MINT Review & Workshop 24-25 Oct. 2006 Anisotropy Distributions in Patterned Magnetic Media Tom Thomson Hitachi San Jose Research Center Page 1
Acknowledgements Manfred Albrecht (Post-doc) Tom Albrecht Maggie Best Liz Dobisz Guohan Hu (Post doc) Charlie Rettner (IBM) Bruce Terris Henry Yang/Dan Kercher Summer students Katie Humphry James Williams Laura Hirsch Page 2
Patterned Media Overview Patterned Media -- Very Small Bits Individual magnetic islands can be created on the disk Each island would represent a single bit of information 200 Gb/in 2 = 56 nm period Patterned media will extend magnetic recording to > 1 Tb/in 2 Signal [a.u.] 0.2 0.0-0.2 0 200 400 600 800 x-position [nm] 1Tb/in 2 = 25 nm period 5 TB 3.5-inch drive 1.2 TB 2.5-inch drive 80 GB 1-inch drive Page 3
Critical role of anisotropy distributions H.J. Richter Manfred Schabes ΔSNR(dB) IBM-Almaden σ HA /H A Continuous media: small change in width of anisotropy distribution has a large impact 1dB ~ 10 2 BER MRAM: must be able to reliably address only the target cell (Oe field or spin torque) Patterned media: Head field must not switch neighbouring islands Page 4
Addressing the correct island in patterned media Manfred Schabes Narrow Hk distribution Write only the intended track Pole head Map of field strength at top of islands Wide Hk distribution Unintentional writing of adjacent islands by stray fields from the head Page 5
Anisotropy distributions are always present! Nucleation and domain wall motion masks distribution Nucleation of only a few very low anisotropy sites Magnetisation (emu/cm 3 ) 400 Co/Pd multilayer (E4085) 200 0-200 -400-1500 -1000-500 0 500 1000 1500 Applied field (Oe) Nanofabrication allows full distribution to be probed Nucleation Domain formation Domain growth Page 6
Co/Pd islands for this study Motivated by patterned magnetic storage media Require smooth, clean, surface capable of supporting flying head Method of choice: Pattern substrate first, then deposit film Process Steps SiO 2 /Si patterned by e-beam lithography and RIE etch Perpendicular Co/Pd multilayers deposited onto islands Island sizes down to 30 nm (300 Gbit/in 2 ) req. > 500 Gbit/in 2 for applications Pd cap layer [12Å] Pd Co Pd Co 8 bilayers Pd Co Pd [9.6Å] Co [3.3Å] Pd seed layer [30Å] SiO 2 /Si SEM MFM 1μm Page 7
Magnetic measurements on islands DCD Remanence curves: Saturate with an applied field Apply an incremental reverse field Return to zero applied field and measure Kerr rotation Kerr rotation measurements Median reversal field Hr Width of switching field distribution (SFD) 0.3 SEM Two distinct reversals Trenches Islands Measure as a function of 1,2 : Island size Applied field angle Initial state of island Kerr Rotation [a.u.] 1 G. Hu, T.Thomson et al. J. Appl. Phys. 97 10J702 (2005) 2 G. Hu, T.Thomson et al. IEEE Trans. Magn. 41 3589 (2005) 0.15 0-0.15-0.3 SFD Islands Trench Material The MOKE measurements (20μm spot) have been verified by VSM and MFM measurements -6000-4000 -2000 0 Magnetic Field [Oe] Page 8
Kerr rotation remanence curves Restrict Kerr DCD curves to island reversal region Systematic increase in reversal field with decrease in island size Increase in SFD with decrease in island size Angle dependence? 1.0 Kerr Rotation (normalised) 0.5 0.0-0.5 film 1 μm 5 μm 500 nm 200 nm 50 nm 100 nm -1.0 0 2000 4000 6000 8000 Applied Field (Oe) Page 9
Domain structure in islands SEM images of 50nm -> 5μm islands 5μm 1μm 200nm 50nm MFM images of islands in a.c. demagnetized states Single domain behavior is found in islands with a diameter < 100nm. 5um 1um 200nm 50nm Page 10
