The effect of the Schrieffer-Wolff interaction on magnetic ordering in (Ba, K)(Zn, Mn)₂As₂ and MnAu₂

James K. Glasbrenner (Naval Research Laboratory)

The Schrieffer-Wolff interaction, which was first put forth in the context of Kondo theory, describes the magnetic interaction between a localized moment and an itinerant charge carrier of differing electronic character. This scenario is possible, for example, when magnetic impurities are present. This interaction is commonly modeled with the Hamiltonian H≃J_s(0)∙S, and is also referred to as the s-d or p-d model. The interaction can be either ferromagnetic or antiferromagnetic and can compete with other magnetic interactions, yielding intriguing physics. There are two interesting materials where the Schrieffer-Wolff interaction plays a key role in the magnetic physics, the dilute magnetic semiconductor (Ba, K)(Zn, Mn)₂As₂ and the spin-spiral material MnAu₂. In the following seminar I will discuss these two materials, and show that in the former, the Schrieffer-Wolff interaction is key to understanding the ferromagnetic ordering in the system, while in the latter, magnetic spirals form when electronic correlations suppress the interaction.

The material (Ba, K)(Zn, Mn)₂As₂ is a recently discovered dilute magnetic semiconductor where spin and charge doping are decoupled. Using density functional theory (DFT) calculations, I show that (i) conventional DFT accurately describes this material, and (ii) the magnetic interaction emerges from the competition of the short-range superexchange and a longer-range interaction mediated by the itinerant As holes, coupled to Mn via the Schrieffer-Wolff p-d interaction representing an effective Hund's rule coupling, J_H^eff. This leads to a magnetic interaction that depends on the Mn d-band position with respect to the Fermi level, and thus is, in principle, tunable. The DFT calculations also reveal a statistical preference for the formation of nearest-neighbor singlets, which explains a puzzling reduction of the magnetization of this material observed in experiment.

The material MnAu₂ is one of the oldest known spin-spiral materials, yet the nature of the spiral state has eluded explanation. Here I propose that the spin spiral state is induced by a competition between the short-range antiferromagnetic exchange and a long-range interaction induced by the polarization of the Au bands. Using DFT calculations, I show that the spin-spiral state becomes stable when Coulomb corrections via a Hubbard U are included, which suppresses the Schrieffer-Wolff-type s-d magnetic interaction between Mn and Au. For realistic values of U, the resulting spiral wave vector is in close agreement with experiment.

Back to Seminar Home Page

Last modified 27-March-2015 by website owner: NCNR (attn: )