Bottom-up magnetic systems

Transcription

1 Bottom-up magnetic systems Olivier Fruchart Institut Néel (CNRS-UJF-INPG) Grenoble - France Institut Néel, Grenoble, France.

2 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

3 Motivation for magnetism Modern applications of magnetism Where can 'nano' contribute? Materials Magnets ( motors and generators) Transformers Magnetocaloric Data storage Hard disk drives Tapes Magnetic RAM? Sensors Nanoparticles Compass Field mapping HDD Read heads Ferrofluids IRM contrast Hyperthermia Sorting & tagging Olivier Fruchart CNano IdF School Paris, June 2013 p.3

4 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

5 Introduction The hysteresis loop Manipulation of magnetic materials: Application of a magnetic field Zeeman energy: E Z = μ0 H. M Spontaneous magnetization Remanent magnetization Coercive field Losses W =μ0 (H. d M) Magnetic induction B=μ0 (H+ M) Other notation J=μ0 M Olivier Fruchart CNano IdF School Paris, June 2013 p.5

6 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

7 Magnetization reversal of nano-objects Framework Approximation: r m=0 (uniform magnetization) 2 E= =EV H ) ] EV =V [ K eff sin θ μ 0 M S H cos cos(θ θ K eff =K mc K d Dimension-less units: e=sin2 2 h cos H θh H Magnetic anisotropy e = E / KV h = H /Ha H a =2K / 0 M S θ M 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

8 Magnetization reversal of nano-objects e=sin 2 θ+2 h cos θ Example for H =180 H>0 Energy barrier Switching field Δ e = e(θmax ) e(0) Switching field ~ coercive field = ( 1 h ) h c 1 2 Δ E = KV (1 H / H a ) H c 2K /μ0 M S ( ln (τ/ τ0 )k B T 2K 1 μ0 M S KV s H c (T )= ) M. P. Sharrock, J. Appl. Phys. 76, 6413 (1994) Hc Blocking temperature T b KV /25k B T Blocked state Superparamagnetism Magnetic anisotropy and volume crucial for thermal stability Olivier Fruchart CNano IdF School Paris, June 2013 p.8

9 A short view on hard disk drives Magnetic bits on hard disk drive Underlying microstructure Co-based Hard disk media : bits 50nm and below B. C. Stipe, Nature Photon. 4, 484 (2010) S. Takenoiri, J. Magn. Magn. Mater. 321, 562 (2009) 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

10 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

11 Go from 2D to 3D through physical routes Growth Step 1 The 2D seed Co/Au(111) dots 300nm FoV Step 2 etc. Vertical replication + Au + Co etc. 300nm FoV 7.5nm 3nm 6nm Olivier Fruchart CNano IdF School Paris, June 2013 p.11

12 Go from 2D to 3D through physical routes Magnetism Increase blocking temperature TB (K) B A 0 Pillar volume C D v (nm ) O. Fruchart et al., Phys. Rev. Lett. 23 (14), 2769 (1999) O. Fruchart et al., J. Cryst. Growth , 2035 (2002) O. Fruchart et al., J. Magn. Magn. Mater. 239, 224 (2002) Olivier Fruchart CNano IdF School Paris, June 2013 p.12

13 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 CoO2 matrix Multifunctional metamaterials CoFe2O4 columns BaTiO3 matrix F. Vidal et al., Appl. Phys. Lett. 95, (2009) F. Vidal et al., Phys. Rev. Lett. 109, (2012) H. Zheng et al., Science 303, 661 (2004) Olivier Fruchart CNano IdF School Paris, June 2013 p.13

14 CLUE#1 Olivier Fruchart CNano IdF School Paris, June 2013 p.14

15 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

16 Domains and domain walls Length scales Magnetic domains Length scales (eg domain wall width) Numerous and complex magnetic domains 2 E= A ( x m j ) + K sin 2 θ i Exchange J /m Anisotropy J /m3 Anisotropy exchange length: Δu = A/ K Δu 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

17 Engineering epitaxial self-assembly Fe/W-Mo(110) Tr <370K, Θ >6AL (for Mo) Overview of Fe(110) growth by PLD Tr >400K, Θ >6AL (for Mo) Tr =700K, Θ [2AL,6AL] t=6al (for Mo) Θ ~3.5AL Nominal coverage (atomic layers, AL) 2 µm Compact 3D dots [-110] >6AL Not explored (110) 4AL [001] 5µm 3AL 2AL Flat islands Compact 3Ddots t~1nm t>30nm (for Fe/Mo) 1AL 1µ m ~750K? 300K 500K 700K 900K Deposition temperature, TS (K) O. Fruchart et al., J. Phys.: Condens. Matter 19, , Topical Review (2007). Olivier Fruchart CNano IdF School Paris, June 2013 p.17

18 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 Magnetization // [001] // [1-10] // [110] Typical length: 1 micron (110) 300K Applied field µ0h (T) P.-O. Jubert et al., Phys. Rev. B64, (2002) 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

19 Magnetization process inside a domain wall Theory first (+,-) H Remanent state 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, (2009) Olivier Fruchart CNano IdF School Paris, June 2013 p.19

