First-principles study of exchange stiffness constant of half-metallic Heusler alloys Co2MnZ (Z= Si, Al) at finite temperatures: Spin fluctuation-induced effective half metallicity

TL;DR

This study uses first-principles calculations to explore how exchange stiffness constants and electronic structures of Co2MnZ (Z= Si, Al) alloys change with temperature, revealing insights into their magnetic stability and Curie temperatures.

Contribution

It provides the first detailed analysis of finite-temperature magnetic properties and electronic structure effects in Co2MnZ Heusler alloys, highlighting the role of spin disorder.

Findings

Co2MnAl has a high Curie temperature similar to Co2MnSi.

Exchange stiffness constant of Co2MnAl remains robust with temperature.

Spin polarization increases with temperature due to electronic structure changes.

Abstract

We performed first-principles calculations at finite temperatures to investigate the temperature dependence of the magnetic properties, such as exchange stiffness constants and Curie temperatures, of Co2MnZ (Z= Si, Al) assuming L21 and B2 structures. In L21 structures, we confirmed a relatively high Curie temperature for Co2MnAl, compatible with that of Co2MnSi; however, its exchange stiffness constant and single site magnetic excitation energy at zero temperature are much smaller than those of Co2MnSi. This might indicate that the Curie temperature of itinerant magnets cannot be determined by the exchange interaction at zero temperature. We also investigated the temperature dependence of the exchange stiffness constants of both alloys, and we found robustness in the temperature dependence of the exchange stiffness constant for Co2MnAl, assuming an L21 structure. This might lead to a high Curie temperature, contrary to the small exchange stiffness constant. Finally, we examined the temperature dependence of the electronic structure to investigate the origin of the behavior of the exchange stiffness constant at finite temperatures. We confirmed that the spin polarization at chemical potential effectively increases with an increasing temperature due to the altered electronic structure induced by the spin disorder. This might contribute to the robustness of the exchange stiffness constant at finite temperatures. Our results might indicate that renomarization of the electronic structure due to spin disorder at finite temperature influences the exchange interactions of Co2MnAl.