Tunable high-field magnetization in strongly exchange-coupled freestanding Co/CoO core/shell coaxial nanowires

German Salazar-Alvarez, Julian Geshev, Sebastia Agramunt-Puig, Carles Navau, Alvar Sanchez, Jordi Sort, and Josep Nogués

ACS Applied Materials and Interfaces, 2016
DOI: 10.1021/acsami.6b05588

Abstract:
The exchange bias properties of Co/CoO coaxial core/shell nanowires have been investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. Firstly, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting pre-defined magnetization states to the AFM, as corroborated by micromagnetic simulations. Secondly, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops, attributed to the AFM easy-axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, is thus experimentally and theoretically demonstrated.

3D visualization of iron oxidation state in FeO/Fe3O4 core-shell nanocubes from electron energy loss tomography

Pau Torruella, Raul Arenal, Francisco de la Peña*, Zineb Saghi, Lluís Yedra, Alberto Eljarrat, Lluis Lopez-Conesa, Marta Estrader*, Alberto López-Ortega, German Salazar-Alvarez, Josep Nogués Sanmiquel, Caterina Ducati, Paul A. Midgley, Francesca Peiró, and Sonia Estrade*

Nano Letters, 16 (2016) 5068-5073

DOI: 10.1021/acs.nanolett.6b01922

Abstract:
The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe2+ and Fe3+ ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.

[OPEN ACCESS] Origin of the Large Dispersion of Magnetic Properties in Nanostructured Oxides: FexO/Fe3O4 Nanoparticles as a Case Study

M. Estrader, A. López-Ortega, I. V. Golosovsky, S. Estradé, G. Salazar-Alvarez, Ll. López- Conesa, D. Tobia, E. Winkler, J. D. Ardisson, W.A.A. Macedo, M. Vasilakaki, K. N. Trohidou, R. D. Zysler, F. Peiró, L. Bergström, and J. Nogués

Nanoscale (2015)
DOI: 10.1039/C4NR06351A

Abstract:
The intimate relationship in transition-metal oxides between stoichiometry and physiochemical properties makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.

Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles

A. López-Ortega, M. Estrader, G. Salazar-Alvarez, A.G. Roca, J. Nogués
Physics Reports, 2014, accepted manuscript
DOI: 10.1016/j.physrep.2014.09.007

Abstract:
The applications of exchange coupled bi-magnetic hard/soft and soft/hard ferromagnetic core/shell nanoparticles are reviewed. After a brief description of the main synthesis approaches and the core/shell structural-morphological characterization, the basic static and dynamic magnetic properties are presented. Five different types of prospective applications, based on diverse patents and research articles, are described: permanent magnets, recording media, microwave absorption, biomedical applications and other applications. Both the advantages of the core/shell morphology and some of the remaining challenges are discussed.