1. Synthesis and Characterization of Nanoparticulate Magnetic Materials Georgia C. Papaefthymiou BuildMoNa Workshop University of Leipzig Leipzig, Germany October 28-29, 2010 Villanova University, Villanova, PA 19085

2. Outline 1.General concepts on the synthesis, stabilization and assembly of magnetic nanoparticles; examples 2.Fundamentals of nanoparticle magnetism 3.Macroscopic vs. microscopic magnetic characterization of Fe-based magnetic nanostructures; SQUID magnetometry, Mössbauer spectroscopy 4.Isolated vs. interacting magnetic nanoparticles 5.Conclusion 6.Acknowledgements

3. Top → Down 100 nm1 nm Macroscopic Mesoscopic Microscopic Bulk Classical behavior Nanoparticles Quantum-size effects Molecules Quantum behavior Up ← Bottom Synthesis by Chemical Methods 1.High Energy Ball Milling 2.Laser Ablation 3.Ion Sputtering 4.Thermal Evaporation 5.etc……. 1.Reduction of Metal Salts in Solution 2.Thermal Decomposition Reactions 3.Hydrolysis in Aqueous Solutions 4.Hydrolysis in Nonaqueous Solutions 5.etc…….. Synthesis by Physical Methods Metal and Metal-Alloy Nanoparticles Metal and Metal-Oxide Nanoparticles

4. Nanostructured nanoparticle matrix Abundance of grain boundariesAbundance of interfaces Nanocomposite H. Gleiter, Acta Mater. 48, 1 (2000) Nanoparticulate Magnetic Materials Novel magnetic properties engineered through tailoring of the grain boundary or interfacial region and through interparticle magnetic interactions. Particles can interact via short-range magnetic exchange through grain boundaries or long-range dipolar magnetic interactions.

5. r rcrc r0r0 + 0  G n = 4  r 2  G s  (4/3)  r 3  G v GnGn General Concepts in Nucleation and Growth of Magnetic Nanoparticles Nucleation and Critical Radii Variation of Gibb’s free energy of nucleation with cluster radius during synthesis. r c is the kinetic critical radius and r 0 the thermodynamic critical radius Stabilization of nanoclusters of various size requires a competitive reaction chemistry between core cluster growth and cluster surface passivation by capping ligands that arrests further core growth. V. K. LaMer and R. H. Dinegar, J. Am. Chem. Soc. 72 (1950) 4847

6. Supramolecular Clusters Controlled hydrolytic polymerization of iron. Iron-core growth arrested via surface passivation with benzoate ligands. Observation of novel magnetic behavior. ~1 nm Fe 16 MnO 10 (OH) 10 (O 2 CPh) 20 Fe 11 O 6 (OH) 6 (O 2 CPh) 15. 6THF ~2 nm G. C. Papaefthymiou, Phys. Rev. B 46 (1992) 10366

7. Block Copolymer Nanotemplates Principles of synthesis Blocks of sequences of repeat units of one homopolymer chemically linked to blocks of another homopolymer sequence. Microphase separation due to block incompatibility or crystallization of one of the blocks. Templates for synthesis and arraying of metal oxide nanoclusters within space confined nanoreactors 0 - 21 %21 - 34 %34 - 38 %38 - 50 % Increasing Volume Fraction of Minority Component B-Block A-Block Chemical Link

8. Cobalt Ferrite Nanocluster Formation within Block Copolymers G.C. Papaefthymiou, S.R. Ahmed and P. Kofinas Rev. Adv. Mater. Sci. 10 (2005) 306

9. Transmission Electron Microscopy Morphology of block copolymer films: ensemble of polydispersed CoFe 2 O 4 nanoparticles, oval in shape and of average diameter of 9.6 ± 2.8 nm. CoFe 2 O 4 Block Copolymer Films Ahmed, Ogal, Papaefthymiou, Ramesh and Kofinas, Appl. Phys. Letts 80 (2002) 1616

10. Self-assembly within Protein Cages: Ferritin Iron Mineralization in Ferritin ferrihydrite 2Fe 2+ + O 2 + 4H 2 O →2FeOOH (core) + H 2 O 2 + 4H + (1) 4Fe 2+ + O 2 + 6H 2 O →4FeOOH (core) + 8H + (2) 2Fe 2+ + H 2 O 2 + 2H 2 O →2FeOOH (core) + 4H + (3) Demineralization followed by metathesis mineralization leads to biomimetic synthesis of various nanoscale particles. A large number of nanostructures and mono-layer films on various supports have been produced including metal oxide (Fe 3 O 4, Co 3 O 4 ), iron sulfide, metallic (Co, Mn, U, Co/Pt, Ni, Cr, Ag) and semi-conducting (CdS, CdSe) structures, and FeOOH(MO4)x, where M=P, As, Mo or V. FerritinApoferritin → ←7nm → 24 amino acid subunits form a robust protein cage → The Ferroxidase and Nucleation sites of Human H-chain Ferritin →

