Size-dependent exchange bias of core|shell nanoparticles
Salazar-Alvarez, German; Sort, Jordi; Surinyach, Santiago; Baro, Maria Dolors; Nogues, Josep
Spain

Magnetic nanostructures are receiving a great deal of attention due to their wide range of current and potential applications. Particularly, since the report on the use of exchange bias to overcome the superparamagnetic limit,[1] the amount of experimental and theoretical work on such nanostructures has increased considerably.[2] Core-shell structures involving two or more different types of magnetic materials are interesting due to their novel magnetic properties. In this context, we have investigated the size-dependent exchange bias properties in two different core|shell configurations. Namely, a heterogeneous one with ferrimagnetic (FiM) cores composed of γ-Fe2O3 and antiferromagnetic (AFM) shells composed of MnO and the inverse type, i.e., AFM cores composed of MnO with FiM shells composed of Mn3O4. The heterogeneous and inverse core|shell particles have been prepared by subsequent thermolysis of metal salts in organic solvents, using the cores as seeds for the heterogeneous growth of the shell or by allowing the cores to passivate under air, respectively. Such preparation allows us to study for the first time the dependence of the magnetic properties as a function of the antiferromagnetic core size,[3] as opposed to the archetypical ferromagnetic metal-core| AFM metal oxide-shell configuration in which the dependence of magnetic properties on the size of the ferromagnet is studied. The results show that the coercivity, Hc, and the loop shift, He, present a non-monotonic behaviour with the thickness of the AFM. The large coercivity and loop shift values are ascribed to the exchange-coupling between the different phases and size effects of the AFM (i.e., uncompensated surface spins and size-dependent transition temperature).
References: [1] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues, Nature 423 (2003) 850.
[2] J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J. S. Muñoz and M. D. Baró, Phys. Rep. 422 (2005) 65.
[3] G. Salazar-Alvarez, J. Sort, S. Suriñach, M.D. Baró, J. Nogués, submitted (2007).
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