[OPEN ACCESS] 2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystals - Nanoscale (RSC Publishing)

Nanoscale 5 (2013) 953-960.

Bertrand Faure , Erik Wetterskog , Klas Gunnarsson , Elisabeth Josten , Raphaël P. Hermann , Thomas Brückel , Jens Wenzel Andreasen , Florian Meneau , Mathias Meyer , Alexander P Lyubartsev , Lennart Bergström , German Salazar-Alvarez and Peter Svedlindh

DOI: 10.1039/C2NR33013JD

http://pubs.rsc.org/en/Content/ArticleLanding/2013/NR/c2nr33013j

Abstract:

The magnetic 2D to 3D crossover behavior of well-ordered arrays of monodomain γ-Fe2O3 spherical nanoparticles with different thicknesses has been investigated by magnetometry and Monte Carlo (MC) simulations. Using structural information of the arrays obtained from grazing incidence small-angle X-ray scattering and scanning electron microscopy together with experimentally determined values for the saturation magnetization and magnetic anisotropy of the nanoparticles, we show that MC simulations can reproduce the thickness-dependent magnetic behavior. The magnetic dipolar particle interactions induce a ferromagnetic coupling that increases in strength with decreasing thickness of the array. The 2D to 3D transition in the magnetic properties is mainly driven by a change in the orientation of the magnetic vortex states with increasing thickness, becoming more isotropic as the thickness of the array increases. Magnetic anisotropy prevents long-range ferromagnetic order from being established at low temperature and the nanoparticle magnetic moments instead freeze along directions defined by the distribution of easy magnetization directions.

25 Positions as Postdoctoral Research Fellow - Stockholm University

25 Positions as Postdoctoral Research Fellow - Stockholm University

25 Positions as Postdoctoral Research Fellow in subject areas within the Faculties of Science, Humanities, Law, and Social Sciences of Stockholm University. 

Ref. No. SU 619-2974-12. 
Deadline for applications: December 17, 2012.

More information on how to apply here.

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If you are interested in applying with a project related to our topics (click here for a description of the activities and list of related publications) and have a strong CV, please contact me directly at german@mmk.su.se.

4 Open PhD positions at MMK - Stockholm University

The Department of Materials and Environmental Chemistry offers 4 new places for graduate students

Application to the graduate research program (PhD 

studies) at MMK, November 2012

The Department of Materials and Environmental Chemistry (MMK,http://www.mmk.su.se/), offers 4 new places for graduate students after an application procedure as described below.

General information:

The extensive research activities of MMK, hosting the Berzelii Center EXSELENT for development of nanoporous materials for catalysis, span over Materials and Solid State Chemistry focusing on different classes of materials; e.g. ceramics and glasses, self-assembled and porous materials, and soft matter. The work often encompasses synthesis, characterisation by x-ray diffraction and electron microscopy, NMR studies, modelling with computer simulations of materials with a potential for various applications. Environmental aspects are an important part of the research activities, where refined natural or anthropogenic inorganic and organic chemicals and materials are studied in relation to their sole or combined impact on, and interaction with biological and non-biological systems in our global environment.

More information on the positions and application procedure here.

The deadline is November 20, 2012

For more information on the project HT12-3: Fabrication of biopolymer–nanoparticle hybrids contact directly the project leader Germán Salazar-Álvarez at german@mmk.su.se .

Stora anslag från Knut och Alice Wallenbergs stiftelse till forskning inom naturvetenskap - Stockholms universitet

Från http://www.wallenberg.com/docs/kaw/pressrelease-2012-10-05.pdf

Drygt 700 miljoner kronor till forskning från Knut och Alice Wallenbergs Stiftelse

 

25 forskningsprojekt som har potential att leda till nya vetenskapliga genombrott beviljas drygt 700 miljoner kronor av Knut och Alice Wallenbergs Stiftelse i årets ansökningsomgång.

