New Band-gap Measurement Technique for a Half-Metallic Ferromagnet

Spintronics is a new and emerging field in nanotechnology, which has been evolving rapidly. It aims to exploit the spin degree of freedom in order to realise advanced electronic devices. With the recent improvement in the storage media devices following to the discovery of giant magnetoresistance effect, it is envisioned that the electronic devices have several advantages over the conventional electronics in respect to the storage capacity, speed and power consumption. One avenue towards next generation spintronic devices is to develop half-metallic ferromagnets (HMFs) with 100% spin polarisation (P) and Curie temperature (TC) above room temperature (RT). HMFs have unique properties, in which the majority spins have a metallic band structure, whereas the minority spins have a semiconducting band with the Fermi level (EF) lying within an energy gap. P of HMFs has been predominantly estimated using Julliere’s formula in a magnetic tunnel junction (MTJ) or measured by the Andreev reflection (AR) at low temperature. Both methods are very sensitive to the surface/interface spin polarisation. Alternative optical methods such as photoemission have also been employed. However, these methods require a complicated and expensive set-up. Therefore, it is of paramount importance to directly and easily measure the band-gap of HMFs.
The main aim of this study is to develop a new technique to directly measure the band-gap (Eg) of HMFs at RT. For that, a simple experimental set-up has been designed utilising circularly-polarised infrared (IR) light to excite minority spins and measure the corresponding absorption. The validity of our technique was demonstrated by measuring Eg of polycrystalline and epitaxial full-Heusler-alloy films and single-crystal half-Heusler alloy bulk. Eg measurements have revealed relevant figures to the calculated and expected reported values.
This technique allows measuring Eg of HMFs at RT for the first time. It can therefore be used to provide simple optimisation of growth conditions.