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 similar to 0.094 eV at RT. This technique allows us to characterize the spin bandgap of HMFs directly.