T. S. Herng

Origin of room temperature ferromagnetism in ZnO:Cu films

Copper-doped ZnO (ZnO:Cu) films were prepared on silicon substrates by filtered cathodic vacuum arc technique at room temperature using a Zn target containing 5at.% of Cu. Room temperature ferromagnetism was observed in the ZnO:Cu films with saturation magnetization of 0.037μB∕Cu atom. The origin of the ferromagnetism in ZnO:Cu was mainly due to Cu ions substituted into the ZnO lattice. X-ray diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy revealed that no ferromagnetic-related secondary phase could be detected in ZnO:Cu.

Magnetic anisotropy in the ferromagnetic Cu-doped ZnO nanoneedles

Copper-doped ZnO (ZnO:Cu) nanoneedles exhibiting room-temperature ferromagnetism were fabricated by an ion beam technique using Cu plate and ZnO film. A saturated magnetization moment of 0.698emu∕cm3 was found in the nanoneedles when a field of 10kOe was applied perpendicular to the substrate, which was 15% larger than the field applied parallel to the substrate. The magnetic ordering of the nanoneedles was enhanced significantly to 0.968emu∕cm3 after annealing of 400°C for 20min. However, the magnetic anisotropy at high field is vanished but an “easy plane” ferromagnetism becomes apparent at low field region. The possible mechanisms of the magnetic ordering and anisotropy in the ZnO:Cu nanoneedles are discussed.

Magnetotransport properties of p-type carbon-doped ZnO thin films

Carbon-doped ZnO (ZnO:C) thin films exhibiting Curie temperature above room temperature were fabricated using ion beam technique. The magnetic moment of the ZnO:C films was found to be around 1.35u2002μB per carbon atom. The ZnO:C films showed p-type conduction with a hole concentration of ∼5×1017u2002cm−3. In addition, the anomalous Hall effect and negative magnetoresistance can be detected in the ZnO:C films. The magnetotransport properties of the ZnO:C suggested that the films possessed charge carrier spin polarization.

Stable ferromagnetism in p-type carbon-doped ZnO nanoneedles

The authors report the synthesis and magnetic properties of carbon-doped ZnO (ZnO:C) nanoneedles. A saturated magnetic moment of 2.16u2002emu/cm3 was found in the ZnO:C nanoneedles. The samples showed anomalous Hall effect and p-type conduction with a hole concentration of 1.8×1018u2002cm−3. The ferromagnetism in the ZnO:C nanoneedles could be attributed to C substitution on the O site which introduces hole, so the p-p interaction leads to the strong spin interaction between the C atoms and carriers. It was found that the ferromagnetism and p-type conduction in the ZnO:C nanoneedles were stable in ambient air over a period of 1 year and annealing temperature of up to 100u2009°C.

Ultraviolet photoluminescence from ferromagnetic Fe-doped AlN nanorods

Fe-doped AlN (AlN:Fe) nanorods on silicon substrates were fabricated using a catalysis-free vapor phase method. The AlN:Fe nanorods exhibited high crystalline quality and preferred c-axis orientation. The spontaneous saturated magnetization of the AlN:Fe nanorods was determined to be ∼0.64μB∕Fe at room temperature. Room temperature photoluminescence measurement of the AlN:Fe nanorods revealed two strong ultraviolet emissions at 3.69 and 6.02eV which could be attributed to Fe3+-related and band edge emissions, respectively. The Fe-doped AlN nanorods not only exhibited ferromagnetism but also significantly enhanced the band edge emission as compared to the undoped AlN nanorods.

Ferromagnetic Cu-doped AlN nanorods

Copper-doped AlN (AlN:Cu) nanorods were grown on catalyst-free Si substrates by chemical vapour deposition. The AlN:Cu nanorods exhibited the wurtzite structure with a growth direction along the c-axis. Ferromagnetic ordering of the AlN:Cu nanorods was observed at room temperature by an alternating gradient magnetometer. The spontaneous magnetization and the coercivity of the AlN:Cu nanorods are about 0.38?emu?cm?3 and 100?Oe, respectively. The results reveal that Cu is a potential nonmagnetic dopant for AlN.

Ferromagnetic Cu doped ZnO as an electron injector in heterojunction light emitting diodes

Ferromagnetic and highly conductive copper doped ZnO (ZnO:Cu) films were prepared by filtered cathodic vacuum arc technique. By employing a biasing technique during growth, the electron concentration and resistivity of the ZnO:Cu films can be as high as 1020u2002cm−3 and 5.2×10−3u2002Ωu2009cm, respectively. The ferromagnetic behavior is observed in all the conductive films, but its magnetization is quenched with an increment in carrier concentration, suggesting that carrier induced exchange is not directly responsible for the ferromagnetism. Heterojunction light emitting diodes have been fabricated using the conductive ZnO:Cu layer as an electron injector and a p-type GaN as hole injector. Electroluminescence can be detected from the devices.

Enhancement of ferromagnetism and stability in Cu-doped ZnO by N2O annealing

Copper-doped ZnO (ZnO:Cu) films were prepared by the filtered cathodic vacuum arc technique at room temperature on Si substrates. The as-grown ZnO:Cu exhibited a saturated magnetic moment (Ms) of 0.36xa0μB/Cu at room temperature. After an annealing treatment at 500u2009°C for 1xa0h under N2O ambient, the Ms of the ZnO:Cu increased by 100% to 0.73xa0μB/Cu. The ferromagnetism of the annealed samples was relatively stable as compared to the as-grown sample in terms of temperature and storage time. In situ x-ray photoelectron spectroscopy of the N2O annealed sample revealed that N was incorporated into the sample. The presence of N promotes the substitution of high-spin Cu2+ ions into Zn2+ sites, which leads to the enhanced ferromagnetism in ZnO:Cu. The cyclic annealing experiments of the ZnO:Cu films in air, N2O and Zn vapor suggested that Zn interstitial defects were crucial to the N incorporation.

Ultraviolet Laser Action in Ferromagnetic Zn1−xFexO Nanoneedles

Fe-doped ZnO nanoneedles (NDs) were fabricated by an Ar+ ion sputtering technique operated at room temperature. The as-grown samples show both ferromagnetic and lasing properties. The saturated magnetization moment was measured from 0.307 to 0.659 emu cm−3 at the field of 10 kOe with various Fe concentrations. Intense ultraviolet random lasing emission was observed from Zn1 − xFexO NDs at room temperature. The X-ray photoelectron spectroscopy result reveals that the doped Fe atoms occupy the Zn sites and lead to a decrease in oxygen deficiency.

Ferromagnetic copper-doped ZnO deposited on plastic substrates

Copper-doped ZnO (ZnO:Cu) films with Cu content of up to 8.2?at.% were prepared by filtered cathodic vacuum arc technique on plastic substrates. The ZnO:Cu films with 1.3?at.% of Cu exhibited the highest saturated magnetic moment of 0.28??B/Cu. The binding species of the Zn 2p3/2, and Cu 2p3/2 peaks are located at around 1022.1 and 933.6?eV respectively, as measured by x-ray photoelectron spectroscopy respectively. This implies that the Cu ions in the films are mainly in divalent states. The magnetic anisotropy of the ZnO:Cu samples under strain were also studied by bending the flexible samples into various directions. The results indicate that strain plays a role in the magnetic anisotropy of the ZnO:Cu films. The electrical transport and annealing measurements of the ZnO:Cu films suggested that Zn interstitial defects have a significant role in the observed ferromagnetism in the ZnO:Cu.