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Chapter 14: Spin Electronics and Magnetic Recording

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Chapter 14: Spin Electronics and Magnetic Recording Chapter 14: Spin Electronics and Magnetic Recording 1. Spin currents 2. Sensors 3. Memory 4. Logic 5. Spin transistors 6. Magnetic recording Comments and corrections please: [email protected] Dublin April 2007 1 Further reading • Michael Ziese and Martin Thornton (editors), Spin Electronics , Springer, Berlin 2001, 493 pp. A multiauthor volume which treats topics at an introductory level, with some emphasis on oxide spin electronics. • Uwe Hartmann (editor) Magnetic Multilayers and Giant Magnetoresistance , Springer, Berlin 1999, 321pp. Readable articles focussed on magnetic multilayers and giant magnetoresistance. • Mark Johnson (editor), Magnetoelectronics , Elsevier Amsterdam 2004, 396 pp. Covers magnetoelectronics in a series of articles, from an introduction to chapters on logic, tunelling and biochips. • Sadamichi Maekawa (editor), Concepts in Spin Electronics , Oxford 2006, 398 pp. A monograph with a focus on theoretical aspects. • Lawrence Comstock, Introduction to Magnetism and Magnetic Recording, Wiley-Interscience 1999, 485 pp. A n extensive and useful introduction for engineers. • M. L. Plumer, J. van Eck and D. Weller (editors) The Physics of Ultra-high Density Magnetic Recording , Springer, Berlin 1999, 355 pp. A series of articles covering micromagnetic and dynamic aspects of recording with a focus on media. Dublin April 2007 2 Modern Electronics Logic; CMOS - Complementary Metal-Oxide Semiconductor. Uses p and n type silicon, carriers are electrons or holes in FETs. It consumes power only when switching, and it is scalable. p-type NAND gate n-type Memory; SRAM - Static Random-Access Memory. 6T Volatile DRAM - Dynamic Random-Access Memory 1T Volatile, refreshed every few ms. FLASH - Nonvolatile; limited rewritability Dublin April 2007 3 Dublin April 2007 4 Conventional electronics has ignored the spin in the electron : The electron is a mobile particle with a charge e = -1.6 10-19 C ! It also has quantized angular momentum ms where m s = ±1/2 spin up ! or spin down " ! The associated magnetic moment is m = e /2m = 1 Bohr magneton (µB). Information can be coded into the ! and " channels • Manipulate the ! and " electrons independently • Exploit magnetic and electric fields Dublin April 2007 5 14.1 Spin Currents ! Pure charge currents; charge flow !Spin-polarized charge currents charge and angular momentum flow ! Pure spin currents angular momentum flow Charge is conserved; Spin is not Dublin April 2007 6 Charge transport Modes of electron transport in solids: ! Ballistic ; transport in a conductor with no scattering ! Diffusive; transport in a conductor with multiple scattering ! Tunneling; transport across an insulator or vacuum by chance Conductors have electrons in extended states: # = eik.r Insulators have electrons in localised states: # = e -ix/x 0 Dublin April 2007 7 BallisticBallistic transporttransport lead contact conductor L # = eik.r L << $ Dublin April 2007 8 DiffusiveDiffusive transporttransport 1/2 l = (De%sf) D = (1/3) v l = ((1/3) !" 2)1/2 lead F $ sd ≈ 100 contact lsd conductor L L >> $ # = eik.r lsd >> $ Dublin April 2007 9 TunnellingTunnelling insulator lead contact # t -ix/x # = e 0 ! t x0 Dublin April 2007 10 ConductivityConductivity nm -1 k 0.07 s - electrons Cu s - electrons EF Cu EF x0 ~0.1 $! 20 $" $" Energy (eV) Energy d - electrons d - electrons $" 20 (eV) Energy & = 1.7 10-8 'm $sd 200 " Conduction in Cu is by the s electrons. The mean free path $! = $" " 20 nm. The spin diffusion length $sd is much longer, 200 nm " -8 -1 & = &0 + &(T) &0 10 'm % Dublin April 2007 11 LengthLength scalesscales $ lsd nm k-1 0.07 Ni s - electrons EF x0 ~0.1 d - electrons $! 5 (eV) Energy $ 1 " & = 7.0 10-8 'm $sd 30 Conduction is mainly by the s electrons. The s" electrons are strongly " scattered by the large d electron density at E F. Hence the mean free path > . The conductivity ratio = / " 5 Mott two-current $! $" ( )! )" model The spin diffusion length $sd is much longer. Dublin April 2007 12 Spin-polarisedSpin-polarised chargecharge transporttransport B TWO-TERMINAL DEVICES; Magnetoresistors Source of spin- Medium with long spin-sensitive polarized electrons spin-diffusion length detector $sd Normal metal; Cu Ferromagnetic metal; Ferromagnetic metal NiFe, CoFe NiFe, CoFe Dublin April 2007 13 HowHow spin-polarisedspin-polarised ?? What is the degree of spin polarization of common ferromagnetic metals? P can be determined from the calculated density of states, but it usually has to be weighted by the Fermi velocity, or the square of the Fermi