Controlling the Dominant Magnetic Relaxation Mechanisms through the shell composition of Fe3O4/ZnxCo1-xFe2O4 nanoparticles

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

Svetlogorsk

Congreso; International Baltic Conference on Magnetism (IBCM 2021); 2021

Immanuel Kant Baltic Federal University

The magnetic losses in magnetic fluid hyperthermia (MFH) is originatedfrom the phase shift between the nanoparticle?s magnetic moment and the magneticvector H of the applied alternating magnetic field, where the magneticrelaxation dynamic depends on the relaxation time of two different concurrentmechanisms. One mechanism consists in the mechanical or Brownian relaxation,where the particles physically rotate with a characteristic time τB,in presence of the alternating magnetic field. The second mechanism is the Néelrelaxation, with a characteristic time τN, which involves theinversion of the magnetic moment within the crystal lattice through itsanisotropy energy barrier KeffV,where Keff is theeffective anisotropy constant and Vis 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 withsmall magnetic anisotropy energy and/or high viscosity, whereas Brown mechanismgoverns in the opposite situation. To improve the heating power, it isnecessary to optimize the nanoparticles characteristics as the size,morphology, magnetic anisotropy, saturation magnetization and surfacefunctionalization. Usually, the improvement of one parameter leads to thechange in another (not always in a positive sense). Then, it is crucial to beable to perform a selective optimization of individual parameters that controlthe relaxation mechanisms to improve the power absorption for each particularexperimental condition. One way to reach this goal is using bimagneticcore/shell nanoparticles, which allows the systematic modification of themagnetic anisotropy, preserving the size, morphology and high values ofmagnetization. In this way, the magnetic relaxation time can be modifiedrespect to the mechanical relaxation and the dominant Brown or Néel heatingmechanism could be tuned. With this in mind, we design bimagnetic core/shell nanoparticles synthesizedby a thermal decomposition method, and we have studied them regarding the shellcomposition [1]. We report a simple and effective way to control the heatgeneration of a magnetic colloid under alternate magnetic fields by changingthe shell composition of bimagnetic core?shell Fe3O4 / ZnxCo1−xFe2O4nanoparticles, and the effective anisotropy that can be tuned by thesubstitution of Co2+ by Zn2+ ions in the shell [2,3]. MFHexperiments were performed with nanoparticles dispersed in hexane and butteroil. Increasing the Zn concentration of the shell diminishes the magneticanisotropy, changing the relaxation mechanism from Brown?s to Néel one. Wedemonstrate that tuning the Zn contents at the shell of these exchange-coupledcore/shell nanoparticles provides a way to control the magnetic anisotropywithout 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