TEM for magnetism: challenges and competitors Olivier Fruchart - PowerPoint PPT Presentation

TEM for magnetism: challenges and competitors Olivier Fruchart Institut Néel (CNRS-UJF-INPG) Grenoble - France http://neel.cnrs.fr Institut Néel, Grenoble, France. Institut Néel, Grenoble, France. http://perso.neel.cnrs.fr/olivier.fruchart/ http://perso.neel.cnrs.fr/olivier.fruchart/

MOTIVATION FOR MICROSCOPY / Length scales (fundamental) Anisotropy exchange length Magnetic domains (domain wall width) 2 + K sin E = A ( ∂ x i m j ) 2 θ Numerous and complex magnetic domains Exchange Anisotropy 3 J / m J / m Δ u = √ A / K Anisotropy exchange length: Δ u ≈ 1 nm → Δ u ≥ 100 nm Hard Soft (History : Weiss domains) Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.2 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MOTIVATION FOR MICROSCOPY / Length scales (technology) Magnetic bits on hard disk drives CoPtCrTaB Hard disk (old…) Relevant spatial resolution  10-100nm S. Takenoiri, J. Magn. Magn. Mater. 321, 562 (2009) Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.3 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MOTIVATION FOR MICROSCOPY / Time scale Relevant time resolution μ : thermally-activated magnetization processes  >1 s  1 ns : precession of magnetization  1 ps : ultrafast demagnetization Demonstration: 1999 Basics of precessional switching Magnetization dynamics: Landau-Lifshitz-Gilbert equation: [ ] α   d M d M = − γ 0 × + × M H M   eff dt M  dt  s γ 0 Gyromagnetic factor gq γ 0 = µ γ γ = 0 2 m γ π = / 2 28 GHz/T Effective field H e f f (including applied) α Damping coefficient (10 -3 → 10 -1 ) C. Back et al., Science 285, 864 (1999) Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.4 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MOTIVATION FOR MICROSCOPY / Link with structure Example : domain wall to be moved along a 1d system Without applied field With applied field F ( x )= E ( x )− 2 μ 0 M S H x E ( x ) x F ( x )=− d E F ( x ) =− d F d x =− d E d x + 2 μ 0 M S H d x E. Kondorski, On the nature of coercive force and irreversible changes in magnetisation, Phys. Z. Sowjetunion 11, 597 (1937) Relevant information  Microstructure  Chemical composition Spectroscopy, diffraction etc.  Crystal structure Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.5 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MOTIVATION FOR MICROSCOPY / Practical considerations Versatility Speed of acquisition  Samples made with lithography  Sample preparation needed ? or ex situ OK ?  How much time for one image ?  Need for sample preparation ?  Compatible with various environments ? (temperature, field etc.) Access What is probed  Large-scale instrument or in-lab ?  Surface or volume technique ?  Expensive or cheap ?  Sensitivity ?  Magnetization, stray field, other ? Conclusion No universal technique Many criteria to be balanced Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.6 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MAGNETISM over time 1956 RAMAC, IBM, first hard disk drive Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.7 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MAGNETISM over time 1986 1998 Discovery of Giant Magneto-Resistance GMR implemented in HDD read heads X10 6 increase of density after 1956 → Nobel prize in Physics 2007 Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.8 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

MAGNETISM over time – What is ahead ? Physics (recent) Technology (to come)  Spin-transfer torques.  Magnetic Random Access Induces magnetization Memories (dots) switching, oscillations  Logic and memory devices  Interfacial effects, based on domain walls. 3D chirality memories ? Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.9 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

PROSPECTS FOR APPLICATIONS MRAM = Magnetic Random Access Memory Overview Detail of a cell (nb : old design) ‘Bit’ ligne I 2 Current Spin valve Current Magn flux lines Conductor (Cu) I 1 Flux guides Transistor Main features : Solid state memory Non-volatile and fast Still issues remaining Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.10 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

PROSPECTS FOR APPLICATIONS – MAGNETIC MEMORIES IN 3D ? Race-track memory in arrays of vertical nanowires Advantages  Goes beyond areal density  Only 2D R/W devices IBM, patents (2004) Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.11 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

