Magnetic properties of Mn-doped ZnO (Zn0.98Mn0.02O) bulk materials prepared by the solid-state reaction method were investigated by measuring magnetization as functions of temperature and magnetic field. The special feature of our sample preparation was the low-temperature processing. When high-temperature (T>700°C) was used, secondary phase was found. The results indicate that the samples sintered in Ar gas show ferromagnetic behavior at room temperature, but it disappears in samples sintered in air. Even for ferromagnetic samples, the obtained saturation value of magnetization is much smaller than the theoretical value, suggesting the possibility that there is a strong antiferromagnetic exchange coupling in this kind of compound.

A series of Ni(1−x)FexO (x=0, 0.015, 0.03, 0.05, and 0.1) bulk samples was synthesized by the chemical concentration-precipitation method. Phase composition analysis was carried out, which showed that trace amounts of ferromagnetic phase NiFe2O4 could not be detected by x-ray diffraction in these bulk samples with x≤0.03. When x>0.03, NiFe2O4 ferrite is detected easily. The magnetic properties of all the bulk samples were investigated by measuring their magnetization as a function of temperature and magnetic field. The results indicated that all the bulk samples sintered in air exhibited large room-temperature ferromagnetic behavior ascribed to a ferromagnetic impurity phase. Simultaneously, an exchange bias and training effect were also observed in all the bulk samples, suggesting the possibility of the existence of a strong ferromagnetic/antiferromagnetic exchange coupling in this kind of compound. Specifically, the exchange bias field could be tuned by changing the concentration of the Fe dopant.

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We calculated the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the magnetic impurities mediated by electrons in nanoribbons. It was shown that the RKKY interaction is strongly dependent on the width of the nanoribbon and the transverse positions of the impurities. The transverse confinement of electrons is responsible for the above size effect of the RKKY interaction. It provides a potential way to control the RKKY interaction by changing nanostructure geometry.

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