Here we report our study of the electronic properties of [100]-textured gadolinium nitride (GdN) thin films synthesized using a chemical vapor deposition (CVD) method. The electronic properties of the films were investigated using photoemission and inverse photoemission spectroscopy coupled with computational modeling. Our density functional theory (DFT) calculations suggest that the theoretically predicted half-metallic electronic structure of GdN is likely due to its low density of states (DOS) at the Fermi level. These calculations are supported by our photoemission and inverse photoemission spectroscopic measurements which show a band gap for the prepared films of a few milli-electron volts, seemingly consistent with the predicted electronic structure. Additionally, the use of a CVD gallium nitride capping layer was found to decelerate the surface oxidation of our GdN samples.

Multifunctional Pr3+-doped GdPO4 nanocrystals possessing UV, visible emissions and magnetic properties were synthesized via co-precipitation and following annealing. It was revealed that as-prepared GdPO4 nanorods were transformed to nanoparticles after heat treatment at 900 °C for 2 hours. Ultimately, GdPO4:Pr3+ nanoparticles have strong UV emission under the excitation of 275 nm and visible emission under the excitation of 445 nm, which offers the optical modalities in both UV and visible range. Moreover, Gd3+-containing nanoparticles are efficient T2-weighted (negative) magnetic resonance (MR) contrast agents due to the presence of Gd3+ ions, which can be potentially applied as multimodal, photoluminescence-magnetic resonance imaging contrast agent.

We present an in-depth transmission electron microscopy (TEM) study about the character of the Gd atom distribution in epitaxial GaN:Gd thin films grown by molecular beam epitaxy. High-resolution TEM (HRTEM) imaging reveals local lattice distortions of dimensions of a few atom planes only. Geometric phase analysis of HRTEM lattice images quantifies the associated displacement field. The results are explained by means of thin coherently strained GdN clusters with platelet shape being located along the basal plane. This is consistent with the observations obtained from strain contrast dark-field TEM images. Theoretically derived structure models provided by calculations based on density functional theory are used to simulate the HRTEM contrast and to determine the corresponding displacement field for matching the experimental data. Best fit is achieved in case of a coherent GdN bi-layer cluster that conclusively reflects the energy favorable configuration. The formation of the platelet clusters is explainable in the framework of spinodal decomposition.

The mechanism leading to RT ferromagnetism in Gd-doped GaN is not agreed upon, despite many experimental and theoretical reports. Oxygen impurities have been proposed as a possible contributor to ferromagnetic behavior in GaN:Gd films. In this report, GaN:Gd thin films grown by MOCVD using two different metalorganic Gd precursors are examined. The two precursors are (TMHD)3Gd, which contains oxygen, and Cp3Gd, which does not. The films have been characterized by XRD, VSM, and EDS. EDS measurements indicate that the TMHD3Gd samples contain oxygen, while the Cp3Gd samples do not, and VSM scans show that the TMHD3Gd samples exhibit much higher magnetic moments than the Cp3Gd samples, supporting the theory that oxygen enhances the ferromagnetic behavior of GaN:Gd.

In order to realize the magnetic refrigeration system, it is necessary to develop a 100 W class refrigerator with COP > 7.5. This requires us to find new magnetic refrigerant materials, of which cooling capacity is 2.5 times higher than that of Gd. In this paper, first we discuss the cooling capacity of magnetic refrigerant materials to achieve COP = 7.5. Then, we compare the experimental results of MnAsSb, MnFe(PGe) and La(FeCoSi)13 compounds with the calculated cooling capacity. It is suggested that a composite layer material of MnFe(PGe) would show excellent cooling capacity in the temperature span of 20 K.

In this work, a room temperature spin-polarized LED based on ferromagnetic Ga1-xGdxN is reported. The device was grown by metalorganic chemical vapor deposition (MOCVD) and is the first report of a spin-LED based on Ga1-xGdxN. Electroluminescence from this device had a degree of polarization of 14.6% at 5000 Gauss and retained a degree of polarization of 9.3% after removal of the applied magnetic field. Ga1-xGdxN thin films were grown on 2 μm GaN templates and were co-doped with Si and Mg to achieve n-type and p-type materials. Co-doping of the Ga1-xGdxN films with Si produced conductive n-type material, while co-doping with Mg produced compensated p-type material. Both Si and Mg co-doped films exhibited room temperature ferromagnetism, measured by vibrating sample magnetometry.

Electronic structure calculations are reported for Gd in GaN on Ga as well as on N site and for pairs of Gd on neighboring Ga and N sites, using the full-potential linearized muffin-tin orbital method in the local spin density approximation with Hubbard U corrections (LSDA+U). The energy of formation for the N site is found to be much higher than for the Ga site even after relaxations are included. The GdN configuration is found to be at best metastable (in ZB). In WZ and in the pair configurations, the Gd is found to move toward an interstitial site leaving a nitrogen vacancy behind. The electronic structure of these structures and their magnetic moments are discussed.

