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Samples of ZnO:Mn nanocrystals with Mn concentrations of 2 and 4 at.% were synthesized by ultrasonic aerosol pyrolysis. The synthesis was carried out at 550°C using aqueous solutions of zinc and manganese nitrates. The samples obtained were subjected to heat treatment at 550°C and 850°C in air for 1 hour. The study of the samples by XRD and EPR methods shows that during the synthesis the process of doping ZnO nanocrystals with manganese occurs only partially, on the surface, in the near-surface layer. Residues of the Mn impurity are located on the surface of nanocrystals and appear during annealing at 550°С in the form of manganese oxides (Mn2O3). During heat treatment at 850°C, decomposition of Mn2O3 and bulk doping of ZnO:Mn nanocrystals occur.

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In the work, the nanocrystals ZnO and ZnO:Mn with a concentration of Mn 2 and 4 at.% were obtained by the low-temperature freeze-drying method. For this purpose, solutions of zinc acetate Zn(CH3COO)2∙2H2O and manganese one Mn(CH3COO)2∙4H2O were used. By means of XRD, it is established that nanocrystals (NC’s) have a pure phase and wurtzite-type hexagonal lattice, their size is d ~ 65 nm. The EPR spectra of the samples have a broad absorption line. It is due to the presence of a large number of intrinsic and impurity defects in the NC’s. These defects are the result of the destructive action of hydrogen, which is a product of the thermal decomposition of zinc and manganese acetates. It is shown that there is a relationship between the number of crystal lattice defects in the ZnO:Mn NC’s and their ferromagnetic properties at room temperature. Samples of ZnO:Mn with a concentration of Mn 2 and 4 at.%. have a specific magnetization value of Мs equal to 0.089 and 0.045 emu/g, respectively. The results can have great potential in spintronic devices and spin-based electronics.

Samples of ZnO and ZnO:Mn nanocrystals with Mn 2 and 4 at.% impurity concentrations obtained by ultrasonic spray pyrolysis were studied by X-ray diffraction. It was determined that the reflexes of the X-ray diffractions of the samples had a shift relative to the standard position towards large diffraction angles. This indicated the nonequilibrium state of the crystal lattice, which caused the appearance of strain stresses. Debye–Scherrer and Williamson–Hall methods were used to calculate the parameters of the crystal lattice, the size of nanocrystals, and strain stresses with different concentrations of Mn. When the concentration of Mn increased to 4 at.%, there was an increase in strain stresses in nanocrystals by 10 times in comparison with undoped ZnO. The unit cell volume of ZnO:Mn nanocrystals was significantly smaller in comparison with single-crystal ZnO, which indicated that they had their intrinsic defects.

A method for computer processing of experimental EPR spectra using the derivative spectroscopy method is proposed. Application of the proposed method to a wide, structureless line of the EPR spectrum in ZnO: Mn nanocrystals obtained by the cryochemical method made it possible to isolate the components of the spectrum, associate them with Mn2+ ions. Application of the proposed method to the analysis of the EPR spectrum ZnO: Mn nanocrystal's obtained by ultrasonic pyrolysis of an aerosol with a different activator concentration after annealing made it possible to reveal the presence of an hyperfine structure of the EPR spectrum of Mn2+ ions, which are located in different local environments. The  constants of the detected spectra are determined.

The results of studies of samples of ZnO:Mn nanocrystals with a Mn concentration of 2 at. % obtained by ultrasonic spray pyrolysis and cryochemical synthesis methods are presented. The precursors were zinc and manganese nitrates and acetates, respectively. Samples were studied by XRD, EPR and vibrational sample magnetometry. It was found by XRD that the crystal lattice parameters of nanocrystals in the samples are much lower than those of single-crystal ZnO. This indicates the presence of defects and deformation stresses in nanocrystals. The EPR method shows that in the samples obtained by ultrasonic spray pyrolysis, defects and Mn 2+ impurity ions are located not in the bulk, but in the surface layer of nanocrystals. Samples obtained by cryochemical synthesis method have a large number of acceptor-type defects. The magnetization of these samples is significantly higher than that of the samples obtained by ultrasonic aerosol pyrolysis. This may be due to the fact that in the samples obtained by ultrasonic spray pyrolysis ferromagnetism occurs on the surface, and in the samples obtained by cryochemical synthesis -in the bulk of nanocrystals. It is caused by the destructive action of special gaseous medium, which is formed during the thermal decomposition of zinc acetate. Such conditions of cryochemical synthesis methods lead to the appearance of a large number of bulk defects in nanocrystals.

The magnetic properties of ZnO:Mn(2at%) nanocrystals synthesized by ultrasonic aerosol pyrolysis were studied. It has been established that short-term thermal treatment in hydrogen does not affect the magnetization of the synthesized sample, which had ferromagnetic and paramagnetic components. The sample, which underwent heat treatment in air at T = 850°C and acquired paramagnetic properties, after heat treatment in hydrogen again became ferromagnetic without a paramagnetic phase. It has been established by the EPR method that the structure of defects in the synthesized ZnO:Mn(2%) NCs is inhomogeneous. It changes after heat treatment in hydrogen. It is shown that the controlled thermal treatment of the samples, first in air and then in hydrogen, makes it possible to predictably change their magnetic properties. The results obtained are explained using the model of coupled magnetic polarons. During thermal treatment in hydrogen, the ratio of the number of oxygen vacancies Vo and interstitial Mn2+ ions changes in the samples.