GMR Read head. Eric Fullerton ECE, CMRR. Introduction to recording Basic GMR sensor Next generation heads TMR, CPP-GMR UCT) Challenges ATE

Transcription

1 GMR Read head Eric Fullerton ECE, CMRR Introduction to recording Basic GMR sensor Next generation heads TMR, CPP-GMR UCT) Challenges ATE 1

2 Product scaling 5 Mbyte 100 Gbyte mobile drive 8 Gbyte UCT) ATE RAMAC kbits/in 2 70 kbits/s 50x 24 in dia disks $10,000/Mbyte 130 Gbits/in Mb/s 2 x 2.5 glass disks <$0.01/Mbyte Microdrive 78 Gbits/in 2 1 x 1 dia disk 2

3 Areal density trends Areal Density (Gb/sq.in.) % / yr Products Demos 60% / yr Standard FF MR head PRML channel Thin film disk 230 Gb/in 2 demo 100% / yr GMR head 40% / yr Perpendicular IBM / Hitachi, highest AD products Competitors, mobile }20%-40% 3

4 1975 4

5 Recording basics Slider Disk Suspension Actuator Spindle / Motor Data track Trackwidth 5

6 Magnetic recording components Inductive Write Element ~8 nm GMR Read Sensor t W Grain Structure and Magnetic Transition B Head-media 10 nm Track of Recording Media B = 25 nm (σ<3 nm), W=140 nm, t = 14 nm ~ GHz data rates / 10 year retention Direction of Disk Motion 6

7 Magnetic recording components Inductive Write Head P2 Layer Pole Width Copper Write Coils Throat Height Inductive Write Head P1 Layer & Top Shield Write Gap Width GMR Contacts & Hard Bias GMR Read Sensor Bottom Shield 7

8 Magnetic recording components Inductive Write Element GMR Read Sensor Compass that responds to local magnetic field and varies the resistance W t B B = 25 nm (σ<3 nm), W=150 nm, t = 14 nm data rate ~ GHz Direction of Disk Motion 8

9 Recording basics signal tim e [n s ] 9

10 Recording basics signal tim e [n s ] 10

11 Magnetic resolution PW signal tim e [n s ] PW 50 /T = signal tim e [ n s ] 11

12 Anisotropic Magneto-resistance (AMR) 1-2 % effects M I high resistance M I resistance low resistance angle between I and M Bulk property of magnetic materials 12

13 AMR Sensor 13

14 Giant Magneto-resistance (GMR) % effects M I high resistance M I resistance low resistance Interface property of magnetic materials angle between M 1 and M 2 Baibich et al. Phys. Rev. Lett (1988) Binasch et al. Phys. Rev. B 39, 4828 (1989); P. Grunberg, U.S. patent # 4,949,039 14

15 GMR sensor 15

16 GMR sensors and scaling MR and GMR/Spin Valve Head Characteristics Spin dependent scattering in a single alloy Grochowski/Gurney 16

17 GMR sensors Pinned layer Spacer layer Free layer Track of data 17

18 GMR sensors Fringe field from bits rotates free layer magnetization Track width 18

19 GMR sensors The reference ferromagnetic layer magnetization is pinned by an antiferromagnetic layer and does not rotate in small magnetic fields ΔR/R Moment 19

20 GMR sensors Pinned layer is AP pinned to obtain flux closure to minimize magneto-static coupling to free layer Utilize AP coupling property of Ru, Ir J (arb. units) F coupling AF coupling thickness (Ru) 20

21 GMR sensors External hard magnet bias free reference H applied X X 21

22 GMR sensors 1) Produce undercut resist structure (193nm photolithography) 2) Ion Mill, then IBD HB/leads 3) Lift-off Resist 22

23 GMR sensors and scaling Total gap shield shield Current-in-plane (CIP) downtrack direction 23

24 GMR sensors NiFe CoFe Cu CoFe Ru CoFe IrMn 70 nm Critical lithography feature State-of-the-art magnetic hard disk drives 24 I. R. McFadyen, E. E. Fullerton and M. J. Carey, MRS Bulletin 31, 379 (2006).

25 GMR sensors The height of the sensor is controlled by lapping (polishing) not by lithography. The smallest feature in a thin film head is determined mechanically sensor Shield 2 Disk Side Shield nm Sensor height = 100 nm Sensor width = 160 nm lapped air bearing surface 25

26 GMR read head Works great, what s the problem ΔR not increasing R increasing Shorting to the shields Edge damage for small features 11 GMR Ratio (%) Model Data J. A. Katine, et al., Appl. Phys. Lett., 83, 401 (2003) Width (nm)

27 New sensor geometry GMR spin-valve I CIP-GMR (Current-in-plane) Magnetic tunnel-valve Tunnel-valve head I I GMR spin-valve CPP-Tunnel Magnetoresistance (high R) (Current-perpendicular-toplane) CPP-GMR (low R) (Currentperpendicular-toplane) 27

