Magnetic properties of Mn-doped ZnO diluted magnetic semiconductors

A series of Mn-doped ZnO films have been prepared in different sputtering plasmas by using the inductively coupled plasma enhanced physical vapour deposition. The films show paramagnetic behaviour when they are deposited in an argon plasma. The Hall measurement indicates that ferromagnetism cannot be realized by increasing the electron concentration. However, the room-temperature ferromagnetism is obtained when the films are deposited in a mixed argon-nitrogen plasma. The first-principles calculations reveal that antiferromagnetic ordering is favoured in the case of the substitution of Mn^2＋ for Zn^2＋ without additional acceptor doping. The substitution of N for O （NO^-） is necessary to induce ferromagnetic couplings in the Zn-Mn-O system. The hybridization between N 2p and Mn 3d provides an empty orbit around the Fermi level. The hopping of Mn 3d electrons through the empty orbit can induce the ferromagnetic coupling. The ferromagnetism in the N-doped Zn-Mn-O system possibly originates from the charge transfer between Mn^2＋ and Mn^3＋ via NO^-, The key factor is the empty orbit provided by substituting N for O, rather than the conductivity type or the carrier concentration.

Regulating the dopant clustering in LiZnAs-based diluted magnetic semiconductor

Tuning of the magnetic interaction plays the vital role in reducing the clustering of magnetic dopant in diluted magnetic semiconductors(DMS).Due to the not well understood magnetic mechanism and the interplay between different magnetic mechanisms,no efficient and universal tuning strategy is proposed at present.Here,the magnetic interactions and formation energies of isovalent-doped(Mn) and aliovalent(Cr)-doped LiZnAs are studied based on density functional theory(DFT).It is found that the dopant–dopant distance-dependent magnetic interaction is highly sensitive to the carrier concentration and carrier type and can only be explained by the interplay between two magnetic mechanisms,i.e.,superexchange and Zener’s p–d exchange model.Thus,the magnetic behavior and clustering of magnetic dopant can be tuned by the interplay between two magnetic mechanisms.The insensitivity of the tuning effect to U parameter suggests that our strategy could be universal to other DMS.

Magnetism induced by nonmagnetic dopants in zinc-blende SiC: First-principle calculations

Magnetism induced by the nonmagnetic dopants in the zinc-blende SiC (3C-SiC) is investigated by first-principle calculations. The atoms of the first 20 elements in the periodic table except inert gas are used to replace either Si or C atoms as dopants. We find that some nonmagnetic substitutional dopants (mainly the Group IA, Group IIA, Group IIIB, and Group VIIB elements) prefer the spin-polarized ground states with local magnetic moments. In general, the condition for obtaining the local magnetic moments and the magnetic ground state requires that the dopants are p-type and have large electronegativity difference from the neighboring host atoms. The magnetic moments can be tuned over a range between 1 μ B and 3 μ B by doping with the nonmagnetic elements. The nearest-neighbor exchange couplings J 0 between the local magnetic moments are quite large and the codoping method is proposed to increase the dopant concentration. These imply that the nonmagnetic doping in SiC may exhibit collective magnetism. Moreover, the Group IIA Mg and Ca atoms substituting the preferred Si atoms favor the ferromagnetic ground states with the half-metallic electronic properties, which suggests that Mg or Ca substitutional doping on the Si sites in SiC could be a potential route to fabricating the diluted magnetic semiconductors.

Magnetic properties of Co-doped SnO:first-principles calculations

Based on density functional theory calculations, the electronic and magnetic properties oi Co-duped SnO are investigated. It is found that the spin-polarized state, with a magnetic moment of about 1.0 μB per Co-dopant, is more favorable in energy than the non-spin-polarized state. Moreover, the origin of the ferromagnetism in Co-doped SnO is found to be the double exchange mechanism. Our results indicate that Co-doped SnO is a possible candidate of the u-type snintronics material.

Defect-induced ferromagnetism in rutile TiO_2 : A first-principles study

Based on first-principles calculations, the electronic and magnetic properties of undoped and Li-doped rutile TiO2 have been studied. The results demonstrate that a cation vacancy can arouse ferromagnetism in TiO2 and the magnetic moment mainly comes from p orbitals of O atoms around the Ti vacancy. However, the Ti vacancy under normal conditions is very difficult to form due to its high formation energy. Our calculations indicate that Li-doped TiO2 can reduce the formation energy while keeping the magnetism. The large magnetization energy indicates that Li-doped TiO2 is a promising room-temperature diluted magnetic semiconductor.