A 100 micron fragment of a b-axis oriented single crystal Gd5Si2Ge2 has been studied using microcalorimetry, enabling the separate measurement of the heat capacity and the latent heat. The sample was taken from the same crystal previously studied with Hall probe imaging, which showed that the phase transition is seeded by a second phase of Gd5Si1.5Ge1.5 nanoplatelets on the increasing field sweep direction only. The multiple transition features observed in the latent heat signature suggests a nucleation size of approximately 20 μm, consistent with the lengthscale suggested by Hall imaging. The difference in nucleation and growth process with field sweep direction is clearly identified in the latent heat. We show that the latent heat contribution to the entropy change is of the order of 50% of the total entropy change and unlike other systems studied, the transition does not broaden (and the latent heat contribution does not diminish significantly) as magnetic field and temperature are increased within the parameter range explored in these experiments.

We developed ultra-violet field-emission devices using rare-earth nitrides of Al1-xGdxN grown by a reactive radio-frequency magnetron sputtering technique. The Al1-xGdxN phosphor film excited by high-energy electrons shows a resolution limited, narrow intra-orbital luminescence from Gd3+ ions at 318 nm. The devise characteristics depend on injected current and acceleration voltage, which were analyzed by considering multiple excitation process of injected high-energy electrons.

The electronic structure of nitrogen vacancies in gadolinium nitride are studied using the full-potential linearized muffin-tin orbital method in the local spin density approximation with Hubbard U corrections (LSDA+U). The vacancy is found to have two localized defect levels in the gap, one of each spin. The third electron of each vacancy in the neutral state dopes the conduction band. The single positive state is found to be the ground state for Fermi levels located anywhere within the band gap. The vacancy has a net magnetic moment of 1 μB in the neutral charge state. The presence of the vacancy is found to increase the average exchange interactions between Gd atoms and hence the Curie temperature but only by about a factor 2 compared to GdN without vacancies.

Solid-state neutron detectors from heterostructures that incorporate Gd intrinsically or as a dopant may significantly benefit from the high thermal neutron capture cross section of gadolinium. Semiconducting devices with Gd atoms can act as a neutron capture medium and simultaneously detect the electronic signal that characterizes the interaction. Neutron capture in natural isotopic abundance gadolinium predominantly occurs via the formation of 158mGd, which decays to the ground state through the emission of high-energy gamma rays and an internal conversion electron. Detection of the internal conversion electron and/or the subsequent Auger electron emission provides a distinct and identifiable signature that neutron capture has occurred. Ensuring that the medium responds to these emissions is imperative to maximizing the efficiency and separating out other interactions from the radiation environment. A GEANT4 model, which includes incorporation of the nuclear structure of Gd, has been constructed to simulate the expected device behavior. This model allows the energy deposited from the decay of the meta-stable state to be localized and transported, providing for analysis of various device parameters. Device fabrication has been completed for Gd doped HfO2 on n-type silicon, Gd2O3 on p-type silicon and Gd2O3 on SiC for validation of the code. A preliminary evaluation of neutron detection capabilities of these devices using a GEANT4 modeling approach is presented.

The primary objective of this study is to discuss the optimum operating conditions of magnetocaloric heat pumps according to the fundamental heat transfer characteristics of an active magnetic regenerator (AMR) bed. The AMR cycle has four sequential processes: magnetization, heat exchange fluid flow, demagnetization, and heat exchange fluid blow. The fundamental heat transfer characteristics of each process of the AMR cycle is investigated minutely. Moreover, the cooling power and the overall system performance are evaluated when the system is running continuously.

In addition to the aforementioned investigation, we have developed a prototype rotational magnetocaloric heat pump having a compact component arrangement and an uncomplicated control system. A performance evaluation has been conducted to obtain the optimum conditions for practical operation. The operation parameters such as the heat transfer fluid flow rate, rotational frequency, and initial temperature of the heat transfer fluid are examined, and the variations of the maximum temperature span between the inlet and outlet for the heat transfer fluid are discussed. As a result, the values of the optimum rotational frequency and flow rate are obtained to obtain the maximum temperature span between the inlet and outlet of the present magnetocaloric heat pump.

This work reports on the epitaxial growth of crystalline high-k Gd2O3 on Si (111) by Molecular Beam Epitaxy (MBE) for CMOS gate application. Epitaxial Gd2O3 films of different thicknesses have been deposited on Si (111) between 650°C~750°C. Electrical characterizations reveal that the sample grown at the optimal temperature (700°C) presents an equivalent oxide thickness (EOT) of 0.73nm with a leakage current density of 3.6×10-2 A/cm2 at |Vg-VFB|=1V. Different Post deposition Annealing (PDA) treatments have been performed for the samples grown under optimal condition. The Gd2O3 films exhibit good stability and the PDA process can effectively reduce the defect density in the oxide layer, which results in higher performances of the Gd2O3/Si (111) capacitor.

