MAGNETIC AND ELECTRONIC PROPERTIES OF IRON OXIDE NANOPARTICLES OF CONTROLLED SIZE AND SHAPE

Pablo Guardia,  Nicolás Pérez, Amílcar Labarta, Xavier Batlle
Departament de Física Fonamental and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, 08028 Barcelona, Catalonia, Spain

Magnetic nanoparticles (NP) are an ideal system to study finite-size and surface effects, all these yielding new interesting phenomena and enhanced properties with respect to their bulk counterpart [1]. Besides, the potential application of magnetic NP for biomedical purposes relies on the synthesis of high quality materials, from both the crystalline and magnetic points of view. In this work, we report on the influence of a variety of parameters on the synthesis of iron oxide nanoparticles (magnetite/maghemite Fe3O4/Fe2O3) by both thermal decomposition of an organic iron precursor in an organic media [2] and co-precipitation methods. It is well known that the former allows the preparation of highly crystalline NP with excellent magnetic parameters [3, 4]. We study the role of the reductor and surfactant on the shape, size distribution and the magnetic and electronic properties. Iron oxide NPs in the 5-50 nm range were synthesized in the presence of a variety of coatings (oleic and decanoic acids, PVA, TMAOH, dextrane…), with controlled shape from pseudo-spherical to cubic and cube-octahedral, thus opening a new range of sizes not reachable before. All the materials show a narrow size distribution with high crystal quality, as revealed by transmission electron microscopy (TEM). Surprisingly enough, saturation magnetization Ms is size independent in the 5-20 nm range and almost reaches the expected value for bulk magnetite at low temperatures, being higher in those NP for which the surfactant is covalently bonded to the surface. A variety of colloidal suspensions of quasi non-interacting NP and mean diameter of about 5 nm were studied in further detail. We have developed a new analytical model to account for the surface contribution to the effective energy barrier distribution of anisotropy, which is still under hot debate in literature. X-ray absorption spectra (XAS) in the L2,3 edges suggest charge transfer to the surfactant due to covalent bond. X-ray magnetic circular dichroism (XMCD) confirms the dependence of the magnetic moment on the surface bond and suggests that the orbital momentum is more effectively quenched in covalently bonded NP. High resolution TEM suggests that all the foregoing is related to the crystal quality of the NP associated with the temperature of the synthesis procedure. All in all, covalently bonded NP show bulk-like magnetic and electronic properties, while NP with just protective coatings shows particle-like properties. The funding from the Spanish MEC (NAN2004-08805-CO4-02, NAN2004-08805-CO4-01, CONSOLIDER CSD2006-12 and MAT2006-03999), and from the Catalan DURSI (2005SGR00969) is acknowledged. 

 
[1] For example, see the review paper, X. Batlle and A. Labarta, J. Phys D: Appl. Phys 35 (2002) R15.
[2] S. Sun et al, J. Am. Chem. Soc. 126  (2004) 273.
[3] A. G. Roca et al., Nanotechnology. 17 (2006) 2783
[4] P. Guardia et al., J. Magn. Magn. Mat. 316 ( 2007) e756