Off-stoichiometry effect on magnetic damping in thin films of Heusler alloy C o2MnSi

We investigated the effect of off-stoichiometry on the Gilbert magnetic damping constant (α) of Heusler alloy Co2MnSi (CMS) thin films by employing time-resolved magneto-optical Kerr effect (TR-MOKE) measurements along with first-principles calculations based on the linear-response theory for magnetic damping. Because of the contribution of extrinsic damping arising from two-magnon scattering, the effective α (αeff) extracted from the TR-MOKE responses showed dependences on the in-plane magnetic-field angle (θH) and the field strength (H). Then, we obtained the smallest αeff (α0) for each sample under the most reduced contribution from two-magnon scattering realized through varying θH and H values, which is the closest value to α. The thus obtained α0 values of epitaxially grown off-stoichiometric Co2MnβSiγ (γ=0.82) films with various (Mn + Si) compositions, (β+γ), decreased with increasing (β+γ) from α0=0.0057 for (Mn + Si)-deficient (β+γ)=1.44 to α0=0.0036 for (β+γ)=1.90 being close to the stoichiometric one of (β+γ)=2.0 at 300 K. It was also demonstrated that a half-metallic (Mn+Si)-rich CMS film with β=1.30 and γ=0.90 showed a low α0 of 0.0035. The dependence of α0 on (β+γ) in Co2MnβSiγ (γ=0.82) was well explained by the first-principles calculations. Through the systematic investigations of off-stoichiometric CMS with various values of (β+γ), it was clarified that the total density of states (DOS) at the Fermi level, D(EF), plays the key role for determining the damping constant of CMS. Furthermore, it was revealed that the reduced minority-spin DOS at EF, caused by decreasing harmful CoMn antisites, is essential for reducing the damping constant of CMS. These findings demonstrate that appropriately controlling off-stoichiometry and film composition is thus promising for achieving half-metallicity and a low α simultaneously for CMS thin films.