Bottom-up magnetic systems Olivier Fruchart Institut Néel (CNRS-UJF-INPG) Grenoble - France http://neel.cnrs.fr Institut Néel, Grenoble, France. http://perso.neel.cnrs.fr/olivier.fruchart/ http://perso.neel.cnrs.fr/olivier.fruchart/
Table of contents 1. Introduction 2. Magnetic anisotropy in nanodots 3. Magnetization processes inside domain walls 4. Towards 3D spintronics ? Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.2 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Motivation for magnetism Modern applications of magnetism Where can 'nano' contribute ? Materials Data storage Magnets Hard disk drives (→ motors and Tapes generators) Magnetic RAM ? Transformers Magnetocaloric Sensors Nanoparticles Compass Ferrofluids Field mapping IRM contrast HDD Read heads Hyperthermia Sorting & tagging Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.3 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Motivation for bottom-up Where can 'bottom-up' contribute ? Personal views Lowest size Highest quality Low-cost and/or mass production 3D self-assembly Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.4 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Introduction – The hysteresis loop Manipulation of magnetic materials: Application of a magnetic field Zeeman energy: E Z =−μ 0 H . M Spontaneous magnetization Remanent magnetization Losses W =μ 0 ∮ ( H .d M ) Coercive field Magnetic induction B =μ 0 ( H + M ) Other notation J =μ 0 M Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.5 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Table of contents 1. Introduction 2. Magnetic anisotropy in nanodots 3. Magnetization processes inside domain walls 4. Towards 3D spintronics ? Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.6 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Magnetization reversal of nano-objects Framework ∂ r m = 0 (uniform magnetization) Approximation : E = EV = V [ K eff sin E = EV = V [ K eff sin 2 − 0 M S H cos − H ] 2 θ−μ 0 M S H cos (θ−θ H ) ] H θ H K eff = K mc K d Magnetic anisotropy e = E / KV Dimension-less units: θ h = H / H a M 2 − 2 h cos − H e = sin H a =2K / 0 M S L. Néel, Compte rendu Acad. Sciences 224, 1550 (1947) E. C. Stoner and E. P. Wohlfarth, Phil. Trans. Royal. Soc. London A240, 599 (1948) IEEE Trans. Magn. 27(4), 3469 (1991) : reprint Names used Uniform rotation / magnetization reversal Coherent rotation / magnetization reversal Macrospin etc. Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.7 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Magnetization reversal of nano-objects 2 θ+ 2 h cos θ H = 180 ° e = sin Example for H>0 Energy barrier Switching field Δ e = e (θ max )− e ( 0 ) Switching field ~ coercive field 2 = ( 1 − h ) h c ≈ 1 -90° 0° 90° 180° 270° Δ E = KV ( 1 − H / H a ) 2 H c ≈ 2K /μ 0 M S μ 0 M S ( 1 − √ ) ln (τ/ τ 0 ) k B T H c ( T )= 2 K KV − 10 s 0 ≈ 10 M. P. Sharrock, J. Appl. Phys. 76, 6413 (1994) H c Blocking temperature T b ≃ KV / 25k B T Superparamagnetism Blocked state Magnetic anisotropy and volume crucial for thermal stability Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.8 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
A short view on hard disk drives Magnetic bits on hard disk drive Underlying microstructure Co-based Hard disk media : bits 50nm and below S. Takenoiri, J. Magn. Magn. Mater. 321, 562 (2009) B. C. Stipe, Nature Photon. 4, 484 (2010) Questions and dreams Engineer (increase) magnetic anisotropy in nano-objects Self-organize grains for one-grain-per-bit concept Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.9 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Example – Go from 2D to 3D through physical routes Litterature – Stacking dots InAs Q.Xie, Phys.Rev.Lett.75(13), 2542 (1995) Driving forces Strain Surface / Interface energy Thermodynamics and kinetics Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.10 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Go from 2D to 3D through physical routes – Growth Step 1 – The 2D seed Step 2 etc. – Vertical replication Co/Au(111) dots – 300nm FoV + Au + Co etc. – 300nm FoV 7.5nm 3nm 6nm Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.11 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Go from 2D to 3D through physical routes – Magnetism Increase blocking temperature T (K) B D C 300 B 200 100 A 0 0 100 200 3 v (nm ) Pillar volume O. Fruchart et al., Phys. Rev. Lett. 23 (14), 2769 (1999) O. Fruchart et al., J. Cryst. Growth 237-239, 2035 (2002) O. Fruchart et al., J. Magn. Magn. Mater. 239, 224 (2002) Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.12 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Other examples of 3D/columnar growth Stacked dots GeMn2 columns inside a Ge matrix Q.Xie, Phys.Rev.Lett.75(13), 2542 (1995) M. Jamet et al., Nature Mater. 5, 653 (2006) → Co columns inside a CoO 2 matrix Multifunctional metamaterials CoFe 2 O 4 columns BaTiO 3 matrix F. Vidal et al., Appl. Phys. Lett. 95, 152510 (2009) H. Zheng et al., Science 303, 661 (2004) F. Vidal et al., Phys. Rev. Lett. 109, 117205 (2012) Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.13 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
CLUE#1 Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.14 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Table of contents 1. Introduction 2. Magnetic anisotropy in nanodots 3. Magnetization processes inside domain walls 4. Towards 3D spintronics ? Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.15 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Domains and domain walls – Length scales Magnetic domains Length scales (eg 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) Nanomagnetism ~ Mesomagnetism Need to adapt size of nanostructure to seek new effects Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.16 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Engineering epitaxial self-assembly – Fe/W-Mo(110) Θ Θ ~3.5AL Θ T >400K, >6AL T <370K, >6AL Θ T =700K, [2AL,6AL] Overview of Fe(110) r r r (for Mo) (for Mo) t=6AL (for Mo) growth by PLD 2 µ m Nominal coverage (atomic layers, AL) Compact 3D dots Not explored [-110] (110) >6AL 4AL [001] 5 µ m 3AL 2AL F la t is la n d s C o m p a c t 3 D d o ts t t ~1nm >30nm µ 1 m (for Fe/Mo) 1AL ~ 750K ? 300K 500K 700K 900K Deposition temperature, T (K) S O. Fruchart et al., J. Phys.: Condens. Matter 19, 053001, Topical Review (2007). Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.17 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Single-crystalline Fe(110) dots – Flux closure and Bloch domain wall Hysteresis loops Magnetization states Landau states: two antiparallel domains (110) [001] [-110] 1.0 // [001] Typical length: 1 micron // [1-10] 0.5 Magnetization // [110] 0.0 (110) -0.5 300K -1.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 P.-O. Jubert et al., Phys. Rev. B64, 115419 (2002) Applied field µ 0 H (T) P.-O. Jubert et al., Europhys. Lett. 63, 135 (2003) Flux-closure domains Domain wall in a box Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.18 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
Magnetization process inside a domain wall – Theory first ( + , - ) Remanent state H H ( + , + ) ( - , - ) H=0 H=0 ( + , - ) ( - , + ) Remanent state Remanent state Remanent state can be switched: makes one more controlable ‘bit’ Remanence of Néel cap is opposite to applied field F. Cheynis et al., Phys. Rev. Lett. 102, 107201 (2009) Olivier Fruchart – CNano IdF School – Paris, June 2013 – p.19 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides
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