First-principles modeling of longitudinal spin fluctuations in itinerant electron antiferromagnets: High Néel temperature in theV3Alalloy

The ${\mathrm{V}}₃\mathrm{Al}$ alloy with $\mathrm{D}{\mathrm{O}}₃$ crystal structure belongs to the family of the very few metallic materials that exhibit a magnetically ordered state with a high ordering temperature (\ensuremath{\sim}600 K) and consist only of nonmagnetic elements. We show that, similarly to the ferromagnetism in the fcc Ni (with ordering temperature at about 630 K), the antiferromagnetism in ${\mathrm{V}}₃\mathrm{Al}$ has itinerant character, and the high value of the N\'eel temperature is the result of the strong longitudinal spin fluctuations in the paramagnetic state. In order to develop an ab initio--based theory of the magnetic ordering at finite temperatures, we employ an effective magnetic Heisenberg-like Hamiltonian with varying values of the on-site magnetic moments. Using a set of approximations we map this model onto the results of the first-principle-based disordered local moment formalism and the magnetoforce theorem applied in the framework of the Korringa-Kohn-Rostoker method. Our high-temperature approach is shown to describe the experimental N\'eel temperature of ${\mathrm{V}}₃\mathrm{Al}$ very well and thus underlines the importance of the longitudinal spin-fluctuation mechanism of formation of the vanadium magnetic moment at high temperatures.