Tuning the magnetic anisotropy of nanostructures

The magnetism in monometallic ferromagnetic 3d transition-metal (TM) nanoparticles containing less than 1000 atoms has been the subject of numerous experimental and theoretical studies. However, very little is known about the behaviour of magnetic nanoalloys. Is it possible to tune the magnetic properties of small particles, in particular the magnetic anisotropy, by alloying as it has been done in bulk TMs? Is it possible to synthesize very hard or very soft nanomaterials in a controlled way?

 

(a) Anisotropy energy and (b) average moment per CoRh unit in CoxRh1-x 2 nm nanoparticles. Crosses refer to experiment while dots to theory (N=489 atoms). The inset shows the average moment induced at the Rh atoms.

In collaboration with the experimental groups at Laboratoire de Chimie de Coordination and at Laboratoire de Physique et Chimie de Nanoobjets in Toulouse, France, we have performed theoretical studies on bimetallic TM nanoparticles in order to determine their structural and magnetic properties and the correlation between them. In the case of CoRh particles, we observe a strong enhancement of the average magnetic moment per Co atom that indicates a significant induced polarisation on the Rh. This is confirmed by calculations and by independent XMCD measurements of the spin and orbital Rh moments. The measured MAE of CoRh alloy clusters is in general higher than in pure Co clusters and can be optimized by varying the Rh concentration. Our results are in good quantitative agreement with experiment and reproduce the observed non-monotonous concentration dependence. A local theoretical analysis shows that the orbital and spin moments at the Co-Rh interface are largely responsible for the increase of the magnetic moments and magnetic anisotropy. Comparison between theory and experiment suggests that a Rh-core covered by a Co-shell with a moderate degree Co-Rh mixing at the interface is the most likely chemical arrangement. In addition, tailoring the Co-Rh interfaces provides new possibilities of optimizing the blocking temperature. Simple general trends are derived in order to achieve a microscopically controlled material design based on 3d-4d and 3d-5d clusters.

This project is developed in collaboration with Prof J. Dorantes-Davila of the Institute of Physics of the University of San Luis Potosi (Mexico) and Prof. M. Respaud from Laboratory of the Physics and Chemistry of Nano-objects (INSA, Toulouse, France) and is supported by the DAAD-EGIDE international exchange program PROCOPE.

References:

"Magnetic properties of CoNRhM nanoparticles: experiment and theory," M. Muñoz-Navia, J. Dorantes-Dávila, D. Zitoun, C. Amiens, B. Chaudret, M.-J. Casanove, P. Lecante, N. Jaouen, A. Rogalev, M. Respaud and G. M. Pastor, Faraday Discuss. 138, 181-192 (2008).
 
"Tailoring the magnetic anisotropy in CoRh nanoalloys,'' M. Muñoz-Navia, J. Dorantes-Dávila, D. Zitoun, C. Amiens, N. Jaouen, A. Rogalev, M. Respaud and G. M. Pastor, Appl. Phys. Lett. 95, 233107-1–3 (2009).