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GdN is a ferromagnetic semiconductor which has seen increasing interest in the preceding decades particularly in the areas of spin- and superconducting- based electronics. Here we report a detailed computational study and optical spectroscopy study of the electronic structure of stoichiometric and nitrogen vacancy doped GdN. Based on our calculations we provide the effective mass tensor for undoped GdN, and some indicative values for electron doped GdN. Such a property is valuable as it can directly affect device design, and be directly measured experimentally to validate the existing computation results.

Nitrogen vacancy doped SmN$_{1-\delta}$ is a semiconductor which lies in the intermediary between insulating-ferromagnetic SmN and metallic-anti-ferromagnetic Sm. The dopant electrons resulting from nitrogen vacancies have recently been predicted to lie in a band precipitated by a majority-spin 4$f$ level on the six Sm ions neighbouring each nitrogen vacancy, an in-gap state $\sim$1 eV below the 4$f$ states in stoichiometric SmN. Optical data reported here corroborate the prediction along with an extended computational study. Electrical transport measurements show the transition from an insulating to metallic state with a hopping type conductivity in dilutely doped films. We provide strong evidence that electron transport is mediated by a dispersion-less majority spin defect band implying triplet type superconductivity, the location of which in the SmN-Sm phase diagram suggests the location of a quantum critical point.

The rare-earth nitrides form a series of structurally simple $intrinsic$ ferromagnetic semiconductors, a rare class of both fundamental interest and application potential. Within the series there is a wide range of magnetic properties relating to the spin/orbit contributions to the ferromagnetic ground states. We report an x-ray magnetic circular dichroism investigation of the spin/orbit magnetic dipole alignments of Sm and Gd ions in epitaxial Gd$_x$Sm$_{1-x}$N films. The Sm spin-alignment expectation value $\langle S_{z} \rangle$ is seen to be strengthened by the Gd/Sm exchange interaction, providing guidance concerning the composition for an angular momentum compensation point (where the volume-averaged total angular momentum of a film is zero)., Comment: 7 Pages, 7 Figures. Submitted to Phys. Rev. B

We report the growth of films of the intrinsic ferromagnetic semiconductor NdN, and an investigation of their optical and transport properties. There is clear evidence of a strong anomalous Hall effect as expected from a 4f conduction channel, supported by an optical absorption into a 4f or 4f/5d hybridized tail at the base of the conduction band. The results reveal a heavy-fermion 4f/5d band lying where it can be occupied at controllable levels with nitrogen-vacancy donors., Comment: 6 pages, 5 figures

The rare-earth nitride ferromagnetic semiconductors owe their varying magnetic properties to the progressive filling of 4f shell across the series. Recent electrical transport measurements on samarium nitride, including the observation of superconductivity, have been understood in terms of a contribution from a 4f transport channel. Band structure calculations generally locate an empty majority 4f-band within the conduction band although over a wide range of possible energies. Here we report optical reflection and transmission measurements on samarium nitride between 0.01 eV to 4 eV, that demonstrate clearly that the 4f band forms the bottom of the conduction band. Results at the lowest energies show no free carrier absorption, indicating a semiconducting ground state, and supporting earlier conclusions based on transport measurements., Comment: 5 pages, 4 figures

We report perpendicular magnetic anisotropy in the ferromagnetic Heusler alloy Co$_2$MnGa in a MgO/Co$_2$MnGa/Pd trilayer stack for Co$_2$MnGa thicknesses up to 3.5 nm. There is a thickness- and temperature-dependent spin reorientation transition from perpendicular to in-plane magnetic anisotropy which we study through the anomalous Hall effect. From the temperature dependence of the anomalous Hall effect, we observe the expected scaling of $\rho_{xy}^{AHE}$ with $\rho_{xx}$, suggesting the intrinsic and side-jump mechanisms are largely responsible for the anomalous Hall effect in this material.

SmN is a ferromagnetic semiconductor with the unusual property of an orbital-dominant magnetic moment that is largely cancelled by an antiparallel spin contribution, resulting in a near-zero net moment. However, there is a basic gap in the understanding of the ferromagnetic ground state, with existing density functional theory calculations providing values of the $4f$ magnetic moments at odds with experimental data. To clarify the situation, we employ an effective $4f$ Hamiltonian incorporating spin-orbit coupling, exchange, the crystal field, and $J$-mixing to calculate the ground state $4f$ moments. Our results are in excellent agreement with experimental data, revealing moderate quenching of both spin and orbital moments to magnitudes of $\sim 2~\mu_B$ in bulk SmN, enhanced to an average of $\sim 3~\mu_B$ in SmN layers within a SmN/GdN superlattice. These calculations provide insight into recent studies of SmN showing that it is an unconventional superconductor at low temperatures and displays twisted magnetization phases in magnetic heterostructures.

