Abstract:Since the prediction by Dietl et al. in the year 2000, extensive research activities have been focused on the ZnObased diluted magnetic materials (DMSs). Earlier works were mainly performed using bulk or thin film materials with transition-metal doping, and diverse magnetic properties including ferromagnetism, paramagnetism, and diamagnetism have been reported. Identifying the dominant mechanism of the ferromagnetic ordering and realizing reproducible ferromagnetism above room temperature have been the main research issues, but a consensus is still lacking even today.Recently, reports on ZnO nanostructures with ferromagnetic behavior have been growing due to the novel properties brought about by the high surface-to-volume ratio and surfaces rich in defect states. In this review, we focus primarily on the recent progress in experimental studies of nanostructure ZnO DMSs. We shall separate our discussion into 2 categories as Mn-and Co-doped ZnO and undoped ZnO nanostructures reported in the literature.

We report the preparation of (111) preferentially oriented CoFe2O4 thin films on Pt(111)/TiO2/SiO2/Si substrates using a spin-coating process. The post-annealing conditions and film thickness were varied for cobalt ferrite (CFO) thin films, and Pt/CFO/Pt structures were prepared to investigate the resistance switching behaviors. Our results showed that resistance switching without a forming process is preferred to obtain less fluctuation in the set voltage, which can be regulated directly from the preparation conditions of the CFO thin films. Therefore, instead of thicker film, CFO thin films deposited by two times spin-coating with a thickness about 100 nm gave stable resistance switching with the most stable set voltage. Since the forming process and the large variation in set voltage have been considered as serious obstacles for the practical application of resistance switching for non-volatile memory devices, our results could provide meaningful insights in improving the performance of ferrite material-based resistance switching memory devices.

Ni 0.6 Zn 0.4 Fe 2 O 4 prepared using solid state reaction and sol-gel methods were compared for their structural and magnetic properties. Due to the much higher annealing temperature used in solid state reaction, the crystalline size was much larger than that of the nanoparticles prepared by sol-gel. The saturation magnetization of sol-gel nanoparticles was higher, and the coercivity was about 2 times larger, compared to the solid state reaction sample. By analyzing the integration intensity of x-ray diffraction peaks (220) and (222), we proposed that the difference in the saturation magnetization might be due to the inversion of cation distribution caused by the different preparation techniques used.

We fabricated textured polycrystalline CoFe 2 O 4 thin films on Pt(111)/TiO 2 /SiO 2 /Si substrate through a sol-gel method. We varied the thickness of the films, by using precursor solutions with different concentrations of 0.1, 0.2, and 0.3 M, and by depositing 5, 8, or 10 layers on the substrate by spin-coating. X-ray diffraction spectra indicated that when the precursor concentration of the solution was higher than 0.1 M, the spin-coated films were preferentially oriented in the direction. Inspection of the surface morphology by scanning electron microscopy revealed that CoFe 2 O 4 thin films prepared with 0.2 M solution and 5-time spin-coatings had smoother surface, as compared to the other conditions. Each coating had an average thickness of about 50 nm. The magnetic properties measured by vibrating sample magnetometer showed magnetic anisotropy, as evidenced from the difference in the in-plane and out-of-plane hysteresis loops, which we attributed to the textured orientation of the CoFe 2 O 4 thin films.

A study has been conducted on the effects of partial, 10 % Co substitution by Fe in the perovskite Sr0.775Y0.225CoO3-δ compound. The XRD data show that the resulted samples of Fe-free and Fe-doped compounds exhibit good 314 single phase quality with a tetragonal I4/mmm crystal structure. The measured M-T curves display the typical feature of ferromagnetic (FM) transition at 335 K for the Fe-free sample, while showing significantly degraded FM ordering for the 10 % Fe-doped sample. The oxygen deficiency determined fro|m our 10 % Fe doped sample is found to decrease by only 0.025. Further detailed analysis of the XRD data also reveals distinctly different structural changes in the Co-O slabs compared to the 50 % Fe-doped sample which exhibits a complete suppression of FM order. The results of this study, thus, have revealed the close relations among the changes induced by Fe doping in the magnetic ordering of crystal, and its oxygen content as well as the associated local structure in the Co-O layers responsible for the magnetic properties in the compound.