Nguyen Hoa Hong

Observation of ferromagnetism at room temperature in ZnO thin films

Room-temperature ferromagnetism (FM) has been observed in laser-ablated ZnO thin films. The FM in this type of compound does not stem from oxygen vacancies as in the case of TiO2 and HfO2 films, but from defects on Zn sites. Magnetization of very thin films is much larger than that of the thicker films, showing that defects must be located mostly at the surface and/or the interface between the film and the substrate. Results on Fe-doped ZnO and Mn-doped ZnO films reveal clearly that the metal-transition doping does not play any essential role in introducing the magnetism in ZnO.

Transparent Cr-doped SnO2 thin films: ferromagnetism beyond room temperature with a giant magnetic moment

Laser ablated Cr-doped SnO2 thin films grown on various kinds of substrates all show ferromagnetism well beyond room temperature. Surprisingly, films of Sn0.95Cr0.05O2 grown on LaAlO3 substrates have a giant magnetic moment of 6 μB/Cr, which is 20–30 times larger than that of films grown under the same conditions on SrTiO3 and R-cut sapphire substrates. All films are highly transparent.

Ferromagnetism at room temperature with a large magnetic moment in anatase V-doped TiO2 thin films

V-doped TiO2 thin films were grown by laser ablation on LaAlO3 substrates. In the chosen range of the growth conditions, all V:TiO2 films have an anatase structure and exhibit semiconducting and ferromagnetic behaviors at room temperature. V:TiO2 films have a giant magnetic moment and they seem to be far better ferromagnetic than Co/Fe/Ni-doped TiO2 films. This study has proved that a few percent of V substituting for Ti in TiO2 can result in a potential diluted magnetic semiconductor.

Room temperature ferromagnetism in laser ablated Ni-doped In2O3 thin films

Ni-doped In2O3 thin films were fabricated by laser ablation on sapphire and MgO substrates under various conditions. All Ni:In2O3 films are well-crystallized, single phase, and show clear evidences of room temperature ferromagnetism (FM). Ni atoms were well substituted for In atoms, and distributed very uniformly over the whole thickness of the films. However, the films grown at 550 °C have the Ni concentration exactly the same as in the synthesized target, and as the results, they have the best crystallinity and the largest magnetic moment (maximum about 0.7μB∕Ni). The observed FM in this type of wide-band gap semiconductors has proved that by applying appropriate growth conditions, doping few percent of Ni into In2O3 could indeed result in a potential magnetic material.

Magnetism in Ni-doped SnO2 thin films

Transparent Ni-doped SnO2 thin films grown by the pulsed laser deposition technique on LaAlO3, SrTiO3 as well as R-cut Al2O3 substrates all show room-temperature ferromagnetism (FM). While the Ni-doped SnO2 films on LaAlO3 substrates have a large magnetic moment of about 2 µB/Ni, films grown under the same conditions on SrTiO3 and Al2O3 substrates have a magnetic moment of one order smaller. Magnetic force microscopy measurements confirmed that the Ni:SnO2 films on LaAlO3 are magnetically homogeneous at nanometre-scales, and the FM in the films comes from the doped matrix.

Mn-doped ZnO and (Mn, Cu)-doped ZnO thin films: Does the Cu doping indeed play a key role in tuning the ferromagnetism?

Zn0.9Mn0.1O and Zn0.85Mn0.1Cu0.05O thin films were grown by the pulsed laser deposition technique on R-cut Al2O3 substrates under various conditions. Both Zn0.9Mn0.1O and Zn0.85Mn0.1Cu0.05O films that were fabricated at 650 °C under an oxygen pressure of 0.1 Torr show ferromagnetism (FM) above room temperature. It appears that by applying appropriate conditions, doping Mn alone can induce FM in ZnO itself, while co-doping with Cu might enhance the magnetic moment for some extent in some specific cases, but not very crucially as theories have predicted. Growth conditions likely play more important roles to result in ferromagnetic samples.

Magnetism in V-doped ZnO thin films

Ferromagnetism at room temperature, along with a spin-glass-like behaviour at low temperatures, has been observed in laser ablated V-doped ZnO thin films. It is found that V atoms were well substituted for Zn atoms and resulted in a very uniform distribution among the ZnO matrices.

Evidence for magnetism due to oxygen vacancies in Fe-doped HfO2 thin films

Fe-doped HfO2 thin films are room temperature ferromagnetic. In comparison with results of the undoped HfO2 films, it seems that the Fe doping is not the main cause for the ferromagnetism but only acts as a catalyst. Experimental results of oxygen annealing and vacuum heat treatments have proven that in this family of compounds, magnetism might originate from oxygen vacancies or defects. Removing oxygen enhances the magnetic moment, while reversibly filling up oxygen vacancies can destroy the ferromagnetic ordering of the system.

Fe- and Ni-doped TiO2 thin films grown on LaAlO3 and SrTiO3 substrates by laser ablation

Room temperature ferromagnetic Fe- and Ni-doped TiO2 thin films were grown by the laser ablation on both LaAlO3 and SrTiO3 substrates. Most of the films are pure anatase, and only the films of Ni content of 3.6% and 4.6% are rutile. Films on LaAlO3 substrates are more crystallized than films on SrTiO3 substrates resulting from the lattice mismatch. Our magnetic measurements also suggest that the ferromagnetism in Fe/Ni:TiO2 films is not due to Fe/Ni segregations but due to Fe/Ni:TiO2 matrices.

Co distribution in ferromagnetic rutile Co-doped TiO2 thin films grown by laser ablation on silicon substrates

Pure rutile Co-doped TiO2 films were fabricated by the pulsed-laser-deposition technique on silicon substrates from a ceramic target. Under the right fabrication conditions, Co concentration in the films could be almost the same as in the target, and films under various conditions all are ferromagnetic well above room temperature. Even though Rutherford backscattering spectroscopy measurements show that Co atoms mostly localize near the surface of the films and exist less in deeper levels, other experimental evidence shows that the ferromagnetism does not come from Co segregations, but from the Co-doped TiO2 matrix. Rutile Ti1−xCoxO2 thin films grown by a very simple technique on low-cost silicon substrates showing a Curie temperature (TC) above 400 K appear to be very attractive to applications.