Efficiency of Energy Dissipation in Nanomagnets: A theoretical study of ac susceptibility
Résumé
In this chapter, we present a summary of published and new results of our theoretical investigations of the dynamic response of an array of nanomagnets (NMs), taking account of dipolar interactions and DC magnetic field, in addition to the AC field. This investigation is relevant to research on magnetic hyperthermia as a potential application in cancer treatment that exploits the heat generated in alternating magnetic fields (AMFs). We developed an analytical formalism for the key quantity, the specific absorption rate (SAR), based on AC susceptibility and applied it to two prototypical systems: (i) a two dimensional (2D) monodisperse array of nanomagnets (NMs) with oriented anisotropy and (ii) a three dimensional (3D) polydisperse assembly with randomly distributed easy axes. This formalism also accounts for nonlinear effects by including corrections to the AC susceptibility (up to third order for the 2D system and fifth order for the 3D assembly) which become necessary for high AC field intensities. Using the analytical expressions we obtained in terms of the relevant physical parameters such as the DC field intensity, the assembly concentration, the AC field intensity and frequency, and temperature, we investigate in a relatively straightforward manner the effect and role of these parameters in the optimization of the SAR. In particular, the calculation of the nonlinear AC susceptibility makes it possible to study the effect of increasing the AC field intensity, as is done in several works for optimizing heat generation, in addition to varying the frequency. Furthermore, the nonmonotonous behavior of the SAR, induced by the competition between the DC field and the dipolar interactions, carries over to the nonlinear regime. Besides, the nonlinear contributions to the SAR bring a nonnegligible enhancement and may also contribute to redefining the biological limit on the AC field specifications.