Microwave Assisted Magnetic Recording

Transcription

1 Microwave Assisted Magnetic Recording, Xiaochun Zhu, and Yuhui Tang Data Storage Systems Center Dept. of Electrical and Computer Engineering Carnegie Mellon University IDEMA Dec. 6, 27

2 Outline Microwave assisted magnetic recording Generation of localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary

3 Outline Microwave assisted magnetic recording Generation of localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary

4 Recording Field Availability Fe 65 Co 35 M s = 2.5 T M s H.7 M s

5 Sub-coercivity Recording H =. 1 ac H k 1.5 M r -.5 r H AC = r H ac cos ( ωt) -1 1 H r r H r =.5H k

6 Switching Dynamics Normalized Magnetization M/M s 1. H r =.17H k.5 H ac =.1H k ω=.69γh k. θ =3 o Time t (ns) θ M r H r AC H r r

7 Damping: r dm dt r r α = γm H + M α =.2 α =.1 α =. 2 r M r dm dt

8 Effect of Reversal Field Angle Normalized Switching Field H/H k H ac =.1H k θ=2 o θ=1 o θ=3 o Normalized Frequency ω (γh k ) r H AC r = H θ ac M r H r AC H r cos ( ωt)

9 Reversal Field Angular Dependence Normalized Switching Field H/H k Stoner-Wohlfarth Model With AC Field (Broadband).2.1 H ac =.1H k Reversal Field Angle θ (degree) θ M r H r AC H r

10 Outline Microwave assisted magnetic recording Generating localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary

11 Basic Oscillator Structure X. Zhu & J. Zhu, Intermag 26, Paper EF-9 Field Generation Layer (FGL)

12 About Oscillation Frequency Field Generating Layer αh eff sinθ H r = H r exchange 4πM S cosθ hεj sin θ emδ m r θ dmˆ dt γ 1+ α mˆ r H cα mˆ γ αmˆ mˆ r H c + mˆ γα = 2 If c mˆ γα r = H dmˆ dt = γmˆ r H f γh = 2 π

13 Spin Torque Driven Oscillation J = P J Magnetic easy axis Ku = 5 x 1 6 erg/c.c. Field generating layer/ spin torque driven layer 1. Oscillator size: 35 nm x 35 nm Single Domain In-Plane Magnetization M x /M s Time (ns)

14 Frequency Tuning by Current Ku = 5 x 1 6 erg/cc Frequency Tuning Window θ 2 θ 1 Excited AC Frequency (GHz) X. Zhu & J. Zhu, Intermag 26, Paper EF-9 J 5 θ 1 θ 2. 5.x1 7 1.x x1 8 2.x x1 8 Injected Current Density (A/cm 2 ) Tilting Angle θ

15 Novel Write Head Design

16 Outline Microwave assisted magnetic recording Generating localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary

17 Generating AC Field Field Generating Layer (FGL) Fe 65 Co 35 4πM s =2.5T δ =1 nm Calculated Magnetization Precession Ι H K U =1.5x1 8 erg/cm 3

18 Precession in FGL M z I =.55 ma I =.85 ma I = 1.1 ma I = 1.4 ma I = 1.7 ma I = 1.95 ma M y M x

19 Power Spectral Density 1.4 I=1.95 ma Power Spectral Density Oscillating Layer δ=1nm 4πM s =2.5T Area=35x35nm Frequency f (GHz)

20 Interlayer Exchange Coupling Perpendicular layer γ wall = 2 δ wall = A K AK The effective interlayer exchange coupling on FGL is: σ = 2 AK H effective H effective H exchange = σ M S t FGL M

21 Field and Injected Current AC Field Amplitude H (x1 3 Oe) 3.2 2GHz 35GHz 42GHz 2.9 δ = 15 nm 4πM 2.5 s =2.5T K=3x1 8 erg/cm 3 K=2x1 8 erg/cm 3 K=1x1 8 erg/cm Current Density J (x1 8 A/cm 2 ) H effective γ wall = 2 H effective AK M S t FGL AK M

22 Higher Modes Excitation When the current is too large, the magnetization precession becomes spatially non-uniform..12 Power Spectral Density Oscillating Layer δ=1nm 4πM s =2.5T Area=35x35nm 2 I = 4.5 ma Frequency f (GHz)

23 Frequency and Current 6 Frequency f (GHz) K=1x1 8 erg/cm 3 K=2x1 8 erg/cm 3 K=3x1 8 erg/cm 3 δ=15nm 4πM s =2.5T Injected Current I (ma)

24 AC Field Strength AC Field Amplitude H ac (x1 3 Oe) 3.5 f = 35 GHz δ = 2 nm δ = 15 nm 2. δ = 1 nm δ = 5 nm Current Density J (x1 8 A/cm 2 )

25 Outline Microwave assisted magnetic recording Generating localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary

26 Recording Fields Physical write head track width: 12 nm Trailing Shield present

27 Recording Footprint Write head field: Rise time.2 ns, Duration 1 ns, Magnitude 12 koe Conventional: M s = 3 emu/cc H k = 8 koe H AC =. MAMR: M s = 3 emu/cc H k = 3 koe H AC = 3 koe f = 32 GHz No dispersion

28 AC Field Frequency M s = 3 emu/cc, H k = 3 koe H AC = 3 koe H head = 12 koe f = 24 GHz f = 32 GHz f = 6 GHz

29 Comparison at 1 MFCI Conventional Recording H head = 12 koe H k = 1 koe MAMR: H head = 12 koe H k = 3 koe f = 32 GHz = 3 Oe H ac

30 Head: Medium: H head = 12 koe H = 3 koe k H ac 1.6 MFCI = 3 koe f = 32 GHz D nm δ = 1 nm grain = 5 C * =. 5 easy axis and % H k dispersion 3 easy axis and 3% H k dispersion

31 Exchange Coupling Head: H head = 12 koe H ac = 3 koe f = 32 GHz Medium: H = k 3 koe D grain = 5 nm δ = 1 nm 1.6 MFCI C * =.5 C * =.

32 Random Bits All "1"s pattern Random bits No nonlinear transition shift is observed.

33 Summary An in-plane ac field at the FMR frequencies results significant reduction of perpendicular switching field. The scheme enables recording at a write field that is significantly below the medium coercivity. Spin momentum transfer can be used to generate localized ac field. Micromagnetic simulation of recording shows very promising results.