M H N Assadi

First-principles calculations of enhanced ferromagnetism in ZnO codoped with cobalt and nitrogen

Using first-principles calculations based on density functional theory, N-codoped ZnO:Co has been demonstrated to be potentially a p-type diluted magnetic semiconductor. By investigating 13 geometrically distinct configurations, Co and N dopants are found to have a tendency toward staying close to each other with most stable –O–Co–N–Co–O– complexes. The dominant ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. The ferromagnetic coupling strongly relies on the distance of N from Co, while it weakly depends on the direction of aligned Co ions.

Intrinsic ambient ferromagnetism in ZnO:Co induced by Eu codoping

We manipulate the interaction of Co’s 3d and Eu’s 4f electrons to design and fabricate ZnO:Co+Eu, which possesses intrinsic ferromagnetism at ambient temperature. The results show that the Eu ions tend to neighboring Co ions in order to eliminate the lattice distortion caused by the larger Eu ions via strain coupling. It was also revealed that the preference of parallel spin alignment between Eu and Co ions results in ferromagnetism. The theoretical analyses and our experimental results evidenced that the induced ferromagnetism in the Eu and Co codoped ZnO is intrinsic at ambient temperature.

Enhancement of Co substitution induced by Eu codoping in ZnO-based diluted magnetic semiconducting thin films

To avoid the occurrence of doped magnetic ion clustering is a challenge in fabrication of diluted magnetic semiconductors (DMSs). In this work, we report the intrinsic ferromagnetic behavior in Co-doped ZnO DMSs induced by Eu codoping. Both structural parameters and magnetic properties demonstrate the existence of an interaction between Co and Eu ions. The observation of multiplet structures for the localized Co 3d states in x-ray absorption and x-ray magnetic circular dichroism characterization evidences that the codoped Eu plays an important role in facilitating the Co substitution of Zn, leading to intrinsic ferromagnetism.

Substantial stabilization of ferromagnetism in ZnO:Mn induced by N codoping

Electronic structures and magnetic properties of ZnO:Mn and ZnO:Mn+N systems are investigated using first-principles density functional calculations with generalized gradient approximation. The results provide a fundamental theoretical understanding in the substantial ferromagnetic stability induced by N codoping in the ZnO:Mn system observed experimentally. They demonstrate that the ferromagnetic interaction is due to the hybridization between N 2p and Mn 3d states and is very sensitive to the geometrical configurations of dopants in the ZnO host lattice. The most stable ferromagnetic configuration corresponds to the Mn-N-Mn cluster, energetically strong enough to lead to hole-mediated ferromagnetism at room temperature.

Subtle interplay between native point defects and magnetism in ZnO:Co

Distribution of Co ions and its effect on magnetic properties of ZnO:Co in the presence of native point defects, oxygen vacancy, and interstitial hydrogen, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the correlation between magnetism and the distribution of magnetic ions and the native point defects in the semiconducting host. Results show that Co ions have a strong tendency toward aggregation via oxygen within ab plane in the presence of point defects. The room temperature ferromagnetism observed experimentally in ZnO:Co is mainly dominated by the interstitial hydrogen rather than oxygen vacancy.

The feeble role of oxygen vacancies in magnetic coupling in ZnO based dilute magnetic semiconductors

The role of the oxygen vacancy (V(O)) as the dominant defect in ZnO in magnetic interactions of ZnO based dilute magnetic semiconductors (DMSs) was examined in detail using density functional theory. It was found that V(O) does not lead to a thermally activated carrier mediated magnetism or form magnetic centers in the ZnO lattice. However, neutral V(O) may facilitate the ferromagnetism, but has a limited influence on the original antiferromagnetic coupling of the magnetic ions in oxygen stoichiometric ZnO DMSs. As a result, the ferromagnetism observed in previous experiments should be attributed to other defects such as hydrogen contamination or zinc interstitials.

Hydrogen multicenter bond mediated magnetism in Co doped ZnO

Magnetism in Co doped ZnO (ZnO:Co) is strongly affected by the presence of the ZnOs extrinsic impurities, such as unintentional hydrogen dopants. Our ab initio investigation reveals that in ZnO:Co the formation of substitutional H (H(O)) with four-fold hydrogenic bonds is favored over interstitial hydrogen (H(I)) by 0.4 eV. It is found that H(O) is trapped by Co ions to form highly stable Co-H(O)-Co complexes. H(O) also mediates a strong short-ranged ferromagnetic interaction between Co dopants via short-range exchange interaction which induces room temperature ferromagnetism.

Structural and magnetic stability of dopants in ZnO-based dilute magnetic semiconductors

Structural and magnetic configurations of Co/Mn ions, the most widely studied transition metal dopants in ZnO-based dilute magnetic semiconductors, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the distribution of the magnetic ions in the ZnO host and its corresponding magnetism. Results show that the substituent magnetic ions at the Zn site strongly tend to aggregate chain-like via oxygen on the ab plane with an antiferromagnetic coupling in contrast to paramagnetic isolated free Co/Mn. Substitutional Cu codoping is found theoretically to reduce the magnetic dopants tendency towards chain-like aggregation, in good agreement with recent experimental observations.

N codoping induced ferromagnetism in ZnO:Co (101¯0) thin films

By first-principles density functional calculations with generalized gradient approximation ZnO:Co (101¯0) thin films have been demonstrated not to exhibit a ferromagnetic behavior in the absence of any additional carrier doping. Virtual crystal approximation is employed to introduce controlled hole concentrations by N codoping in dilute concentrations of 1–5 at. %. The results show that N codoping can change the ground state from antiferromagnetic to ferromagnetic while maximum ferromagnetic state stability occurs for a N concentration of 2 at. % which is practically achievable. Density of states analyses confirm that the substitutional N in the ZnO:Co thin film provides shallow acceptor states. N codoping is suggested to be a promising technique to realize p-type dilute magnetic semiconducting thin films in the dominantly paramagnetic ZnO:Co (101¯0) system.

Predominant role of defects in magnetic interactions in codoped ZnO:Co.

The roles of codoping ions (Li, Ga and Cu) and defects (oxygen vacancy and hydrogen impurity) in magnetic interactions in ZnO:Co systems have been studied systematically using an ab initio method with density functional theory and the standard molecular field model. The results show that where defects are not included in ZnOs lattice carrier mediated magnetism is only achievable in shallow p-type codoping, such as ZnO:Co + Cu. However, in deep p-type codoping (ZnO:Co + Li) and deep n-type codoping (ZnO:Co + Ga), the carriers generally do not induce spontaneous magnetism. It was also found that the oxygen vacancy, due to its deep donor nature, has a minor favoring effect on ferromagnetic ordering among Co ions. The observed ferromagnetism in such systems can be attributed to the interaction of Co ions with unintentional hydrogen contamination rather than codopants or oxygen vacancies.