Voyage au pays du Nanomagnétisme

Transcription

1 Voyage au pays du Nanomagnétisme Olivier Fruchart Institut Néel (CNRS / Univ. Grenoble-Alpes) Grenoble - France

2 Magnetism in products Modern applications of magnetism Where does 'nano' contribute? Materials Magnets Data storage Hard disk drives Tapes MRAM ( motors and generators) Transformers Magnetocaloric Sensors Nanoparticles Compass Field mapping HDD read heads Ferrofluids MRI contrast Hyperthermia Sorting & tagging Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.2

3 Introduction to magnetism 1. Fields, moments, materials 2. Magnetization processes 3. Low dimensions Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.3

4 FIELDS and MOMENTS Currents produce magnetic field Maxwell equation : B=μ0 j Electric currents produce magnetic fields Oersted field Magnetic dipole Magnetic material μ Bθ= μ0 I 2πr B= μ0 4 π r3 [ ] 3 (μ.r)r μ 2 r Magnetic dipole A.m2 B=μ0 (H+ M) Magnetization A/m Magnetic moment A.m2 Induction B, Magnetic field H, Magnetization M Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.4

5 FIELDS and MOMENTS Types of magnetic order Magnetic ordering Magnetic exchange Ordering temperature between microscopic moments E = 2 J i, j S i. S j i> j Ferromagnet Antiferromagnet J i, j 0 Ferrimagnet J i, j 0 J i, j 0 Helical J1, J2 Fe T C=1043 K CoO T N =292 K Fe3O4 T C=858 K Dy T K M s = A /m J =3/2 M s =480 ka /m μ=10.4μ B Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.5

6 FIELDS and MOMENTS Magnetic periodic table Magnetic properties at room temperature, single elements Periodictable.com Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.6

7 Introduction to magnetism 1. Fields, moments, materials 2. Magnetization processes 3. Low dimensions Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.7

8 MAGNETIZATION PROCESSES Hysteresis loops 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 Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.8

9 MAGNETIZATION PROCESSES Types of magnetic materials Hard magnetic materials Soft magnetic materials M M Hext Hext Transformers Flux guides, sensors Permanent magnets, motors Magnetic shielding Magnetic recording Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.9

10 MAGNETIZATION PROCESSES From bulk to nano Bulk material Mesoscopic scale Nanometric scale Numerous and complex magnetic domains Small number of domains, simple shape Magnetic single-domain FeSi soft sheet Microfabricated dots Kerr magnetic imaging Nanofabricated dots MFM A. Hubert, Magnetic domains A. Hubert, Magnetic domains Sample courtesy : N. Roug le, I. Chioar Nanomagnetism ~ mesoscopic magnetism How small is 'Nano'? Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.10

11 Introduction to magnetism 1. Fields, moments, materials 2. Magnetization processes 3. Low dimensions Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.11

12 MAGNETIZATION PROCESSES Micromagnetism, continuous approach Magnetization () () Mx mx M= M y =M S my Mz mz Magnetization vector M Can vary in time and space. Modulus is constant (hypothesis in micromagnetism) m x +m y+m z =1 Mean-field approach possible: M S= M S (T ) Exchange interaction Atomistic view : Micromagnetic view : E = 2 J i, j S i.s j i> j (total energy) S i. S j=s 2 cos θi, j S 2 (1 θ2i, j /2) E= A(. m)2 (energy per unit volume) Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.12

13 MAGNETIZATION PROCESSES Micromagnetism, various types of magnetic energy Exchange energy Magnetocrystalline anisotropy energy 2 2 E ex = A (. m ) E mc = K sin θ Zeeman energy Dipolar energy 2 1 E d= μ0 M. H 1 E d= μ0 M. Hd 2 Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.13

14 MAGNETIZATION PROCESSES Dipolar energy Magnetic charges Maxwell equation div (Hd )= div (M) ρ(r)= M S div [m(r)] volume density of magnetic charges σ (r)=m S m (r). n(r) surface density of magnetic charges Examples of magnetic «charges» + Notice: no charges + + and E=0 for infinite + cylinder Charges on surfaces Take-away message Surface and volume charges - Dipolar energy favors alignement of magnetization with longest direction of sample Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.14

15 MAGNETIZATION PROCESSES From bulk to nano Bulk material Mesoscopic scale Nanometric scale Numerous and complex magnetic domains Small number of domains, simple shape Magnetic single-domain FeSi soft sheet Microfabricated dots Kerr magnetic imaging Nanofabricated dots MFM A. Hubert, Magnetic domains A. Hubert, Magnetic domains Sample courtesy : N. Roug le, I. Chioar Nanomagnetism ~ mesoscopic magnetism Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.15

16 MAGNETIZATION PROCESSES Magnetic characteristic length scales Dipolar exchange length Anisotropy exchange length E= A ( x θ )2 + K d sin 2 θ E= A ( x θ )2 + K sin2 θ Exchange J /m Anisotropy J /m3 Exchange J /m Dipolar energy J /m3 1 K d= μ 0 M 2S 2 Dipolar exchange length: Anisotropy exchange length: Δu 1 nm Δ u 100 nm Soft Hard = 2A /μ0 M 2s Δd 3 10 nm Single-domain critical size relevant for nanoparticules made of soft magnetic material Often called Exchange length Often called Bloch parameter or domain-wall width Notice: Other length scales: with field etc. Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.16

17 MAGNETIZATION PROCESSES Domain walls and related low-dimensional objects Bloch domain wall in the bulk (2D) Domain walls in thin films (2D 1D) Bloch wall t w No magnetostatic energy Width Δ = A/ K Areal energy γ =4 AK u W Other angles & anisotropy F. Bloch, Z. Phys. 74, 295 (1932) Néel wall 1000x2000nm t w Contains magnetostatic energy No exact analytics L. Néel, C. R. Acad. Sciences 241, 533 (1956) Magnetic vortex (1D 0D) Bloch point (0D) Point with vanishing Constrained walls (eg : in stripes) Permalloy (15nm) Strip 500nm 300x800nm magnetization T. Shinjo et al., Science 289, 930 (2000) W. Döring, J. Appl. Phys. 39, 1006 (1968) Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.17

18 Some examples of recent work 1. Low-dimensional structures in nanomagnetism 2. What could come next for magnetism in data storage? 3. An example of academic study towards 'next' Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.18

19 An example of academic study Dimensionality crossover Bloch wall Vortex A. Masseboeuf et al., Phys. Rev. Lett. 104, (2010) Elongated dots Breaking of symmetry (2nd order transition) Order parameter Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.19

20 How does an ultra-high vacuum setup for epitaxy (single-crystal) growth look like? Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.20

21 Choice of a model system Single crystalline self-assembled structures Wulff Kaishev s theorem Growth and structure Stacking: Al2O3(11-20) \ W[10nm] \ Co \ Au Growth at 450 C AFM view, 6 mm FoV Co[11-2] Supported crystal (growth on surfaces) hint i hi {001} {110} {111} Truncated crystal Co[1-10] O. Fruchart et al., J. Phys. : Condens. Matter 25, (2013) P. Müller and R. Kern, Surf. Sci. 457, 229 (2000) (and incl. ref.) Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 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 Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.22

23 Evidence for the dimensionality cross-over Vortex Bloch wall Vortex Domain wall A. Masseboeuf et al., Phys. Rev. Lett. 104, (2010) Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.23

24 How does a transmission electron microscope look like? Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.24

25 Some reading (general magnetism) Repository of lectures of the European School on Magnetism: Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.25

26 Some examples of recent work 1. Low-dimensional structures in nanomagnetism 2. What could come next for magnetism in data storage? 3. An example of academic study towards 'next' Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.26

27 What is this? Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.27

28 Current (past?) technology for magnetic data storage 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) Technological / Physical breakdown Compromise time stability, writability, readibility Disruptive solutions sought Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.28

29 Magnetic recording over time 1956 RAMAC, IBM, first hard disk drive Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.29

30 Magnetic recording over time Discovery of Giant Magneto-Resistance Nobel prize in Physics 2007 GMR implemented in HDD read heads X106 increase of density from 1956 Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.30

31 Magnetic recording over time Physics (recent) Spin-transfer torques. Induces magnetization switching, oscillations Interfacial effects, chirality Technology (to come) Magnetic Random Access Memories (dots) Logic and memory devices based on domain walls. 3D memories? Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.31

32 Magnetic Random Access Memories MRAM = Magnetic Random Access Memory Overview Detail of a cell (very old design) Bit ligne I2 Current Spin valve Current Magn flux lines I1 Conductor (Cu) Flux guides Transistor Main features: Solid state memory Non-volatile and fast Issues being fixed entered semiconductor roadmap Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.32

33 What is next? What shall we prepare in labs? Proposal for a race-track memory in 3D Magnetic logic with domain walls (Field driven) D. A. Allwood et al., Science 309, 1688 (2005) Limitation: Requires homogeneous rotating field S. S. P. Parkin, Science 320, 190 (2008) + patents (IBM) Potentially 3D storage, however technologically challenging Cylinders are the 'natural' geometry Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.33

34 Some examples of recent work 1. Low-dimensional structures in nanomagnetism 2. What could come next for magnetism in data storage? 3. An example of academic study towards 'next' Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.34

35 Some of the bottom-up routes implemented at Institut NEEL The basics Specific aspects Anodization of aluminum template ALD to reduce pore diameter 100nm S. Da Col et al., Appl. Phys. Lett. 98, (2011) H. Masuda, Science 268, (1995) Decrease dipolar interactions Electroplating magnetic nanowires Modulation of pore diameter S. Allende et al., Phys. Rev. B 80, (2009) Simple metals and alloys : Co, Ni, Fe20Ni80 Landscape for domain walls 100nm Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.35

36 Nucleate domain-walls Modulated diameters Principle Nucleation of domain walls SEM AFM MFM Provides energy landscape for domain walls Tail-to-tail Head-to-head NB : reports with modulations however no domain walls K. Pitzschel et al., J. Appl. Phys. 109, (2011) Protrusions create a 'box' for domain walls Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.36

37 Propagation of domain walls Experiments Straight sections SEM AFM MFM Distribution of propagation fields, range 1-10mT Similar to the case of strips J. Sampaio et al., J. Appl. Phys. (2013) Protrusions and constrictions Domain-wall motion demonstrated (range 1-10mT) Pinning at modulations of diameter Pinning field ~30mT Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.37

38 Details of domain walls Domain walls in one-dimensional systems Transverse Vortex Bloch Néel Transverse (TW) Bloch-point (BPW) Need for advanced magnetic microscopies to identify domain walls Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.38

39 Experiments on a large scale instrument : synchrotron radiation facility Elettra synchrotron, Trieste, Italy Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.39

40 Inside a synchrotron ring Swiss Light Source (SLS), close to Zürich Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.40

41 XMCD-PEEM microscopy : probing surface magnetization XMCD PEEM X-ray Magnetic Circular Dichroism Photo-Emission Electron Microscopy Element selectivity Magnetic sensitivity Courtesy: W. Kuch Features Secondary electrons surface sensitive Spatial resolution : 25nm Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.41

42 What does a Photo-Emission Electron Microscope look like? Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.42

43 Experimental contrast (XMCD-PEEM) Two examples Beam along wire Locate domain walls Beam across wire Inspect domain wall Several non-trivial patterns Need for modelling Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.43

44 Test case : uniform transverse magnetization Schematics Above wire : surface magnetization In the shadow : volume magnetization Non-linear, even change of sign Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.44

45 Formal identification of domain walls Bloch-Point Wall (BPW) Transverse Wall (TW) Wire diameter : 95nm Experiment Simulation Wire diameter : 70nm Experiment Wire Wire Shadow Shadow Simulation 300nm 500nm Orthoradial curling Symmetry with respect to plane Transverse curling Loss of symmetry with respect to perpendicular to axis plane perpendicular to axis Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.45

46 Synthesis Other systems with chemical routes Long-range ordered templates Nanotubes Pre-defined bits Multilayered structures Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.46

47 Figures Strength and weaknesses («business plan») Areal density Prospects for HDD :1 Tbit/in Power Current for spin-torque 10 A/m Resistivity 10 Ω.m Power consumption 10 mw Energy per bit pj bit/cm 2 Pitch 100nm 10 wire/cm Need 100 bits/wire to overcome HDD Goal 1bit/200nm 20μ m Speed Cost per bit Shift wire registers in parallel 2D patterning similar to flash not a limiting factor 2 Determined by R/W (planar) device Similar to 2D competitors Scalability 2D cost + 3D scalability cost/bit decreases No limitation for 3D fabrication Practical limitation for energy consumption Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.47

48 Collaborative work! Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.48

49 Daily work in an academic lab Perform experiments (or simulations, theory) Develop instrumentation, codes etc. Reading recent and less recent publications, books etc. Go to conferences : learn and disseminate Seeking funding Grants, equipment, travel etc. Do some teaching Organize the lab : scientific life, technical versus scientific activity etc. Olivier Fruchart Séminaire de Physique, PHELMA Grenoble, 2014/12/08 p.49