Tailoring the structural and magnetic properties of Cu-doped ZnO by c-axis pressure

The structural and magnetic properties of the Cu-doped ZnO（ZnO：Cu） under c-axis pressure were studied using first-principle calculations. It was found that the ZnO：Cu undergoes a structural transition from Wurtzite to Graphite-like structure at a c-axis pressure of 7–8 GPa. This is accompanied by an apparent loss of ferromagnetic stability, indicating a magnetic transformation from a ferromagnetic state to a paramagnetic-like state. Further studies revealed that the magnetic instability is closely related to the variation in crystalline field originated from the structural transition, which is in association with the overlapping of spin–charge density between the Cu^2＋ and adjacent O^2-.

Dominant ferromagnetic coupling over antiferromagnetic in Ni doped ZnO： First-principles calculations

The low magnetic moment （MM） in diluted magnetic semiconductors （DMS） at low impurity doping levels has triggered considerable research into condensed magnetic semiconductors （CMS）.This work reports an ab-initio investigation of the electronic structures and magnetic properties of ZnO in a zinc-blende （ZB） structure doped with nickel ions. Ni-doped ZnO-based DMS and CMS exhibit a dominance of ferromagnetic coupling over antiferromagnetic. A robust increase in the magnetization has been observed as a function of Ni impurity levels. This material favors short-range magnetic interactions at the ground state, suggesting that the observed ferromagnetism is defined by the double exchange mechanism. The spin-polarized density of states （DOS） of Ni-doped ZnO characterizes it as half-metallic with a considerable energy gap for up-spin components and as metallic for-down spins. Half metallic Ni：ZnO based magnetic semiconductors with high magnetization are expected to have potential applications in spintronics.

First-principles calculations on magnetic property of Cu-doped Zn O tuned by Na and Al dopants

The effect of a second dopant on the magnetic property of Cu-doped ZnO by first-principles calculations based on the density functional theory was studied. It is found that the Cu-doped ZnO shows ferromagnetism due to the hybridization between Cu-3d and O-2p orbitals. When Na is introduced to the Cu-doped ZnO system, Cu cations tend to take on a bivalent state. Therefore, the magnetic moments on both Cu and coordinated oxygen sites increase due to Na doping. On the contrary, the magnetic moments decrease dramatically in the （Cu, A1） co-doped ZnO, which can be attributed to the fully occupied 3d states of Cu＋ and O-2p states.

The effects of Zn vacancies on ferromagnetism in Cu-doped ZnO films controlled by oxygen pressure and Li doping

Zn0.99Cu0.01O films were studied experimentally and theoretically. The films were prepared by pulsed-laser deposition on Pt（111）/Ti/SiO2/Si substrates under various oxygen pressures to investigate the growth-dependence of the ferromagnetic properties. The structural, magnetic, and optical properties were studied, and it was found that all the samples possess a typical wurtzite structure, and that the films exhibit room-temperature ferromagnetism. The sample deposited at 600 ℃ and an oxygen pressure of 10 Pa showed a large saturation magnetization of 0.83 μB/Cu. The enhanced ferromagnetism in the （Cu, Li）-codoped ZnO is attributable to the existence of Zn vacancies （Vzn）, as shown by first-principles calcu- lations. The photoluminescence analysis demonstrated the existence of Vzn in both Zn0.99Cu0.01 O and （Cu, Li）-codoped ZnO thin films, and this plays an important role in the increase of ferromagnetism, according to the results of first-principles calculations.

氧空位对钴掺杂氧化锌半导体磁性能的影响

从实验和理论上阐述了氧空位对Co掺杂ZnO半导体磁性能的影响.采用磁控溅射法在不同的氧分压下制备了Zn0.95Co0.05O薄膜,研究了氧分压对薄膜磁性能的影响.实验结果表明,高真空条件下制备的Zn0.95Co0.05O薄膜具有室温铁磁性,提高氧分压后制备的薄膜铁磁性逐渐消失.第一性原理计算表明,在Co掺杂ZnO体系中引入氧空位有利于降低铁磁态的能量,铁磁态的稳定性与氧空位和Co之间的距离密切相关.