Introduction to nanomagnetism I. Magnetization reversal Olivier - PowerPoint PPT Presentation

Introduction to nanomagnetism – I. Magnetization reversal Olivier Fruchart Institut Néel (CNRS-UJF-INPG) Grenoble - France http://neel.cnrs.fr Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

Common language Advanced point: fasten seat belt Slippery topic: be cautious Blackboard explanation Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.2 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MOTIVATING THE LECTURE – Miniaturization is a key for technology Telegraphone (1898, W. Poulsen)  Same concept over 100 years RAMAC (IBM, 1956)  Technological innovation? 2 kbit/in 2  New science? 50 disks Ø 60 cm Total 5Mo Modern hard disk drive (>1 To) Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.3 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MOTIVATING THE LECTURE – The principle of magnetic recording (hard disk drives) Principle of hard-disks Magnetic bits CoPtCrTaB Hard disk (old…) Read/Write head ~100nm Substrate Shielding Shielding Coils ~7-8nm MR Disk (rotation 7000-10000 rpm) See lecture: L. Ranno S. Takenoiri, J. Magn. Magn. Mater. 321, 562 (2009) Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.4 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MOTIVATING THE LECTURE – The need for nanomagnetism Fundamental issues for nanomagnetism  Is a small grain (ferro)magnetic? Count number of surface atoms  Is a small grain stable against thermal fluctuations? − 21 J ≈ 25 meV k B T  300 K ≈ 4 × 10 100 k B T  300 K ≈ 2.5 eV Derive from macroscopic arguments  Decades-old (yet still modern) topic  Are there domains, domain walls? → See later on... Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.5 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MOTIVATING THE LECTURE – The need for spin electronics The technological need for spin electronics ~100nm  How to read information? → Convert magnetic information to electric signal Substrate Shielding Shielding  Field of spin electronics Coils ~7-8nm • Magneto-transport. MR • Requires nanometer length scales  Official birth: discovery of magneto-resistance N. M. Baibich, Phys. Rev. Lett. 61, 2472 (1998) Disk G. Binach, Phys. Rev. B 39, 4828 (1989) (rotation 7000-10000 rpm)  Novel prize 2007: A. Fert & P. Grünberg See lecture: L. Ranno Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.6 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MOTIVATING THE LECTURE – Controlling magnetization reversal Magnetic domains: from macroscopic to small systems Bulk material Mesoscopic scale Nanometric scale Numerous and complex Small number of domains, Magnetic magnetic domains simple shape single-domain Microfabricated dots Co(1000) crystal – SEMPA Kerr magnetic imaging A. Hubert, Magnetic domains A. Hubert, Magnetic domains R.P. Cowburn, Micromagnetism ~ mesoscopic magnetism J.Phys.D:Appl.Phys.33, R1 (2000) Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.7 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

SKETCH OF THE LECTURES Part I – Magnetization reversal Part II – Techniques Part III – Atomic-scale properties Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.8 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

Techniques → See local excellent expertise Part II – Techniques Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.9 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MAGNETIZATION REVERSAL → ToC 0. Introduction 1. Energies and length scales in magnetism 2. Single-domain magnetization reversal 3. Magnetostatics 4. Magnetization reversal in materials Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.10 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

1. ENERGIES AND LENGTH SCALES – Hysteresis and magnetic materials Manipulation of magnetic materials:  Application of a magnetic field S pontaneous ≠ S aturation = − µ E H.M Zeeman energy: Z 0 s Spontaneous magnetization M s Remanent magnetization M r Another notation M M J s = 0 M s Coercive field H c Hext Hext Losses ∫ = µ E H d M 0 ext Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.11 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

1. ENERGIES AND LENGTH SCALES – Soft and hard magnetic materials Soft materials Hard materials M M Hext Hext Transformers Flux guides, sensors Permanent magnets, motors Magnetic shielding Magnetic recording Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.12 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

1. ENERGIES AND LENGTH SCALES – Sources of magnetic energy Echange energy Magnetocrystalline anisotropy energy = − E J S . S 1 2 M Ech 1 , 2 = ∇ θ 2 A ( ) = θ sin 2 E K ( ) mc Hext Zeeman energy (enthalpy) Dipolar energy 2 1 1 = − µ E M . H S d = − µ d 0 E M . H 2 S Z 0 Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.13 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

1. ENERGIES AND LENGTH SCALES – Magnetic characteristic length scales Typical length scale: Numerical values Domain wall width =  A / K Bloch parameter: =  A / K Bloch wall width: ≈ 1 nm  ≥ 100 nm Hard Soft 2  K sin E = A  ∂/∂ x  2  Exchange Anisotropy J/m 3 J/m Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.14 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

1. ENERGIES AND LENGTH SCALES – Magnetic domains Bulk material Mesoscopic scale Nanometric scale Numerous and complex Small number of domains, Magnetic magnetic domains simple shape single-domain Microfabricated dots Co(1000) crystal – SEMPA Kerr magnetic imaging A. Hubert, Magnetic domains A. Hubert, Magnetic domains R.P. Cowburn, Nanomagnetism ~ mesoscopic magnetism J.Phys.D:Appl.Phys.33, R1 (2000) Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.15 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

MAGNETIZATION REVERSAL → ToC 0. Introduction 1. Energies and length scales in magnetism 2. Single-domain magnetization reversal 3. Magnetostatics 4. Magnetization reversal in materials Olivier Fruchart – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.16 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

2. SINGLE-DOMAIN REVERSAL – Coherent rotation (1/4) Framework ∂ r m = 0 (uniform magnetization) Approximation : (strong!) H θ H E = EV = V [ K eff sin E = EV = V [ K eff sin 2 − 0 M S H cos − H  ] 2 − 0 M S H cos − H  ] K eff = K mc  K d θ M e = E / KV Dimensionless units: h = H / H a 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 – IWOS MASENA – Hanoi, Vietnam, Nov.2010 – p.17 Institut Néel, Grenoble, France http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/