11. Two-dimensional Array of Ferritin I. Yamashita Thin Solid Films, 391 (2001) 12 Ensemble of monodispersed magnetic nanoparticles

12. Monodispersed γ-Fe 2 O 3 nanoparticles Thermal decomposition of iron pentacarbonyl, Fe(CO) 5, in the presence of oleic acid produced monodispersed metal iron particles. Controlled oxidation using trimethylamine oxide, (CH 3 ) 3 NO, as a mild oxidant produced highly crystalline γ-Fe 2 O 3 particles. The particles were in the size range 4 nm to 16 nm diameter depending on experimental conditions. Highly uniform, oleic acid covered, magnetic nanoparticles of γ-Fe 2 O 3, ~(11.8 ± 1.3) nm diameter are shown. XRD patterns confirm the presence of Fe 2 O 3. D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying, Chem. Mater. 18 (2006) 614

13. Schematic of the synthesis of MP/SiO 2 /MS nanoarchitectures D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying, Chem. Mater. 18 (2006) 614 MP = Magnetic Particle SiO 2 =Solid Silica MS = Mesoporous Silica

14. Solid-silica coated γ-Fe 2 O 3 nanoparticles TEM micrographs of ~12 nm γ-Fe 2 O 3 particles covered with solid silica shell. Shell thickness from 1.8 nm to 25 nm was achieved. Scale bar 20 nm D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying, Chem. Mater. 18 (2006) 614

15. TEM micrographs of γ-Fe 2 O 3 core- solid silica shell-mesoporous silica shell nanocomposites ~ 12 nm maghemite particles were used as templates (a)A thick mesoporous layer (~21nm) was obtained using a mixture of TEOS and C18TMS, 260 μl and (b)a thinner mesoporous layer (~10nm) was obtained using a mixture of TEOS and C18TMS, 120 μl. In both cases, (a) and (b), ca. 25 nm solid silica shell coated Fe 2 O 3 core- solid silica shell nanocomposites were used as templating cores. Higher Nanoarchitectures

16. Fundamentals of Magnetic Ordering Direct Exchange Indirect Exchange Magnetic ordering in solids is due to Quantum Mechanical Exchange and the Pauli Exclusion Principle Bethe-Slater Curve Curie temp T c in °C, Iron (Fe) 770, Cobalt (Co) 1130, Nickel (Ni) 358, Iron Oxide (Fe 2 O 3 ) 622

17. Magnetic Anisotropy Uniaxial Magnetic Anisotropy Minimization of magnetostatic energy Moment rotation at a Bloch Wall Bulk Co in its demagnetized, multi-domain state ↘ Anisotropy Field Exchange energy per unit area of Bloch wallwhere for a simple cubic lattice with lattice constant a. leads to domain wall formation

18. Process of Magnetic Saturation of a Multi-domain Particle Hysteresis Loop The hysteresis loop defines the technological properties of the magnetic material Saturation Magnetization Remnant Magnetization Coercivity Hard process in a single domain system Easy process in a multi- domain system

19. Critical Size for SMD Particles ← ← Magnetostatic vs. wall energy as a function of particle size for a spherical particle of radius r Below R c the particle is a Single Magnetic Domain, and thus permanently magnetized. The demagnetized state cannot be formed. R c ~ 100 nm

20. Coercivity as a function of particle size F. E. Luborsky J. Appl. Phys.32 (1961) S171

21. DpDp DsDs Particle Diameter D Unstable SP S-D Nanomagnetism: Coercivities and Spin Reversal Mechanisms M-D Single-magnetic domain particle Coherent spin rotation HcHc 0 Multi-magnetic domain structure Magnetic wall movement Bulk K ~10 3 J/m 3 Nanoparticle K ~ 10 5 J/m 3 Maximum coercivity

22. F. Bødker, S. Mørup, S. Lideroth, Phys. Rev. Lett. 72 (1994) 282 Origin of magnetic anisotropy enhancement in nanoparticles c = core s = surface σ = stress sh = shape

23. Nanoparticle coercivity for coherent spin rotation (Stoner and Wohlfarth model) Maximum coercivity for coherent spin rotation of a single magnetic domain particle with uniaxial total effective anisotropy E.C. Stoner, E.P. Wohlfarth, Trans. Roy. Soc. Lond. A 240 (1948) 599 coherent moment rotation

24. Spin Dynamics in Magnetic Nanoparticles Temperature dependence of coercivity Superparamagnetic relaxation time Due to fast moment reversals at elevated temperatures the internal magnetic order of the particle escapes detection. You must either lower the temperature or use ultrafast measuring techniques that can record the moment before it flips. (thermally assisted spin reversals) Easy axis

25. Superparamagnetism of Small Magnetic Particles Relaxation Time t RELAX = t 0 exp (K u V/kΤ) Energy barrier Δ E = K u V where K u is the effective uniaxial magnetic anisotropy Energy density and V is the particle volume Observe net magnetic moment when t MEAS < t RELAX Magnetocrystalline Anisotropy Shape Anisotropy Surface effects

26. Micro-magnetics and Spin Dynamics - Mössbauer spectroscopic measurements Probe local magnetic moments and internal magnetic fields, with a response time of  m =  Möss = 10 ns -DC Magnetization measurements Probe global magnetic properties in an applied field, with a response time of τ m = τ SQUID = 10 s

27. Hysteresis Loops for CoFe 2 O 4 Block Copolymers The temperature at which the coercivity vanishes defines the blocking temperature T B for SQUID magnetometry. Ahmed, Ogal, Papaefthymiou, Ramesh and Kofinas, Appl. Phys. Letts 80 (2002) 1616 Hysteresis due to particle moment rotation away from the particle’s easy axis to the direction of the applied magnetic field.