 

– Det är mycket glädjande att få bevilja anslag till så många projekt som har bedömts ligga i den internationella forskningsfronten. Att spännvidden på ämnena dessutom sträcker sig från jordens uppkomst till nanomaterial visar att svensk forskning fortfarande håller hög kvalitet på bred front, säger Peter Wallenberg Jr, vice ordförande i Knut och Alice Wallenbergs Stiftelse.

Genom programmet ger Knut och Alice Wallenbergs Stiftelse Sveriges främsta forskare resurser och villkor så att de kan angripa komplexa och svåra forskningsproblem.

I huvudsak stödjer Stiftelsen grundforskning inom medicin, teknik och naturvetenskap.

– Starka grundforskningsmiljöer är fundamentet för framgångsrik tillämpad forskning. Det är ett gott betyg att de internationella bedömargrupperna som utvärderat ansökningarna funnit att så många håller hög klass. För att nå banbrytande rön behövs en långsiktig trygghet som också tillåter misslyckanden, vi hoppas att denna anslagsform medger detta säger Göran Sandberg, verkställande ledamot, Knut och Alice Wallenbergs Stiftelse.

Anslagen delas ut inom tre huvudområden: Teknik/fysik/matematik, övrig naturvetenskap samt medicin.

Mer info finns på www.su.se webbsidan:

Stora anslag från Knut och Alice Wallenbergs stiftelse till forskning inom naturvetenskap - Stockholms universitet

[OPEN ACCESS] Hard and transparent films formed by nanocellulose-TiO2 nanoparticle hybrids

http://dx.doi.org/10.1371/journal.pone.0045828

PLoS ONE 2012, 7, e45828

Christina Schütz, Jordi Sort, Zoltán Bacsik, Vitaliy Oliynyk, Eva Pellicer, Andreas Fall, Lars Wågberg, Lars Berglund, Lennart Bergström, German Salazar-Alvarez

DOI: 10.1371/journal.pone.0045828

Abstract

The formation of hybrids of nanofibrillated cellulose and titania nanoparticles in aqueous media has been studied. Their transparency and mechanical behavior have been assessed by spectrophotometry and nanoindentation. The results show that limiting the titania nanoparticle concentration below 16 vol% yields a homogeneous hybrids with a very high Young’s modulus and hardness, of up to 44 GPa and 3.4 GPa, respectively, and an optical transmittance above 80 %. Electron microscopy shows that higher nanoparticle contents result in agglomeration and an inhomogeneous hybrid nanostructure with a concomitant reduction of hardness and optical transmittance. Infrared spectroscopy suggests that the nanostructure of the hybrids is controlled by electrostatic adsorption of the titania nanoparticles on the negatively charged nanocellulose surfaces.

High strength, flexible and transparent nanocellulose/vermiculite biohybrid films with tunable oxygen and water vapor permeability

http://pubs.rsc.org/en/content/articlelanding/2012/NR/C2NR31726E

Nanoscale, 2012, Accepted Manuscript
DOI: 10.1039/C2NR31726E

Christian Aulin , German Salazar-Alvarez and Tom Lindström

Abstract

A novel, technically benign procedure to combine vermiculite nanoplatelets with nanocellulose fibre dispersions into functional biohybrid films is presented. Nanocellulose fibres of 20 nm diameters and several µm in length are mixed with high aspect ratio exfoliated vermiculite nanoplatelets through high-pressure homogenization. The resulting hybrid films obtained after solvent evaporation are stiff (tensile modulus of 17.3 GPa), strong (strength up to 257 MPa), and transparent. Scanning electron microscopy (SEM) shows that the hybrid films consist of stratified nacre-like layers with a homogenous distribution of nanoplatelets within the nanocellulose matrix. The oxygen barrier properties of the biohybrid films outperform commercial packaging materials and pure nanocellulose films showing an oxygen permeability of 0.07 cm3·μm/m2·day·kPa at 50 % relative humidity. The oxygen permeability of the hybrid films can be tuned by adjusting the composition of the films. Furthermore, the water vapor barrier properties of the biohybrid films were also significantly improved by the addition of nanoclay. The unique combination of excellent oxygen barrier behavior and optical transparency suggest the potential of this biohybrid materials as an alternative in flexible packaging of oxygen sensitive devices like thin-film transistors or organic light-emitting diode displays, gas storage applications and as barrier coatings/laminations in large volume packaging applications.

