Defang Shen

Formation of p-type MgZnO by nitrogen doping

A wurtzite N-doped MgZnO film with 20at.% Mg (MgZnO:N) was grown by plasma-assisted molecular beam epitaxy on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown MgZnO:N film behaves n-type conduction at room temperature, but transforms into p-type conduction after annealed for 1h at 600°C in an O2 flow. The p-type MgZnO:N has a hole concentration of 6.1×1017cm−3 and a mobility of 6.42cm2∕Vs. X-ray photoelectron spectroscopy measurement indicates that substitution of N for O site is in forms of N atom (N)O and N molecule (N2)O for the as-grown MgZnO:N, but almost only in a form of (N)O for the annealed MgZnO:N. The mechanism of the conduction-type transition induced by annealing is discussed in the present work.

Zn0.76Mg0.24O homojunction photodiode for ultraviolet detection

Zn0.76Mg0.24O p-n photodiode was fabricated on (000l) Al2O3 substrate by plasma-assisted molecular beam epitaxy. Ni∕Au and In metals deposited using vacuum evaporation were used as p-type and n-type contacts, respectively. Current-voltage measurements on the device showed weak rectifying behavior. The photodetectors exhibited a peak responsivity at around 325nm. The ultraviolet-visible rejection ratio (R325nm∕R400nm) of four orders of magnitude was obtained at 6V bias. The photodetector showed fast photoresponse with a rise time of 10ns and fall time of 150ns. In addition, the thermally limited detectivity was calculated as 1.8×1010cmHz1∕2∕W at 325nm, which corresponds to a noise equivalent power of 8.4×10−12W∕Hz1∕2 at room temperature.

p-type conductivity and donor-acceptor pair emission in Cd1−xFexS dilute magnetic semiconductors

Cd1−xFexS thin films with different Fe contents were grown on c-plane sapphire by low-pressure metal organic chemical vapor deposition. The resistivity of the thin films was found to increase with the addition of more Fe contents into the Cd1−xFexS thin films by elevating the growth temperature, which was attributed to the ionization of holes from trivalent Fe ions. High density Fe doping eventually reversed the conductivity of Cd1−xFexS thin film from n to p type. With increasing Fe content, the band-to-band transition at 2.5eV was suppressed while the emission from the donor-acceptor pairs at 2.0–2.4eV finally dominated the photoluminescence spectra.

High-coercivity CoPt alloy films prepared at ultra-high Ar pressure

Large coercivity of 4.8 KOe with fine grains and smooth surface was obtained in Co0.27Pt0.73 alloy film deposited directly on heated glass by sputtering an alloy target at ultrahigh Ar pressure of 20 Pa. The effect of Ar pressure on the microscopic structure and magnetic anisotropy was investigated. The increase of the coercivity with the sputtering pressure was attributed to the increase of the perpendicular magnetic anisotropy but not the change of microscopic structure. The similarity of Kerr hysteresis loops of a film on and without substrate showed clearly that the perpendicular magnetic anisotropy did not result from external strain. Strong evidence is shown that there was Go-rich phase segregation in the films with strong perpendicular magnetic anisotropy

CoPt alloy films with saturated remanence ratio prepared by sputtering a composite target

Abstract CoPt alloy films with perpendicular magnetic anisotropy, 100% remanence, large coercivity and Kerr rotation were prepared by sputtering a composite target without any buffer layer. Effects of sputtering pressure, substrate temperature, film thickness and Co content on the magnetic and magnero-optical properties of the films are studied. X-ray diffraction analysis shows that the perpendicular magnetic anistropy in CoPt alloy films is not directly related to the ordered CoPt3 L12.

Ferromagnetic resonance study of Co/Pt multilayers

Abstract In this paper the results of ferromagnetic resonance study of sputtered Co/Pt multilayers, with a fixed Co layer thickness of 15 A, are presented. For these multilayers with small Pt layer thickness, in addition to a uniform resonance mode, a spin wave resonance mode is found to exist, which confirms the existence of an interlayer coupling between the neighboring Co layers. Simultaneously, the effective magnetization 4πMeff and the resonance linewidth ΔH are found to change correlatively with varying Pt layer thickness. This phenomenon can be explained as a result of an interplay between the interlayer coupling and the low-dimensional effect.

Apparatus for statically testing the properties of magnetic coupling and hybrid recording thin films

We designed and built up an apparatus for measuring the properties of magnetic coupling thin films and hybrid recording media. The temperature of the tested points of the thin film samples change as irradiated by focused laser beam through adjusting laser power. By calculation, we also simulated the distribution of temperature in TbFeCo magneto-optical films irradiated by laser, in order to get the relationship between the film coercive force and the irradiating laser power. Using this apparatus, we can determine the Curie temperature and compensation temperature of the films. The apparatus might present an effective approach for studying the variation of magneto-optical characteristics of films at different temperatures and the properties of magnetic coupling multilayer films, and it could be useful for studying hybrid recording.

Analysis on the magnetic capping effect in heat assisted magnetic recording

The use of magnetic capping effect can enhance the head sensitivity of the heat assisted magnetic recording (HAMR). The principle of this effect was analyzed theoretically. The GdFeCo film was used as the capping layer and TbFeCo film was used as the recording layer. Through theoretical calculation we found that a large capping field in the same direction with the external field can be generated at the interface because of the exchange coupling effect, which had increased the effective strength of the external field. Thus, the head sensitivity was improved greatly and the data transfer rate could be faster. We expected that this idea can be used to help write the medium with much larger cocervity such as L10FePt.

Magnetic and magneto-optical properties of Co-Pt alloy films before and after vacuum annealing

Co-Pt alloy films with perpendicular magnetic anisotropy and 100 percent remanence ratio were successfully prepared using e-beam coevaporation. A series of vacuum annealing was done on these alloy samples to study their thermal stability. The experimental results were as follows: a) For annealing temperature at 200 to approximately 300 degrees C, the Kerr rotation angle was almost unchanged or presented a slow decrease with the lasting of annealing, and the increase in coercivity was also small; b) For annealing temperature above 300 degrees C, both coercivity and Kerr rotation angle dropped prominently, and in the mean time magnetization became partly in-plane; c) Thermal stability is also probably a problem that needs to be solved for CoPt alloy films. The change of magnetic and magneto-optical properties of Co-Pt alloy films during annealing was considered to be related to the strength variation of different textures in films.

Study on the coercivity variation in Pt/Co multilayer films after vacuum annealing

Effects of vacuum annealing on the coercivity in sputtered and evaporated Pt/Co multilayer films were detailedly investigated. Our experimental results showed that the coercivity experienced an initial increase and a subsequent decrease with the process of annealing. The saturation magnetization decreased slowly at initial annealing stage and then dropped drastically when the annealing temperature was further raised. The coercivity variation after vacuum annealing can be explained by the improvement of f.c.c. (111) texture and the reduction of Co layer thickness.