Two-, Three-, and Four-Component Magnetic Multilayer Onion Nanoparticles Based on Iron Oxides and Manganese Oxides

http://pubs.acs.org/doi/abs/10.1021/ja205810t
Journal of the American Chemical Society, 2011, 133, 16738–16741

German Salazar-Alvarez, Hans Lidbaum, Alberto López-Ortega, Marta Estrader, Klaus Leifer, Jordi Sort, Santiago Suriñach, Maria Dolors Baró, and Josep Nogués 

DOI: 10.1021/ja205810t

Magnetic multilayered, onion-like, heterostructured nanoparticles are interesting model systems for studying magnetic exchange coupling phenomena. In this work, we synthesized heterostructured magnetic nanoparticles composed of two, three, or four components using iron oxide seeds for the subsequent deposition of manganese oxide. The MnO layer was allowed either to passivate fully in air to form an outer layer of Mn₃O₄ or to oxidize partially to form MnO|Mn₃O₄ double layers. Through control of the degree of passivation of the seeds, particles with up to four different magnetic layers can be obtained (i.e., FeO|Fe₃O₄|MnO|Mn₃O₄). Magnetic characterization of the samples confirmed the presence of the different magnetic layers.

A transparent hybrid of nanocrystalline cellulose and amorphous calcium carbonate nanoparticles

http://pubs.rsc.org/en/Content/ArticleLanding/2011/NR/c1nr10681c

Nanoscale, 2011, 3, 3563-3566

Denis Gebauer, Vitaliy Oliynyk, Michaela Salajkova, Jordi Sort, Qi Zhou, Lennart Bergström and German Salazar-Alvarez 

DOI: 10.1039/C1NR10681C 

 Abstract:
Nanocellulose hybrids are promising candidates for biodegradable multifunctional materials. Hybrids of nanocrystalline cellulose (NCC) and amorphous calcium carbonate (ACC) nanoparticles were obtained through a facile chemical approach over a wide range of compositions. Controlling the interactions between NCC and ACC results in hard, transparent structures with tunable composition, homogeneity and anisotropy.

Hamaker Constants of Iron Oxide Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/la201387d

Langmuir, 2011, 27 (14), pp 8659–8664
Bertrand Faure, German Salazar-Alvarez, and Lennart Bergström

DOI: 10.1021/la201387d

Abstract:

The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory. Expressions for the dielectric responses of three iron oxide phases (magnetite, maghemite, and hematite) were derived from recently published optical data. The nonretarded Hamaker constants for the iron oxide nanoparticles interacting across water, A_1w1 = 33 – 39 zJ, correlate relatively well with previous reports, whereas the calculated values in nonpolar solvents (hexane and toluene), A₁₃₁ = 9 – 29 zJ, are much lower than the previous estimates, particularly for magnetite. The magnitude of van der Waals interactions varies significantly between the studied phases (magnetite < maghemite < hematite), which highlights the importance of a thorough characterization of the particles. The contribution of magnetic dispersion interactions for particle sizes in the superparamagnetic regime was found to be negligible. Previous conjectures related to colloidal stability and self-assembly have been revisited on the basis of the new Lifshitz values of the Hamaker constants.

[OPEN ACCESS] Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes

http://pubs.acs.org/doi/abs/10.1021/nl200126v [OPEN ACCESS]

Nano Lett., 2011, 11 (4), pp 1651–1656
Sabrina Disch, Erik Wetterskog, Raphaël P. Hermann, German Salazar-Alvarez, Peter Busch, Thomas Brückel, Lennart Bergström, and Saeed Kamali

DOI: 10.1021/nl200126v

Abstract:

Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.

Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates

http://www.nature.com/nnano/journal/v5/n8/abs/nnano.2010.155.html
Nature Nanotechnology 5, 584–588 (2010)
R. T. Olsson, M. A. S. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L. A. Berglund, O. Ikkala, J. Nogués & U. W. Gedde

doi:10.1038/nnano.2010.155

Abstract:
Nanostructured biological materials inspire the creation of materials with tunable mechanical properties^{1, 2, 3}. Strong cellulose nanofibrils derived from bacteria⁴ or wood^{5, 6} can form ductile or tough networks^{7, 8} that are suitable as functional materials^{9, 10}. Here, we show that freeze-dried bacterial cellulose nanofibril aerogels can be used as templates for making lightweight porous magnetic aerogels, which can be compacted into a stiff magnetic nanopaper. The 20–70-nm-thick cellulose nanofibrils act as templates for the non-agglomerated growth of ferromagnetic cobalt ferrite nanoparticles¹¹ (diameter, 40–120 nm). Unlike solvent-swollen gels¹² and ferrogels^{13, 14, 15}, our magnetic aerogel is dry, lightweight, porous (98%), flexible, and can be actuated by a small household magnet. Moreover, it can absorb water and release it upon compression. Owing to their flexibility, high porosity and surface area, these aerogels are expected to be useful in microfluidics devices and as electronic actuators.

