Au quantum dots engineered room temperature crystallization and magnetic anisotropy in CoFe2O4 thin films.

For the first time, this work presents a novel room temperature time-effective concept to manipulate the crystallization kinetics and magnetic responses of thin films grown on amorphous substrates. Conventionally, metal-induced crystallization is adopted to minimize the crystallization temperature of the upper-layer thin film. However, due to the limited surface area of the continuous metal under-layer, the degree of crystallization is insufficient and post-annealing is required. To expose a large surface area of the metal under-layer, we propose a simple and novel approach of using an Au nanodots array instead of a continuous metallic under-layer to obtain crystallization of upper-layer thin films. Spinel cobalt ferrite (CFO) thin film as a 'model' was deposited on an Au nano-dots array to realize this methodology. Our findings revealed that the addition of quantum-sized Au nano-dots as a metal under-layer dramatically enhanced the crystallization of the cobalt ferrite upper layer at room temperature. The appearance of major X-ray diffraction peaks with high intensity and well-defined crystallized lattice planes observed via transmission electron microscopy confirmed the crystallization of the CFO thin film deposited at room temperature on 4 nm-sized Au nano-dots. This crystallized CFO thin film exhibits 18-fold higher coercivity (Hc = 4150 Oe) and 4-fold higher saturation magnetization (Ms = 262 emu cm-3) compared to CFO deposited without the Au under-layer. The development of this novel concept of room-temperature crystallization without the aid of additives and solvents represents a crucial breakthrough that is highly significant for exploring the green and energy-efficient synthesis of a variety of oxide and metal thin films.

Magnetism and spin entropy in Ru doped Na0.5CoO2.

The effect of Ru doping on the magnetic coupling among Co ions and on the Seebeck effect in Na0.5CoO2 was systematically studied using density functional theory. It was found that the Ru dopant takes the 4+ oxidation state and replaces a Co4+ ion. In addition, the remaining Co4+ ions in Na0.5CoO2:Ru were stabilized in a low spin state. Magnetically, the Ru dopants couple in a ferrimagnetic manner with Co ions in the host lattice. Due to the higher electronic degeneracy of Ru4+ dopants, the Seebeck coefficient in Na0.5CoO2 is predicted to be higher than that of the pristine compound.

Dopant incorporation site in sodium cobaltate's host lattice: a critical factor for thermoelectric performance.

NaxCoO2 that comprises alternating Na and CoO layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped NaxCoO2 (x = 0.50, 0.625, 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA + U method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid-like Na layer and, therefore, improve the crystallinity and decrease the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.

Ferromagnetism in ZnO:Co originating from a hydrogenated Co-O-Co complex.

The effects of hydrogen interstitials and oxygen vacancies on the overall ferromagnetic behaviour of Co doped ZnO (ZnO:Co) have been closely examined using different density functional calculations. The results demonstrate the importance of correcting the bandgap problem of the ZnO host as well as the lack of correlation in Co's 3d states which can severely affect the coupling of H and Co's impurity bands. Our results show that in hydrogenated ZnO:Co, hydrogen interstitial can also stabilize the ferromagnetic interaction at low Co concentrations, but this requires the formation of the in-plane O-H-Co-O-Co complex. In this structure, the hydrogen interstitial forms an anionic complex with the neighbouring oxygen, which polarizes the surrounding oxygen to mediate the ferromagnetism through the superexchange mechanism. An oxygen vacancy by itself would not cause ferromagnetism in ZnO:Co. On the other hand, in the presence of hydrogen interstitials, oxygen vacancies can significantly enhance the magnetic coupling between H and Co-O-Co as a shallow donor if it is far away from the in-plane O-H-Co-O-Co complex. However, the total energy results show that this is much more favoured when the oxygen vacancy is near the in-plane O-H-Co-O-Co complex, which can inhibit the ferromagnetic interaction between Co ions.

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 dopant's tendency towards chain-like aggregation, in good agreement with recent experimental observations.

Hydrogen multicenter bond mediated magnetism in Co doped ZnO.

Magnetism in Co doped ZnO (ZnO:Co) is strongly affected by the presence of the ZnO's 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.

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.

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 ZnO's 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.

Isolation and aggregation of substituent Co in ZnO:Co diluted magnetic semiconductors.

Distribution of magnetic ions in a semiconducting host is critical for the functionality of diluted magnetic semiconductors. By investigating the temperature- and field-dependent magnetization of single-phase polycrystalline ZnO:Co oxides, the substitution of Co at the Zn site is found not to occur randomly but Co ions appear to have a tendency for aggregation via oxygen with an antiferromagnetic coupling, in contrast to paramagnetic isolated free Co. The experimental findings are justified through first-principles density functional calculations based on the generalized gradient approximation. It suggests that Co dopants in ZnO:Co have a tendency towards staying close to each other along the ab plane.

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.