Dynamic response of magnetic nanoparticles arranged in a tubular shape

J. CURIALE; R. D. SÁNCHEZ; C. A. RAMOS; A. G. LEYVA; A. BUTERA

Rio de Janeiro, Brazil

Workshop; VIII LAW3M; 2007

We have developed a model that describes the ferromagnetic resonance spectra recently observed in nanotubes formed by assembled LaSr manganite nanoparticles.[1] Although the tubes have an elongated shape, magnetic measurements showed that the interparticle interactions are small. The resonance lineshape in these tubes resembles that of a system of randomly oriented flat particles instead of what is usually seen in elongated samples. In order to describe the experimental data we have assumed that each individual grain (or small group of grains) has an easy plane effective anisotropy which could be randomly oriented, or partially aligned in a preferential direction.  The magnetization of each grain is also distributed around a mean value, and hence the magnitude of the anisotropy varies from grain to grain. Within this framework we have been able to fit reasonably well the resonance field and the overall lineshape of nanotubes that were deposited on a glass substrate and were aligned in a strong magnetic field. From the simulations we have deduced an average effective magnetization M=180 gauss with a width of 150 gauss. This value of magnetization is smaller than that of bulk LaSr manganites at room temperature, and the reduction is probably due to the granular nature of the tubes.  To explain the in-plane and out of plane angular variations of the resonant spectra it was necessary to assume a preferential orientation of the particles in the direction of the tube axis and parallel to the plane of the substrate.