Interplay of local moment and itinerant magnetism in cobalt-based Heusler ferromagnets: Co2TiSi, Co2MnSi and Co2FeSi

Abstract

Heusler ferromagnets based on Co are important materials for spintronics. This is due to the exceptional combinations of high Curie temperature and strong spin polarization, including half-metallicity, found in some of these. We investigate the full Heusler compounds, ${\mathrm{Co}}_{2}\mathrm{TiSi}$, ${\mathrm{Co}}_{2}\mathrm{MnSi}$, and ${\mathrm{Co}}_{2}\mathrm{FeSi}$ using first principles calculations. ${\mathrm{Co}}_{2}\mathrm{TiSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnSi}$ are half metals, while ${\mathrm{Co}}_{2}\mathrm{FeSi}$ is not. The trends in the Curie temperatures are reproduced by the calculated spin wave dispersions. Remarkably, ${\mathrm{Co}}_{2}\mathrm{TiSi}$ is a very itinerant magnet but ${\mathrm{Co}}_{2}\mathrm{FeSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnSi}$ show local moment behavior regarding the Fe and Mn, while retaining the itinerancy of the Co magnetism. These materials can therefore be described as itinerant systems with embedded local moment atoms. This provides an explanation for their exceptional behavior. Our results do not support the half-metallic character proposed for ${\mathrm{Co}}_{2}\mathrm{FeSi}$, but they are consistent with a higher Curie temperature relative to the Mn compound. The density of states and transport spin polarizations ${\mathrm{Co}}_{2}\mathrm{FeSi}$ have opposite signs. Importantly, although there is a large minority spin density of states at the Fermi level, leading to a low density of states spin polarization, we find a very strong transport spin polarization in ${\mathrm{Co}}_{2}\mathrm{FeSi}$. This, combined with the large moment, cubic structure, and high Curie temperature supports the further investigation of ${\mathrm{Co}}_{2}\mathrm{FeSi}$ for spintronic applications that make use of the transport spin polarization.