Chapter 14: Spin Electronics and Magnetic Recording

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1 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: Dublin April

2 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

3 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 n-type NAND gate 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

4 Dublin April

5 Conventional electronics has ignored the spin in the electron: The electron is a mobile particle with a charge e = C It also has quantized angular momentum m s where m s = ±1/2 spin up or spin down The associated magnetic moment is m = e /2m = 1 Bohr magneton (µ B ). Information can be coded into the and channels Manipulate the and electrons independently Exploit magnetic and electric fields Dublin April

6 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

7 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: = e ik.r Insulators have electrons in localised states: = e -ix/x 0 Dublin April

8 Ballistic transport lead contact conductor L = e ik.r L << Dublin April

9 Diffusive transport lead contact l = (D e sf ) 1/2 D = (1/3) v F l sd = ((1/3) 2 ) 1/2 l sd = e ik.r conductor L L >> l sd >> Dublin April

10 Tunnelling lead contact insulator = e -ix/x 0 t t x 0 Dublin April

11 Conductivity nm k x 0 ~ d - electrons d - electrons sd 200 Energy Energy (ev) (ev) Cu Cu s - electrons s - electrons E E F F = m Conduction in Cu is by the s electrons. The mean free path = 20 nm. The spin diffusion length sd is much longer, 200 nm = 0 + (T) m % -1 Dublin April

12 Length scales l sd nm k x 0 ~ sd 30 Mott two-current model Energy (ev) Ni d - electrons s - electrons 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 The spin diffusion length sd is much longer. E F = m Dublin April

13 Spin-polarised charge transport TWO-TERMINAL DEVICES; Magnetoresistors Source of spinpolarized electrons Medium with long spin-diffusion length B spin-sensitive detector sd Normal metal; Cu Ferromagnetic metal; Ferromagnetic metal NiFe, CoFe NiFe, CoFe Dublin April

14 How spin-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 M AlO x Al H J Moodera, G Mathon JMMM I Fe Co Ni P % Fe 20 Ni Fe 50 Co P = (N v n - N v n )/(N v n + N v n ) n = 0 for photoemission n = 1 for ballistic transport n = 2 for diffusive or tunneling transport P depends on materials combination and bias Dublin April

15 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 pinned I free af I pinned 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

GMR spin valve af spin valve I Free pinned R/R% 10 8 6 4 5 10 8 sensors per year read heads 5 nm Ta 10 nm IrMn 2.5 nm CoFe 2.9 nm Cu 1.5 nm CoFe 3.

16 GMR spin valve af spin valve I Free pinned R/R% sensors per year read heads 5 nm Ta 10 nm IrMn 2.5 nm CoFe 2.9 nm Cu 1.5 nm CoFe 3.5 nm NiFe 5 nm Ta 5 nm Ta 3.5 nm NiFe 1.5 nm CoFe 2.9 nm Cu 2.5 nm CoFe 10 nm IrMn 5 nm Ta Field(mT) Magnitude of the effect 10 % Dublin April

17 Single MgO Tunnel Junctions CoFeB 3 /MgO t/ CoFeB 4 Cu 50 Ta 5 CoFeB3 MgO2.5 CoFeB4 Ru0.85 CoFe2 Artificial antiferromagnet R/R % IrMn10 NiFe5 Ta5 Ru50 Ta5 µ 0 H (mt) Dublin April

18 TMR Spin valves free pinned af planar magnetic 200 tunnel junction per year for MRAM? I TMR ( % ) First-generation devices use a nanolayer of disordered aluminium oxide as the tunnel barrier, giving TMR of up to 70% (dark blue). Crystalline MgO barriers improve the sensitivity of the device by a factor of three (red), changing MRAM architecture Others AlOx MgO Jullier 14%(4.2K) GeO Suezawa 1% NiO Maekawa 2.5%(2.5K) NiO Year Miyazaki 2.7%(RT) Moodera 22%(RT) Miyazaki 18%(RT) Sousa 37%(RT) Parkin 220% (RT) Yuasa 188% (RT) Nakashio 55% (RT) Bowen 27%(RT) 355% Ikeda 2006 Wang 70% (RT) CoFeB SSP Parkin et al, Nature Materials 3, 862 Dublin April 2007 (2004). H. Ohno, J.App. Phys. (2996(. 18