28 CPP sensor Sensor deposition Photo/Ion mill Insulator/hard bias deposition/insulator deposition Top lead and shield current x bottom lead and shield 28

29 TMR sensor 29

30 TMR sensor Overlayer(s) Magnetic oxidation electrode 2 Tunnel Barrier barrier material Magnetic electrode 1 Underlayer(s) Shield S1 1) Barrier + magnetic electrode interfaces determine maximum TMR AlOx 70%TMR and MgO >500% TMR 2) Thickness of given barrier material Determines RA= Resistance x Area 3) TMR is independent of specific materials in sensor stack Sensor design is highly flexible Key goals: - Smooth substrate+underlayers - continuous ultrathin barrier growth - Stable barrier interfaces - No shunting of barrier during lithography 30

31 TMR sensor The resistance of the device depends on RA product R (per unit area) depends exponentially on the barrier thickness A is set by size of the bits (decreases with time). To keep the resistance of the device constant you need to thin the barrier over time (or find lower R barrier materials). 31

32 TMR sensor Gb/in2 65 MB/s 150 Gb/in 2 50 MB/s 70 Gb/in 2 40 MB/s 35 Gb/in 2 31 MB/s ΔR/R (%) 10 1 CPP spin-valves Spin-valves CPP tunnel-valves Tunnel 26.5 db head S/ N 15% utilization Vop = 180 mv Low RA (high current): +Amplifier noise +Ampere curling field + Thermal/electromigration [+spin-torque] RA (Ω-μm 2 ) Johnson noise High RA (low current): +Shot noise +Voltage bias limit +RC time constant 32

33 TMR sensor CoFeB/MgO/CoFeB "knee" TMR (%) Typical behavior RA (Ω-μm 2 ) Y. Nagamine et al. (Anelva corp.) Intermag 06 33

34 CPP GMR Sensor In absence of low RA tunnel barrier move to metal devices. Shunting Gap 2 Cap Free layer Parasitic resistance Parasitic resistance CPP-GMR Shield 2 Cap Free layer Lead Shunting Spacer GMR Electron mean-free path λ (10-100A) Spin-diffusion length l sf ( A) GMR Spacer Shunting Pinned layer Underlayer Gap 1 CIP-GMR Shunting effect limits maximum signal Parasitic resistance Pinned layer Underlayer Shield 1 34

35 CPP GMR Sensor S2 Free layer (M=30-50A NiFe) Cap layer Spacer CoFe/Cu interfaces Total gap < 500A AP-pinned layers (ΔM = 0) Ru Spin mixing layers J.Y. Gu et al., JAP 87, 4831 (2000) Antiferromagnet (J ex > 0.3 erg.cm 2 ) AFM Underlayer S1 35

36 CPP GMR Sensor Want high resistance high spin polarized materials nm Hex 17.8 at.% Al 24.9 at.% Al Heusler alloys ΔR/R (%) at.% Al (Co 45.5 Fe 54.5 ) (1-x) Al x CoFeAl in FL CoFeAl in FL and AP at.% Al 36.4 at.% Al 40.9 at.% Al e.g. NiMnSb, CrFeAl, x (at.%) Maat et al., JAP Hirohata et al., Current Opinion in Solid State and Mat. Sci. 10, 93 (2006). 36

37 CPP GMR Sensor Compared to TMR, CPP GMR has lower R, ΔR & ΔR/R So how to get a signal? Current: > 10 8 A/cm 2 Heating Electromigration Spin transfer torques 37

38 Spin transport R ~ R 0 + ΔR (1-cos(θ)) GMR metallic TMR insulator 38

39 Spin transfer effect J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) L. Berger, Phys. Rev. B 54, 9353 (1996) Current polarized by F1 Transfer of spin angular momentum to M J 10 7 A/cm 2 39

40 Magnetization dynamics dm dt = γ 0 H m Field torque (precession) + α m dm dt Damping torque (dissipation) H m IPg i μ em t s B ( ( )) m x m x p Spin torque (negative friction ) 40

41 Magnetization dynamics F1 F2 e- e- F1 F2 GMR H Nano contact nano-pillar M. Tsoi et al., Phys. Rev. Lett. 80, (1998) W. H. Rippard et al., Phys. Rev. Lett. 92, (2004) J.A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000) I P AP C AαM S V g(0) p ( H + H + H 2πM ) dip K // + 41 S

42 GMR sensor Thermal fluctuations fluctuation-dissipation theorem S k B T/ M V free Smith and Arnett, APL 78, 1448 (2001). 42

43 Sensors Sensors need a lot of properties not just ΔR/R Opportunities for new materials and new phenomena 43

44 Non-magnetic magnetic sensors High-mobility SC Metal shunt Metal dot embedded in high mobility low carrier density semiconductor (i.e. InSb). Solin, APL 80, 4012 (2001) At low field E is _ _ to metal/sc boundary and j follows E low R. At high fields because of the Lorentz force the angle between j and E can approach 90 degrees with little current flowing through metal high R. Attractive because immune from magnoise, spin-torque However geometry is challenging for a slider-type sensor 44