Magnetic ordering in two-dimensional. nanoparticle assemblies

Transcription

1 Magnetic ordering in two-dimensional nanoparticle assemblies Pedro Zeijlmans van Emmichoven Faculty of Science, Utrecht University Leiden, June 18 th, 2007

2 Collaborators Mirela Georgescu Mark Klokkenburg Ben Erné Daniël Vanmaekelbergh Peter Liljeroth

3 Outline Introduction Experiments and simulations - Magnetite - Cobalt ferrite Conclusions

4 Introduction Goal: understand fundamental interactions in 2D assemblies of magnetic nanoparticles Two systems: I. 21-nm magnetite (Fe 3 O 4 ) particles - Single domain with large magnetic moment - Superparamagnetic II. 21-nm cobalt ferrite (CoFe 2 O 4 ) particles - Single domain - Cubic anisotropy - Anisotropy energy small at room temperature, large at low temperature (related to k B T) 6.7-nm CoFe 2 O 4 Liu et al., Pure Appl. Chem. 72 (2000) 37

5 I. Superparamagnetic magnetite Dipole-dipole interactions E dd ~ -500 mev II. Cobalt ferrite (cubic anisotropy) Room temperature Low temperature

6 Experiments and simulations Sample preparation - Wet chemical preparation of nanoparticles - Particles capped with oleic acid - Drop casting on graphite (HOPG) - 2D islands of nanoparticles Measurements - Scanning probe methods - Ultra high vacuum < torr - Temp. 25 K 1000 K Simulations - Monte Carlo

7 Non-contact Atomic Force Microscopy Principles Sample df Forces z A V z Detector Scanner tube LASER Van der Waals Repulsive magnetic Attractive magnetic VdW + repulsive magn. Oscillate tip at resonance frequency f 0 Close to sample: force field shifts f 0 by df Keep df constant and scan sample: Topography Scan at large distance: Magnetic image Force spectroscopy

8 Results for magnetite Topography Magnetic image 1x1 µm 2 ; Inset 170x130 nm 2 Room temperature Dark: 0 nm Bright: 25 nm Large islands ( particles) Single monolayer Tip-substrate distance: 70 nm Substrate: force ~ 0 Attraction with small contrast Dark: -5 Hz Bright: 0 Hz

9 Force spectroscopy (Force vs. distance) x1 µm 2 ; Room temperature 1 df (Hz) z (nm) Observations: Above substrate: weak attraction (1; VanderWaals) Above island: strong attraction (6; magnetic) At edge of island: repulsion (2-3-4; magnetic)

10 Interpretation On top of island: strong attraction - Tip very close (large field) - Dipoles reoriented tip Edge of island: repulsion - Tip-particle distance large (small field) - Strong dipole-dipole interactions: minimum energy configuration (blocking)

11 Monte-Carlo simulations Nanoparticles fixed on substrate Calculate minimum energy - Start with arbitrary configuration of dipoles and vary them while minimizing energy - Energy = sum of all dipole-dipole energies - Stop when minimum is found Experiment Simulation Results: - Dipoles in plane - Flux-closure configuration - E = -650 mev/particle M. Georgescu et al., Phys. Rev. B 73, (2006)

12 Experimental and calculated Force Experiment Simulation Image D shows, for comparison, flux closure in hexagonal arrangement of particles Solutions not unique

13 Results for cobalt ferrite Experiments at room temperature Topography Force spectroscopy Dark: 0 nm Bright: 25 nm 500x500nm 2 Magnetic image Tip-substr. dist. 40 nm Dark: -2 Hz Bright: 0.2 Hz

14 Experiments at low temperature Temperature 100 K Topography Magnetic image 700x400nm 2 Bright: repulsion; dark: attraction

15 Experiments on small islands (~25 nanoparticles) Temperature 100 K Topography Spectroscopy 100nm Magnetic image nm Bright: repulsion; dark: attraction df [Hz] z [nm]

16 Monte-Carlo simulations Minimize energy Room temperature: - anisotropy energy small - dipole-dipole interactions Low temperature: - large cubic anisotropy - Dipole-dipole interactions and anisotropy energy Calculate arrangements of dipoles and MFM images

17 Simulations at room temperature E dd = ev E dd = ev E dd = ev Flux-closure arrangements Large contrast at edges ( escaping field lines)

18 Simulations at low temperatures Room temperature Temp. 100 K Room temperature E dd = ev Temp. 100 K E dd = -7.9 ev E dd = ev Temp. 100 K E dd = -8.8 ev E dd = -8.8 ev

19 Comparison with experiments 100nm Experiments, above islands: Areas with small contrast Areas with repulsion/attraction Simulations, above islands: Areas with flux-closure type arrangements Areas where moments point away from flux closure

20 Conclusions Superparamagnetic nanoparticles: moments arrange in flux-closure structures Nanoparticles with large anisotropy: only partial flux closure