GdN Films Research Articles

X-ray reciprocal space mapping analysis of ferromagnetic GdN films grown by pulsed laser epitaxy

Epitaxial thin films of ferromagnetic rare-earth nitride, GdN, were synthesized using pulsed laser deposition (PLD) on (001) MgO, pseudo-cubic (001) YAlO3, and (001) TiN buffered (001) MgO substrates. X-ray high-resolution reciprocal space mappings confirmed the epitaxial relationship between GdN and the substrates. The use of a TiN buffer layer changed the growth direction of the GdN films from (001) to (111). The ferromagnetic behavior of the films was characterized, and it was found that the magnetic easy axis could be tuned according to the crystal growth direction. These results suggest that PLD is a viable method for synthesizing epitaxial GdN films with tunable magnetic properties. The ability to control the crystal growth direction and magnetic easy axis of GdN films could be useful for developing spintronic devices.

Spin-Split Joint Density of States in GdN

We present an investigation of the optical constants of the near stoichiometric GdN films. Transmission and reflection spectra are collected for the paramagnetic and the ferromagnetic GdN in the photon energy range of 0.5–5.5 eV. In the ferromagnetic phase, behaviors of minority and majority spin states are specifically focussed on, which indicate spin-split joint density of states. The results confirm the LSDA+U estimates of energy gap associated with the majority-spins and also the magnitude of spin splitting.

Highly resistive epitaxial Mg-doped GdN thin films

We report the growth by molecular beam epitaxy of highly resistive GdN, using intentional doping with magnesium. Mg-doped GdN layers with resistivities of 103 Ω cm and carrier concentrations of 1016 cm−3 are obtained for films with Mg concentrations up to 5 × 1019 atoms/cm3. X-ray diffraction rocking curves indicate that Mg-doped GdN films have crystalline quality very similar to undoped GdN films, showing that the Mg doping did not affect the structural properties of the films. A decrease of the Curie temperature with decreasing the electron density is observed, supporting a recently suggested magnetic polaron scenario [F. Natali, B. J. Ruck, H. J. Trodahl, D. L. Binh, S. Vézian, B. Damilano, Y. Cordier, F. Semond, and C. Meyer, Phys. Rev. B 87, 035202 (2013)].

Molecular beam epitaxy of ferromagnetic epitaxial GdN thin films

The structural, magnetic and electrical properties of a series of GdN thin films grown by molecular beam epitaxy on AlN on silicon substrates have been studied. In situ scanning tunneling microscopy and reflection high energy electron diffraction evidence the formation of three-dimensional GdN triangle-shaped islands in the early stage of the growth which coalesce as growth develops. It is then shown by X-ray diffraction that the thickness and the growth temperature play a key role in determining the structural properties of the GdN films and optimum growth conditions are found. A significant structural dependence of the magnetic properties is observed with results suggesting superparamagnetism in very thin films while thicker films show uniform ferromagnetism.

Giant optical splitting in the spin-states assisting a sharp magnetic switching in GdN thin films

The spin-states splitting and spin ordering in the band structure of GdN thin films have been studied using optical absorbance spectroscopy and magnetometer. Remarkably, the optical absorbance measurements indicate a giant splitting in the spin-states near-infrared frequency regime, through which the spin-up and spin-down states of the GdN films have been evaluated. Both the spin-up and spin-down states split subsequently wider, which attributes to a combining effect from the nitrogen vacancies, and the population of electronic states pertaining to the spins associated with 4f states of Gd3+ ions. While spin-splitting energy enhances, magnetic-hysteresis loops suggest a sharp magnetic switching feature.

Tuning optical and ferromagnetic properties of thin GdN films by nitrogen-vacancy centers

AlN/GdN/AlN double heterostructures were grown on c-sapphire substrates using a reactive rf sputtering method under high vacuum conditions. The optical absorption spectrum of the GdN shows a clear fundamental band edge of GdN around 800 nm; this transition is attributed to the minority spin band energy of GdN at the X point. Nitrogen vacancy centers cause a blue-shift of the optical band edge of GdN, which could be ascribed to both the band filling, and the electron-hole interactions resulting from the free carriers generated by nitrogen vacancies. Temperature-dependent magnetization measurements demonstrate a clear change in the magnetization values of GdN with respect to the N2 partial pressure. Nitrogen vacancy centers in the thin GdN film raise the Curie temperature from 31 K to 39 K, which has been accurately measured by the Arrott plots.

Enhanced Curie temperature in N-deficient GdN

Polycrystalline GdN thin films have been grown at room temperature with varying N2 pressure. By varying the nitrogen pressure during growth we alter the carrier concentrations of the films. Films grown at low nitrogen pressures display onset of magnetization at temperatures as high as 200 K and a resistivity of 0.3 mΩ cm, whereas films grown at high nitrogen pressures all show a Curie temperature very close to 70 K and resistivity ranges over 1–1000 Ω cm are observed. For all GdN films a peak in the resistivity occurs at TC.

