Co$_2$FeAl full Heusler compound based spintronic terahertz emitter

TL;DR

This paper demonstrates the successful fabrication of Co$_2$FeAl full Heusler compound-based spintronic terahertz emitters with optimized THz radiation bandwidth and amplitude, addressing previous challenges in achieving high spin polarization in ultrathin films.

Contribution

It reports the first successful deposition of ordered Co$_2$FeAl Heusler films at room temperature and their application in efficient THz emitters, expanding the material options for spintronic THz sources.

Findings

Achieved L2$_1$ and B2 ordering of Co$_2$FeAl films using room temperature sputtering.

Generated THz radiation with a bandwidth of 0.1-4 THz from Co$_2$FeAl/Pt spintronic emitters.

Optimized THz amplitude through thickness and growth parameter adjustments.

Abstract

To achieve a large terahertz (THz) amplitude from a spintronic THz emitter (STE), materials with 100\% spin polarisation such as Co-based Heusler compounds as the ferromagnetic layer are required. However, these compounds are known to loose their half-metallicity in the ultrathin film regime, as it is difficult to achieve L2$_1$ ordering, which has become a bottleneck for the film growth. Here, the successful deposition using room temperature DC sputtering of the L2$_1$ and B2 ordered phases of the Co$_2$FeAl full Heusler compound is reported. Co$_2$FeAl is used as ferromagnetic layer together with highly orientated Pt as non-ferromagnetic layer in the Co$_2$FeAl/Pt STE, where an MgO(10 nm) seed layer plays an important role to achieve the L2$_1$ and B2 ordering of Co$_2$FeAl. The generation of THz radiation in the CFA/Pt STE is presented, which has a bandwidth in the range of 0.1-4 THz. The THz electric field amplitude is optimized with respect to thickness, orientation, and growth parameters using a thickness dependent model considering the optically induced spin current, superdiffusive spin current, inverse spin Hall effect and the attenuation of THz radiation in the layers. This study, based on the full Heusler Co$_2$FeAl compound opens up a plethora possibilities in STE research involving full Heusler compounds.