Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles

Nature Communications 4 (2013) 2960.

doi:10.1038/ncomms3960

M. Estrader, A. López-Ortega, S. Estradé, I. V. Golosovsky, G. Salazar-Alvarez, M. Vasilakaki, K. N. Trohidou, M. Varela, D. C. Stanley, M. Sinko, M. J. Pechan, D. J. Keavney, F. Peiró, S. Suriñach, M. D. Baró & J. Nogués

Abstract:

The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide—manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles.

PhD Position in Analytical Transmission Electron Microscopy::Stockholm University, Sweden

Project title: Studies and characterization of interfaces in nanoscale materials using novel 3DEM techniques. 

Reference number: SU FV-3142-13 (project HT13-1).

Deadline: November 20, 2013

General information.

Stockholm University (http://www.su.se) is a modern university with a multicultural environment and is one of the world’s top 100 higher education institutes. Here more than 60,000 students and 5,000 staff are active within science, humanities and social sciences. The Department of Materials and Environmental Chemistry (MMK, http://www.mmk.su.se) is a leading institution with research spanning over Materials and Solid State Chemistry focusing on different classes of materials, from ceramics and glasses to self-assembled and porous materials. The work often encompasses synthesis, characterisation by X-ray diffraction and electron microscopy, NMR studies, and modelling with computer simulations. Environmental aspects are also an important part of the research activities, where refined natural or anthropogenic chemicals and materials are studied in relation to their impact in our global environment.

Project description.

Synthetic and naturally occurring nanostructured materials are the building blocks of nanoscience and nanotechnology. These nanostructures often exhibit novel properties as their physical dimensions become comparable to certain characteristic length scales, which situate them at the border between quantum effects and bulk properties.

Nanomaterials composed of two phases often shown interesting interfacial phenomena. This project focuses on developing element specific three-dimensional electron microscopy (3DEM) to obtain compositional and crystallographic information at the interface of such nanoscale materials and correlate it with optical and magnetic properties. The project will be carried out in close collaboration with other groups at Stockholm University and international partners in Spain, Denmark and USA.

Relevant publications:

1.   E. Wetterskog, C.W. Tai, J. Grins, L. Bergström and G. Salazar-Alvarez, ACS Nano 2013 7, 7132.

2.   G. Salazar-Alvarez, H. Lidbaum, A. López-Ortega, M. Estrader, K. Leifer, J. Sort, S. Suriñach, M. D. Baró, and J. Nogués, Journal of the American Chemical Society 2011 133, 16738.

3.   K.L. Krycka, J. A. Borchers, G. Salazar-Alvarez, A. López-Ortega, M. Estrader, S. Estradé, E. Winkler, R.D. Zysler, J. Sort, F. Peiró, Maria Dolors Baró, C.C. Kao, and J. Nogués, ACS Nano 2013 7, 921.

4.   S. Disch, E. Wetterskog, R. P. Hermann, G. Salazar-Alvarez, P. Busch, T. Brueckel, L. Bergström, S. Kamali, Nano Letters 2011, 11, 1651.

Funding: The project will be funded by the recently approved KAW project “3DEM-
NATUR” (http://www.mmk.su.se/page.php?pid=945).

More information.

About the project: Doc. Germán Salazar-Alvarez, Group leader, german@mmk.su.se (http://www.mmk.su.se/page.php?pid=155&id=1303)

About MMK: Prof. Gunnar Svensson, Head of Department, gunnar.svensson@mmk.su.se.

Starting date: the anticipated starting date is February 1, 2014, or later.

How to apply.

Interested candidates should send their application and supporting material to: registrator@su.se with reference number SU FV-3142-13 in the subject field.

Your application should contain:

• An application on the form (can be found at www.mmk.su.se/page.php?pid=413) together with CV and documentation of study merits, where your eligibility is clearly documented (see below).
• A "Letter of intent", describing your expectations of the PhD studies connected to the project.
• IMPORTANT: Please combine all your documents into a single, self-contained pfd-file, including the cover letter.

Eligibility.

To be eligible for PhD studies in chemistry, an education at the undergraduate level of at least 240 credits is required (corresponding to three years of full-time studies on undergraduate level and one year on advanced level) with at least 60 credits in chemistry or physics. Those studies should include at least one specialized course or a thesis in the research subject. In order to facilitate the evaluation of merits and suitability for the PhD studies your curriculum vitae (CV) should contain information about the extent and focus of the academic studies. The quantity (as part of an academic year) and the quality mark of courses in chemistry and physics are of particular interest. Please, state titles of undergraduate theses and project works. Further information is found in the home page, www.mmk.su.se/page.php?pid=413.

A selection committee will assess the candidate´s ability to successfully complete the PhD program and invite short-listed candidates to an interview in person or via internet. The study merits are an important selection criterion. Economic support for the graduate studies is guaranteed for full time studies during the time agreed in the individual study syllabus (study plan), normally for four years of full time studies, see  www.mmk.su.se/page.php?pid=413.

