Engineering Co2MnAlxSi1-x Heusler compounds as a model system to correlate spin polarization, intrinsic Gilbert damping and ultrafast demagnetization

Abstract

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond time scale. Here, we use engineered Co2MnAlxSi1-x Heusler compounds grown by molecular beam epitaxy to adjust the degree of spin polarization at the Fermi energy P and investigate how it correlates with the damping. We first demonstrate experimentally that substituting Si by Al in Co2MnAlxSi1-x Heusler compounds allows us to get a tunable spin polarization at EF from ~60% in Co2MnAl to 100% in Co2MnSi, indicating the transition from metallic to half metallic behaviors. Second, a strong correlation between the spin polarization and the Gilbert magnetic damping is established in these films. The damping decreases when increasing the spin polarization from 1.1 10-3 for Co2MnAl (with 63% spin polarization) to an ultra-low value of 4.10-4 for the half-metallic ferromagnet Co2MnSi [1]. This allows us investigating the relation between these two parameters and the ultrafast demagnetization time characterizing the loss of magnetization occurring after femtosecond laser pulse excitation. The demagnetization time is observed to be inversely proportional to 1-P and as a consequence to the magnetic damping, which can be attributed to the similarity of the spin angular momentum dissipation processes responsible for these two effects. Altogether, our high quality Heusler compounds allow controlling the band structure and therefore the channel for spin angular momentum dissipation [2].