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Enhancing the superparamagnetic blocking temperature of nanoparticles with bi-magnetic core-shell structures

German Salazar-Álvarez 1Jordi Sort 2,3Abdusalam Uheida 4Mamoun Muhammed 4Santiago Suriñach 3María Dolores Baró Josep Nogues 2,3

1. Institut Catala de Nanotechnologia (ICN), Campus UAB, Barcelona 08913, Spain
2. Institucio Catalana de Recerca i Estudis Avançats (ICREA), Campus UAB, Barcelona 08913, Spain
3. Universitat Autonoma de Barcelona, Departament de Fisica, Barcelona 08913, Spain
4. Royal Institute of Technology, Dept of Materials Science and Engineering, Materials Chemistry Division (KTH), Brinellvaegen 23, 2tr., Stockholm SE10044, Sweden


Magnetic nanoparticles are being extensively studied due to their wide range of potential applications, spreading from biomedicine to magnetic recording. While some applications require superparamagnetic particles, others, like recording, need the particles to remain ferromagnetic. Moreover, the use of magnetic nanoparticles for sensing purposes calls for the particles to be magnetically soft without being superparamagnetic. In this context, we have investigated oxide bi-magnetic core-shell nanoparticles with ferrimagnetic cores and antiferromagnetic (or ferrimagnetic) shells. Two examples of core-shell structures will be discussed. These have been fabricated by two different post-synthesis methods using previously prepared ferrimagnetic γ-Fe2O3 cores: i) Co(II) ions were adsorbed on the cores to form a shell with a composition near CoFe2O4 and ii) a MnO layer was formed by using the cores as seeds for the heterogeneous growth. It is shown that both ferrimagnetic and antiferromagnetic shells allow to controllably increase the blocking temperature, TB, of the system with respect to that of untreated particles. The presence of the shells also brings about a remarkable increase of the coercivity. Moreover, in the case of antiferromagnetic shells a shift of the hysteresis loop along the field axis, i.e., exchange bias, is also observed after field cooling from above the Néel temperature. These results open the door to the possibility to synthesize a single, generic, type of particle, which can later be modified to adapt to the needs of a specific application.


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Presentation: Invited oral at E-MRS Fall Meeting 2006, Symposium K, by Josep Nogues
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-04-28 09:18
Revised:   2009-06-07 00:44