See link at http://www.nature.com/naturejobs/science/jobs/351442-phd-position-in-analytical-transmission-electron-microscopy

[OPEN ACCESS] Dynamic growth modes of ordered arrays and mesocrystals during drop-casting of iron oxide nanocubes

Cryst. Eng. Comm. 2013
DOI: 10.1039/C3CE41871E
Michael Agthe, Erik Wetterskog, Johanne Mouzon, German Salazar-Alvarez and Lennart Bergström

Abstract:
The growth modes of self-assembled mesocrystals and ordered arrays from dispersions of iron oxide nanocubes with a mean edge length of 9.6 nm during controlled solvent removal have been investigated with a combination of visible light video microscopy, atomic force microscopy and scanning electron microscopy. Mesocrystals with translational and orientational order of sizes up to 10 μm are formed spontaneously during the final, diffusion-controlled, drop-casting stage when the liquid film is very thin and the particle concentration is high. Convection-driven deposition of ordered nanocube arrays at the edge of the drying droplet is a manifestation of the so called coffee-ring effect. Dendritic growth or fingering of rapidly growing arrays of ordered nanocubes could also be observed in a transition regime as the growth front moves from the initial three-phase contact line towards the centre of the original droplet.

Video abstract:


Cover page:
http://pubs.rsc.org.ezp.sub.su.se/en/content/articlepdf/2014/ce/c4ce90010c?page=search

Application to the PhD programme at the Department of Materials and Environmental Chemistry, Stockholm University, November 2013

Original text at http://www.mmk.su.se/page.php?pid=983

Application to the graduate research program (PhD studies) at MMK, November 2013

Ref number SU FV-3142-13

The Department of Materials and Environmental Chemistry (MMK), offers 6 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.

For project and application information please read the PhD advert

[NEWS] Our review is one of the hotest articles in august!

at the Journal of Materials Chemistry A: http://blogs.rsc.org/jm/2013/08/07/hot-articles-for-august/

Functional hybrids based on biogenic nanofibrils and inorganic nanomaterials

Bernd Wicklein and German Salazar-Alvarez
J. Mater. Chem. A, 2013,1, 5469-5478 
DOI: 10.1039/C3TA01690K, Feature Article

[OPEN ACCESS] Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1-xO|Fe3-δO4 Core|Shell Nanocubes to Single-Phase Particles

ACS Nano, 2013

DOI: 10.1021/nn402487q

Erik Wetterskog , Cheuk-Wai Tai , Jekabs Grins , Lennart Bergström, and German Salazar-Alvarez

Abstract:

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe1-xO|Fe3-δO4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of anti-phase boundaries. Dark-field transmission electron microscopy and powder x-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe1-xO core is removed, but that the anti-phase boundaries remain. The anti-phase boundaries result in anomalous magnetic behavior, i.e., a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

[REVIEW][OPEN ACCESS] Dispersion and surface functionalization of oxide nanoparticles for transparent photocatalytic and UV-protecting coatings and sunscreens

Sci. Technol. Adv. Mater. 14 (2013) 023001
doi:10.1088/1468-6996/14/2/023001

Bertrand Faure, German Salazar-Alvarez, Anwar Ahniyaz, Irune Villaluenga, Gemma Berriozabal, Yolanda R De Miguel and Lennart Bergström

Abstract

This review describes recent efforts on the synthesis, dispersion and surface functionalization of the three dominating oxide nanoparticles used for photocatalytic, UV-blocking and sunscreen applications: titania, zinc oxide, and ceria. The gas phase and liquid phase synthesis is described briefly and examples are given of how weakly aggregated photocatalytic or UV-absorbing oxide nanoparticles with different composition, morphology and size can be generated. The principles of deagglomeration are reviewed and the specific challenges for nanoparticles highlighted. The stabilization of oxide nanoparticles in both aqueous and non-aqueous media requires a good understanding of the magnitude of the interparticle forces and the surface chemistry of the materials. Quantitative estimates of the Hamaker constants in various media and measurements of the isoelectric points for the different oxide nanoparticles are presented together with an overview of different additives used to prepare stable dispersions. The structural and chemical requirements and the various routes to produce transparent photocatalytic and nanoparticle-based UV-protecting coatings, and UV-blocking sunscreens are described and discussed.

Correlating material-specific layers and magnetic distributions within onion-like Fe3O4/MnO/γ-Mn2O3 core/shell nanoparticles

J. Appl. Phys. 113, 17B531 (2013)

K. L. Krycka; J. A. Borchers; M. Laver; G. Salazar-Alvarez; A. López-Ortega; M. Estrader; S. Suriñach; M. D. Baró; J. Sort; J. Nogués

DOI: 10.1063/1.4801423

Abstract
The magnetic responses of two nanoparticle systems comprised of Fe₃O₄/γ-Mn₂O₃ (soft ferrimagnetic, FM/hard FM) and Fe₃O₄/MnO/γ-Mn₂O₃ (soft FM/antiferromagnetic, AFM/hard FM) are compared, where the MnO serves to physically decouple the FM layers. Variation in the temperature and applied field allows for Small Angle Neutron Scattering (SANS) measurements of the magnetic moments both parallel and perpendicular to an applied field. Data for the bilayer particle indicate that the graded ferrimagnetic layers are coupled and respond to the field as a single unit. For the trilayer nanoparticles, magnetometry suggests a Curie temperature (T_C) ≈ 40 K for the outer γ-Mn₂O₃component, yet SANS reveals an increase in the magnetization associated with outer layer that is perpendicular to the applied field above T_C during magnetic reversal. This result suggests that the γ-Mn₂O₃ magnetically reorients relative to the applied field as the temperature is increased above 40 K.

