Self-assembling magnetic nano-particles for advanced applications

1. Self-assembling magnetic nano-particles for advanced applications Ovidiu Crisana,, J. M. Grenéchec, M. Angelakerisb and George Filotia a National Institute for Materials Physics, Bucharest, Romania b LPEC-CNRS UMR 6087, Université du Maine, Le Mans, France c Aristotle University, Dept. of Physics, Thessaloniki, Greece

2. NP’s may be obtained in 2D regular arrays or 3D super-lattices by self-assembly Special Nano-particles • Breakthrough in data storage, biomedicine, catalysis, nano-electronics • Nanometer scale confinement give rise to possible non-crystallographic symmetries for NP’s • Anomalous magnetic behavior driven by finite size effects and / or surface spin disorder

3. Synthesis • Wet (colloidal) chemistry technique and coating with organic surfactants • Decomposition of metallic precursors followed by transmetalation reaction Co2(CO)8+AgClO4 Ag+Co+CO+Co(ClO4)2 • Ag55Co45 and Ag30Co70 bimetallic nano-particles dispersed in toluene

4. Basic of self assembling n-particles • Allowed engineering of regular arrays of nano-entities onto very large sample areas • Extremely sensitive GMR and SDT effects exhibited by these nano-particles provide a detection with very high spatial resolution • Using a suitable substrate for magnetic nano-arrays both the signal conditioning and the logistic capability can be used to optimize the system performance

5. H Nanoparticles Morphology SEM: Ag30Co70 dried on Si(100) substrate under applied field H • Formation of straight stripes of ~20 mm length oriented along the applied field H

6. H Nanoparticles Morphology SEM: Ag30Co70 dried on Si(100) substrate under rotating applied field H • Formation of winding stripes and round shapes when the applied field H rotates in the sample plane

7. Nanoparticles Morphology AFM: Ag30Co70 on 7nm Pt / 200nm PMMA / Si patterned substrate AFM: Ag30Co70 on: a) Si(100) wafer b) Co/Pt multilayer deposited on Si c) 80 nm Pd thin film on kapton d) 100 nm Pt on Si patterned substrate. • substrate choice prevention of clustering during self-assembly • Columnar growth of uniformly dispersed NP’s • Growth modes strongly dependent on the substrate nature • Patterning as factor of controlling 2D arrays

8. Nanoparticles structure • Relatively dispersed • Narrow log-normal size distribution • Bimetallic character with a (incomplete) core-shell structure • Multiphase (polycrystalline) nano-grains TEM images of Ag30Co70 nanoparticles • Mean size: d = 18nm • Distribution width: 12%

9. Nanoparticles structure High resolution TEM images of AgCo nanoparticles Ag core and Co as incomplete shell Single-crystalline hcp Co particle Ag core with (111) twin and Co patches as shell Single-crystalline five-fold twinned Ag particle • Both icosahedral (from MTP) and fcc symmetry co-exist for Ag • Co shells and Co single particles show fcc and/or hcp symmetry

10. Nanoparticles structure XRD of Ag30Co70 on Si(100) Line profile from EDP of Ag30Co70 • Evidence of layering nanoparticles from small angle XRD • Periodical 3D superlattice: 4.5 nm • Multiphase symmetry for Co (fcc and/or hcp)and for Ag (icosahedral and fcc) • Need of a quantitative model to account for multiple symmetries ?

11. Magnetism of Ag30Co70Nanoparticles • Lack of saturation even at 5.5 T • Small hysteresis at RT • M influenced by surface spin disorder and/or finite size effects • Shape of M(H) indicates two-phase behavior • M(H) follows a Langevin law: • Co-existence of interacting SPM NP’s and ferromagnetic clusters

12. Monte Carlo study of nanoparticles magnetic properties • Isolated ferromagnetic nanoparticle R = 6a(905 atoms) and R = 15a (14137 atoms) • Heisenberg-type hamiltonian: • Periodic boundary conditions • Si,j = 1; Jij = 1000; KV = 20; Ks = 0.2  2000; • KV – uniaxial; Ks – normal to the surface • 105 Monte Carlo steps / spin / temperature • Spin configuration energy is minimized using a Metropolis algorithm

13. MCS simulations • R = 15a, Ks/KV = 10:throttled spin configuration • R = 15a, Ks/KV = 60:throttled spin configuration • Vortex-type reversal centers migrate towards lower hemisphere • Surface magnetization reversal at equator

14. R = 15a: M(T) for different Ks MCS simulations • M  as Ks : reduced magnetization due tosurface spin disorder • M(TC)  0 featuresfinite size effects • Instabilities in the transition region • Sharp decrease of magnetization in the transition region • Overall magnetization strongly influenced by the surface spin configuration

15. MCS simulations R = 6a, Ks/KV = 1 collinear R = 6a,Ks/KV = 10 throttled R = 6a, Ks/KV = 40 throttled (reversal centers) R = 6a, Ks/KV = 60 hedgehog (M=0)

16. R = 6a : M(T) for different Ks MCS simulations • M  as Ks :surface spin disorder • Increasedfinite size effects compared to R = 15a • Ks/KV = 10  20: Transition from collinear to throttled spin configuration • Ks/KV = 50: Transition from throttled to hedgehog spin configuration (M=0)

17. Conclusions - Perspectives Ag30Co70 bimetallic nanoparticles: • Exhibit different growth modes depending on substrates nature and depositing parameters • Self-assembly of NP’s onto large 2D arrays imposed themselves for technological applications • Exhibit anomalous magnetic behavior driven by the multiphase character of the sample, finite size effects and surface spin disorder • Their ‘in situ’ as well as self-organized on substrates phase composition, magnetic and magneto-transport properties needs further investigations inorder to promote performing functional materials

18. The PROJECT aims: (on self-assemblingnano-particles) • to develop a new generation of magnetic sensors • to process the self-organization of colloidal nano-particles on a single chip of regular 2D array of magnetic sensors • to optimizesystems able to detect very small magnetic fields with very high spatial resolution • to allow mutual sharing of each partner facilities for deeper and faster research, promoting earlier results at level of functional materials • to promote an improved level of each partner professional abilities by reciprocal training

19. The PROJECT goals: (on new self-assemblingnano-particles) • to select new element-pairs for high performing magnetic sensors • to use alternative procedures in order to obtainthe best self-organization of colloidal nano-particles on a single chip of magnetic sensors • to define the most suitable support which provides the highest spatial resolution • to search for a competitively low cost technology for very efficient bank-notes and credit card survey / check via complex functional devices

20. PARTNERS • ESTABLISHED: • National Institute for Materials Physics, Bucharest • LPEC-CNRS UMR 6087, Université du Maine, Le Mans, • Aristotle University, Dept. of Physics, Thessaloniki, • POTENTIALLY……... • Science ofMaterialsInstitute, Zaragoza • University of Padova, Metal-organic Chemistry • Institutul de Chimie, Chisinau • ICPE- CA and IMT, both in Bucharest • ICF-Bucharest + ICM- Iassy (both Romanian Academy) • CN-IS-FC- University of Timisoara • MAVILOR-motors, Barcelona and Pro-Auto - Bucharest (both are SME)