Epitaxial growth of GdN on silicon substrate using an AlN buffer layer

We report on the epitaxial growth of the intrinsic ferromagnetic semiconductor GdN on Si (1 1 1) substrates buffered by a thick AlN layer, forming a heteroepitaxial system with promise for spintronics. Growth is achieved by depositing Gd in the presence of unactivated N 2 gas, demonstrating a reactivity at the surface that is sufficient to grow near stoichiometric GdN only when the N 2:Gd flux ratio is at least 100. Reflection high-energy electron diffraction and X-ray diffraction show fully (1 1 1)-oriented epitaxial GdN films. The epitaxial quality of the films is assessed by Rutherford backscattering spectroscopy carried out in random and channelling conditions. Magnetic measurements exhibit a Curie temperature at 65 K and saturation magnetisation of 7 μ B/Gd in agreement with previous bulk and thin-film data. Hall effect and resistance data establish that the films are heavily doped semiconductors, suggesting that up to 1% of the N sites are vacant.

Electronic band structure information of GdN extracted from x-ray absorption and emission spectroscopy

The electronic structure of GdN films grown by pulsed laser deposition has been investigated by soft x-ray absorption (XAS) and x-ray emission spectroscopy (XES) at the N K-edge. Density functional calculations within the local spin density approximation with Hubbard-U corrections of the N p weighted bands and density of states are used to extract band information from the spectra. Gd M4,5 XAS and XES spectra are also presented. The XES-XAS separation is shown to give information on the f-band spin splitting and the XAS line shapes are shown to reflect atomic multiplet effects.

GdN (1 1 1) heteroepitaxy on GaN (0 0 0 1) by N 2 plasma and NH 3 molecular beam epitaxy

We report on the heteroepitaxial growth of thin films of rocksalt GdN on c-plane (0 0 0 1) wurtzite GaN by molecular beam epitaxy (MBE) using either an N 2 plasma or NH 3 as the nitrogen source. In both cases, epitaxial films with fully oriented GdN (1 1 1)∥GaN (0 0 0 1) were deposited as demonstrated by θ–2 θ X-ray diffraction. φ scans of GdN peaks demonstrate 6-fold symmetry along the growth axis implying the presence of two 3-fold-symmetric GdN (1 1 1) crystal variants in-plane. Electrical transport and magnetometry measurements on films grown using N 2 plasma show that these GdN films are ferromagnetic below T C=70 K and degenerately doped or metallic from 10 to 300 K with magnetotransport signatures associated with T C.

Ion beam analysis of rare earth nitride thin films

We report compositional measurements on highly disordered GdN, DyN, ErN and SmN thin films, grown using ion-assisted deposition and capped with GaN AlN and Al, grown using the same technique. Ion beam analysis technique of RBS, PIXE and nuclear reaction analysis (NRA) were used to determine the composition of the capped films ex situ, and show that GaN and AlN protects the GdN, DyN and SmN films from oxidation over a timescale of at least a few days. NRA depth profiles indicate that oxygen is incorporated into the films during deposition and is located at the GaN/GdN interface. The ion beam analysis measurements showed that stoichiometric ratios can be obtained and oxygen impurities are significantly reduced by varying the film deposition parameters. The successful protection of the rare earth (RE) nitride films from oxidation allows for a reliable analysis of the RE films in the as-deposited state.

Epitaxial gadolinium nitride thin films

GdN thin films are deposited on MgO(100) by low-energy ion-beam-assisted molecular-beam epitaxy at elevated temperatures. Elemental analysis by secondary-ion mass spectrometry proves that a protective layer is imperative to avoid oxidation of the GdN films in air. In situ surface structural investigation of the growing GdN films by reflection high-energy electron diffraction reveals epitaxial film growth. This result is confirmed by x-ray diffraction structure and texture analysis. Accordingly, the GdN films on MgO(100) exhibit cube-on-cube epitaxy. Due to the epitaxial growth the crystalline quality of the films is by far higher than that of films previously reported of in literature.

Structural and optical properties of sputtered gadolinium nitride films

Rare-earth (RE) materials have high magnetic moments and form a wide range of magnetic structures. There has been speculation in the literature that rare-earth nitrides may form half-metallic ferromagnetics. This is surprising because, based on a simple ionic model, trivalent rare-earth nitrides would be expected to be insulators with a similar electronic structure to the divalent rare-earth chalcogenides. However if it is the case that they are half-metallic or narrow gap insulators, then they have potential applications in spin-filtering devices. In the present investigation, We have deposited GdN films on glass substrate at room temperature by Ar/N2 mixed gas plasma-radio frequency (rf) sputtering method. The structure and the complex optical properties as well as the energy gap of GdN thin films as a function of N2 partial pressure are determined.