Correlation between magnetic and electric properties in the series of Co$_x$Zn$_{1-x}$Fe$_2$O$_4$ nanoparticles

Magnetization dependences of Co$_x$Zn$_{1-x}$Fe$_2$O$_4$ nanoparticles ($x = 0 , 0.1, 0.2, 0.3, 0.4, 0.5$) synthesized with the citrate precursor technique on an external magnetic field and temperature are presented. Ferrimagnetic order in nanoparticles with $x \geqslant 0.2$ appeared at temperatures, $T$, exceeding room temperature, and in nanoparticles with $x = 0$ and $0.1$ at $T$ near $100$ K. The saturation magnetization, $M_s$, remnant magnetization, $M_r$, and the coercive force, $H_c$, increase with $x$ increase and the temperature decrease. $M_s$ reached very high value: $M_s$ of NPs with $x = 0.5$ equals to $106.6$ emu/g at $100$ K while, according to the literature data, $M_s$ of stoichiometric bulk Co ferrite equals to $90$ emu/g at $4.2$ K. Correlations between concentration dependences of magnetic and electric properties has been revealed and explained qualitatively.