Study of ferrite Co$_{1+x}$Ti$_{x}$Fe$_{2-2x}$O$_{4}$ (0.2 $<x<$ 0.5) nanoparticles for magnetic hyperthermia

Magnetic nanoparticles (MNPs) of Co$_{1+x}$Ti$_{x}$Fe$_{2-2x}$O$_{4}$ (0.2 $<x<$ 0.5) ferrite spinel with an average diameter of $\sim$12 nm in a SiO$_2$ shell are obtained by “wet” chemical synthesis and studied by X-ray diffraction, magnetic, and Mössbauer methods. Based on the data on the MNP released heat as a function of the applied external alternating magnetic field (EAMF) strength and frequency, the particle heating mechanisms are studied. The imaginary part of the magnetic susceptibility $\chi''$ identical to the MNP heat release is analyzed at room temperature in an EAMF of strength 1 Oe and a frequency of 100 Hz. The $\chi''$ maximum temperature decreases with increasing Ti content in CoTi spinel. An increase in the temperature by $\sim$10 K was observed in an EAMF of frequency 10 kHz and a strength of 300 Oe. The temperature increase rate $\Delta T/dt$ was measured in the range from 0.001 to 0.008 K/s depending on the EAMF frequency and sample composition. It is found that Co$_{1+x}$Ti$_{x}$Fe$_{2-2x}$O$_{4}$ MNPs synthesized at 0.2 $<x<$ 0.5 satisfy the requirements imposed on materials used as heat sources during magnetic hyperthermia. Based on measurements of the magnetic susceptibility in an EAMF and Mössbauer studies, it is shown that CoTi ferrite MNPs with a titanium ion content $x$ = 0.3, i.e., Co$_{1.3}$Ti$_{0.3}$Fe$_{1.4}$O$_{4}$, are most efficient for magnetic hyperthermia.