The magnetic groundstate of RuO2 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 N\'eel temperature of the (hypothetical) altermagnetic state of bulk RuO2, stabilized via the DFT+U technique, by mapping on a Heisenberg Hamiltonian. The N\'eel temperature scales monotonously with the magnetic moment up to the point where a large +U term opens a band gap and turns RuO2 semiconducting. The maximum N\'eel 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 β-MnO2, which is a prototype helimagnet. Further comparison with calculations on CrO2 and altermagnetic MnF2 taking U as an adjustable parameter supports the validity of the spin model analysis.
@article{arxiv.2602.15606,
title = {Neel temperature and helical spin order of altermagnetic RuO2},
author = {Markus Meinert},
journal= {arXiv preprint arXiv:2602.15606},
year = {2026}
}