Abstract: The invention relates to a method for producing iron oxide-based composite magnetic nanocomposites, for modulating the magnet grade of the magnetic nanocomposites to, for example, a soft magnetic material, or a semi-hard magnetic material, or a hard magnetic material, comprising the following steps: a0) separate dissolutions of precursors and of a base a) introduction at room temperature of an iron-based precursor (F) and of at least one metal precursor (M) other than an iron-based precursor, and of at least one base (B), and optionally of at least one rare earth precursor (R), in a given order of introduction into the autoclave b) hydrothermal and/or solvothermal production, so as to obtain magnetic nanocomposites which have a main phase and one or more secondary phases M?2(OH)2O2 and/or R(OH)3, c) a step of washing the nanocomposites.