mean free path. In order to achieve this GMR effect another key in the construction of this film is that these are manufactured in thin layers of magnetic and non-magnetic materials.( Application Notes for GMR Sensors, 2010) The GMR film is such that the magnetization of the two layers separated by the non magnetic layer creates an anti-ferromagnetic coupling. It means when there is no externally applied magnetic field the magnetization of these two layers is in the opposite direction. Under the influence of an external magnetic field the electrons in each magnetic layer aligns according to its spin. Since the spin direction is opposite in these two layers either electron from each layer scatters off the boundary of the other magnetic layer. This results in a huge decrease of the mean free length of the electron causing an increase in electrical resistance. If however the magnetic field exceeds a certain limit this boundary barrier is overcome as all electrons in both the magnetic layers acquire the same spin. Thus the movement of electrons are eased leading to decrease in resistance.( Application Notes for GMR Sensors, 2010) Application Principle of a GMR The GMR technology is finding widespread application in the electronic industry. This is because the difference in resistances between two alternate layers of ferromagnetic materials can be used to store large quantities of binary information in the form of 1’s and 0’s. Types of GMR 1. Multilayer GMR- This consists of Fe, Ni, Co and its alloys that are separated by thin layers of Ag, Cu or Au and is prepared in ultra high vacuum by the magnetron sputtering technique on silicon crystals. The thin separating layers can at certain thicknesses make the spin electrons in either ferromagnetic layers to align opposite to each other thus increasing the net resistance of the device.( Durusoy et al, 1999) Cross section of magnetic multilayer deposited on a silicon substrate 2. Spin valve GMR- The spin valve GMR is similar in construction but does not have interaction between the conducting electrons of either of the ferromagnetic layers due to the increased thickness of the spacer. This basically consists of anti-ferromagnetic, pinned ferromagnetic, non-magnetic and free ferromagnetic layers arranged as shown in the figure. Conduction electrons can travel freely back and forth between the free layer and the pinned layer via the non-magnetic spacer.( F. Russo et al, 2007) The anti-ferromagnetic layer on the side adjacent to the pinned layer fixes the magnetic orientation while the magnetic orientation of the free layer changes with the impact of the external magnetic field. Schematic illustration of a spin-valve GMR sensor When the magnetization of the free and pinned layer is parallel the spin electrons can move freely while the movement of these electrons are hindered when the magnetizations are anti-parallel leading to an increase in resistance. This is used actively in GMR sensors where the magnetic flux in the free layer is switched between the two alternative arrangements of high and low resistance to obtain an output. 3. Pseudo-spin valve – A Pseudo spin valve is similar to a spin valve GMR but has an increased non-magnetic layer thickness and conduction electron interaction between the two ferromagnetic layers across the non-magnetic spacer layer is minimal. A soft ferro-magnet is used in one layer while the other layer uses a hard ferro-magnet thereby achieving ease with which magnetization can be alternated.(F. Russo et al, 2007) 4. Granular GMR – This is similar to a multilayered GMR but consists of granular structure of ferromagnetic grains separated by a non-magnetic conductive matrix. This kind of GMR effect can be usually seen in copper containing cobalt granules since both the materials are immiscible in each other. The granular GMR nano-composite can be manufactured using the SIP process (surface initiated polymerization) method to stimulate a high degree of grain sandwiching followed by the annealing process. This annealing process creates a granular GMR profile but the performance of the GMR is not as favourable when compared to the other varieties. ( Guo Zhanhu et al, 2007) . Applications of GMR 1. GMR Magnetic Field Sensors (Magnetometers) - This works on the principle of Wheatstone bridge which effectively has four legs forming a bridge. When the ratio of opposite resistances across a bridge are equal i.e. R1/R2= R3/Rx then the voltage across points B and D is zero. This configuration thus allows two legs of the opposite bridge as shown in the sensor layout to be exposed to the magnetic field. Since the GMR resistors are effectively shielded by a thick layer of magnetic material placed on the sensor substrate which directs all magnetic fields away from the shielded resistors there is no change in its resistance. However the electrical resistance of the other two resistors in an external field decrease thus generating a signal output. This application is thus used effectively in sensors in generating output signals.( Application Notes for GMR Sensors, 2010) 2. GMR Magnetic Gradient Sensors (Gradiometers) - This employs the Wheatstone bridge principle in a similar fashion as the Magnetometer. However the device is unshielded and all the four legs of the bridge are exposed to external magnetic fields. The resistors react actively to external magnetic fields and these are primarily used in detecting magnetic or ferrous targets. 