Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

• Published in: Journal of magnetism and magnetic materials 2022-10, Vol.560, p.169615, Article 169615
• Main Authors: Khmelevskyi, Sergii, Mohn, Peter
• Format: Article
• Language: English
• Subjects: Ab-intio modelling; Heusler alloys; Magnetic alloys; Magnetic ordering; Spin-fluctuations; Temperature simulations
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2022.169615

•The main conclusion of the given work is that for an itinerant electron magnetic system a simple comparison of the calculated magnetic ordering temperatures with experiment cannot be used as a judgment for the validity if the proposed ab-initio model.•The exchange interactions calculated in the magnetically ordered state and the application of the straightforward Heisenberg model might give an excellent result purely by coincidence.•Here we have shown that the itinerant character of the Co moments in Co2FeSi and dominant role of the longitudinal spin fluctuations together with an enhancement of the inter-atomic exchange interactions in the magnetically disordered PM state, is a requirement for the formation of very high Curie temperature. The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.