19 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 (2001) Dublin April

20 Bias-dependence AlO x tunnel junction; Signal 180 mv Dublin April

21 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

22 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) thin film M I 2.5 % Discovered by W. Thompson in 1857 = 0 + cos 2 Magnitude of the effect / < 3% The effect is usually positive; > Maximum sensitivity d /d occurs when = 45. Hence the barber-pole configuration used for devices. AMR is due to spin-orbit s-d scattering µ 0 H(T) H Dublin April

23 Giant magnetoresistance (GMR) and tunnel magnetoresistance (MR) Discovered by A. Fert in 1988 MR = Csin 2 /2 Sensitivity is maximum when = /2 Easy axis H I The bottom layer is pinned by exchange bias. The free layer has a weak easy axis at = /2. magnetic tunnel junction: tunnel magnetoresistance TMR Dublin April

14.2.1 Noise Four types of noise; Johnson (thermal) noise Shot

24 Noise Four types of noise; Johnson (thermal) noise Shot noise 1/f (flicker) noise Random telegraph noise Dublin April

25 log S V -6 1/f noise Random telegraph noise -7 Thermal noise Shot noise log f Johnson (thermal) noise. S V (f) = 4k B TR There are voltage fluctuations with no imposed current: <V 2 > = 4k B TR f Dublin April

26 Shot noise. A non-equilibrium effect associated with electric current S I (f) = 2eI There are current fluctuations, first seen in vacuum tubes I shot = (2eI f) 1/2 Operating a TMR sensor at a high bias, to increase the signal also increases the noise. Dublin April

27 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 S V (f) = Cf -1 The power spectral density is S V (f) = H v a /N e f Hooge constant H = 10-3 for pure metals and semiconductors. It can be as high as 10 3 in some magnetic films 1/f noise in CrO 2 Dublin April

28 Random telegraph noise. Fluctuations between two distinct levels. The noise presents itself as a broad peak in the noise spectrum. Dublin April

29 Noise in a CoFe/AlO x /CoFe MTJ; Currents range from 0 to 36 microamps. Modulate the signal at 10 khz to avoid the 1/f noise Dublin April

30 14.3 Memory Magnetic Random Access Memory 400 bit ferrite core half-select memory (1965) bit lines word lines H y Freescale 4 Mbit MRAM (2006) Dublin April H x

31 Magnetic Random Access Memory Stoner-Wohlfarth asteroid Dublin April

32 Toggle-switching H y H x Dublin April

33 Spin transfer torque Electron current Transverse spin component absorbed Electron current Torque exerted as electrons cross F2 F1 F2 Electron current Backscattered electrons exert torque on F2 F1 F2 L. Berger Phys Rev B (1996) J. Slonczewski JMMM 159 L1 (1996) Dublin April

34 Spin transfer torque B damping m spin torque Torque on a single-domain nanomagnet of moment m produce magnetization reversal move domain walls emit microwaves m/ t = m B - m (m B) Favourable scaling: Rate of transfer of angular momentum from electron current r 2 j /e; change of angular momentum on flipping free layer is 2m = 2 r 2 tm Dublin April

35 Competing memory technology. PCRAM Medium Dublin April

36 Data storage Semiconductor Memory Price Storage Hierarchy DRAM & Co. MRAM Magnetic Disk Optical Disc Decision criteria: Access time Frequency of use Concurrent access Archive requirements Permanent media Cost per megabyte Capacity Performance Costs (USD/GB) SRAM The Comparison of Storage Media MRAM DRAM FRAM OUM Flash HD Magnetic Tape 1 DVD RAM Source: IBM Capacity 1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 Access time (ns) Source: Beerenberg Bank/Singulus Technology W Maas, Singulus Dublin April

Vertically stacked memory Magnetic Race-track Memory Japanese car-park A novel 3-dimensional spintronic storage class memory - The capacity of a hard disk drive but the reliability and performance of

37 Vertically stacked memory Magnetic Race-track Memory Japanese car-park A novel 3-dimensional spintronic storage class memory - The capacity of a hard disk drive but the reliability and performance of solid state memory - A disruptive technology based on recent developments in spintronic materials and physics S.S.P.Parkin, US patents , , , Current pulses move domains along racetrack shift register TMR sensor to read bit pattern Special current pulse-driven domain wall element to re-write a bit Dublin April

