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UTK PHYS 672 - Giant MagnetoResistance

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Giant MagnetoResistanceJason TherrienSolid State II Spring 2009March 6, 2009AbstractIn this paper I give an overview of the history of Giant MagnetoRe-sistance. I then go on to describe what Giant MagnetoResistance isand how it works. I then talk about its major commercial applications,and finally talk about GMR’s legacy.1 HistoryIn 1988 there was considerable interest in layered magnetic structure.Independetly two groups were able to produce GMR, one in a multi-layered sample, the other in in tri-layer sample. Qualitatively this ideagoes all the way back to 1936. Mott proposed a mechanism for thechange in resistivity of ferromagnetic materials. In the 1960’s AlbertFert wrote his Ph.D. thesis on spin dependent resistivity, howeverit wasn’t until the mid 1980’s that experimental methods had ma-tured enough for scientists to create materials that exhibited GiantMagnetoresistance. Fert’s and Peter Gr¨unberg’s groups used Molecu-lar Beam Epitaxy to grow their samples which was the only methodavailable at the time to grow the materials with the required thickness,1nm. Later on sputtering would be used in commercial applications.Both Fert and Gr¨unberg would recieve the 2007 Nobel prize for theirindependent discoveries of Giant Magnetoresistance.2 PhysicsFor now this is goin to be qualitative with mathematics to come at alater date. Mott’s arguement goes as follows. Consider two currents1Figure 1: Gr¨unberg’s sampleFigure 2: Fert’s Samplepassing through a sample. The sample is ferromagnetic with all thespins aligned up. One current consists of spin up electrons, the otherspin down electrons. Due to the energy needed flip a spin scatteringfrom | ↑i → | ↑i will be nearly 1. One the other hand the spin downelectrons have a much higher chance of being scattered cause theyinteract much more strongly with the spin ups. Fert’s and Gr¨unberg’sgroups both used a layered sample. The reason for this is if you passa current through a sample, lets say tri layered where the first andthird layer are aligned in a spin up arrangement, with the middlelayer made of some non-magnetic material just so that the seperationof the aligned layers is less than the mean free path of the electron.The spin up current will see very few scattering events(resistance).The spin down current though will see a scattering event in bothlayers. Now if you flip the orientation of the third material you sothat the first layer is anti-alligned with the second and so that thespin up’s see the same number of scattering events in the third layeras the spin down’s in the first layer you have drastically increased thetotal resistance. For example, if in the original configuration the spin2Figure 3: These are four examples of systems that show GMR. A) is layeredmagnetic,non-magentic layers B)and C) are both examples of spin valves D)is a granular sample.down’s see 10 times as much resistance as the spin ups, when you flipthe orientation of the third layer you will increase the total resistanceby roughly a factor of 3.The first samples that exhbited GMR did it with the Current inPlane(CIP), by 1993 samples were produced with the Current Per-pindicular to the Plane (CPP). More research is required.3 ApplicationsAs far as applications of science to consumer electronics few thingshave made as great of an impact as quicly as GMR did. GMR’ most fa-3mous application is the Hard Disk Drive.The way a hdd works is the head senses modulations in the magneticfield. The problem is as you decrease the magnetic domains(increasethe memory density) you also decrease the magnetic field strength.In the early 90’s anisotropic magnetoresistance was used to createhard drives. But as the need for more storage increased the size ofthe domains decreased and as such the field strength decreased too.Roughly a decade after GMR was discovered the effect was under-stood well enough hard drive manufacturers were willing to put theeffect to use commercially. During the life span of GMR based harddrives the storage capacity increased almost 40,000%(I know it wassome were from 1 gb to roughly 400 gbs I will find an exact number).Today hard drives are based on TMR. Some would argue that thediscovery of Giant Magnetoresistance created the field of Spintronics.TMR is one effect that was born out of this research and has usurpedGMR as the dominate technology in hdd. It is based upon quantummechanical tunneling through layers. Another promising applicationof spintronics is MRAM or Magnetoresitive Random Access Memory.MRAM is a non-volatile form of memory meaning that it doesn’t de-grade over time like hdd based on gmr and tmr as well as flash memory.The memory cells in MRAM are tunneling junctions which have theirmagnetic field changed by the Word and Bit lines.4 Conclusion and FutureGMR has had a succesful run in the industrial market place and itstime has ended, but we still see its legacy, and will continue to. GMRresearch gave birth to TMR, CMR, and in general Spintronics. Tun-4Figure 4: A comparison of GMR versus that of AMRFigure 5: A schematic diagram of mram5neling Magnetoresistance has allowed us to see the price per gigabytefall below 1 dollar. As of this writing Western Digital has announcedthe world’s first 2 TB hdd.5 References*I have been fighting with Bibtex for days now, I don’t know why it isn’tworking for me right now. For now this is a place holder. My referencesinclude Fert’s and Gr¨unberg’s nobel lectures, I took the first two pic-tures from their presentation. I also used a GMR review which can befound here http : //physics.unl.edu/ tsymbal/tsymbal f iles/P ublished%20papers/GMR−review −


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