6 PhD positions in Physical, Inorganic, Materials and Environmental Chemistry

6 PhD positions in Physical, Inorganic, Materials and Environmental Chemistry at the Department of Materials and Environmental Chemistry. Reference number SU FV-1026-13 (project numbers – see below). Deadline for applications: May 2, 2013.

Application to the graduate research program (PhD studies) at MMK, April 2013
The Department of Materials and Environmental Chemistry (MMK), www.mmk.su.se, offers 6 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 about available projects, requirements, and details about the application procedure at:

http://www.su.se/english/about/vacancies/phd-studies/6-phd-positions-in-physical-inorganic-materials-and-environmental-chemistry-1.129642

[OPEN ACCESS] Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation

Nanoscale, 2013, Just Accepted Manuscript

Sabrina Disch,  Erik Wetterskog,  Raphaël P. Hermann,  Denis Korolkov,  Peter Busch,  Peter Boesecke,  Olivier Lyon,  Ulla Vainio,  German Salazar-Alvarez,  Lennart Bergström and Thomas Brückel

DOI: 10.1039/C3NR33282A

Abstract:

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure - similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

Preparation of dry ultra-porous cellulosic fibres: Characterization and possible initial uses

Carbohydrate Polymers, 92 (2013) 775–783

Anna Svensson, Per Tomas Larsson, German Salazar-Alvarez, Lars Wågberg

DOI: 10.1016/j.carbpol.2012.09.090

Abstract


Dry ultra-porous cellulose fibres were obtained using a liquid exchange procedure in which water was replaced in the following order: water, methanol, acetone, and finally pentane; thereafter, the fibres were dried with Ar(g). The dry samples (of TEMPO-oxidized dissolving pulp) had a specific surface area of 130 m2 g−1as measured using BET nitrogen gas adsorption. The open structure in the dry state was also revealed using field emission scanning electron microscopy.

This dry open structure was used as a scaffold for in situ polymerization. Both poly(methyl methacrylate) and poly(butylacrylate) were successfully used as matrix polymers for the composite material (fibre/polymer), comprising approximately 20 wt% fibres. Atomic force microscopy phase imaging indicated a nanoscale mixing of the matrix polymer and the cellulose fibril aggregates and this was also supported by mechanical testing of the prepared composite where the open fibre structure produced superior composites. The fibre/polymer composite had a significantly reduced water absorption capacity also indicating an efficient filling of the fibre structure with the matrix polymer.

[REVIEW] [OPEN ACCESS] Functional hybrids based on biogenic nanofibrils and inorganic nanomaterials

Bernd Wicklein and German Salazar-Alvarez

J. Mater. Chem. A, 2013, Accepted Manuscript

DOI: 10.1039/C3TA01690K

Abstract:

This feature article reviews some of the recent work on the fabrication of functional hybrids based on biogenic nanofibers and inorganic nanomaterials with an emphasis on the functional properties and suggested potential applications. We also discuss some of the work oriented towards the formation of ordered materials in the pursuit of achieving a hierarchical construction. Besides the academic interest in biogenic nanomaterials, it is anticipated that the use of natural, abundant nanomaterials, e.g., cellulose, chitin, collagen, and silk, could provide affordable functional nanomaterials in developing countries.

Resolving Material-Specific Structures within Fe3O4|γ-Mn2O3 Core|Shell Nanoparticles Using Anomalous Small-Angle X-ray Scattering

ACS Nano 2013, 7, 921-931.

Kathryn L. Krycka, Julie A. Borchers, German Salazar-Alvarez, Alberto López-Ortega , Marta Estrader, Sonia Estradé, Elin Winkler, Roberto Daniel Zysler, Jordi Sort, Francesca Peiró, Maria Dolors Baró, Chi-Chang Kao and Josep Nogués

DOI: 10.1021/nn303600e

http://pubs.acs.org/doi/abs/10.1021/nn303600e

Abstract:

The material specific structure of monodispersed Fe3O4|γ-Mn2O3 core|shell nanoparticles is determined using multiple energy, anomalous, small-angle x-ray scattering (ASAXS). The contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe3O4|γ-Mn2O3 core|shell nanoparticles with a diameter of 8.2 nm ± 0.2 nm consist of a core with a composition near Fe3O4 surrounded by a (MnXFe1−X)3O4 shell with a graded composition, i.e., ranging from X ≈ 0.40 at the inner shell toward X ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe3O4cores without a coating shell. Importantly, the present analysis enhances the sensitivity of the method with regard to the chemical boundaries and internal nanoparticle morphology compared with traditional approaches. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multi-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.