The discovery of materials that simultaneously host different phases of matter has often initially confounded, but ultimately enhanced, our basic understanding of the coexisting types of order. The associated intellectual challenges, together with the promise of greater versatility for potential applications, have made such systems a focus of modern materials science. In particular, great efforts have recently been devoted to making semiconductors ferromagnetic and metallic ferromagnets superconducting. Here we report the unprecedented observation of a heavily donor-doped ferromagnetic semiconductor, SmN, becoming superconducting with ferromagnetism remaining intact. The extremely large exchange splitting of the conduction and valence bands in this material necessitates that the superconducting order hosted by SmN is of an unconventional triplet type, most likely exhibiting p-wave symmetry. Short range spin fluctuations, which are thought to be the cause of pairing interactions in currently known triplet superconductors, are quenched in SmN, suggesting its superconductivity to be the result of phonon- or Coulomb-mediated pairing mechanisms. This scenario is further supported by the inferred heavy mass of superconducting charge carriers. The unique near-zero magnetisation associated with the ferromagnetic state in SmN further aids its coexistence with superconductivity. Presenting this novel material system where semiconducting, ferromagnetic and superconducting properties are combined provides a versatile new laboratory for studying quantum phases of matter. Moreover it is a major step towards identifying materials that merge superconductivity and spintronics, urgently needed to enable the design of electronic devices with superior functionality., Comment: 8 pages, 5 figures, main text plus supplemental material

The strong spin-orbit interaction in the rare-earth elements ensures that even within a ferromagnetic state there is a substantial orbital contribution to the ferromagnetic moment, in contrast to more familiar transition metal systems, where the orbital moment is usually quenched. The orbital-dominant magnetization that is then possible within rare-earth systems facilitates the fabrication of entirely new magnetic heterostructures, and here we report a study of a particularly striking example comprising interfaces between GdN and SmN. Our investigation reveals a twisted magnetization arising from the large spin-only magnetic moment in GdN and the nearly zero, but orbital-dominant, moment of SmN. The unusual twisted phase is driven by (i) the similar ferromag- netic Gd-Gd, Sm-Sm and Gd-Sm exchange interactions, (ii) a SmN Zeeman interaction 200 times weaker than that of GdN, and (iii) the orbital-dominant SmN magnetic moment. The element specificity of X-ray magnetic circular dichroism (XMCD) is used in seperate modes probing both bulk and surface regions, revealing the depth profile of the twisting magnetization., Comment: 15 pages, 4 figures, supplementary material

We report the growth by molecular beam epitaxy of highly resistive GdN, using intentional doping with magnesium. Mg-doped GdN layers with resistivities of 1000 {\Omega}.cm and carrier concentrations of 10E16 cm-3 are obtained for films with Mg concentrations up to 5 x 10E19 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 et al., Phys. Rev. B 87, 035202 (2013)].

Europium nitride is semiconducting and contains non-magnetic \3+, but sub-stoichiometric EuN has Eu in a mix of 2+ and 3+ charge states. We show that at \2+ ~concentrations near 15-20% EuN is ferromagnetic with a Curie temperature as high as 120 K. The \3+ ~polarization follows that of the \2+, confirming that the ferromagnetism is intrinsic to the EuN which is thus a novel diluted magnetic semiconductor. Transport measurements shed light on the likely exchange mechanisms., Comment: 5 pages

We present both electric-field and photo-excited control of the carrier concentration in GdN. There is no evidence in the results of a carrier-mediated contribution to the Gd-Gd exchange interaction that has been suggested to explain a measured Curie temperature that is much higher than obtained within theoretical treatments. Persistent carrier concentrations seen in both the field-effect and photo-induced conductivities point to a distribution of long-lived trap states below the conduction band, very likely centred at nitrogen vacancies

SmN is ferromagnetic below 27 K, and its net magnetic moment of 0.03 Bohr magnetons per formula unit is one of the smallest magnetisations found in any ferromagnetic material. The near-zero moment is a result of the nearly equal and opposing spin and orbital moments in the 6H5/2 ground state of the Sm3+ ion, which leads finally to a nearly complete cancellation for an ion in the SmN ferromagnetic state. Here we explore the spin alignment in this compound with X-ray magnetic circular dichroism at the Sm L2,3 edges. The spectral shapes are in qualitative agreement with computed spectra based on an LSDA+U (local spin density approximation with Hubbard-U corrections) band structure, though there remain differences in detail which we associate with the anomalous branching ratio in rare-earth L edges. The sign of the spectra determine that in a magnetic field the Sm 4f spin moment aligns antiparallel to the field; the very small residual moment in ferromagnetic SmN aligns with the 4f orbital moment and antiparallel to the spin moment. Further measurements on very thin (1.5 nm) SmN layers embedded in GdN show the opposite alignment due to a strong Gd-Sm exchange, suggesting that the SmN moment might be further reduced by about 0.5 % Gd substitution.