3. Tunneling magnetoresistance (TMR) The revolutionary application of this principle is in the Hard Disk Drives. This is advancement into the GMR technology and consists of alternating metallic and insulating layers. The challenge for future generations is to develop ways to store the maximum amount of information into the smaller spaces in such a manner that the information stored retains its individuality and space. The output from storing such a large volume of energy also needed to be definitive and refined. TMR is a step into that direction. The insulating material consists of a thickness of few atomic layers that are juxtaposed between two ferromagnets. The performance of TMR is higher compared to GMR. Use of TMR in magnetic access memories (MRAM) have already begun. (The Royal Swedish Academy of Sciences, 2007) Storage of information in Disks 4. GMR magnetic Sensors are used widely in measuring relative linear displacement. The actual position of parts in a machinery and location of other transducers in a system can be predicted with a GMR sensor. The below sketch illustrates the manner in which this displacement is measured. In the first case the displacement of the magnet is along the y axis and in the second case the displacement is along the x axis. The values of magnetic flux generated are different which can be calibrated to create a displacement graph or a plot showing location of a particular component with respect to a reference point. (Application Notes for GMR Sensors, 2010) Typical examples where this system is used for measurement and detection include (i) The cylinder stroke position of a Hydraulic pressure cylinder. (ii) Fluid level of various storage tanks. The increasing and decreasing level of fluid stored in system generates different magnetic flux and hence a different voltage output. This when calibrated gives the exact location of the fluid in the tank. (iii) The aircraft once off the ground is controlled in mid air by its flight control surfaces. The wing and the tail of the aircraft can be adjusted in such manner so as to achieve minimum turbulence. The exact location and the degree to which these flaps need to rotate are accurately are sensed using a GMR. (iv) These are also used along unmanned crossings and areas near international borders to detect and intercept any vehicles that may have intruded into our territory.( Smith Carl and Schneider Robert, n.d) 5. GMR magnetic field sensors can also be used in current measurement detecting the magnetic field that is generated by the current flowing through the conductor wire. In this, the sensor which is placed over the current carrying wire in no way meddles with the circuit. The sensor chip can be placed above or below the wire but should be aligned in a perpendicular direction to the wire. (Application Notes for GMR Sensors, 2010) The principle of operation is very simple as the magnetic field that is created by the current that is flowing axially along the wire engulfs the GMR radially. This magnetic field affects the resistance of GMR material and an output is generated from the sensor. The increase and decrease in current also affects the magnetic field which changes the sensor output proportionately. The areas where GMR is used in current sensing applications are (i) AC or DC current detection and sensing (ii) Industrial Instrumentation (iii) Industrial Process control where detection of this current is of paramount importance as an increase or decrease in current would affect the outcome of the process (iv) These are also used in making eddy current probes used for measuring subsurface cracks and is brought about by the realignment of magnetic flix at the cracked areas.( Smith Carl and Schneider Robert, n.d) Output of GMR eddy current sensor when scanning a subsurface crack. Crack is 15mm long and placed 1mm below the surface of an aluminium specimen. (Courtesy of UNC-Charlotte) 6. Magnetic Media Detection- Materials that have a definitive magnetic signature can be detected using GMR. The GMR sensors are swept across these materials and any disturbance in the materials own magnetic field is picked up by the sensor. The output is thus calibrated as a function of the magnetic properties of the substrate, working gap and the kind of sensor used. Ink used nowadays has a small amount of iron oxide and when used on sensitive documents can be detected and verified using a GMR. ATM cards that have a password in scripted into the magnetic tape are read and verified by a GMR sensor in the ATM machine. Detection of fine magnetic particles and any defects in substrates containing magnetic particles can be detected using GMR. (Application Notes for GMR Sensors, 2010) 7. Detection and Validation of Currency – This is a very important technique used in validating true and fake currencies. Currency notes have fine magnetic bar strips inserted in between. The magnetic field that is emitted by this magnet will have a vertical and horizontal component. The sensor placed in parallel with either of the component will generate a output which is used for validating currencies.( Application Notes for GMR Sensors, 2010) Conclusion The discovery of the GMR effect has lead to the rise of a host of related technologies and has opened a new path for research into nanotechnology. This resurgence in magnetoelectronics which deals with the handling of charge and spin of a particular substrate has lead to a huge increase of commercial products. These products compete with each other primarily in the area of offering more storage capacity in a smaller area. However, rapid progress is being made in the use of this material for other applications and new technologies are being developed with improved versions of the GMR. Bibliography 1. Application Notes for GMR Sensors, 2010, Available at: www.nve.com, [Accessed 27th November 2010] 2. Durusoy et al, “A magnetization and GMR study on multilayered Fe/Ag/Co Thin Film”, Tr. J. Of Physics, 1999 Print. 3. “The Discovery of Giant Magnetoresistance”, Scientific Background on the Nobel Peace Prize in Physics in 2007, The Royal Swedish Academy of Sciences, October, 2007, Print. 4. F. Russo et al, “Pseudo spin valves with Al or Nb as spacer layer: GMR and search for spin switch behaviour”, The European Physics Journal B, September 2007, Print 5. Guo Zhanhu et al, Behaviour and Mechanics of Multifunctional and Composite Materials, 2007, Print 6. Smith Carl and Schneider Robert, “GMR and SDT sensors and arrays for Low-Field Magnetic Applications”, Nonvolatile Electronics Inc, n.d, Print. Read More