Experimental techniques for the study of small magnetic objects

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1 Experimental techniques for the study of small magnetic objects Olivier Fruchart Institut Néel (CNRS-UJF-INPG) Grenoble - France France.

2 Foreword Large overview of characterization techniques. Therefore remains handwavy Not all techniques currently used for nanoalloys (yet?) Personal views on prospects Experts in the audience: please interrupt any time Non-experts in the audience: please interrupt any time Olivier Fruchart GdR Nanoalliages Lyon Jan p.2

3 Nano Sensitivity Spatial resolution Restricted amount of material Depth sensitivity Relevance of measurements Volume versus surface properties Single-object measurements Characterize non-homogeneities Alloys Chemical sensitivity Spatial / depth resolution Olivier Fruchart GdR Nanoalliages Lyon Jan p.3

4 TABLE OF CONTENTS (1/4) 1. Microscopy 2. Flux and direct local probes 3. Reciprocal space 2 1 fα ', β ' µ (α ', β ' ) = 2 α ' β ' M 4. Interactions in assemblies Hext β α Olivier Fruchart GdR Nanoalliages Lyon Jan p.4

Down to 5nm lateral resolution Fresnel: fine resolution of domain walls and vortices Collaboration:

5 Lorentz microscopy Fresnel mode Fresnel imaging mode Self-assembled fcc Co dots (vortex state) Sensitive mainly to in-plane components of induction integrated over the sample s + air thickness Down to 5nm lateral resolution Fresnel: fine resolution of domain walls and vortices Collaboration: P. Bayle-Guillemaud, A. Masseboeuf (CEA - Grenoble) Olivier Fruchart GdR Nanoalliages Lyon Jan p.5

6 Lorentz microscopy Foucault mode Foucault imaging mode FoV: 1500nm Foucault: highlights domains 2D/3D vectorial maps of induction can be computed (also possible from series of focus Collaboration: P. Bayle-Guillemaud, A. Masseboeuf (CEA - Grenoble) Olivier Fruchart GdR Nanoalliages Lyon Jan p.6

Compensation measurement needed to get rid of electrical contribution Courtesy: B.

7 Lorentz microscopy - Holography Working principle Setup Phase-shift of electrons through matter Computational procedure to retrieve the integrated map of induction Compensation measurement needed to get rid of electrical contribution Courtesy: B. Warot-Fonrose, CEMES-Toulouse Olivier Fruchart GdR Nanoalliages Lyon Jan p.7

cases): Probes magnetization AND stray/internal fields E. Snoeck et al.

8 Lorentz microscopy - Holography Vortex state (plane view) TEM Magnetization Single-domain (side view) Phase shift Micromagnetic Simulation Notice (both cases): Probes magnetization AND stray/internal fields E. Snoeck et al., Nano letters 8 (12), 4293 (2008) Olivier Fruchart GdR Nanoalliages Lyon Jan p.8

9 Microscopies de Lorentz et holographie Features Ultimate lateral resolution ~5nm Minimum thickness: a few nm (best: holography) to ten nm Rate (video). Preparation (spreading, thinning) but versatile ('ex situ') Sensitive to total induction: magnetization AND demagnetizing / stray fields Probes the two in-plane components of induction; tiltable sample holder. Field applied through dedicated Lorentz lens: mainly perpendicular Trends / future? Micromagnetic simulations are required to fully benefit from the resolution More combined HREM / Lorentz studies 'Environmental': moderate stand-alone external field; electrical connections etc. Magnetization dynamics Olivier Fruchart GdR Nanoalliages Lyon Jan p.9

SEMPA or spin-sem SEMPA = Scanning Electron Microscope with Polarization Analysis Setup Polarization of secondary electrons Max values Fe: 50% Co: 35% Ni: 10% Spin detectors General feature: low

10 SEMPA or spin-sem SEMPA = Scanning Electron Microscope with Polarization Analysis Setup Polarization of secondary electrons Max values Fe: 50% Co: 35% Ni: 10% Spin detectors General feature: low efficiency or low energy window Mott detector, LEED detector (W(001), Low-energy diffuse scattering R. Allenspach, Spin-polarized scanning electron microscopy, IBM J. Res. Develop. 44, 553 (2000) Olivier Fruchart GdR Nanoalliages Lyon Jan p.10

