Calculation of the biquadratic spin interactions based on the spin cluster expansion for ab initio tight-binding models

We devise a calculation scheme for ab initio tight-binding Hamiltonians to evaluate the biquadratic spin interaction. This scheme employs the spin cluster expansion with the disordered local moment method and was originally developed within the Korringa-Kohn-Rostoker theory. By applying it to the two-orbital Hubbard model, we show that the evaluated biquadratic interactions agree well with those in the effective quantum spin model derived for the limit of strong correlation. This result suggests the broad applicability of the method to various magnets with large local moments. We then apply it to the ab initio tight-binding models for bcc Fe and fcc Ni and obtain consistent results with previous studies. The present scheme offers a convenient ab initio tool for understanding or predicting magnetic properties arising from the biquadratic interaction.