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ALS UM 2009 Stochastic Behavior of Magnetic Processes on a Nanoscale Mi-Young Im Center for X-ray Optics, LBNL Berkeley, CA, USA mim@lbl.gov

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ALS UM 2009 Challenge in Nano-magnetism 1 cm 1 mm 1 µm 1 nmUltra-small Ultrathin FilmsNanowireNanoparticles Novel Manuplating Technique B-fieldSpin currentThermal Controllability Vortex switching Domain wall motion Nano-Magnetism 1 ms 1 ns 1 ps 1 fsUltra-fast Thermal activationDampingThermalizationPrecession

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ALS UM 2009 Contents Background Statistical Behavior of Magnetic Processes --- Domain Nucleation Process in Ultra Thin Magnetic Film (2D) --- Domain Wall Depinning Process in Notch Patterned Nanowires (1D) --- Vortex- State (chirality) Creation Process in Circular Nanodot Arrays (0D) Summary

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ALS UM 2009 Statistical Behavior Whether the magnetic process is deterministic or stochastic Scientific Point of View Scientific Point of View : century old long-standing question - Is there any unifying physical mechanism? - Is there any specific law, which governs the complicate magnetic phenomena? - Which is dominant factor for determination of statistical nature? Technological Point of View Technological Point of View : substantial issue for application - Is the spin reversal phenomena repeatable? - Is the domain wall motion controllable? - What is the way to acheive the tunable and repeatable spin reversal and dynamics?

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ALS UM 2009 Review Irreversible Reversal Reproducible Hysteresis loop J. M. Deutsche et al., Phys. Rev. Lett.(2004) M. S. Pierce et al., Phys. Rev. Lett. (2003) Macroscopic or k-space Contradictory DWM at Single Time A. J.Zambano et al., Appl. Phys.Lett. (2004) Switching Field Distribution Justin M. Shaw et al., J. Appl. Phys. (2007) Theoretical approach Single measurement Simulation for DW Process E. Martinez et al., Phys. Rev. Lett. (2007) Reversal Process in Nanodot V. Novosad, et al., Phys. Rev. B, (2002) Direct observation in real space Statistical measurement

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ALS UM 2009 Our Goal Observation S. Parkin US Patent 309, 6,834,005 (2004). G. Meier et al. PRL (2007) UnderstandingControlling S. Parkin US Patent 309, 6,834,005 (2004). Nanodot (0D) Ultrathin Films Nanowires Ultra Thin Film (2D) Nanowire (1D) Nanodot possibility for controllable spin process solution for unsolved-question

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ALS UM 2009 Magnetic soft X-ray microscopy at XM-1 H max = 5 kOe (perp.) = 2 kOe (long.) CCD 2048x2048 px 2 Mag ~ 2000 FOV ~ 10-15 m m t<70 ps 3rd generation synchrotron source E = 250 eV - 1.8 keV l = 0.7 nm - 5 nm E/E=500 element specificity time resolution XMCD contrast polarization lateral resolution r < 25 nm

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ALS UM 2009 Domain Nucleation Process in Ultra Thin Magnetic Film

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ALS UM 2009 Sample: 50–nm (Co 82 Cr 18 ) 87 Pt 13 / 40-nm Ti / 200-nm Si 3 N 4 Magnetic Domain Evolution Patterns +H -H +400 Oe+600 Oe 2m2m +200 Oe 0 Oe -200 Oe Nucelation-mediated magnetization reversal behavior that originated from individual switching of grain M.-Y. Im et al., APL 83, 4589 (2003)

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ALS UM 2009 Stochastic Nature 1st cycle 2nd cycle Stochastic and asymmetric nature of magnetic domain nucleation process 1st cycle (left branch) 2nd cycle (right branch) Both cycles (branches) Magnetic domain configurations in repeated hysteretic cycles and different branches

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ALS UM 2009 Degree of Stochastic Nature X and Y : same size matrices 1 : existence 0 : nonexistence of domain nucleation in each pixel r=0 : totally different r=1: completely identical Average correlation coefficient among domain configurations Correlation coefficient in both cases increases as magnetization reversal is progressed M.-Y. Im et al., Adv. Mater 20, 1750 (2008)