11 SEMPA or spin-sem Reconstruction of magnetization map Ex: 3ML Fe/Cu(001) Channel up Sum: topography Channel down Difference: Mz R. Allenspach, Spin-polarized scanning electron microscopy, IBM J. Res. Develop. 44, 553 (2000) Olivier Fruchart GdR Nanoalliages Lyon Jan p.11

(scanning, efficiency of spin detectors) External field require special setups Prospects More in-field

12 SEMPA or spin-sem Example: Fe/W(001) Fields of view: 1.5 mm W. Wulfhekel et al., Phys. Rev. B 68, /1-9 (2003) Features Potential lateral resolution ~ 5nm Surface sensitive (<1nm) Low rate (scanning, efficiency of spin detectors) External field require special setups Prospects More in-field setups See improved spin detectors Use energy filtering for elemental sensitivity Olivier Fruchart GdR Nanoalliages Lyon Jan p.12

SPLEEM SPLEEM = Spin-Polarized Low-Energy Electron Microscope = Spin-Polarized LEEM Working principle of LEEM Features Full-field (video imaging rate) 5-10nm lateral resolution Resolution of atomic

13 SPLEEM SPLEEM = Spin-Polarized Low-Energy Electron Microscope = Spin-Polarized LEEM Working principle of LEEM Features Full-field (video imaging rate) 5-10nm lateral resolution Resolution of atomic steps Some elemental or thickness resolution through working energy High voltage column, however low energy electrons on sample See: growth of Fe/W(110) E. Bauer, Rep. Prog. Phys 57, 895 (1994) Olivier Fruchart GdR Nanoalliages Lyon Jan p.13

14 SPLEEM Electron spin polarization and manipulation Features Polarization over 80% achievable 3D manipulation of spin direction using combined magnetic/electrostatic optics 2D maps of magnetization with 3 components Olivier Fruchart GdR Nanoalliages Lyon Jan p.14

Rougemaille & A. K. Schmid, J. Appl. Phys.

15 SPLEEM SPLEEM: example 14MLFe/W(110), deposited at RT After annealing at 350 C Field of view: 7 mm N. Roug le & A. K. Schmid, J. Appl. Phys. 99, 08S502 (2006) Olivier Fruchart GdR Nanoalliages Lyon Jan p.15

16 SPLEEM Features UHV technique Full-field (video imaging rate) Prospects / future / dreams Use dark-field imaging for magnetism? Investigate samples from ex situ? 5-10nm lateral resolution Vertical resolution: atomic steps Surface sensitivity 2D maps of magnetization with 3 components Some elemental or thickness resolution through working energy No applied field Olivier Fruchart GdR Nanoalliages Lyon Jan p.16

17 XMCD magnetometry XMCD = X-ray Magnetic Circular Dichroism XMLD = X-ray Magnetic Linear Dichroism Intensity (a.u.) Fe 700 Co 800 Photon Energy (ev) Ni 900 Courtesy: W. Kuch Olivier Fruchart GdR Nanoalliages Lyon Jan p.17

Electron Microscope e e e hν LEEM e PEEM Note: simpler XMCD-PEEM

18 SPELEEM : PEEM + LEEM XMCD-PEEM: X-ray Magnetic Circular Dichroism PhotoElectron Emission Microscope Based on LEEM instrument: Low-Energy Electron Microscope e e e hν LEEM e PEEM Note: simpler XMCD-PEEM instruments exist, not based on dual LEEM/PEEM. Olivier Fruchart GdR Nanoalliages Lyon Jan p.18

19 XMCD-PEEM - Examples Surface sensitivity Topography (LEEM) For nanoparticles? Co, diameter 8nm Height = 180nm Length = 1650 nm X-PEEM µm 11µm Surface magnetization (XMCD-PEEM) 1 µm SEM Magnetization sensititivity direction 1 µm R. Hertel et al., Phys. Rev. B 72, (2005) A. Fraile Rodriguez, JMMM316, 426 (2007) Olivier Fruchart GdR Nanoalliages Lyon Jan p.19

20 XMCD-PEEM : time resolution Field-driven magnetization reversal Spin-polarized driven domain wall motion FeNi/Al2O3/Co spin valves x-rays 400 nm wide stripes: FeNi(4nm)/Cu(8nm)/ Co(7nm) B A 5 µm J. Vogel et al. (2005) C I I S. Pizzini et al., APEX, in press (2009) Olivier Fruchart GdR Nanoalliages Lyon Jan p.20