OBJECTS TO INVESTIGATE Spin textures : 2D/3D Constrained walls Skyrmions and helix (eg : in stripes) : 1D/2D Cu\Fe\Ni stackings, interfacial Fe 0.5 Co 0.5 Si, bulk X. Z. Yu et al., Nature 465, G. Chen et al., Phys. Rev. Permalloy (15nm) - Stripe 500nm 901 (2010) Lett. 110, 177204 (2013) Bloch point (0D) Magnetic vortices (1D/0D)  Point with vanishing magnetization W. Döring, J. Appl. Phys. 39, 1006 (1968) Diameter ~ 10nm T. Shinjo et al., Science 289, 930 (2000) Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.12 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

COMPETITORS / Scanning probe Spin-polarized STM Magnetic Force Microscopy NV center microscopy Fe(1ML)/W(001) FePt thin films Square Fe20Ni80 dot 2 μ m 5 μ m Magnetic domain Antiferromagnetic Signature of flux-closure walls domain O.F. M. Bode et al., Nat. Mater. 5, L. Rondin et al., arXiv:1302.7307 Sample courtesy: A. Marty 477-481 (2006) REVIEW : REVIEW : R. Wiesendanger, Rev. Mod. R. Proksch et al., Modern Phys. 81, 1495 (2009) techniques for characterizing magnetic materials, Springer, Overview p.411 (2005) Others : BEMM, scanning Hall probe, near-field  Large variety optical etc.  Rather slow Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.13 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

COMPETITORS / Electron-based SPLEEM SEMPA Spin-Polarized Low Energy Scanning Electron Microsc. Electron Microscopy with Polarization Analysis 7 μ m 1.5 μ m Stripes of Fe/W(110) Stripes of Fe/W(110) Maze of Fe/W(001) REVIEW: W. Wulfhekel et al., N. Rougemaille et al., Eur. Phys. Rev. B 68, Phys. J. Appl. Phys. 50, 20101 144416/1-9 (2003) (2010)  Requires sample preparation Overview  Good spatial resolution  Some information about structure Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.14 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

COMPETITORS / Synchrotron-light TXM XMCD-PEEM X-ray Magnetic Circular Dichroism Transmission X-ray Microscopy Photo-Emission Electron Microsc. Co\Cu\FeNi trilayer FeNi 6μm square dot → time resolution → elemental resolution J. Vogel et al., J. Phys. : Condens. Matter 19, J. Raabe et al., Phys. Rev. Lett. 94, 217204 (2005) 476204 (2007) Others : holography, scattering  Elemental sensitivity Overview  Compatible with time resolution  Rather versatile Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.15 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

COMPETITORS / Combining surface and projection PEEM Understanding the contrast  Above wire → inform about surface magnetization  In the shadow → inform about volume magnetization Previous PEEM in shadow : J. Kimling et al., Phys. Rev. B 84, 174406 (2011) Identification Beam along wire Beam across wire  Locate domain walls  Inspect domain wall O.F., unpublished Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.16 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides

Overview of some magnetic microscopies with spatial resoltuion better than 50nm Sp-STM MFM NV BEMM SEMPA SPLEEM TEM XMCD XMCD- -PEEM microscopy (Fresnel ZP) 5-10 1-2nm 25nm Resolution <1nm 15nm 1-5nm 10nm 10nm 15nm nm → 10nm Sensitivity Sensitivity High Med High Med Med High Low High High In-field YES Limited Limited YES local No? Limited No? YES No Versatile* YES No Yes Yes UHV Limited Yes Limited Dynamics No No No No New Yes Yes No No Element- Limited No No No No Limited Limited Yes Yes sensitive Probes m i m i m x , y m k m k m m H d H d * Versatile may mean: Conclusions Sample preparation, measurement of brought-in  Worse and best come together samples etc.  Need for combining various instruments Olivier Fruchart – Inauguration I2TEM – Toulouse, 17 June 2013 – p.17 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides