Energies and Lifetimes of Magnons in Complex Ferromagnets: A First-Principle Study of Heusler Alloys

The energies and lifetimes of magnons in several Mn-based Heusler alloys are studied using linear response density functional theory. The number of the spin wave branches in ${\mathrm{Co}}_2\mathrm{MnSi}$ corresponds to the number of its magnetic sublattices in contrast with the NiMnSb case in which the induced Ni sublattice cannot support optical magnons. The half-metallicity of these systems results in long-living acoustic spin waves. The example of non-half-metallic ${\mathrm{Cu}}_2\mathrm{MnAl}$ shows that the hybridization with Stoner continuum leads not only to the damping of magnons but also to a renormalization of their energies.

Figure 1

Enhanced susceptibility of ${\mathrm{Co}}_2\mathrm{MnSi}$, an example of the spectrum of loss tensor for $\mathbf{q}=0.28(1,1,0)2\pi /a$. $a$ stands for the lattice constant. Three clear peaks (EV 1, 2, and 3 corresponding to the labels in Fig. 2) can be discerned. The panels show corresponding eigenvectors; arrows indicate the deviations of magnetic moments. The basis atoms are Co at $(a/4,a/4,a/4)$ and $3(a/4,a/4,a/4)$ and Mn at $(a/2,a/2,a/2)$.

Figure 2

Energies (a) and inverse lifetimes (b) of three SW modes in ${\mathrm{Co}}_2\mathrm{MnSi}$. Parameter ${\beta }_1(\mathbf{q})$ of EV 1 does not exceed 5 meV and it is not shown.

Figure 3

Energy of the only spin wave mode in NiMnSb extracted from dynamical susceptibility is compared to experiment [13] and adiabatic SW calculations (solid lines) based on the DLM state, as described in the text.

Figure 4

Energies (●) and inverse lifetimes (◆) of the SW mode in ${\mathrm{Cu}}_2\mathrm{MnAl}$ along the [100] direction obtained in this study compared to the experimental (▲) [20] and adiabatic SW energies (■) [19].