21 XMCD-PEEM : overview Features UHV technique, however nanofabricated and capped samples OK Full-field (medium imaging rate) Prospects / future Resolution improved to 10nm (energy filtering; abberation correction) Manipulator for multi-component imaging 25nm lateral resolution Surface sensitivity 2D maps of magnetization with 1 component only Elemental resolution Compatible with time resolution No significant applied field Olivier Fruchart GdR Nanoalliages Lyon Jan p.21

22 TXM / STXM (S)TXM = (Scanning) Transmission X-ray Microscope Working principle (TXM) Focusing zone plate (detail) P. Fischer et al., Rev. Sci. Instrum. 72, 2322 (2001) Olivier Fruchart GdR Nanoalliages Lyon Jan p.22

72, 2322 (2001) Pump-probe time-resolved measurements 6mm Permalloy

23 Time-resolved TXM Static imaging CoPtCr recording media P. Fischer et al., Rev. Sci. Instrum. 72, 2322 (2001) Pump-probe time-resolved measurements 6mm Permalloy squares J. Raabe et al., Phys. Rev. Lett. 94, (2005) Olivier Fruchart GdR Nanoalliages Lyon Jan p.23

24 (S)TXM overview Features Compatible with nanofabricated and capped samples Down to 15nm lateral resolution Volume sensitivity (transmission) 2D maps of magnetization with 1 component only. Samples can be tilted Elemental resolution Compatible with time resolution Compatible with applied field Olivier Fruchart GdR Nanoalliages Lyon Jan p.24

25 X-ray Holography Working principle Example [Co(4Å)/Pt(7Å)]50 with perpendicular magnetization STXM Holography S. Eisebitt et al., Nature 332, 885 (2004) improving Features Resolution Requires nanofabrication Olivier Fruchart GdR Nanoalliages Lyon Jan p.25

26 Sp-STM Working principle spstm = Spin-Polarized Scanning Tunneling Microscope Spin-dependent spectroscopy Modulation of tip magnetization + lock-in tunneling current W. Wulfhekel et al., APL 75, 1944 (1999) O. Pietzsch et al., PRL 84, (2000) Olivier Fruchart GdR Nanoalliages Lyon Jan p.26

5, 477-481 (2006) U. Schlickum, Phys. Rev.

27 SpSTM - Examples Ultimate magnetic resolution Antiferromagnetic Fe(1ML)/W(001) Frustration at a ferro/antiferro interface Mn/Fe(001) Antiferromagnetic domain Antiferromagnetic domain wall M. Bode et al., Nat. Mater. 5, (2006) U. Schlickum, Phys. Rev. Lett. 92, (2004) Olivier Fruchart GdR Nanoalliages Lyon Jan p.27

28 SpSTM - Overview Features UHV technique, very few samples from ex situ Slow Ultimate lateral resolution Prospects / future Magnons and anisotropy with spectroscopy Local manipulation with spin-polarized current or electrical field Compatible with field and low temperature Olivier Fruchart GdR Nanoalliages Lyon Jan p.28

29 TABLE OF CONTENTS (2/4) 1. Microscopy 2. Flux and direct local probes 3. Reciprocal space 2 1 fα ', β ' µ (α ', β ' ) = 2 α ' β ' M 4. Interactions in assemblies Hext β α Olivier Fruchart GdR Nanoalliages Lyon Jan p.29

Micro-SQUID Principle Micro-SQUID: a microfabricated loop with

monitored through the measurement of the critical current V(I)

transition in the loop W. Wernsdorfer et al., Adv. Chem. Phys.

30 Micro-SQUID Principle Micro-SQUID: a microfabricated loop with Josephson junction Critical current versus flux Flux in the loop monitored through the measurement of the critical current V(I) characteristics Critical current measured by triggering the S N transition in the loop W. Wernsdorfer et al., Adv. Chem. Phys. 118, (2001) Olivier Fruchart GdR Nanoalliages Lyon Jan p.30

Micro-SQUIDs Examples Mode: flux monitoring Cold mode: switching monitoring T < 7K 0.3 Fe dots 0.008 0.006 0.04K 0.2 µ 0 H z (T) 0.1 0.004 0-0.1 0.002-0.3 0.0 0.3 Applied field O. Fruchart et al., J.

