Ferromagnetic redshift of the optical gap in GdN

Abstract

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 figure

Electronic structure of EuN: Growth, spectroscopy, and theory

“…However, optical absorption is dominated by direct transitions, whereas the minimum gap is expected to be between Γ and X in the band structure, as found previously for GdN. 17 We thus turn to an examination of the band structure calculated using QSGW , as shown in Figure 10. The calculation does indeed return a semiconducting and ferromagnetic ground state, with a direct gap at X of 0.94 eV and a minimum indirect gap between Γ and X of 0.31 eV.…”

“…33 We have previously shown that for GdN the optical absorption edge in the paramagnetic state is reasonably well described by taking an average of the spin-down and spin-up direct band gaps. 17 Following this approach we obtain a spin averaged direct QSGW gap at X of 1.59 eV for EuN, somewhat higher than the FM gap and the optical absorption onset. However, the QSGW method is known to provide a slight overestimation of band gaps for many semiconductors.…”

“…Recent progress has been made, especially in the case of GdN, with the demonstration of epitaxial film growth, [10][11][12][13][14] and studies of its electronic and magnetic properties. [15][16][17][18][19][20][21] Far fewer experimental data concerning the rest of the series are available. [22][23][24][25][26] Of particular interest are the properties of EuN.…”

“…However, the separate spin and orbital angular momenta are both large, and thus it is possible that magnetic ordering could exist in the form of "hidden ferromagnetism". 27 Accurate calculations of the electronic structure have proven difficult; for example, an LSDA+U treatment that has been shown to be accurate for GdN 3,17 was unable to determine whether EuN has Eu in the 2+ or 3+ charge state, due to the presence of Eu 4f states close to the Fermi level. 3 Thus, even the fundamental question of whether EuN is metallic or semiconducting remains unsettled.…”

Growth and oxidization stability of cubic Zr1−xGdxN solid solution thin films

“…The foremost issue related to growth has been the high tendency to oxidization, which (when needed) has been solved with buffer and/or capping layers. 37,43,44,48 GdN single crystals have been shown to have a metallic electrical conductivity. 49 For use in a neutron detector, we are seeking a solution for a stable enough compound that is electrically conducting and contains high amounts of Gd.…”