Angle Dependence of Reversal Coherent rotation (Stoner-Wohlfarth reversal) H sw ( θ ) = H sw( 0) [ cos 2/3 ( θ ) + sin 2/3 ( θ )] 3/ 2 Domain wall motion (Kondorsky reversal) M K u θ H a H M ( θ ) K u M θ sw = H cos sw H a V wall ( 0) ( θ ) 1 0.75 H sw (θ) /H sw (0) 0.5 H sw (θ) /H sw (0) 3 2.5 2 1.5 1 0 15 30 45 60 75 90 Field Angle θ [ ] 0 20 40 60 80 Field Angle θ [ ] Page 11
Islands: Angle dependent switching Hc/Hc(0) 1 0.9 0.8 0.7 easy axis θ H appl. 5um 1um 500nm 200nm 100nm 50nm 30nm 0.6 0.5 0 20 40 60 80 Angle [degree] Island arrays with sizes ranging from 5μm to 30nm exhibit Stoner- Wohlfarth like angle dependent switching with a minimum at 45. Page 12
Size dependent reversal mechanism 4 2.2 3.5 Hc/Hc(0) 3.5 3 2.5 2 Continuous film Hc/Hc(0) 2 1.8 1.6 1.4 100μm Hc/Hc(0) 3 2.5 2 50μm 1.5 1.2 1.5 1 0 20 40 60 80 Angle [degree] 1.8 1.6 20μm 1 0 20 40 60 80 Angle [degree] 1.1 1 10μm 1 0 20 40 60 80 Angle [degree] 1.2 1μm Hc/Hc(0) 1.4 1.2 Hc/Hc(0) 0.9 Hc/Hc(0) 1 1 0.8 0.8 0.8 0 20 40 60 80 Angle [Degree] 0.7 0 20 40 60 80 Angle [Degree] 0 20 40 60 80 Angle [degree] With the decrease of island size, the island switching behavior changes from a wall motion dominant process to a nucleation controlled one. The reversal characteristics of continuous film are only recovered in islands with a diameter of 50μm to 100μm. Page 13
Introduction of nucleation sites the experiments Saturate perpendicular H H=H sat perp H=0 1um island arrays Nucleate with inplane field H H=H sat in-plane H=0 M H=H in-plane H=0 H H in-plane =-2.16kOe H in-plane =-2.41kOe H in-plane =-2.54kOe Page 14
Reversal changes with nucleation sites Kerr Rotation [arb.] 0-0.1-0.2-0.3-0.4 Norm -1.91kOe -2.16kOe -2.41kOe -2.54kOe 1 2 3 0kOe -2.16kOe -0.5-3600 -2400-1200 0 Magnetic Field [Oe] 1--- Islands without nucleation sites; 2--- Trench; 3 --- Islands with nucleation sites. -2.41kOe -2.54kOe The field required for wall propagation is much lower than for nucleation. Page 15
Reversal mechanism comparison Nucleation controlled reversal processes follows the Stoner- Wohlfarth angle dependent switching model. 1 After introducing nucleation sites, the angle dependent switching behaviour follows the Kondorsky model. 3 0.9 2.5 Hc/Hc(0) 0.8 0.7 Hc/Hc(0) 2 1.5 0.6 0.5 0 20 40 60 80 Angle [degree] 1 1um Round 0 20 40 60 80 Angle [degree] Page 16
Model of reversal in Co/Pd islands Model of island reversal from full remanence: A small volume reverses by rotation Rapid domain wall motion then ensures the entire island reverses from the nucleation site Initial reversal depends on local anisotropy and total effective field Nucleation field (Hn) >> pinning field (Hp) Consequences Angle dependence has Stoner-Wohlfarth form Distribution of nucleation (anisotropy) fields can be determined by varying island size Reversal field depends on initial state i.e. on the presence of reversed regions Tests Compare experimental and simulated results for Hr and σhr Functional form of SFD Time dependence of coercivity Page 17
Length scale for nucleation of reversal Length scales in magnetism (Co/Pd) Exchange length sets limits on coherence 1.25μm l A π M ex = = 17 2 2 24 s nm assuming A ~3-6 x 10 6 erg/cm Activation volume (length) 2 act π l t = V = kt M s χ irr S l act ~ 30nm Domain wall width (180º Block wall) DWW = π A K u ~ 30nm Page 18
Gaussian distribution of intrinsic anisotropy Assume intrinsic anisotropy is spatially varying Variation described by a Gaussian function No correlation between regions 10-9 Probability 400 300 200 100 0 Gaussian anisotropy distribution 6 8 + 10 12 14 Anisotropy Constant K 1 (10 6 erg/cm 3 ) Length scale is set by nucleation (activation) volume Page 19
Distribution of nucleation fields Assume: Nucleation field (H n ) >> pinning field (H p ) Nucleation volume 25 x 25 x t nm Island reversal occurs when the lowest nucleation site switches H n = 1000 Oe 50 nm islands σ = 100 Oe 500 nm island Nucleation site Page 20