31 Micro-SQUIDs Examples Mode: flux monitoring Cold mode: switching monitoring T < 7K 0.3 Fe dots K 0.2 µ 0 H z (T) Applied field O. Fruchart et al., J. Magn. Magn. Mater , 228 (1999) µ 0 H y (T) W. Wernsdorfer et al., Phys. Rev. Lett. 78, 1791 (1997) M. Jamet et al., Phys. Rev. Lett., 86, 4676 (2001) Olivier Fruchart GdR Nanoalliages Lyon Jan p.31

32 Micro-SQUID - Overview Features Prospects / future Monitors flux or magnetization switching 3 Sensitivity 10 mb Improved resolution ; nanotube -SQUID Limited to low temperature Olivier Fruchart GdR Nanoalliages Lyon Jan p.32

Sensitivity 10 mb No temperature limitation Prospects Magnetization dynamics? M. Rahm et al.

33 Micro-Hall probes Working principle and example Local flux monitored via a nanofabricated micro-hall cross of a 2D electron gas Features Monitors flux 4 Sensitivity 10 mb No temperature limitation Prospects Magnetization dynamics? M. Rahm et al., Appl. Phys. Lett. 82, (2003) Olivier Fruchart GdR Nanoalliages Lyon Jan p.33

Magneto-transport Hard-disk-drive GMR heads GMR /

[CoPt]n/Si(111) using a hard-disk drive head on a translator

Moritz et al., Appl. Phys. Lett. 84, 1519 (2004) P. P. Freitas, Magnetoresistive sensors, J.

Matter 19, 165221 (2007) Also: direct magneto-transport:

34 Magneto-transport Hard-disk-drive GMR heads GMR / TMRelements for biochips Read-Write a patterned media [CoPt]n/Si(111) using a hard-disk drive head on a translator magnetic microscopy and manipulation Sensitivity: 10pT DC J. Moritz et al., Appl. Phys. Lett. 84, 1519 (2004) P. P. Freitas, Magnetoresistive sensors, J. Phys. Condens. Matter 19, (2007) Also: direct magneto-transport: GMR, Extraordinary Hall Effect (EHE) etc. Olivier Fruchart GdR Nanoalliages Lyon Jan p.34

35 TABLE OF CONTENTS (3/4) 1. Microscopy 2. Flux and direct local probes 3. Reciprocal space 2 1 fα ', β ' µ (α ', β ' ) = 2 α ' β ' M 4. Interactions in assemblies Hext β α Olivier Fruchart GdR Nanoalliages Lyon Jan p.35

Reflectometry and small angle scattering with X-rays Idea: probe depth and lateral magnetic

250nm wide AFM MFM, demagnetized state Off-specular reflectivity to probe lateral order K.

B 66, 024435 (2002) Probe lateral order on muchy smaller structures (no other microscopy

36 Reflectometry and small angle scattering with X-rays Idea: probe depth and lateral magnetic correlations with reflectivity and scattering Test case: perpendicularly-magnetized stripes, 250nm wide AFM MFM, demagnetized state Off-specular reflectivity to probe lateral order K. Chesnel et al., Phys. Rev. B 66, (2002) Probe lateral order on muchy smaller structures (no other microscopy tool available) Prospects Use self-organization as a tool to learn about magnetic domains at the nanoscale? Olivier Fruchart GdR Nanoalliages Lyon Jan p.36

37 Reflectometry and small angle scattering with polarized neutrons GISANS: Grazing Incidence Small Angle Neutron Scattering Test sample and geometry Results FeCoV/TiN neutron supermirror E. Kentzinger et al., Phys. Rev. B 77, (2008) Recent improvement in neutron flux Apply to nuried material and assembles of nanoparticles Olivier Fruchart GdR Nanoalliages Lyon Jan p.37

38 TABLE OF CONTENTS (4/4) 1. Microscopy 2. Flux and direct local probes 3. Reciprocal space 2 1 fα ', β ' µ (α ', β ' ) = 2 α ' β ' M 4. Interactions in assemblies Hext β α Olivier Fruchart GdR Nanoalliages Lyon Jan p.38

39 Estimating interactions between nano-objects in assemblies Hysteresis loops to estimate interactions in assemblies Olivier Fruchart GdR Nanoalliages Lyon Jan p.39

40 COLLECTIVE PROPERTIES Interactions and distributions Expected hysteresis loop for macrospins Hysteresis for assemblies of dots M M Hext Hext Possible effects Distribution of coercive fields (Dipolar, RKKY) interactions Olivier Fruchart GdR Nanoalliages Lyon Jan p.40