20 XMCD-PEEM : high resolution magnetic imaging XMCD PEEM X-ray Magnetic Circular Dichroism Photo-Emission Electron Microscopy Element selectivity Courtesy: W. Kuch Magnetic sensitivity Features Collection of electrons surface sensitive Spatial resolution : 20-25nm Hardly compatible with applied field SOLEIL ELETTRA Olivier Fruchart CNano IdF School Paris, June 2013 p.20

21 XMCD-PEEM Switching of Néel caps Topography Field of view: 5µm LEEM Magnetic contrast PEEM Population of Néel cap XMCD-PEEM Positive Experiments: 90% switching. F. Cheynis et al., JAP103, 07D915 (2008) Negative F. Cheynis et al., Phys. Rev. Lett. 102, (2009) Olivier Fruchart CNano IdF School Paris, June 2013 p.21

22 High-resolution magnetic imaging Lorentz and holography (TEM based) Fresnel imaging mode Self-assembled fcc Co dots (vortex state) Sensitive mainly to in-plane components of magnetization integrated over the sample s thickness Pascale Bayle-Guillemaud (INAC) Aurélien Masseboeuf et al. (INAC CEMES) Olivier Fruchart CNano IdF School Paris, June 2013 p.22

23 LORENTZ Dimensionality cross-over from domain-wall to vortex Vortex Bloch wall Vortex Domain wall A. Masseboeuf et al., Phys. Rev. Lett. 104, (2010) Olivier Fruchart CNano IdF School Paris, June 2013 p.23

24 Various magnetic objects in low dimensions to have (more) fun Spin textures : 2D/3D Skyrmions and helix Constrained walls (eg : in stripes) : 1D/2D Cu\Fe\Ni stackings, interfacial Fe0.5Co0.5Si, bulk X. Z. Yu et al., Nature 465, 901 (2010) Magnetic vortices (1D/0D) G. Chen et al., Phys. Rev. Lett. 110, (2013) Permalloy (15nm) - Stripe 500nm Bloch point (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 CNano IdF School Paris, June 2013 p.24

25 CLUE#2 Olivier Fruchart CNano IdF School Paris, June 2013 p.25

26 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.26

27 Prospects Dreams for domain-wall devices Magnetic logic with domain walls (Field driven) Magnetic memories with domain walls (Current driven) D. A. Allwood et al., Science 309, 1688 (2005) Limitation: Requires homogeneous rotating field S. S. P. Parkin, Science 320, 190 (2008) + patents Makes use of spin transfer effect Potentially 3D storage, however technologically challenging Olivier Fruchart CNano IdF School Paris, June 2013 p.27

28 Some of the bottom-up routes implemented at Institut NEEL The basics Specific aspects Anodization of aluminum template ALD to reduce pore diameter 100nm H. Masuda, Science 268, (1995) Electroplating magnetic nanowires S. Da Col et al., Appl. Phys. Lett. 98, (2011) Decrease dipolar interactions Modulation of pore diameter S. Allende et al., Phys. Rev. B 80, (2009) Simple metals and alloys : Co, Ni, FeNi 100nm Landscape for domain walls Olivier Fruchart CNano IdF School Paris, June 2013 p.28

29 Other bottom-up routes (collaborators) Long-range ordered templates Nanotubes Planar structures Multilayered and core-shell K. Nielsch et al., Univ. Hamburg J. Bachmann et al., Univ. Erlangen J. P. Araujo, Univ. Porto Smartmembrane GmbH, Halle Olivier Fruchart CNano IdF School Paris, June 2013 p.29

30 Domain-walls in one-dimensional systems Stripes, in-plane magnetization Transverse Vortex Bloch Néel Stripes, out-of-plane magnetization Wires, longitudinal magnetization Domain-wall transformation Walker limit, low speed (~100m/s) Experiments and theory BPW in wires Stripes Transverse (TW) Bloch-point (BPW) Theory predictions ; no experiments No domain-wall transformation High speed (>1000m/s) Olivier Fruchart CNano IdF School Paris, June 2013 p.30

31 Bottleneck : how to stabilize a domain-wall? Nucleation Propagation mechanism Sequence of magnetization reversal Single-domain wire (MFM) Y. Henry et al., Eur. Phys. J. B 20, 35 (2001) R. Hertel et al., J. Magn. Magn. Mater. 249, 251 (2002) Explains why domain walls hardly reported in cylindrical nanowires Olivier Fruchart CNano IdF School Paris, June 2013 p.31

32 Collaborative work! Olivier Fruchart CNano IdF School Paris, June 2013 p.39

33 Domain wall nucleation and propagation in cylindrical nanowires S. Da Col, S. Jamet, N. Roug le, R. Afid, M. Darques, L. Cagnon, J. C. Toussaint, O. Fruchart Institut NEEL Grenoble - France A. Locatelli, T. O. Mentes, B. Santos Burgos Sincrotrone Elettra Trieste - Italy The research leading to these results has received funding from the European Unions's 7th Framework Programme under grant agreement n (M3d) Olivier Fruchart CNano IdF School Paris, June 2013 p.40