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ALS UM 2009 gyromagnetic ratio dimensionless damping coefficient parameter h fluc fluctuating magnetic field LLG equation incl. thermal term Micromagnetic simulation of magnetization reversal patterns in repeated hysteretic cycles at 300 K Thermal flucutation effect play a role on stochastic nature in domain nucleation process Thermal Fluctuation Effect

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ALS UM 2009 Domain Wall Depinning Process in Notch Patterned Nanowires

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ALS UM 2009 Permalloy (Ni 80 Fe 20 ) SEM images Wire width (w): 150, 250, 450 nm Notch depth (N d ): 30, 50 % Film thickness (t): 50, 70 nm Notch Patterned Permalloy Nanowire +H -H MTXM image

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ALS UM 2009 -47 Oe -383 Oe -413 Oe -430 Oe -489 Oe -141 Oe -189 Oe -236 Oe -259 Oe -371 Oe -24 Oe -106 Oe -124 Oe -129 Oe -319 Oe w= 150 nm w= 250 nm w= 450 nm Domain walls are stopped at precise position Domain Wall Evolution Patterns

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ALS UM 2009 Depinning field of domain wall in repeated hysteretic cycles -100 Oe H -530 Oe DW depinning process shows stochastic behavior in repeated measurements Stochastic Nature DW depinning process is not completely governed by DW pinning mechanism

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ALS UM 2009 Multiplicity of Domain-wall Types The multiplicity of domain-wall type generated in the vicinity of a notch is responsible for the observed stochastic nature Vortex wall Transverse wall courtesy S. Parkin -440 Oe-450 Oe -485 Oe -490 Oe

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ALS UM 2009 Degree of Stochastic Nature Standard deviation of DW depinning field M.-Y. Im et al, Phys. Rev. Lett. 102, 147204 (2009) Standard deviation of the depinning field is minimized to below 7 Oe The DW depinning process can be controllable in properly designed nanowire

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ALS UM 2009 Vortex State (chirality) Creation Process in Nanodot Arrays

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ALS UM 2009 Permalloy Nanodot Arrays MTXM Image Dot Size (r): 200, 400, 600, 800, 1000 nm Film Thickness (t): 40, 70, 100 nm 800 nm Chirality in-plane circular domain structure Polarity out-of-plane component of magnetization Vortex State Ni 80 Fe 20 :t=100 nm, r=800 nm Normalized Images In-plane Out-of-plane 1000 nm 800 nm 600 nm 400 nm 200 nm

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ALS UM 2009 Statistical Behavior of Vortex State (chirality) Creation Process Ni 80 Fe 20 (t=40 nm, r=1000 nm, d=200 nm) 2nd1st +x saturation Overlapped images Switched Dots In-plane domain state in repeated measurements and changing the field direction Stochastic nature of creation process of chirality in repeated (different saturation field direction) measurements M.-Y. Im, Peter Fischer, et al., in preparation +x H -x saturation

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ALS UM 2009 Summary Statistical Behavior of Magnetic Processes on a Nanoscale Direct observation of stochastic behavior - Domain nucleation process in ultra thin ferromagnetic system - Domain wall depinning process in nanowire system - Vortex state creation process in nanodot system Investigation of the origin (thermal fluctuation, multiplicity, aspect ratio, etc.) of stochastic behavior Answering for long-standing fundamental question on nanomagnetism Providing of controllable magnetic process

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ALS UM 2009 Thanks to… Peter Fischer, B. Mesler, A.E. Sakdinawat, W. Chao, R. Oort, B. Gunion, S.B. Rekawa, P. Denham, E.H. Anderson, D.T. Attwood (CXRO Berkeley USA) S.-C. Shin (KAIST, Taejeon), S.-K. Kim (SNU, Seoul), S.B. Choe (SNU, Seoul), D.-H. Kim (Chungbuk U) L. Bocklage, Judith Moser, A. Vogel, R. Eiselt, M. Bolte, G. Meier, B. Krüger (U Hamburg, Germany) S. Kasai (NIMS in Jap.), K. Yamada, K. Kobayashi, T. Ono (U Kyoto), A. Thiaville (U Paris-Sud) ALS and CXRO staff Thank you for attention!

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