38 14.4 Logic M. Johnson, IEEE Trans Magn (2000) A ferromagnetic element with a square hysteresis loop is an ideal bistable logic and memory element. M/V +M r H/I V + I I + I - V B + V W t 2deg InAs Hall sensor R = 170-1, R H = CoFe 350 T -1 -M r Equivalent surface pole density, M Am/m 2 Line of poles =Mt A. H = /2 r = Mt/2 r If r=2t, H = M/4 If M = 1 MAm -1, H = 80 kam -1 (100 mt) regardless of scale. Dublin April

39 Logic A C B I R 0 1 Nonvolatile switch output Generic logic device Inputs A and B set the state of the magnetic layer (0) or (1). The state of the element is read out at another terminal with a current pulse I R which produces a voltage V 0 or V 1. A clock pulse is applied at control terminal C. All four logic operations AB, A+B, AB, A+B are complete in two clock cycles (reset/evaluate) The normalised write current has one of two values I w (5 mt) or I w ( (10 mt) and either polarity + or - Dublin April

40 Domain wall logic Logic elements made from transistors Interconnect made from copper / aluminium ELECTRONIC LOGIC Data represented by magnetisation direction. SPINTRONIC MEMORY NAND Magnetic dw logic elements. NOT AND Interconnect made from permalloy C Allwood, R Cowburn et al. Science 309, 1688 (2005) Dublin April

41 4-element domain wall circuit IV AND III II B (mt) NOT Fan Fan Cross I Kerr signal Time (sec) 0.25 Dublin April

42 Ultimate computing technology? Magnetic Logic non volatile, fast, error resistance, low power, easy to integrate, low cost MTJ s -signal -stability -switching Dublin April

43 Perspectives System-on-a-chip. Sensing + signal processing. Digital signal processing Nonvolatile switches programmable gate arrays; ASICs Integration of memory and logic a) MRAM + CMOS b) Universal magnetoelectronic device memory and logic, with the possibility of flipping between them, Dublin April

44 A new generation? First-generation spin electronics was based on passive 2-terminal devices magnetoresistors for sensors and memory. CMOS dominates 99 % of the world semiconductor market: Circuits have sufficient gain to permit fanout Inputs are tolerant of fluctuations High signal/noise ratio Output isolated from input Fast, scaleable and cheap. BUT Charge leaks away; memory is volatile and needs refreshing 100 times s -1 Quiescent power requirement Dublin April

45 Number of Terminals\\\\ / 3+ 4 / 4+ Classical Devices Switch Resistor Diode Photodiode Varistor Transistor Filter Amplifier Wheatsone Bridge 2-gate MOSFET Tetrode Multiplier Spin Electronic Devices Spin Switch Magnetic switch (MTJ) Spin transistors Hall Probe Magneto-resistor Magnetic Gradiometer (bridge) Spin Diode Magnetic Photodiode Dublin April

46 Spin diffusion lengths (nm) semiconductors (nm) 200 sd (nm) >2000 semimetals >50 >500 s-band metals d-band metals sd Dublin April

47 Mobility of semiconductors, semimetals and metals Curie Point (K) Mobility (cm 2 V -1 s -1 ) Semiconductors Si InSb GaAs (GaMn)As Semimetals Graphite Bi Metals Cu - 44 Au - 48 Fe Co Ni Half Metals CrO Fe 3 O Dublin April

48 Magnetic semiconductors Desiderata for a magnetic semiconductor Curie temperature > 500 K Ferromagnetism should be coupled to the carriers cb p or n type conductivity spin-polarized electrons or holes Useful spin diffusion length and mobility Magnetoresistance in heterostructures Anomalous Hall effect Magneto-optic Faraday effect; MCD vb Dublin April

49 Magnetic semiconductors - overview EuO T c = K (GaMn)As T c < 175 K ZnO:Co T c > 400 K Spin-split conduction band Spin-split valence band Spin-split impurity band Dublin April

50 14.5 Spin Transistors F 1 N F 2 emitter collector sd base V I Johnson transistor. Metal-base transistor where conditions at and determine. Collector is floating. It samples µ or µ. No power gain. V - V nanovolts. Dublin April

