Synthesis and magnetic properties of cobalt ferrite nanoparticles formed under hydro and solvothermal conditions

Cobalt ferrite nanoparticles were synthesized both in hydro and in solvothermal conditions from Co and Fe salts of different chemical nature (nitrates and chlorides) taken in a 1:2 cation ratio. Varying the chemical prehistory, synthesis temperature, isothermal holding time, and the reaction medium composition made it possible to obtain a set of Co$_x$Fe$_{3-x}$O$_4$ nanoparticles of different average diameters and isomorphic substitution degree $x$. The resulting nanoparticles' elemental composition, crystal structure, sizes, and magnetic properties were determined using EDX analysis, X-ray diffraction, transmission electron microscopy, and SQUID magnetometry. The temperature dependences of the coercivity, remanent magnetization, squareness on both the average diameter and the stoichiometry of nanoparticles are considered. Co$_x$Fe$_{3-x}$O$_4$ nanocrystals ranged from 11 to 29 nm were single magnetic domains and showed ferrimagnetic behavior at room temperature. The hardest magnetic nanoparticles with maximum squareness values in the high-temperature region were formed with $x$ = 0.79 using solvothermal treatment of cobalt and iron nitrates at 250$^\circ$C, 7 MPa for 10 hours.