Selfassembling magnetic nanoparticles for advanced applications Ovidiu Crisana

Self-assembling magnetic nanoparticles for advanced applications Ovidiu Crisana, , J. M. Grenéchec, M. Angelakerisb and George Filoti 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 a

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

Synthesis Wet (colloidal) chemistry technique and coating with organic surfactants v Decomposition of metallic precursors followed by transmetalation reaction v Co 2(CO)8+Ag. Cl. O 4 Ag+Co+CO+Co(Cl. O 4)2 Ag 55 Co 45 and Ag 30 Co 70 bimetallic nanoparticles dispersed in toluene v

Basic of self assembling n-particles v v Allowed engineering of regular arrays of nanoentities onto very large sample areas v Extremely sensitive GMR and SDT effects exhibited by these nano-particles provide a detection with very high spatial resolution v Using a suitable substrate for magnetic nanoarrays 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 ~20 mm length oriented along the applied field H v Formation of straight stripes of

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

Nanoparticles Morphology 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 7 nm Pt / 200 nm PMMA / Si patterned substrate v Columnar growth of uniformly dispersed NP’s v substrate choice prevention of clustering v Growth modes strongly during self-assembly dependent on the substrate v Patterning as factor of controlling 2 D arrays nature

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

Nanoparticles structure High resolution TEM images of Ag. Co 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 v Both icosahedral (from MTP) and fcc symmetry co-exist for Ag v Co shells and Co single particles show fcc and/or hcp symmetry

Nanoparticles structure XRD of Ag 30 Co 70 on Si(100) Line profile from EDP of Ag 30 Co 70 v Evidence of layering nanoparticles from small angle XRD v Periodical 3 D superlattice: 4. 5 nm v Multiphase symmetry for Co (fcc and/or hcp) and for Ag (icosahedral and fcc) v Need of a quantitative model to account for multiple symmetries ?

Magnetism of Ag 30 Co 70 Nanoparticles v Lack of saturation even at 5. 5 T v Small hysteresis at RT v M influenced by surface spin disorder and/or finite size effects v Shape of M(H) indicates two-phase behavior v M(H) follows a Langevin law: v Co-existence of interacting SPM NP’s and ferromagnetic clusters

Monte Carlo study of nanoparticles magnetic properties v Isolated ferromagnetic nanoparticle R = 6 a (905 atoms) and R = 15 a (14137 atoms) v Heisenberg-type hamiltonian: v Periodic boundary conditions v Si, j = 1; Jij = 1000; KV = 20; Ks = 0. 2 2000; v KV – uniaxial; Ks – normal to the surface v 105 Monte Carlo steps / spin / temperature v Spin configuration energy is minimized using a Metropolis algorithm

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

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

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

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

Conclusions - Perspectives Ag 30 Co 70 bimetallic nanoparticles: v Exhibit different growth modes depending on substrates nature and depositing parameters Self-assembly of NP’s onto large 2 D arrays imposed themselves for technological applications v v Exhibit anomalous magnetic behavior driven by the multiphase character of the sample, finite size effects and surface spin disorder v. 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) v to develop a new generation of magnetic sensors v to process the self-organization of colloidal nanoparticles on a single chip of regular 2 D array of magnetic sensors v to optimize systems able to detect very small magnetic fields with very high spatial resolution v to allow mutual sharing of each partner facilities for deeper and faster research, promoting earlier results at level of functional materials v to promote an improved level of each partner professional abilities by reciprocal training

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

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