## 3 Results
### Room Temperature Absorption and Emittance  
Complementary information and estimates of the sample emittance can be obtained from optical measurements at room temperature (RT). The absorption spectra measured at RT in the single crystals are given in Figure 6. The MgO:Co spectra show strong absorption bands corresponding to the intraconfigurational transitions of octahedrally coordinated Co\({}^{2+}\)[17]. The bands correspond to transitions within crystal field levels from the \({}^{4}\)F free ion term, namely, \({}^{4}\)T\({}_{1}\) to \({}^{4}\)T\({}_{2}\) at 1180 nm and \({}^{4}\)T\({}_{1}\) to \({}^{4}\)A\({}_{2}\) at 510 nm. MgAl\({}_{2}\)O\({}_{4}\)Co absorption spectra correspond to tetrahedrally coordinated Co\({}^{2+}\) in MgAl\({}_{2}\)O\({}_{4}\)[17]. The band in the visible range (around 590 nm) corresponds to transition from the \({}^{4}\)A\({}_{2}\) ground state to the \({}^{4}\)T\({}_{1}\)(\({}^{4}\)P) and the one around 1528 nm to the transition \({}^{4}\)A\({}_{2}\) to \({}^{4}\)T\({}_{1}\)(\({}^{4}\)F). The absorption cross sections can be estimated by calculating the absorption coefficient per Co ion in the sample, \(\alpha\)/\(N_{Co}\), where \(\alpha\) is the absorption coefficient in cm\({}^{-1}\) and \(N_{Co}\) is the concentration of Co\({}^{2+}\) ions in ions/cm\({}^{3}\) in the respective crystals. This is an approximation as long as we do not know for sure the proportion of Co\({}^{2+}\) ions sitting in tetrahedrally coordinated sites in MgAl\({}_{2}\)O\({}_{4}\), but according to the literature [27; 28] it is the vast majority of cobalt introduced in the spinel. The absorption cross-section in the NIR band amounts to 2.7 \(\times\) 10\({}^{-19}\) cm\({}^{2}\) at 1528 nm in MgAl\({}_{2}\)O\({}_{4}\):Co and to 5.8 \(\times\) 10\({}^{-21}\) cm\({}^{2}\) at 1180 nm in MgO:Co. At 1180 nm, the value for MgAl\({}_{2}\)O\({}_{4}\):Co is 1.1 \(\times\) 10\({}^{-19}\) cm\({}^{2}\). Even if five times more Co\({}^{2+}\) ions were located in MgO inside the eutectic samples, only 20% of the absorption at 1180 nm would be due to cobalt in MgO. Moreover, the bandwidth is around two times larger (in nm) in MgAl\({}_{2}\)O\({}_{4}\), the band is red-shifted and structured. In fact, the weighed sum spectrum given in Figure 7, calculated with cobalt concentrations in each phase corresponding to sample Co0.48-750, is essentially the same as the spectra measured in Spinel0.17-50 (green lines in Figure 7). Figure 5: Thermal emission of MgO:Co (blue dash-point line), Spinel0.06-50 (red dotted line) and Co0.22-50 (green solid line) measured at 1000 \({}^{\circ}\)C. Spectra are presented normalized for comparison purposes. A third absorption farther in the IR is also observed in MgAl\({}_{2}\)O\({}_{4}\):Co (Figure 6), with the maximum around 2270 nm and absorption cross-section of 8 \(\times\) 10\({}^{-21}\) cm\({}^{2}\) at room temperature. This is due to the transition \({}^{4}\)A\({}_{2}\)(F) to \({}^{4}\)T\({}_{2}\)(F), electric dipole is forbidden, hence its low absorption cross-section. This band is similar to the one observed in (Mg\({}_{1-x}\)Co\({}_{x}\))Al\({}_{2}\)O\({}_{4}\) solid solutions by [PERSON] et al. [27]. Measured directional-hemispherical reflectance and transmittance of the samples with the least cobalt content grown at 50 and 750 mm/h are given in Figure 8. Note first that the material is not opaque, although transmittance is smaller than reflectance. Non-zero transmittance is expected as the refractive index contrast between MgO and MgAl\({}_{2}\)O\({}_{4}\) is relatively small. The structured absorption bands due to Co\({}^{2+}\) in MgAl\({}_{2}\)O\({}_{4}\) at around 590 nm and 1400 nm give rise to bands of diminished reflectance and transmittance with respect to their background. There is no evidence of the band around 2300 nm, which is a consequence of its small absorption coefficient and the low dopant content of these samples.