Size dependence of the magnetic anisotropy constant in Cr2O3 nanoparticles

DINA TOBIA; EMILIO DE BIASI; ELIN WINKLER; ROBERTO ZYSLER; MARA GRANADA; HORACIO TROIANI; GUILLERMO ZAMPIERI

Manizales

Workshop; Latin American Workshop on Magnetism and Magnetic Materials (LAW3M 2010); 2010

In this work we present a study of the size dependence of the magnetic properties of antiferromagnetic Cr2O3 compound. The samples were synthesized by chemical precipitation and calcined at different temperatures in air or oxygen atmospheres in order to control the particles diameters. The different synthesised samples have diameters (f) from 6 nm to 70 nm [1], [2]. The structural and magnetic characterization was performed by x-ray diffraction, transmission electron microscopy and magnetization experiments. The experimental results show an important enhance of the surface disorder and spin canting as the particle size is reduced. We have found that the lattice parameters and the volume cell present a smooth decrease when the size diminishes. On the contrary, the magnetic anisotropy constant, Keff, shows a non- monotonic behaviour. The Keff value decreases from its bulk value (Kbulk = 2.5 105 erg/cm3) when the particle size diminishes, shows a minimum near f ~ 30 nm (Keff,30nm = 3.8 103 erg/cm3) and displays an important increase for smaller sizes (Keff,10nm = 1.3 105 erg/cm3). We analyze the size dependence of Keff in terms of the magnetocrystalline and surface contributions. We observed that as the particle size is reduced, the surface magnetic disorder enhances and propagates inside the particle and, as a consequence, the magnetocrystalline anisotropy diminishes. As the particle size is further reduced and approaches a critical size, the surface effects start to govern the magnetic properties; the crystalline disorder at the particle surface enhances and the surface anisotropy becomes the main contribution. We fitted the complete size evolution of Keff on the basis of a phenomenological model that considers the size evolution of the magnetically ordered core and the disordered particle surface. References: [1] D. Tobia, E. Winkler, R.D. Zysler, M. Granada and H. Troiani; Phys. Rev. B 78, 104412 (2008) [2] D. Tobia, E. Winkler, R.D. Zysler, M. Granada, H. Troiani;  Superparamagnetism in AFM Cr2O3 nanoparticles . In press (Journal of Alloys and Compounds, available online: doi: 10.1016/j.jallcom.2009.10.060)