Neel temperature and helical spin order of altermagnetic RuO2

Abstract

The magnetic groundstate of RuO${}_2$ remains controversial, with experimental evidence for a nonmagnetic groundstate of ideal bulk material and indications of a magnetic state in strained thin films. Here, I investigate the Néel temperature of the (hypothetical) altermagnetic state of bulk RuO${}_2$, stabilized via the DFT$+U$ technique, by mapping on a Heisenberg Hamiltonian. The Néel temperature scales monotonously with the magnetic moment up to the point where a large $+U$ term opens a band gap and turns RuO${}_2$ semiconducting. The maximum Néel temperature obtained by this procedure is 408\,K at $U=3$\,eV, and much smaller values for smaller $U$. A reciprocal-space eigenvalue analysis reveals a helimagnetic groundstate of the spin model due to intra-sublattice antiferromagnetic coupling. This situation resembles the isostructural $\beta$-MnO${}_2$, which is a prototype helimagnet. Further comparison with calculations on CrO${}_2$ and altermagnetic MnF${}_2$ taking $U$ as an adjustable parameter supports the validity of the spin model analysis.

6 pages, 4 figures