Crystal structure and magnetic properties of Al$_{2}$O$_{3}$ nanoparticles by $^{27}$Аl NMR data

The magnetization of a series of Al$_{2}$O$_{3}$ with different particle sizes and their $^{27}$Аl NMR spectra have been studied at room temperature. The field dependence of the magnetization demonstrated the existence of a long-range ferromagnetic order in a small part of the sample at room temperature; however, the relative volume of this contribution was very small (less than 1%), and this seems likely due to an impurity phase. The NMR spectra did not contain any lines of metallic aluminum the existence of which in these nanooxides was assumed before in a surface layer of the nanoparticles, according to the data of other techniques. The data on the phase composition and the charge distribution in different phases of the Al$_{2}$O$_{3}$ nanoparticles have been obtained. The change in the mean particle size (by a factor of almost three) only insignificantly changed their phase composition.