velocity. Values for an amorphous AlO x tunnel barrier are obtained by tunneling into superconducting Al. Andreev reflection can be used at a ballistic point contact P % P = (N!v!n - N"v"n)/(N!v!n + N"v"n) I Fe 44 M Co 45 AlO x n = 0 for photoemission n = Al Ni 33 1 for ballistic transport n = 2 H for diffusive or tunneling transport Fe20Ni80 48 J Moodera, G Mathon Fe Co 51 P depends on materials combination and JMMM 200 248 50 50 bias Dublin April 2007 14 First-generation spin electronics First-generation spin electronics has been built on spin-valves – sandwich structures using GMR or TMR with a pinned layer and a free layer. These can serve as very sensitive field sensors, or as bistable memory elements Free I pinned free pinned af I af GMR spin valve planar magnetic tunnel junction One layer in the sandwich has its magnetization direction pinned by exchange coupling with an antiferromagnet – exchange bias. Dublin April 2007 15 GMR spin valve 10 Free 5 108 sensors pinned 8 per year — read heads af I 6 spin valve R/R% ! 4 5 nm Ta 5 nm Ta 2 3.5 nm NiFe 10 nm IrMn 1.5 nm CoFe 2.9 nm Cu 2.5 nm CoFe 2.5 nm CoFe 2.9 nm Cu 0 1.5 nm CoFe -100 -50 0 50 100 10 nm IrMn 3.5 nm NiFe Field(mT) 5 nm Ta 5 nm Ta Magnitude of the effect " 10 % Dublin April 2007 16 Single MgO Tunnel Junctions Cu 50 R/R% Ta 5 * CoFeB 3 CoFeB3 /MgO t/ MgO2.5 CoFeB4 CoFeB 4 Artificial Ru0.85 antiferromagnet CoFe2 + 200 100 IrMn10 NiFe5 Ta5 Ru50 Ta5 µ0H (mT) Dublin April 2007 17 TMR Spin valves 355% Ikeda 2006 300 (RT) I % 220 n free Others Parki AlOx pinned af MgO 200 RT) planar magnetic tunnel ( junction uasa 188% Y FeB o 16 C ) ) 10 per year TMR ( % ) ) % (RT) O NiO T ) 55 for MRAM ? 100 e 2%(RT 0% (RT G K) 2 o 5 7 shi iO ng .7%(RT) dera N 2 i 18%(R Wa % ki oo ak Naka First-generation devices use a nanolayer of 2.5%(2. 1 M 14%(4.2K) a ousa 37%(RT S iyaza Miyaz disordered aluminium oxide as the tunnel lier M ul uezawa J aekaw barrier, giving TMR of up to 70% (dark blue). M S (RT) Bowen Crystalline MgO barriers improve the sensitivity 0 27% of the device by a factor of three (red), 1970 1980 1990 2000 2010 changing MRAM architecture. Year SSP Parkin et al , Nature Materials 3, 862 Dublin April 2007 (2004). H. Ohno, J.App. Phys. (2996(. 18 Transmission through an MgO barrier •Majority channel tunneling is dominated by the transmission through a #1 state ! • 1 state decays rapidly in anti-parallel configuration WH Butler et al Phys Rev B 63 054416 (2001) Dublin April 2007 19 Bias-dependence AlO x tunnel junction; Signal 180 mV Dublin April 2007 20 14.2 Sensors >1 billion magnetic sensors of all types are produced every year; half of them for magnetic recording. also in permanent magnet motors to control electronic commutation (classical MR in semiconductors) and in proximity sensors. Dublin April 2007 21 A sensor is most useful if it has a linear response to applied field. Some sensors are inherently linear; - coil, Hall generator, NMR. Others must be specially prepared. Anisotropic magnetoresistance (AMR) $ M Discovered by W. Thompson in 1857 2 I & = & 0 + *& cos , thin film Magnitude of the effect *&/& < 3% The effect is usually positive; &||> &- Maximum sensitivity d&/d, occurs when , = 45°. Hence the ’barber-pole’ 2.5 % configuration used for devices. AMR is due to spin-orbit s-d scattering 0 2 4 µ0H(T) H Dublin April 2007 22 Giant magnetoresistance (GMR) and tunnel magnetoresistance (MR) Discovered by A. Fert in 1988 MR = Csin 2./2 Sensitivity is maximum when . = //2 H The bottom layer is pinned by exchange Easy axis bias. The free layer has a weak easy axis at . = //2. I magnetic tunnel junction: tunnel magnetoresistance TMR Dublin April 2007 23 14.2.1 Noise Four types of noise; 0 Johnson (thermal) noise 0 Shot noise 0 1/f (flicker) noise 0 Random telegraph noise Dublin April 2007 24 log SV -6 1/f noise Random telegraph noise -7 Thermal noise Shot noise 0 1 2 log f 0 Johnson (thermal) noise. SV(f) = 4kBTR There are voltage fluctuations with no imposed current: 2 <V > = 4kBTR#f Dublin April 2007 25 0 Shot noise. A non-equilibrium effect associated with electric current SI(f) = 2eI There are current fluctuations, first seen in vacuum tubes 1/2 Ishot = (2eI#f) Operating a TMR sensor at a high bias, to increase the signal also increases the noise. Dublin April 2007 26 0 1/f noise. A ubiquitous and remarkable effect exhibited by many natural and man-made phenomena - heartbeat (< 0.3 Hz); water level of the Nile; pop music stations ( SV(f) = Cf ( ≈ -1 The power spectral density is SV(f) = 1Hva/Nef Hooge constant -3 1H = 10 for pure metals and semiconductors. It can be as high as 103 in some magnetic films 1/f noise in CrO2 Dublin April 2007 27 0 Random telegraph noise.
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