Controlling the dominant magnetic relaxation mechanisms in magnetic fluid hypertyhermia through the shell composition of Fe3O4/(Zn/Co)Fe2O4 nanoparticles.

FABRIS, FERNANDO; E. LIMA JR.; DE BIASI EMILIO; H. E. TROIANI; VASQUEZ MANSILLA M; T.E. TORRES; FERNÁNDEZ PACHECO, RODRIGO; M RICARDO IBARRA; G. F. GOYA; E. WINKLER; R. D. ZYSLER

Conferencia; 4th International Baltic Conference on Magnetism; 2021

The magnetic losses in magnetic fluid hyperthermia (MFH) is originated from the phase shift between the nanoparticle?s magnetic moment and the vector H of the applied alternating magnetic field, where the magnetic relaxation dynamic depends on the relaxation time of two diffrent concurrent mechanisms. One mechanism consists in the mechanical or Brownian relaxation, where the particles physically rotate with a characteristic time taoB, in presence of the alternating magnetic field. The second mechanism is the Néel relaxation, with a characteristic time taoN, which involves the inversion of the magnetic moment within the crystal lattice through its anisotropy energy barrier Keff V, where Keff is the effective anisotropy constant and V is the volume of the particle. Depending on the different parameters of the system, one of the mechanisms is dominant, i.e. Néel relaxation dominates systems with small magnetic anisotropy energy and/or high viscosity, whereas Brown mechanism governs in the opposite situation. To improve the heating power, it is necessary to optimize the nanoparticles characteristics as the size, morphology, magnetic anisotropy, saturation magnetization and surface funtionalization. Usually, the improvement of one parameter leads to the change in another (not always in a positive sense). Then, it is crucial to be able to perform a selective optimization of individual parameters that control the relaxation mechanisms to improve the power absorption for each particular experimental condition. One way to reach this goal is using bimagnetic core/shell nanoparticles, which allows the systematic modification of the magnetic anisotropy, preserving the size, morphology and high values of magnetization. In this way, the magnetic realaxation time can be modified respect to the mechanical relaxation and the predominat Brown or Néel heating mechanism coud be turned.With this in mind, we desing bimagnetic core/shell nanoparticles synthesized by a thermal decomposition method, and we have studied the regarding the shell composition [1]. We report a simple and effective way to control the heat generation of a magnetic colloid under alternated magnetic fields by changing the shell composition of bimagnetic core/shell Fe3O4/ZnxCo1-xFe2O4 nanoparticles, and the effective anisotropy that can be tuned by the sustitution of Co2+ by Zn2+ ions in the shell [2,3]. MFH experiments were performed with nanoparticles dispersed in hexane and butter oil. Increasing the Zn concentration of the shell diminishes the magnetic anisotropy, changing the relaxation mechanism from Brown?s to Néel one. We demostrate that tuning the Zn contents at the shell of these exchange-coupled core/shell nanoparticles provides a way to control the magnetic anisotropy without loss of saturation magnetization.[1] G. C. Lavorato, E. Lima, H. E. Troiani, R. D. Zysler, E. L. Winkler, Nanoscale 9, 10240 (2017).[2] F. Fabris, et al. Nanoscale 11, 3164 (2019).[3] F. Fabris, et al. Nanotechnology 32 (2021) 065703.