Giant Magnetoresistance

Transcription

1 Giant Magnetoresistance 03/18/2010 Instructor: Dr. Elbio R. Dagotto Class: Solid State Physics 2 Nozomi Shirato Department of Materials Science and Engineering

2 ntents: Giant Magnetoresistance (GMR) Discovery of GMR Physics of GMR Applications and Impacts of GMR Summary

3 the to both both the the anisotropic anisotropiceffect effect thefe-cr-fe Fe-Cr-Fefilm, film,we wehave have an an MR MR effect effect due due to (negative (positive values). values).the Thezero zerolevel level (negativevalues) values)and andantiparallel antiparallel alignment alignment (positive ofofthis thisplot plotisisdefined definedby byr=r R=R.. AtAtthe was well well known known that thatleading leadingcomcomthetime timeofofthe thediscovery discovery of of GMR, GMR, it it was puter so it it could could be beused usedfor forread readheads heads putercompanies companiesplanned plannedto todevelop develop AMR AMR so ininhard between AMR AMRand andthe thenew newef-efharddisk diskdrives drives(hdds). (HDDs). The The comparison comparison between The Nobel Prize of Physics in 2007 fect (later, the term GMR was widely accepted) encouraged us to file a patent fect (later, the term GMR was widely encouraged us to file a patent"for the discovery of Giant Magnetoresistance" for forusing usinggmr GMRininHDD. HDD. Discovery of GMR: Grunberg & Fert (b) (b) 1.5 RR/ /R(H=0) R(H=0) (Fe/ Cr) / Cr) 3030x x(fe 35xx(Fe (Fe/ /Cr) Cr) xx(Fe (Fe/ /Cr) Cr) R/R R/RPP (%) (%) (a)(a) Fe Fe/ /Cr Cr/ /Fe Fe ÅÅFe Fe field comorder to measure the anisotropic magnetoresistance (AMR) effect for-1 Magnetic0 field (kg) 1 Magnetic (kg) Magnetic field (kg) Magnetic field (kg) parison. Laterally, the samples had the shape of a long strip with contacts at Figure 9. GMR effect (a) in a multilayer and (b) double layer of Fe interspaced by Cr. In both ends. Figure 9. GMR effect (a) in a multilayer and (b) double layer of Fe interspaced by Cr. In Albert Fert (b) the AMR effect of a 250-Å-thick film of Fe is also shown for comparison. (b) the AMR effect of a 250-Å-thick film of Fe is also shown for comparison. In Figure 9 [9], we can see a comparison of the measurements using a multiin Figure 9 [9], we can see a comparison of the measurements using a multilayer as in Orsay and a double layer as in Jülich. The value for the multilayer layer as in Orsay and a double layer as in Jülich. The value for the multilayer Magnetoresistance of around 80% obtained in Orsay, which led to the term giant appears to of around 80% obtained in Orsay, which led to the term giant appears to have been surpassed by far by the TMR value of 500% for systems with MgO have been surpassed by far by the TMR value of 500% for systems with MgO barriers. However, one has to be careful. There seems to be an important barriers. However, has to beinsulators, careful. There seems to and be an important difference betweenone dealing with semiconductors metals. The MR: ~0.1 % difference between dealing with insulators, semiconductors and metals. magnetic semiconductor EuS, for example, changes its resistivity below The its magnetic semiconductor EuS,offor example, Ifchanges its resistivity belowtoits Curie point by many orders magnitude. we restrict the discussion Curie point by many orders magnitude. we restrict the discussion purely metallic systems, whichofhas also some Ifimportant implications for ap-to purely metallic systems, which hasjustified. also some important implications for applications, the term giant is still GMR: 3~50 % plications, the term still justified. The value 1.5% giant for theisdouble layer displayed in Figure 9 (b) is even The value 1.5% forcompared the double displayed in Fe Figure (b)the is boteven smaller but still large to layer the AMR effect of (trace9 at smaller butfigure still large the AMRusing effect of Fe configurations (trace at the bottom). In 8, thecompared effect wastomeasured various of tom). In Figure 8, thetoeffect wasofmeasured various external field relative the axis the crystalusing anisotropy to configurations make sure that itof relative to thegrunberg, axis ofp.,the crystal anisotropy to make surevalues that it really due to theb.relative orientation the magnetizations. The low Fert et al., Phy. Rev. Lett. (1988) external Grunbergisetfield al., Phy. Rev. (1989) Nobelof Lecture (2007) Peter A. Grünberg

4 Physics of GMR The splitting between the energies of the majority spin [spin up] and minority spin [spin down] directions are in different states for opposite spin directions and exhibit different conduction properties. Introduced by Mott in Electron spins Ferromagnetic Metal Non-magnetic Metal Cu Cu Cu

5 Physics of GMR R R AP GMR has ~10 % Normal metal has only a few % R P H S H Tsymbal and Pettifor, Academic Press (2001) Electron Ferromagnetic Metal Non-magnetic Metal Cu Cu Cu

6 Physics of GMR r R R r Relative magnetoresistance r R r R Chappert et al., Nature Mat. (2007)

7 Geometry of GMR (a) CIP geometry (b) CPP geometry x z y Leads Current Sample (multilayer) Tsymbal and Pettifor, Academic Press (2001) CIP: In General GMR geometry. It has been used for HDD head. method. A sample (a magnetic multilayer) is placed between semiinfinite perfect leads. The electric current flows in the z direction. Chappert et al., Nature Mat. (2007) CPP: In Sandwitching magnetic multilayer between superconducting electrodes. GMR is higher than CIP because of spin accumulation effects at magnetic/non-magnetic

8 Applications of GMR: HDD Head ~ 1 Gbit/inch 2 => ~15 Gbit/inch 2 => ~100 Gbit/inch 2 (~1992) (~ 2001) (~ 2003) GMR Tunneling magnetoresistance (TMR) Spin-Valve As the raw HDD areal recording density increase, the size and the magnetic strength of each bit gets reduced. --> The reading head needs to give the large change of the magnetoresistance with the small magnetic moment t W Magnetic shield S2 GMR read sensor P2 Inductive write element N S S N N S S N N S S N N S B Chappert et al., Nature Mat. (2007) P1 P1 Recording medium Magnetic shield S1

9 Spin-Valve Head Anti-ferromagnetic Exchange Layer Pinned Ferromagnetic Layer nducting Spacer Layer Free Layer S N Hitachi, rp. GMR head has the spin-valve mechanism. The top ferromagnetic layer is pinned by the attached AF layer. The bottom ferromagnetic layer is free to rotate by the applied magnetic field.

10 Tunneling Magnetoresistance (TMR) In 1994, first observed in RT Spin conservation High MR compared to GMR Crocus Technologies Inc.

11 MRAM (Magnetic Random Access Memory) Cross point architecture Bit lines MRAM Non-volatility, infinite endurance and fast random access (down to 5 ns read/write time) Word lines 1 0 MgO based MTJ (Magnetic Tunnel Junction) using TMR (Tunnel Magnetoresistance) effect 1T/1MTJ cell architecture TMR ratios up to 1800% at low T, 500% at RT Transistor 2006 IEEE Chappert et al., Nature Mat. (2007)

12 Summary Technologies depending on GMR brought huge impact on the mass data storage. GMR, Spin-Valve and TMR led to increase areal recording density by three orders of magnitude within 10 years. MRAM is potential candidate for becoming the universal memory Crocus Technologies Inc.