41 COLLECTIVE PROPERTIES Evaluation of interactions Preisach model Solving G. Biorci et al., Il Nuov. Cim. VII, 829 (1958) I. D. Mayergoyz, Mathematical models of hysteresis, Springer (1991) M Hext β α Distribution function µ (α, β ) with α > β No true link between real particles and β α 2 1 fα ', β ' µ (α ', β ' ) = 2 α ' β ' Long experiments (1D set of hysteresis curves) Better suited to bulk materials with strong interactions Olivier Fruchart GdR Nanoalliages Lyon Jan p.41

42 COLLECTIVE PROPERTIES Evaluation of interactions Henkel plots O. Henkel, Phys. Stat. Sol. 7, 919 (1964) S. Thamm et al., JMMM184, 245 (1998) Measure of dipolar interactions M H ( x) = M d ( x) [1 2 M r ( x)] Long experiments (ac demagnetization) Better physical meaning than Preisach Olivier Fruchart GdR Nanoalliages Lyon Jan p.42

43 COLLECTIVE PROPERTIES Evaluation of interactions Minor loops: negative interactions Example: dipolar interactions -15 mt -185 mt B (T) B (T) Minor loops: negligible interactions Faster than Henkel Other applications: characterization of exchange bias -17 mt -50 mt B (T) B (T) Olivier Fruchart GdR Nanoalliages Lyon Jan p.43

44 COLLECTIVE PROPERTIES Evaluation of interactions Superparamagnetic regime: plot of inverse susceptibility Brillouin 1/2 function m = B½ ( μ0 μco NH eff. / kt ) 0.16 Effective field y = x R = /χ 0.14 H eff. = H + r M s m (Demagnetizing dipolar interactions) 0.02 First order expansion: susceptibility 0 d(μ0h) 1 = = μ0 MSr + k T dm χ μ Co N a + b.t T(K) O. Fruchart et al., PRL 23, 2769 (1999) No need of hysteresis Analogy with Curie-Weiss law Olivier Fruchart GdR Nanoalliages Lyon Jan p.44

45 COLLECTIVE PROPERTIES Distribution of properties Distribution of properties M Reversible Irreversible Hext dm ρ (H r ) = dh irreversible Hc(T) for a given population of the distribution can be studied at a given stage of the reversal (10%, 20% etc.) Effect of distributions and dipolar interactions are sometimes difficult to disentangle Olivier Fruchart GdR Nanoalliages Lyon Jan p.45

46 Not included... MFM-based techniques High-resolution MFM MRFM (O. Klein et al., Saclay) NMR Other microscopy techniques Dichroism in TEM BEM: Ballistic Electron Microscope Direct transport techniques GMR Extraordinary Hall Effect Probes of electronic properties, excitations Spin-resolved Photo-emission and inverse photo-emission SPEELS: Spin-polarized electron energy loss spectroscopy Supra/I/Ferro to measure the spin polarization Discussion of the estimation of lateral resolution etc. Olivier Fruchart GdR Nanoalliages Lyon Jan p.46

47 SOME READING [1] Magnetic domains, A. Hubert, R. Schäfer, Springer (1999) [2] J. Stöhr, H. C. Siegmann, Magnetism, Springer (2006) [3] H. Kronmüller, S. Parkin Eds., Handbook of Magnetism and Advanced Magnetic Materials, vol.3: Noval techniques for characterizing and preparing samples, Wiley (2007). [4] H. Hopster, H. P. Oepen, Eds., Magnetic Microscopy of Nanostructures, Springer (2005). [5] E. Beaurepaire, F. Scheurer, G. Krill, J.-P. Kappler, Magnetism and Synchrotron Radiation, Lecture Notes in Physics, Springer (2001). [6] European School on Magnetism 2005: New experimental approaches to Magnetism, Olivier Fruchart GdR Nanoalliages Lyon Jan p.47

Contents. 1 Imaging Magnetic Microspectroscopy W. Kuch 1

Contents. 1 Imaging Magnetic Microspectroscopy W. Kuch 1 1 Imaging Magnetic Microspectroscopy W. Kuch 1 1.1 Microspectroscopy and Spectromicroscopy - An Overview 2 1.1.1 Scanning Techniques 2 1.1.2 Imaging Techniques 3 1.2 Basics 5 1.2.1 X-Ray Magnetic Circular