The utilization of the charge freedom degree of electrons in semiconductors to describe the transportation properties and of the spin freedom degree of electrons in magnetic materials to store the information is separated. Until Ohno reports that ferromagnetism can be achieved in (Ga,Mn)As system by introducing magnetic ions Mn into GaAs matrix [1
], which invokes great interests in the diluted magnetic semiconductors (DMSs) because of the utilization of both the charge and spin freedom degrees of electrons in these materials [2
]. Both theoretical and experimental investigations are performed extensively due to the great potential applications of DMSs in spintronic devices [4
]. However, one difficulty for the applications is that the Curie temperatures (Tc
) of most DMSs materials are below the room temperature. Many attempts have been performed to find high Tc
DMSs materials [11
]. Sato and Katayama-Yoshida [11
] have systematically investigated the ferromagnetic properties of transition metals (TMs) doped in ZnO based DMSs using first principles calculations, and suggested that the Cr-doped ZnO is one candidate for high Tc
ferromagnetic DMSs. Nano-scaled DMSs have also been reported recently [12
]. The Cr-doped single-crystalline nanowires with the stable room-temperature ferromagnetism have been successfully fabricated [14
]. However, the origin of the ferromagnetism is still under debate. The Zener model has been proposed to explain the interaction between the TM ions [15
]. In addition, wei et al. also proposed a band coupling model based on p-d
repulsions between TM ions and host elements to explain the origin of the ferromagnetism using first-principles band structure calculations [16
Recently, experimental investigations show that the doping is more difficult in the nanosized structures than in the bulk semiconductors, leading to the low solubility of the dopant [18
]. So far, the origin of this doping difficulty in the nanosized structures is still not clear. Since the concentration of the charge carriers in the nanosized ZnO-based DMSs is important for their applications, it is of great interest to investigate the chemical trends of defect formation and magnetic couplings of TMs doped in the ZnO nanowire theoretically.
In this letter, we systematically calculate the defect formation energies and magnetic coupling properties of transition metals (Cr, Fe, and Ni) doped in the ZnO nanowire and bulk. We find that the two nearest Cr atoms favor ferromagnetic state, which is more stable in the nanowire than in the bulk case. We also find that Cr and Fe can be doped in the ZnO nanowire more easily than Ni due to their lower formation energies, and favor ferromagnetic states.
The paper is organized as follows. In Sect. “Calculational Methods and Details”, we describe our calculational methods and details. In Sect. “Results and Discussion”, we discuss the defect formation energies of transition metals in ZnO nanowires and bulk, and the magnetic coupling properties of the TMs in the two systems. A brief summary of the letter is given in Sect. “Summary”.