We report measurements of the optical response of polycrystalline DyN thin films. The frequency-dependent complex refractive index in the near IR-visible-near UV was determined by fitting reflection/transmission spectra. In conjunction with resistivity measurements these identify DyN as a semiconductor with 1.2 eV optical gap. When doped by nitrogen vacancies it shows free carrier absorption and a blue-shifted gap associated with the Moss-Burstein effect. The refractive index of 2.0+/-0.1 depends only weakly on energy. Far infrared reflectivity data show a polar phonon of frequency 280 cm-1 and dielectric strength delta epsilon= 20.

A major challenge for the next generation of spintronics devices is the implementation of ferromagnetic-semiconductor thin films as spin injectors and detectors. Spin-polarised carrier injection cannot be accomplished efficiently from metals, and coupled with the rarity of intrinsic ferromagnetic semiconductors this has driven intensive study of diluted magnetic semiconductors. Chief among these is the doped III-V compound (Ga,Mn)As. These materials suffer from a number of drawbacks; they (i) require magnetic-ion doping well above the solubility limit, and (ii) must be hole doped to above the degenerate limit, preventing independent control of the carrier concentration and charge sign. Here we demonstrate the first epitaxial growth of a recently-characterised intrinsic ferromagnetic semiconductor, GdN, on silicon substrates, providing an essential step on the way to integrate new spintronics functionalities into Si-based technology. The films have been characterised as regards their growth toward fully relaxed GdN, the density and mobility of their carriers, and their magnetic behaviour.

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Soft x-ray emission and absorption spectroscopy of the O K-edge are employed to investigate the electronic structure of wurtzite ZnO(0001). A quasiparticle band structure calculated within the GW approximation agrees well with the data, most notably with the energetic location of the Zn3d - O2p hybridized state and the anisotropy of the absorption spectra. Dispersion in the band structure is mapped using the coherent k-selective part of the resonant x-ray emission spectra. We show that a more extensive mapping of the bands is possible in the case of crystalline anisotropy such as that found in ZnO., Comment: 5 pages, 5 figures

The magnetic behaviour of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature (T_c) of 27 +/- 3 K, evidenced by the occurrence of hysteresis below T_c. The ferromagnetic state is characterised by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetisation at 2 K, measured after cooling in the maximum field, is 0.035 mu_B per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for Sm3+ in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field . The susceptibility in the paramagnetic phase shows temperature-independent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the J=7/2 excited state and the 5/2 ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of 0.4 mu_B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm3+ ions., Comment: 5 pages, 3 figures

We report measurements of the optical gap in a GdN film at temperatures from 300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is 1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split bands below the Curie temperature. The paramagnetic gap is larger than was suggested by very early experiments, and has permitted us to refine a (LSDA+U)-computed band structure. The band structure was computed in the full translation symmetry of the ferromagnetic ground state, assigning the paramagnetic-state gap as the average of the majority- and minority-spin gaps in the ferromagnetic state. That procedure has been further tested by a band structure in a 32-atom supercell with randomly-oriented spins. After fitting only the paramagnetic gap the refined band structure then reproduces our measured gaps in both phases by direct transitions at the X point., Comment: 5 pages, 4 figures

We investigate the electronic band structure of two of the rare-earth nitrides, DyN and SmN. Resistivity measurements imply that both materials have a semiconducting ground state, and both show resistivity anomalies coinciding with the magnetic transition, despite the different magnetic states in DyN and SmN. X-ray absorption and emission measurements are in excellent agreement with LSDA+U calculations, although for SmN the calculations predict a zero band gap., Comment: 2 figures

The structure of disordered GaN:O films grown by ion-assisted deposition is investigated using x-ray absorption near-edge spectroscopy and Raman spectroscopy. It is found that between 4 and 21 % of the nitrogen in the films is in the form of molecular N_2 that interacts only weakly with the surrounding matrix. The anion to cation ratio in the GaN:O host remains close to unity, and there is a close correlation between the N_2 fraction, the level of oxygen impurities, and the absence of short-range order in the GaN:O matrix., Comment: 5 pages, 3 figures

Ion assisted deposition (IAD) has been investigated for the growth of GaN, and the resulting films studied by x-ray diffraction and absorption spectroscopy and by transmission electron microscopy. IAD grown stoichiometric GaN consists of random-stacked quasicrystals of some 3 nm diameter. Amorphous material is formed only by incorporation of 15% or more oxygen, which we attribute to the presence of non-tetrahedral bonds centered on oxygen. The ionic favourability of heteropolar bonds and its strikingly simple constraint to even-membered rings is the likely cause of the instability of stoichiometric a-GaN., Comment: 4 pages, 3 figures

We have developed a continuum model that explains the complex surface shapes observed in epitaxial regrowth on micron scale gratings. This model describes the dependence of the surface morphology on film thickness and growth temperature in terms of a few simple atomic scale processes including adatom diffusion, step-edge attachment and detachment, and a net downhill migration of surface adatoms. The continuum model reduces to the linear part of the Kardar-Parisi-Zhang equation with a flux dependent smoothing coefficient in the long wavelength limit., Comment: 11 pages, 4 figures. Submitted to the Journal of Crystal Growth