Jung-Chun Andrew Huang

Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar∕H2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization (Ms) varied drastically for different annealing processes with Ms∼0.5, 0.2, 0.9, and 1.5μB∕Co for the as-deposited, air-annealed, Ar-annealed, and Ar∕H2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar∕H2-annealed film can be estimated by simulation to be about 1×1021cm−3.

Role of grain boundary and grain defects on ferromagnetism in Co:ZnO films

The annealing effects on magnetism, structure, and ac transport for Co:ZnO films have been systematically investigated. The room temperature saturation magnetization (Ms) varies drastically with Ar or Ar∕H2 annealing processes. By using the impedance spectra, the change in grain boundary and grain defects of these films can be analyzed. The results demonstrate that Ar annealing produces mainly the grain boundary defects which cause the enhancement of Ms. Ar∕H2-annealing creates not only grain boundary defects but also the grain defects, resulting in the stronger enhancement of Ms. Ferromagnetism for Co:ZnO films is influenced by both grain boundaries and grain defects.

Origin of ferromagnetism in ZnO∕CoFe multilayers: Diluted magnetic semiconductor or clustering effect?

Epitaxial growth of (0001) oriented [ZnO(20A)∕Co0.7Fe0.3(xA)]25 multilayers (MLs) with nominal thickness x=1, 2 and 5 has been prepared on α-Al2O3 (0001) substrate by ion-beam sputtering. The magnetic properties over a temperature range of 6–350K and structures probing by x-ray absorption spectroscopy (XAS) are reported. Above room-temperature ferromagnetism has been observed for x=1 and x=2 MLs, while superparamagnetic behavior dominates for x=5 ML. The field-cooled magnetization-temperature M(T) curves of x=1 and x=5 MLs can be fitted by a standard three-dimensional (3D) spin-wave and a Curie–Weiss model, respectively. For x=2 ML, however, neither a 3D spin-wave nor a Curie–Weiss model, but a combination of the two fits the M-T curve. The XAS studies together with the magnetic measurements further reveal that x=1 sample behaves as a diluted magnetic semiconductor (DMS) ML, while x=2 ML shows a mixed structure consisting of a minor component of DMS and a major component of CoFe clusters. A predominant cl...

The origins of ferromagnetism in Co-doped ZnO single crystalline films: From bound magnetic polaron to free carrier-mediated exchange interaction

High-quality Co-doped ZnO single crystalline films with a wide range of carrier concentration and good reproducibility have been grown by molecular beam epitaxy. After the systematic studies of the magnetic and transport properties of the films, we suggest that there are two distinct ferromagnetic mechanisms in different conductivity regimes. In the insulating regime, carriers tend to be localized, favoring the formation of bound magnetic polarons, which leads to ferromagnetism. In the metallic regime, however, most carriers are weakly localized and the free carrier-mediated exchange is dominant. Our experimental observations are well consistent with the recent theoretical description of magnetism in Co-doped ZnO and helpful for understanding the ferromagnetic mechanism in oxide-based diluted magnetic semiconductors.

Effects of hydrogenated annealing on structural defects, conductivity, and magnetic properties of V-doped ZnO powders

The structure, electrical property, and magnetism of 7.5% V-doped ZnO powders that were hydrogenated annealed at 300, 500, and 700°C have been systematically investigated. The saturation magnetization and conductivity of the V:ZnO powders increase with hydrogenated annealing temperature (Tha), and the magnetization-temperature curves transform from paramagnetism to weak ferromagnetism (∼10−4μB∕V atom) as Tha increases. The results suggest that ferromagnetism in V:ZnO powders is highly correlated to the structural defects.

Influence of crystal structure on the perpendicular magnetic anisotropy of an epitaxial CoPt alloy

By molecular beam epitaxy CoPt1.1 alloys were simultaneously prepared on Mo seeding layers on Al2O3(11–20), (1–102), and (1–100) substrates, respectively. Distinct crystal structures and chemical ordering of the CoPt1.1 alloys were observed for substrate temperatures of 300 and 400 °C. Structural and magnetic observations for CoPt1.1 alloys grown on separate sapphire substrates show that the appearance of the ordered L11(111) phase results in an enhancement of the perpendicular magnetic anisotropy and Kerr rotations in the CoPt alloys.

Inspection of magnetic semiconductor and clustering structure in CoFe-doped ZnO films by bias-dependent impedance spectroscopy

Diluted magnetic semiconductor and cluster dominated structure of CoFe-doped ZnO films have been systematically investigated by bias-dependent impedance spectroscopy. The complex impedance spectroscopy of 5mol% CoFe-doped ZnO film can be fitted by an equivalent circuit employing two sets of parallel resistance (R) and capacitance (C) components in series, representing the oxide grain and grain boundary contribution, respectively. For 10mol% CoFe-doped ZnO film, a third RC component together with a single resistance element, which are likely due to the presence of metal clusters and metal-oxide interface, have to be taken into account to fit the impedance spectroscopy. By applying a dc bias of 0∼1.5V, the relaxation contribution from different structural origin can be clearly identified. The bias-dependent impedance spectroscopy demonstrates significant sensitivity to the formation of CoFe clusters in ZnO.

Ferromagnetism in Tb doped ZnO nanocrystalline films

Nanocrystalline Tb-doped ZnO films have been prepared by ion-beam sputtering technique. Magnetic characterization showed that the films are ferromagnetic with Curie temperature (TC) higher than room temperature. By further treated with a rapid thermal annealing process, both the grain size and the carrier concentration of the films increase, while the saturation magnetization of the films decreases. This magnetic behavior can be hardly explained by either bound magnetic polaron model or free carrier mediation model, thus suggests that the grain boundaries play a key role for the origin of ferromagnetism in these films.

Epitaxial growth and characterization of (100) and (110) permalloy films

Abstract High-quality, single-crystal fcc (100) and (110) permalloy films were epitaxially grown on MgO(100) and MgO(110) substrates, respectively, while polycrystalline structures were established on Si(111) substrates. The excellent crystal growth of the permalloy films on the MgO substrates was evidenced by fine streaks in reflection high energy electron diffraction and X-ray diffraction. Low-temperature magnetic hysteresis measurements show that the polycrystalline permalloy films grown on the silicon substrates are magnetically hard with coercive fields of ∼ 15–20 Oe. On the other hand, (100) and (110) permalloy films are magnetically softer with coercive fields of about 1 and 6 Oe, respectively.

Room temperature anomalous Hall effect in Co doped ZnO thin films in the semiconductor regime

Observation of the room temperature (RT) anomalous Hall effect (AHE) and ferromagnetism in semiconducting like (carrier concentration ∼1019cm−3) Co-doped ZnO samples is reported. These small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic diluted magnetic oxide (DMO) effect with spin polarized carriers. The contribution to the DMO effect depends on the types of carriers and how they incorporated into the electric conduction, magnetic coupling, and the coupling between them. These findings can provide useful information in the study of the origin of RT ferromagnetism in ZnO-based DMO and for further application in spintronics.