Simulation Stoner Wohlfarth reversal of nucleation volume Input measured M s and K int (mean) Inputs Shape anisotropy calculated using method of Aharoni 1 Input measured shape of islands Sharrock 2 extrapolation for finite temperatures and times to compare with experiment Output nucleation (volume) constrained by length scales Distribution of intrinsic anisotropy is independent of island size Output σk int Outputs 1 Aharoni J. Appl. Phys. 83 3432 (1998) 2 Sharrock and McKinney IEEE Trans. Magn. 17 3020 (1981) Page 21
Simulation and experiment Optimise simulation to match both measured H cr and SFD with a single set of parameters Find value of σk int across all island sizes and obtain Stoner Wohlfarth equivalent nucleation volume Co/Pd multilayers M s = 400 emu/cm 3 K int = 2.20 x 10 6 erg/cm 3 σk int = 7.6% l nuc = 38 nm Remanent Kerr rotation (norm.) 1.0 0.5 0.0-0.5-1.0 0 10000 8000 6000 4000 2000 T.Thomson, G. Hu and B.D. Terris, Phys. Rev. Lett. 96 257204 (2006) H cr (Oe) 0 film H cr Remanence curves (islands only) 5 μm 2000 500nm 4000 Applied field (Oe) SFD Neighbours 6000 50nm 8000 5 6 7 8 9 100 2 3 4 5 6 7 8 9 1000 2 3 4 5 Island size (nm) SFD 800 600 400 200 0 SFD [Std. Dev.] (Oe) Page 22
Functional form of SFD Simulation correctly reproduces changes in the functional form of the SFD as a function of island size 10-6 Switching Probability 800 600 400 200 0 4000 50 nm islands 5000 6000 7000 8000 Applied field (Oe) 10-3 Switching Probability 2.0 1.5 1.0 0.5 0.0 5 μm 1 μm 500 nm 200 nm 100 nm 2000 4000 6000 Applied field (Oe) 50 nm 8000 10-3 Switching Probability 1.5 1.0 0.5 0.0 1600 1 μm islands 1800 2000 2200 2400 2600 Applied field (Oe) 2800 3000 3200 Page 23
Time dependence of coercivity Good agreement with experimental time dependence of coercivity (VSM) Compare H 0 with hard axis (inplane) field (H n- inplane ) needed to create nucleation sites Reasonable agreement between H0 and H n- (Sanity check) inplane H cr (Oe) Inplane (hard axis) field required for nucleation 8000 7000 6000 5000 4000 3000 2000 1000 10-10 10-8 10-6 10-4 10-2 10 0 10 2 Time (s) 50 nm 500 nm 5 μm Page 24
Summary Controlling the distribution of anisotropy is critical for nanomagnetic technologies Patterning magnetic arrays is a novel approach for determining intrinsic distributions of magnetic properties SFD is due to intrinsic properties of the magnetic film and is not a patterning induced artifact Kerr Rotation (normalised) 1.0 0.5 0.0-0.5-1.0 0 film 1 μm 5 μm 500 nm 2000 200 nm 50 nm 100 nm 4000 6000 Applied Field (Oe) 8000 Significant impact on the direction of patterned media development towards the goal of 1 Tbit/in 2 T.Thomson, G. Hu and B.D. Terris, Phys. Rev. Lett. 96 257204 (2006) Topical Review "Nanofabricated and Self-Assembled Magnetic Structures as Data Storage Media" B.D. Terris and T.Thomson J. Phys. D: Applied Physics 38 (2005) R199 Page 25
Patterned Media: Status and Outlook Patterned magnetic nanostructures: Topographically patterned island arrays have been fabricated by e-beam lithography and nano-imprinting at 300Gbit/in 2 Challenge is to reliable fabricate at low cost and scale towards 1 Tbit/in 2 Most recently we have developed a new understanding of the SFD width in terms of a distribution of intrinsic anisotropy Recording physics Read/Write experiments demonstrated at 200Gbit/in 2-1Tbit/in 2 possible? Noise is ultimately lithography limited, and in test structures is superior to similar continuous media Model of the write process developed to understand synchronisation Topical Review "Nanofabricated and Self-Assembled Magnetic Structures as Data Storage Media" B.D. Terris and T.Thomson J. Phys. D: Applied Physics 38 (2005) R199 Page 26
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