New Bandgap Measurement Technique for a Half-Metallic Ferromagnet

In this paper, we report our new optical method for the measurement of the bandgaps of half-metallic ferromagnets (HMFs) that are sensitive to both surface/interface and bulk regions of these ferromagnets depending on the light incident angles and resulting penetration depth. We measured the bandgap of polycrystalline Co₂FeSi Heusler alloy thin films at room temperature (RT) as an example. The Co₂FeSi thin films were deposited on MgO(001) and Si/SiO₂ substrates by sputter deposition. Circularly, polarized infrared light was used to excite the corresponding spin-polarized electrons, which were effectively absorbed by the bandgap of the Co₂FeSi Heusler alloy. The Co₂FeSi bandgap energy was repeatedly measured and it was found to be ~0.094 eV at RT. This technique allows us to characterize the spin bandgap of HMFs directly.