28. Isomer Shift Interaction of the nuclear charge distribution with the electron cloud surrounding the nuclei in both the absorber and source. Quadrupole Splitting Interaction of the nuclear electric quadrupole moment with the EFG and the nucleus Observed EffectIllustration Observed Spectrum 0 0 v v Nuclear Hyperfine Interactions with Mössbauer Spectroscopy Zeeman Effect (Dipole Interaction) Interaction of the nuclear magnetic dipole moment with the internal magnetic field on the nucleus. 0 I(v) v

29. Velocity (mm/s) Mössbauer spectra of lyophilized, in vitro reconstituted HoSF ferritin. 80 K 40 K 30 K 25 K 4.2 K Modeling Dynamical Spin Fluctuations in Isolated Nanostructures G. C. Papaefthymiou, et. al. MRS Symp. Proc. Fall 2007 Experimentally the temperature at which the Mössbauer spectra pass from magnetic, six- line spectra to paramagnetic or quadrupolar, two-line spectra defines T B for Mössbauer Theoretically T B is defined by: Spectrum Key Magenta: spectral signature of magnetic particle core (internal iron sites) Green: spectral signature of surface layers (surface iron sites) → G. C. Papaefthymiou, Biochim. Biophys. Acta 1800 (2010) 886 Determination of Blocking Temperature T B = 40 K

30. Zero-field cooled and field-cooled magnetization of lyophilized HoSF ferritin Typical ZFC/FC behavior of an ensemble of magnetically isolated superparamagnetic particles 25-nm thick protein shell FC ZFC Note: Saturation magnetization is ~ 0.05 emu/g, weakly magnetic.

31. Determination of K u for an ensemble of superparamagnetic nanoparticles 1.Determine average particle volume by TEM 2.Determine T B with two different techniques, whose measuring response times lie in different time windows 3.Use the Arrhenius equation above to determine τ 0 and K u

32. Collective magnetic excitations below T B Surface Effects:Temperature Dependence of Mössbauer Magnetic Hyperfine Fields S. Mørup and H. Topsøe, Appl. Phys. 11 (1976) 63 CME model, double potential well complex potential energy landscape at the surface Velocity (mm/s) 4.2 K 25 K 30 K 40 K 80 K

33. Mössbauer Spectra of γ-Fe 2 O 3 /Solid Silica Nanoarchitectures Bare 12 nm particles 12 nm particles with 25 nm SiO 2 shell G.C. Papaefthymiou et. al. Phys. Rev. B 80 (2009) 024406 Spectral Key: Blue A-sites, Green B-sites of spinel structure

34. Effect of silica shell on the RT Mössbauer Spectra Bare γ-Fe 2 O 3 nanoparticles γ-Fe 2 O 3 nanoparticles with 4 nm silica shell γ-Fe 2 O 3 nanoparticles with 25 nm silica shell Behavior typical of strongly interacting particles G.C. Papaefthymiou et. al. Phys. Rev. B 80 (2009) 024406

35. Magnetization of γ-Fe 2 O 3 /Solid Silica/Mesoporous Silica Nanoarchitectures A-bare * B-4 nm (S) C-25 nm (S) D-25 nm (S) + 10 nm (MS) E-25 nm (S) +21 nm (MS) Typical behavior of strongly interacting magnetic nanoparticles, spin-glass-like systems. * Bare particles are covered with a very thin layer (~1 nm) of oleic acid. Saturation magnetization of the order of ~ 8 emu/g, strongly magnetic

36. Conclusion Ferrihydrite is an antiferro-magnet. Magnetization of ferritin is due to uncompensated spins at the surface → Weak magnetism. Protein coat of only 2.5 nm thickness sufficient to magnetically isolate the ferritin iron cores Maghemite is a ferri -magnet due to uncompensated spin sublattices in its spinel structure. In small particles uncompensated spins at the surface also contribute → Strong magnetism. Silica coat of 23 nm thickness insufficient to isolate the γ-Fe 2 O 3 cores Dipole-dipole interaction~

37. Acknowledgements Steve Lippard, MIT Peter Kofinas, University of Maryland Dennis Chasteen, University of New Hampshire Jackie Ying, IBN Singapore Eamonn Devlin, NCSR Demokritos, Greece NSF, EU/Marie-Curie