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
Outline Microwave assisted magnetic recording Generation of localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary
Outline Microwave assisted magnetic recording Generation of localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary
Recording Field Availability Fe 65 Co 35 M s = 2.5 T M s H.7 M s
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.5 -.5-1 -1 -.5.5 1 H r =.5H k
Switching Dynamics Normalized Magnetization M/M s 1. H r =.17H k.5 H ac =.1H k ω=.69γh k. θ =3 o -.5-1...2.4.6.8 1. 1.2 1.4 1.6 Time t (ns) θ M r H r AC H r r
Damping: r dm dt r r α = γm H + M α =.2 α =.1 α =. 2 r M r dm dt 1 1 1.5.5.5 -.5 -.5 -.5-1 1.5 -.5-1 -1 -.5.5 1-1 1.5 -.5-1 -1 -.5.5 1-1 1.5 -.5-1 -1 -.5.5 1 1.8.6.4.2 -.2 -.4 -.6 -.8-1.2.4.6.8 1 1.2 1.4 1.6 1.8.6.4.2 -.2 -.4 -.6 -.8-1.2.4.6.8 1 1.2 1.4 1.6 1.8.6.4.2 -.2 -.4 -.6 -.8-1.2.4.6.8 1 1.2 1.4 1.6
Effect of Reversal Field Angle Normalized Switching Field H/H k 1..9.8.7.6.5.4.3.2.1 H ac =.1H k θ=2 o θ=1 o θ=3 o..1.2.3.4.5.6.7.8.9 Normalized Frequency ω (γh k ) r H AC r = H θ ac M r H r AC H r cos ( ωt)
Reversal Field Angular Dependence Normalized Switching Field H/H k 1..9.8.7.6.5.4.3 Stoner-Wohlfarth Model With AC Field (Broadband).2.1 H ac =.1H k. 5 1 15 2 25 3 35 4 45 5 Reversal Field Angle θ (degree) θ M r H r AC H r
Outline Microwave assisted magnetic recording Generating localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary
Basic Oscillator Structure X. Zhu & J. Zhu, Intermag 26, Paper EF-9 Field Generation Layer (FGL)
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
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.5. -.5-1. 2. 2.2 2.4 2.6 2.8 21. Time (ns)
Frequency Tuning by Current Ku = 5 x 1 6 erg/cc Frequency Tuning Window θ 2 θ 1 Excited AC Frequency (GHz) 3 25 2 15 1 X. Zhu & J. Zhu, Intermag 26, Paper EF-9 J 5 θ 1 θ 2. 5.x1 7 1.x1 8 1.5x1 8 2.x1 8 2.5x1 8 Injected Current Density (A/cm 2 ) 18 15 12 9 6 3 Tilting Angle θ
Novel Write Head Design
Outline Microwave assisted magnetic recording Generating localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary
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
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
Power Spectral Density 1.4 I=1.95 ma Power Spectral Density 1.1.1.55.85 1.1 1.7 Oscillating Layer δ=1nm 4πM s =2.5T Area=35x35nm 2 5 1 15 2 25 3 35 4 45 Frequency f (GHz)
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
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 2.2 1.8 1.4 1.1.7.4 K=3x1 8 erg/cm 3 K=2x1 8 erg/cm 3 K=1x1 8 erg/cm 3.. 1. 2. 3. 4. Current Density J (x1 8 A/cm 2 ) H effective γ wall = 2 H effective AK M S t FGL AK M
Higher Modes Excitation When the current is too large, the magnetization precession becomes spatially non-uniform..12 Power Spectral Density.1.8.6.4.2 Oscillating Layer δ=1nm 4πM s =2.5T Area=35x35nm 2 I = 4.5 ma 5 1 15 2 25 3 35 4 45 5 Frequency f (GHz)
Frequency and Current 6 Frequency f (GHz) 5 4 3 2 1 K=1x1 8 erg/cm 3 K=2x1 8 erg/cm 3 K=3x1 8 erg/cm 3 δ=15nm 4πM s =2.5T..5 1. 1.5 2. 2.5 3. 3.5 4. 4.5 5. Injected Current I (ma)
AC Field Strength AC Field Amplitude H ac (x1 3 Oe) 3.5 f = 35 GHz δ = 2 nm 3. 2.5 δ = 15 nm 2. δ = 1 nm 1.5 1. δ = 5 nm.5 1. 1.5 2. 2.5 3. 3.5 Current Density J (x1 8 A/cm 2 ) 3 2 1-1 -2-3 -5-4 -3-2 -1 1 2 3 4 5
Outline Microwave assisted magnetic recording Generating localized microwave field: spin torque Analysis of AC field generation Recording simulation Summary
Recording Fields Physical write head track width: 12 nm Trailing Shield present
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
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
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
Head: Medium: H head = 12 koe H = 3 koe k H ac MAMR @ 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
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 MAMR @ 1.6 MFCI C * =.5 C * =.
Random Bits All "1"s pattern Random bits No nonlinear transition shift is observed.
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.