Observation of ferromagnetism at room temperature in polycrystalline Zn$_{1-x}$Fe$_x$O solid solutions synthesized by the precursor method

Polycrystalline Zn$_{1-x}$Fe$_x$O (0 $\le x\le$ 0.075) solid solutions with the wurtzite structure, which belong to the class of diluted magnetic semiconductors, have been synthesized by the precursor method using a mixed formate of the composition Zn$_{1-x}$Fe$_x$(HCOO)$_2$ $\cdot$ 2H$_2$O as a precursor. It has been found that the iron concentration in Zn$_{1-x}$Fe$_x$O solid solutions, which exhibit ferromagnetic properties with a high degree of magnetization at room temperature, is determined by two factors: (1) the degree of substitution of iron for zinc in the structure of zinc formate Zn(HCOO)$_2$ $\cdot$ 2H$_2$O and, therefore, in the structure of zinc oxide ZnO and (2) the conditions of heat treatment of the precursor. It has been shown using samples with $x$ = 0.025 and 0.05 as an example that the magnetization of Zn$_{1-x}$Fe$_x$O solid solutions regularly increases with an increase in the iron concentration. Based on the results of the X-ray photoelectron spectroscopy analysis of Zn$_{1-x}$Fe$_x$O powders, it has been suggested that the iron concentration increases in the surface layer of particles, and the oxidation state of iron is 3+.