Self-assembling magnetic nano- particles for advanced applications Ovidiu Crisan a,, J. M. Grenéche c, M. Angelakeris b and a M. Angelakeris b and George Filoti a 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

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

Synthesis Decomposition of metallic precursors followed by transmetalation reaction Decomposition of metallic precursors followed by transmetalation reaction Wet (colloidal) chemistry technique and coating with organic surfactants Wet (colloidal) chemistry technique and coating with organic surfactants Ag 55 Co 45 and Ag 30 Co 70 bimetallic nano- particles dispersed in toluene Ag 55 Co 45 and Ag 30 Co 70 bimetallic nano- particles dispersed in toluene Co 2 (CO) 8 +AgClO 4 Ag+Co+CO+Co(ClO 4 ) 2

Basic of self assembling n-particles Allowed engineering of regular arrays of nano- entities onto very large sample areas 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 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 Using a suitable substrate for magnetic nano- arrays both the signal conditioning and the logistic capability can be used to optimize the system performance

Nanoparticles Morphology SEM: Ag 30 Co 70 dried on Si(100) substrate under applied field H H Formation of straight stripes of ~20 m length oriented along the applied field H Formation of straight stripes of ~20 m length oriented along the applied field H

Nanoparticles Morphology SEM: Ag 30 Co 70 dried on Si(100) substrate under rotating applied field H H Formation of winding stripes and round shapes when the applied field H rotates in the sample plane Formation of winding stripes and round shapes when the applied field H rotates in the sample plane

Nanoparticles Morphology Columnar growth of uniformly dispersed NPs Columnar growth of uniformly dispersed NPs Growth modes strongly dependent on the substrate nature Growth modes strongly dependent on the substrate nature AFM: Ag 30 Co 70 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. AFM: Ag 30 Co 70 on 7nm Pt / 200nm PMMA / Si patterned substrate substrate choice substrate choice prevention of clustering during self-assembly Patterning as factor of controlling 2D arrays Patterning as factor of controlling 2D arrays

Nanoparticles structure TEM images of Ag 30 Co 70 nanoparticles Mean size: Mean size: d = 18nm d = 18nm Distribution width: 12% Distribution width: 12% Relatively dispersed Relatively dispersed Narrow log-normal size distribution Narrow log-normal size distribution Multiphase (polycrystalline) nano-grains Multiphase (polycrystalline) nano-grains Bimetallic character with a (incomplete) core-shell structure Bimetallic character with a (incomplete) core-shell structure

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

Nanoparticles structure XRD of Ag 30 Co 70 on Si(100) Multiphase symmetry for Co (fcc and/or hcp) and for Ag (icosahedral and fcc) Multiphase symmetry for Co (fcc and/or hcp) and for Ag (icosahedral and fcc) Evidence of layering nanoparticles from small angle XRD Evidence of layering nanoparticles from small angle XRD Periodical 3D superlattice: 4.5 nm Periodical 3D superlattice: 4.5 nm Line profile from EDP of Ag 30 Co 70 Need of a quantitative model to account for multiple symmetries ? Need of a quantitative model to account for multiple symmetries ?

Magnetism of Ag 30 Co 70 Nanoparticles Lack of saturation even at 5.5 T Lack of saturation even at 5.5 T Small hysteresis at RT Small hysteresis at RT M influenced by surface spin disorder and/or finite size effects M influenced by surface spin disorder and/or finite size effects Shape of M(H) indicates two-phase behavior Shape of M(H) indicates two-phase behavior M(H) follows a Langevin law: M(H) follows a Langevin law: Co-existence of interacting SPM NPs and ferromagnetic clusters Co-existence of interacting SPM NPs and ferromagnetic clusters

Monte Carlo study of nanoparticles magnetic properties Isolated ferromagnetic nanoparticle Isolated ferromagnetic nanoparticle R = 6a (905 atoms) and R = 15a (14137 atoms) R = 6a (905 atoms) and R = 15a (14137 atoms) Heisenberg-type hamiltonian: Heisenberg-type hamiltonian: Periodic boundary conditions Periodic boundary conditions S i,j = 1; J ij = 1000; K V = 20; K s = ; S i,j = 1; J ij = 1000; K V = 20; K s = ; K V – uniaxial; K s – normal to the surface K V – uniaxial; K s – normal to the surface 10 5 Monte Carlo steps / spin / temperature 10 5 Monte Carlo steps / spin / temperature Spin configuration energy is minimized using a Metropolis algorithm Spin configuration energy is minimized using a Metropolis algorithm

MCS simulations throttled spin configuration R = 15a, K s /K V = 10: throttled spin configuration Surface magnetization reversal at equator Surface magnetization reversal at equator throttled spin configuration R = 15a, K s /K V = 60: throttled spin configuration Vortex-type reversal centers migrate towards lower hemisphere Vortex-type reversal centers migrate towards lower hemisphere

MCS simulations R = 15a: M(T) for different K s M as K s : reduced magnetization due to surface spin disorder M as K s : reduced magnetization due to surface spin disorder M(T C ) 0 features finite size effects M(T C ) 0 features finite size effects Instabilities in the transition region Instabilities in the transition region Sharp decrease of magnetization in the transition region Sharp decrease of magnetization in the transition region Overall magnetization strongly influenced by the surface spin configuration Overall magnetization strongly influenced by the surface spin configuration

MCS simulations R = 6a, K s /K V = 1 collinear R = 6a, K s /K V = 10 throttled R = 6a, K s /K V = 40 throttled (reversal centers) R = 6a, K s /K V = 60 hedgehog (M=0)

MCS simulations R = 6a : M(T) for different K s M as K s : surface spin disorder M as K s : surface spin disorder Increased finite size effects compared to Increased finite size effects compared to R = 15a Transition from collinear to throttled spin configuration K s /K V = 10 20: Transition from collinear to throttled spin configuration Transition from throttled to hedgehog spin configuration (M=0) K s /K V = 50: Transition from throttled to hedgehog spin configuration (M=0)

Conclusions - Perspectives Ag 30 Co 70 bimetallic nanoparticles: Exhibit different growth modes depending on substrates nature and depositing parameters Exhibit different growth modes depending on substrates nature and depositing parameters Self-assembly of NPs onto large 2D arrays imposed themselves for technological applications Self-assembly of NPs 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 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 in order to promote performing functional materials Their in situ as well as self-organized on substrates phase composition, magnetic and magneto-transport properties needs further investigations in order to promote performing functional materials

The PROJECT aims: (on self-assembling nano-particles) to develop a new generation of magnetic sensors 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 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 optimize systems 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 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 to promote an improved level of each partner professional abilities by reciprocal training

The PROJECT goals: (on new self-assembling nano-particles) to select new element-pairs for high performing magnetic sensors to select new element-pairs for high performing magnetic sensors to use alternative procedures in order to obtain the best self-organization of colloidal nano-particles on a single chip of magnetic sensors to use alternative procedures in order to obtain the 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 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 to search for a competitively low cost technology for very efficient bank-notes and credit card survey / check via complex functional devices

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