51 Datta Das transistor L gate source drain Spin-polarized electrons are injected into the channel, made of a two-dimensional electron gas, where > L (ballistic transport). They are subject to an electric field on passing under the gate, which looks like a magnetic field from the viewpoint of the relativistic electron (Rashba effect) E = ev B/c 2. The spin precesses, and by adjusting the electric field, the electron arrives with its spin parallel (or antiparallel) to the drain. The drain may be a bistable magnetic element. S. Datta and B. Das, Appl. Phys Letters (1990) Dublin April

Hot-electron spin transistors parallel, passes

52 Hot-electron spin transistors parallel, passes antiparallel Monsma transistor. Injects hot electrons via a Schottky barrier. Different energy-loss processes in the GMR base lead to a field-contollable emitter current. Magnetic tunnel transistor Parkin The emitter/collector current ratio is very small in these devices. Dublin April

53 Spin MOSFET Ferromagnet Tunnel barrier Source Ferromagnet V g Gate oxide Drain Ferromagnet SOI suspended membrane demonstrator Silicon Similar to ordinary field effect transistor, but with ferromagnetic source and drain Why? It combines 1) power amplification (semiconductor) 2) memory (ferromagnets) v v J. F. Gregg et al JMMM (1997) Dublin April

54 Bipolar transistor p-n junctions I Zutic v Dublin April

55 Single-electron spin transistor Dublin April

Pure spin currents Is it possible in principle to separate and mainpulate spin currents independently of charge currents? If so, electronics might avoid resistive losses.

56 Pure spin currents Is it possible in principle to separate and mainpulate spin currents independently of charge currents? If so, electronics might avoid resistive losses. Spin waves. Spin Hall Effect I I Kerr effect image of a 500 x 100 micron n-gaas sample at 30 K. Kato et al Science (2004) due to spin-orbit scattering. Dublin April

57 14.6 Prospects 1st generation passive devices MRAM scaleup Integrated sensors magnetic biochips Magnetically reprogrammable gate arrays 2nd generation active devices Components with spin or field-dependent power gain Integration of memory and logic Dynamic reconfiguration between memory and logic. Coming later? Magnetically-generated microwave chip/chip communication Logic with spin currenta Magnetic quantum computing Dublin April

58 14.7 Magnetic Recording Hard disc drives Magnetic medium Read-write head Spindle motor Voice-coil actuator 8 Gbit 1 drive for cameras 160 Gbit 2.5 perpendicular drive for laptops Dublin April

Technology Timeline In-plane 1950 1960 1970 1980 1990 2000 2010 AMR discovered (1857) RAMAC - first hard-disc drive; inductive head TMR discovered GMR discovered.

59 Technology Timeline In-plane AMR discovered (1857) RAMAC - first hard-disc drive; inductive head TMR discovered GMR discovered.. Spin valve AMR head Spin-valve head (CIP) TMR head perpendicular year caapcity platters size rpm Mb 50x Gb Dublin April

60 A magnetic exponential - Recording AMR TMR perpendicular 1µm 1 2 AMR GMR Superparamagnetic Limit Magnetization blocked when KV/kT > 40 V > 300 nm 3 If record is on 100 grains, medium is 5 nm thick, area/bit is µm Gbit in 2. (155 bit µ m -2 ). Dublin April

61 Scaling Why does magnetism lend itself to miniaturization? A H = (m/4 r 3 )[2cos e r + sin e ] H A = 2Ma 3 /4 r 3; If a = 0.1m, r = 2a, M = 1 MAm -1 H A = M/16 = 20 kam -1 (~25 mt) a m Magnet-generated fields are limited by M. Scale-independent A I H = I/2 r = 8jr H ~ r Current-generated fields are limited by j. Scaling is poor Dublin April

62 More transistors and magnets are produced in fabs Than grains of rice are grown in paddy fields Dublin April

Magnetic Spin Devices: 7 Years From Lab To Product. Jim Daughton, NVE Corporation. Symposium X, MRS 2004 Fall Meeting

Magnetic Spin Devices: 7 Years From Lab To Product Jim Daughton, NVE Corporation Symposium X, MRS 2004 Fall Meeting Boston, MA December 1, 2004 Outline of Presentation Early Discoveries - 1988 to 1995