On the role of tannins and iron in the Bogolan or mud cloth dyeing process

http://trj.sagepub.com/content/early/2012/07/16/0040517512452955.abstract

Textile Research Journal, 2012, In press.


Mukta V Limaye, Zoltán Bacsik, Christina Schütz, Aïssata Dembelé, Mama Pléa, Linnéa Andersson, German Salazar-Alvarez, and Lennart Bergström


DOI: 10.1177/0040517512452955

We have investigated the chemistry of the Bogolan or mud cloth dyeing process, a traditional technique of coloring cotton cloths deeply rooted in Mali. Textiles produced by the traditional Bogolan process, using tannin-rich plant extract and iron-rich clay-based mud, were compared using infrared (IR) spectroscopy, scanning electron microscopy (SEM) and X-ray absorption near-edge spectroscopy (XANES) with cotton fibers that were impregnated with tannin and iron salt solutions. IR spectroscopy in both reflective mode on the cloth and cotton and in transmission mode on single fibers, together with SEM, showed that gallic and tannic acid adsorb and precipitate onto the cotton fiber surface. IR spectroscopy and comparison with tannin and iron solution-impregnated cotton showed that the black color of the traditional Bogolan cloth is dominated by the formation of iron-tannin complexes. The presence of iron in the Bogolan cloth was confirmed using XANES data, supporting the notion that iron has been transferred from the iron-rich clay-based mud to the cloth. The chemistry of Bogolan cloth is not only historically and culturally significant and of importance in textile conservation, but may also inspire future research on sustainable dyeing and processing techniques based on natural products.

Strongly exchange coupled inverse ferrimagnetic soft|hard, MnxFe3-xO4|FexMn3-xO4, core|shell heterostructured nanoparticles

http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr30986f

Nanoscale, 2012,4, 5138-5147

Marta Estrader ,  Alberto López-Ortega ,  German Salazar-Alvarez,  Igor Golosovsky ,  Marianna Vasilakaki ,  Kalliopi Trohidou , David Keavney ,  Randy Dumas ,  Jordi Sort ,  Sonia Estrade , Dolors Baro ,  Santiago Suriñach ,  Francesca Peiro and Josep Nogués

Inverted soft|hard, in contrast to conventional hard|soft, bi-magnetic core|shell nanoparticles of Mn_xFe_3-xO₄|Fe_xMn_3-xO₄ with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (~ 0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft|hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft|hard core|shell nanoparticles can be fine tuned to match specific applications.

[OPEN ACCESS] Quantitative spatial magnetization distribution in iron oxide nanocubes and nanospheres by polarized small-angle neutron scattering

http://stacks.iop.org/1367-2630/14/013025 [OPEN ACCESS]

New Journal of Physics, 2012, 14, 013025

Sabrina Disch, Erik Wetterskog, Raphaël P. Hermann, Albrecht Wiedenmann, Ulla Vainio, German Salazar-Alvarez, Lennart Bergström and Thomas Brückel

DOI: 10.1088/1367-2630/14/1/013025

By means of polarized small-angle neutron scattering, we have resolved the long-standing challenge of determining the magnetization distribution in magnetic nanoparticles in absolute units. The reduced magnetization, localized in non-interacting nanoparticles, indicates strongly particle shape- dependent surface spin canting with a 0.3(1) and 0.5(1) nm thick surface shell of reduced magnetization found for ~9 nm nanospheres and ~8.5 nm nanocubes, respectively. Further, the reduced macroscopic magnetization in nanoparticles results not only from surface spin canting, but also from drastically reduced magnetization inside the uniformly magnetized core as compared to the bulk material. Our microscopic results explain the low macroscopic magnetization commonly found in nanoparticles.