Specific Loss Power of Ferrofluids under Radiofrequency fields

I. BRUVERA; C. LABORDE; P.MENDOZA ZÉLIS; G. PASQUEVICH; S. E. JACOBO; J. C. APHESTEGUY; M. B. FERNÁNDEZ VAN RAAP; F. H. SÁNCHEZ

Constituyentes

Workshop; "At the Frontiers of Condensed Matter" (FCM2010); 2010

Comisión Nacional de Energía Atómica

Ferro uids (FF) are liquid suspensions of magnetic nanoparticles. When FF are submitted to a radiofrequency (RF) magnetic eld, the nanoparticles dissipate energy. In the case of single domain particles two relaxation mechanisms exist, known as Neel and Brown mechanisms. Neel relaxation depends on particle magnetic anisotropy, whereas Brown relaxation depends on uid viscosity. Both types of relaxation depend on temperature and particle size, but through dierent functional expressions. Except for the infrequent situation in which the two relaxation times are equal, relaxation occurs almost entirely by one of the two mechanisms. In this work we study aqueous FFs of ZnxFe3􀀀xO4( 0  x  0:5). We have determined the FF Specic Loss Power (SLP), dened as the power dissipated per mass unit of nanoparticles, as a function of RF eld parameters (amplitude H0 and frequency f) and FF concentrations. H0 was varied up to 700 Oe and discrete f values between 160 kHz and 260 kHz were used. The nanoparticles mass/water volume ratio, R = mNP =VFF , was varied from about 1 g/l to 10 g/l. We observed that measured SLPs did not depend on R. On the other hand, SLP f and H0 dependences deviate slightly but clearly from the behaviors found in the literature. It was reported that SLP depends linearly and quadratically with f and H0, respectively [1]. The experimental results indicate that for eld amplitude larger than about 500 Oe, SLP increases at a slower rate than predicted by the power H2 0 low, suggesting saturation eects. Simulations based on the StonerWholfahrt model modied for nite temperature conditions, give a consistent account of the observations. The simulations illustrate how the MH minor loops shape determines the previously mentioned dependences. [1] Kallumadil, M., Tada, M., Nakagawa, T., Abe, M., Southern, P., Pankhurst, Q.A., Journal of Magnetism and Magnetic Materials 321 (2009) 15091513.