Size-Dependent Passivation Shell and Magnetic Properties in Antiferromagnetic/Ferrimagnetic Core/Shell MnO Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/ja1021798

J. Am. Chem. Soc., 2010, 132 (27), pp 9398–9407
Alberto López-Ortega, Dina Tobia, Elin Winkler, Igor V. Golosovsky, German Salazar-Alvarez, Sònia Estradé, Marta Estrader, Jordi Sort, Miguel Angel González, Santiago Suriñach, Jordi Arbiol, Francesca Peiró, Roberto D. Zysler, Maria Dolors Baró, Josep Nogués

DOI: 10.1021/ja1021798

Abstract:

The magnetic properties of bimagnetic core/shell nanoparticles consisting of an antiferromagnetic MnO core and a ferrimagnetic passivation shell have been investigated. It is found that the phase of the passivation shell (γ-Mn₂O₃ or Mn₃O₄) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly γ-Mn₂O₃ shell, larger ones have increasing amounts of Mn₃O₄. A considerable enhancement of the Nel temperature, T_N, and the magnetic anisotropy of the MnO core for decreasing core sizes has been observed. The size reduction also leads to other phenomena such as persistent magnetic moment in MnO up to high temperatures and an unusual temperature behavior of the magnetic domains.

Direct evidence of imprinted vortex states in the antiferromagnet of exchange biased microdisks

http://apl.aip.org/resource/1/applab/v95/i1/p012510_s1
Appl. Phys. Lett. 95, 012510 (2009)
G. Salazar-Alvarez, J. J. Kavich, J. Sort, A. Mugarza, S. Stepanow, A. Potenza, H. Marchetto, S. S. Dhesi, V. Baltz, B. Dieny, A. Weber, L. J. Heyderman, J. Nogués, and P. Gambardella doi:10.1063/1.3168515

Abstract:
The magnetic domain structure of patterned antiferromagnetic/ferromagnetic Ir₂₀Mn₈₀/Ni₈₀Fe₂₀ bilayer microdisk arrays has been investigated using layer-specific polarized x-ray photoemission electron microscopy and magnetic circular dichroism. Magnetic imaging at the Fe and Mn L-edge resonances provided direct evidence of a vortex state imprinted into the antiferromagnet at the interface. The opposite magnetic contrast between the layers indicated a reversed chirality of the imprinted vortex state, and a quantitative analysis of the magnetic moment from the dichroism spectra showed that uncompensated Mn spins equivalent to about 60% of a monolayer of bulk Ir₂₀Mn₈₀ contributed to the imprinted information at the interface.

Magnetic Proximity Effect Features in Antiferromagnetic/Ferrimagnetic Core-Shell Nanoparticles

Phys. Rev. Lett. 102, 247201 (2009)

URL:

http://link.aps.org/doi/10.1103/PhysRevLett.102.247201

I. V. Golosovsky, G. Salazar-Alvarez, A. López-Ortega, M. A. González, J. Sort, M. Estrader, S. Suriñach, M. D. Baró, and J. Nogués DOI:

10.1103/PhysRevLett.102.247201

Abstract:
A study of “inverted” core-shell, MnO/γ-Mn₂O₃, nanoparticles is presented. Crystal and magnetic structures and characteristic sizes have been determined by neutron diffraction for the antiferromagnetic core (MnO) and the ferrimagnetic shell (γ-Mn₂O₃). Remarkably, while the MnO core is found to have a T_N not far from its bulk value, the magnetic order of the γ-Mn₂O₃ shell is stable far above T_C, exhibiting two characteristic temperatures, at T∼40  K [T_C(γ-Mn₂O₃)] and at T∼120  K [∼T_N(MnO)]. Magnetization measurements are consistent with these results. The stabilization of the shell moment up to T_N of the core can be tentatively attributed to core-shell exchange interactions, hinting at a possible magnetic proximity effect.