The structure and vibrational spectrum of Gd2 and Gd2C2 endofullerenes are studied through Raman spectroscopy and universal force field (UFF) calculations. Hindered rotations, shown by both theory and experiment, indicate the formation of a Gd–cage bond, which reduces the ideal symmetry of the cage. We have conducted Raman studies of Gd2@C90, Gd2@C79N, and Gd2C2@C92. We have also studied Y2C2@C92 for comparison. Several modes have been identified which provide information about the endohedral complex.

Mg–8Gd–2Y–Nd–0.3Zn (wt%) alloy was prepared by the high pressure die-cast technique. The microstructure, mechanical properties in the temperature range from room temperature to 573 K, and strengthening mechanism were investigated. It was confirmed that the Mg–Gd-based alloy with high Gd content exhibited outstanding die-cast character. The die-cast alloy was mainly composed of small cellular equiaxed dendrites and the matrix. The long lamellar-shaped stacking compound of Mg3X (X: Gd, Y, Nd, and Zn) and polygon-shaped precipitate of Mg5RE (RE: Gd, Y, and Nd) were mainly concentrated along the dendrite boundaries. Meanwhile, it was demonstrated that the Zn addition affects the formation of non-equilibrium precipitate Mg3X. The ultimate tensile strength, yield strength, and Young’s modulus were 302 MPa, 267 MPa, and 38 GPa at room temperature, respectively. The outstanding mechanical properties were mainly attributed to the small dendrite spacing, wide skin region, and some dispersed precipitates in the alloy formed by the high-pressure die-cast technique. Designing a novel die-cast Mg alloy with good heat resistance without Al element is a significant accomplishment.

Highly crystalline and transparent Eu-doped Gd2O3 thin films were produced through a modified sol-gel method that did not require the use of any chelating agent. The effect of the atomic fraction of Eu3+ ions (‘x’ =0.05-0.30) on the structural, optical and luminescent properties has been studied. X-ray diffraction studies showed the preferential growth of Gd2-xEuxO3 thin films along the (400) plane corresponding to the cubic phase. UV-vis measurements revealed the high film transparency of the films in the visible region and a band gap value of 5.3eV. It has also been observed that the luminescence properties of the films were strongly dependent on both, the excitation wavelength and Eu concentration; the most efficient excitation conducive to red luminescence was achieved at the absorption band of Gd2O3 host (229nm). Under this condition all films exhibited strong red emission that is characteristic of Eu3+ ions. The emission intensity was also dependent on the doping level; the most intense luminescence was obtained at ‘x’=0.15. The drop in the luminescence intensity observed for ‘x’ values higher than 0.15 was attributed to quenching concentration effect.

Gd2O3 and HfO2 films exhibit photovoltaic charging that depends on temperature. The dielectric properties of the surface and concomitant photovoltaic surface charging increases following oxygen exposure under ultrahigh vacuum conditions suggesting that the surface conductivity plays a role in the surface photovoltage charging.

The magnetic characteristics of the dilute magnetic system GaGdN are investigated by mainly soft-X-ray magnetic circular dichroism (MCD) in energy range of 1160 – 1240 eV. The strong MCD signals up to 30 % at 15K are observed. The temperature dependence of its intensity is not on simple Curie-Weiss curve and depicts three-step curve. A step around 40 – 100K suggests a new magnetic phase. The luminescence spectrum of GaGdN at low temperature is divided into three parts consisting of two broad bands around 432 nm and 503 nm and a sharp peak at 652 nm. This sharp line is assigned to the intra-transition of f – f orbital owing to the weak temperature dependence of the intensity and peak position. AlGdN grown by molecular beam epitaxy produces luminescence at 318.5 nm. X-ray absorption fine structure is examined to survey the occupancy of the Gd ion in the grown specimens.

The present work addresses the synthesis and characterization of red emitting Gd2-xEuxO3 nanocrystalline phosphors by a modified sol-gel based method. The effects of the annealing temperature and atomic fraction of Eu3+ ions, ‘x’, on the structural and luminescence properties of the produced oxides have been systematically investigated. X-ray diffraction analyses revealed that crystalline cubic-Gd2O3 host structure was obtained when the intermediates (x=0.01-0.30) were annealed at different temperatures in air. Photoluminescence spectra of doped Gd2O3 powders showed all transitions of Eu3+ species, being the 5D0→7F2 transition the most intense. On a common sample-weight basis, the highest photoluminescence intensity was obtained at ‘x’ = 0.15. The energy transfer from host to dopant was verified for all evaluated ‘x’ values, which suggest the actual incorporation of Eu species into the Gd-oxide lattice. It was also found that the photoluminescence intensity was strongly dependent on the annealing temperature and dopant concentration.