Shinya Kitayama

Study on spin-splitting phenomena in the band structure of GdN

An exceptional kind of spin splitting in the band structure of AlN/GdN/AlN double heterostructures has been studied by employing temperature-dependent spectroscopy. This spin splitting can be attributed to both the band-gap shrink and the difference between minority and majority band energies in GdN below the Curie point; these results have been established by evaluating the optical band gaps at the X-point. The temperature-dependent magnetization measurements that provide direct evidence of the magnetic ordering below 32 K and it is described by long-range spin correlation in GdN.

Narrow-band deep-ultraviolet light emitting device using Al1-xGdxN

We demonstrated mercury-free narrow-band deep-ultraviolet luminescence from field-emission devices with Al1−xGdxN thin films. The Al1−xGdxN thin films were grown on fused silica substrates by a radio frequency reactive magnetron sputtering method. The deposited film shows a strong c-axis preferential orientation. A resolution limited, narrow intra-4f luminescence line from Gd3+ ions has been observed at 315nm. The luminescence spectrum depends on the growth temperature of the thin film, and the intensity varies as a function of the GdN mole fraction.

Ferromagnetic properties of GdN thin films studied by temperature dependent circular polarized spectroscopy

Magneto-optical properties of GdN thin films have been investigated by employing temperature dependent optical circular dichroism spectroscopy and angular dependent resonance field measurement. Spin-splitting in the band structure of GdN thin films has been evaluated by optical circular dichroism absorbance spectra, and the manifested spin-splitting energy ascribed to the difference between majority and minority spin band states in GdN. The plot of left circular polarization and right circular polarization bandgap reflects the half-hysteresis loop (positive side) trend, which is evidenced by magnetization measurements. The angular dependent resonance field measurements showed strong magnetic anisotropy along in-plane of GdN, which is attributed to the disturbance in the spin alignment in GdN. We demonstrate that the ferromagnetic properties depend on the film thickness. These results provide a pathway to control the spin ordering using circularly polarized light and the magnetic anisotropy.

Influence of local atomic configuration in AlGdN phosphor thin films on deep ultra-violet luminescence intensity

We investigated the narrowband ultraviolet emission properties of Al0.94Gd0.06N phosphor thin films pumped by an electron beam. An extremely narrow luminescence line, which was less than 1 nm from the intra-orbital f-f transition in Gd3+ ions, was confirmed at 318 nm. The corresponding emission efficiency was improved by decreasing the growth temperature. The extended X-ray absorption fine structure analysis of the local atomic structure revealed that a low-temperature growth led to the formation of a uniform atomic configuration around Gd, which was found to play a key role in improving the luminescence intensity of the films.

Narrowband ultraviolet field-emission device using Gd-doped AlN

We developed mercury-free narrow-band deep-ultraviolet luminescence devices which were accomplished by performing field-emission excitation of Al1−xGdxN. The narrow band emission is a typical feature of the intra-orbital electron transition in the rare-earth Gd3+ ions. AlN, GdN and Al1−xGdxN thin films were grown on fused silica substrates by using the reactive sputtering technique. A resolution limited, narrow band luminescence line from Gd3+ ions has been observed around 315 nm. Detailed luminescence characteristics depending on the GdN mole fraction and the growth temperature have been investigated.

Magneto-optical effect in GdN epitaxial thin film

We report the cooperative phenomena of magneto-optical effect in AlN/GdN/AlN heterostructure grown by reactive rf magnetron sputtering technique performed under a base pressure less than 5×10−6 Pa. The ultra-high vacuum conditions during growth of GdN enable to curtail the oxophilicity character. The temperature dependent optical absorbance measurements have been carried out to evaluate the direct optical band gaps at the X point through the Tauc plots for the heterostructure. We observed for the first time that the optical absorbance studies demonstrate an exceptional splitting in valance and conduction bands in GdN below the critical temperature. The splitting energy is slightly increased with increase of temperature range of 30–55 K and the difference is merely ΔE=28 meV. Furthermore, the splitting energy drastically increases from 30 K to 3 K and the difference is ΔE=102 meV. This startling increase convincing that there is rapid increase of magnetic moments caused by long range correlation of spins.

Correlation between local atomic structure and ultraviolet luminescence of AlGdN thin films

The present study reports on the correlation between the local atomic structure and cathodoluminescent properties of Al1−xGdxN thin films grown by reactive rf magnetron sputtering at ultra-high vacuum conditions. Those thin films were characterised using X-ray absorption fine structure (XAFS) and cathodoluminescence (CL). From the CL measurements, we have observed a narrow intense ultraviolet emission at 318 nm which is originated from the intra-orbital f-f transition in Gd3+ ions. In order to understand the local atomic structure around the Al1−xGdxN (x=0.1 to 6.0 mol%) thin film, XAFS measurements have been carried out. Analysis of the local atomic structural results showed that both the large distance among Gd atoms and nitrogen vacancies in Al1−xGdxN lattice significantly contribute to the richness in the ultraviolet emission intensity.

Multiple excitation process in deep-ultraviolet emission from AlGdN thin films pumped by an electron beam

We studied the deep-ultraviolet emission properties of Al0.999Gd0.001 N thin films pumped by an electron beam. The Al0.999Gd0.001 N thin films were grown on fused silica substrates using an ultra-pure reactive sputtering technique. The intra-orbital electron transition of the Gd3+ ions in Al0.999Gd0.001 N showed an extremely narrow luminescence line at 318 nm. We fabricated field-emission devices using an Al0.999Gd0.001 N phosphor thin film and analyzed the dependence of the device characteristics on the injected current and acceleration voltage. The maximum output power was 1.0 mW/cm2. The excitation cross section was of the order of 10−13 cm2 and was found to depend on the acceleration voltage. These results indicate that injected high-energy electrons multiply excite Gd3+ ion.

Microscopic properties of degradation-free capped GdN thin films studied by electron spin resonance

The microscopic magnetic properties of high-quality GdN thin films have been investigated by electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements. Detailed temperature dependence ESR measurements have shown the existence of two ferromagnetic components at lower temperatures, which was not clear from the previous magnetization measurements. The temperature, where the resonance shift occurs for the major ferromagnetic component, seems to be consistent with the Curie temperature obtained from the previous magnetization measurement. On the other hand, the divergence of line width is observed around 57 K for the minor ferromagnetic component. The magnetic anisotropies of GdN thin films have been obtained by the analysis of FMR angular dependence observed at 4.2 K. Combining the X-ray diffraction results, the correlation between the magnetic anisotropies and the lattice constants is discussed.

Evaluation of minority and majority spin band energies of ferromagnetic GdN thin film using optical absorption spectroscopy

Temperature dependent optical Tauc plots of AlN/GdN/AlN heterostructures have showed two optically induced transitions, and those optical transitions could be attributed to the minority and majority spin band energy. In contrast, temperature dependent magnetization measurements of GdN thin film provide direct evidence of spin ordering below 39 K, and which is also evidenced by Arrott plots.