Cold Consolidation of Metal–Ceramic Nanocomposite Powders with Large Ceramic Fractions

http://onlinelibrary.wiley.com/doi/10.1002/adfm.200800456/abstract

Advanced Functional Materials

Volume 18, Issue 20, pages 3293–3298, October 23, 2008

Enric Menéndez, German Salazar-Alvarez, Alexander P. Zhilyaev, Santiago Suriñach, Maria Dolors Baró, Josep Nogués, Jordi Sort

DOI: 10.1002/adfm.200800456

Abstract

Co/α-Al₂O₃ powder mixtures (5, 10, 20, 30, 40, and 50 mass % of α-Al₂O₃) have been ball-milled and, subsequently, consolidated at room temperature by means of a high pressure torsion procedure in order to produce bulk nanostructured composites. For mixtures up to 20% of α-Al₂O₃, the cold-compaction results in roughly fully dense disks with relatively high microhardness values. However, the compaction for 30, 40, and 50% of α-Al₂O₃ is less effective, resulting in a reduction of the microhardness although a structure in the nanoscale range is still preserved. A detailed structural investigation has been also performed.

Cubic versus spherical magnetic nanoparticles: The role of surface anisotropy

http://pubs.acs.org/doi/abs/10.1021/ja0768744

J. Am. Chem. Soc., 2008, 130 (40), pp 13234–13239
G. Salazar-Alvarez, J. Qin, V. Sepelák, I. Bergmann, M. Vasilakaki, K. N. Trohidou, J. D. Ardisson, W. A. A. Macedo, M. Mikhaylova, M. Muhammed, M. D. Baró, J. Nogués

DOI: 10.1021/ja0768744

Abstract:
The magnetic properties of maghemite (γ-Fe₂O₃) cubic and spherical nanoparticles of similar sizes have been experimentally and theoretically studied. The blocking temperature, T_B, of the nanoparticles depends on their shape, with the spherical ones exhibiting larger T_B. Other low temperature properties such as saturation magnetization, coercivity, loop shift or spin canting are rather similar. The experimental effective anisotropy and the Monte Carlo simulations indicate that the different random surface anisotropy of the two morphologies combined with the low magnetocrystalline anisotropy of γ-Fe₂O₃ is the origin of these effects.

Enhanced Coercivity in Co-Rich Near-Stoichiometric CoxFe3-xO4+δ Nanoparticles Prepared in Large Batches

http://pubs.acs.org/doi/abs/10.1021/cm070827t

Chem. Mater., 2007, 19 (20), pp 4957–4963
German Salazar-Alvarez, Richard T. Olsson, Jordi Sort, Waldemar A. A. Macedo, José D. Ardisson, Maria Dolors Baró, Ulf W. Gedde, and Josep Nogués

DOI: 10.1021/cm070827t

Abstract:
The relationship between the structure and composition with the magnetic properties of near stoichiometric cobalt ferrite nanoparticles Co_xFe_3-xO_4+δ (0.85 < x < 1.1) prepared in large batches with average sizes in the range 60−210 nm has been investigated. Chemical analysis and Rietveld refinement of the X-ray diffraction data in conjunction with Mössbauer spectroscopy allowed us to identify an interplay between particle size, microstructure (concentration of interstitial ions, microstrain, cation arrangement in octahedral and tetrahedral sites), and composition, which sensitively controls the magnetic properties such as coercivity and saturation magnetization. In all cases, cobalt-rich compositions resulted in a higher coercivity, whereas lower degrees of inversion and higher iron contents led to slightly higher saturation magnetization values.

Synthesis and Size-Dependent Exchange Bias in Inverted Core−Shell MnO|Mn3O4 Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/ja0714282

J. Am. Chem. Soc., 2007, 129 (29), pp 9102–9108
German Salazar-Alvarez, Jordi Sort, Santiago Suriñach, M. Dolors Baró, and Josep Nogués

DOI: 10.1021/ja0714282

Abstract:
Core−shell nanoparticles of MnO|Mn₃O₄ with average particle sizes of 5−60 nm, composed of an antiferromagnetic (AFM) core and a ferrimagnetic (FiM) shell, have been synthesized and their magnetic properties investigated. The core−shell structure has been generated by the passivation of the MnO cores, yielding an inverted AFM-core|FiM-shell system, as opposed to the typical FM-core|AFM-shell. The exchange-coupling between AFM and FiM gives rise to an enhanced coercivity of 8 kOe and a loop shift of 2 kOe at 10 K, i.e., exchange bias. The coercivity and loop shift show a non-monotonic variation with the core diameter. The large coercivity and the loop shift are ascribed to the highly anisotropic Mn₃O₄ and size effects of the AFM (i.e., uncompensated spins, AFM domains, and size-dependent transition temperature).