Is N-doped SrO magnetic? A first-principles view

TL;DR

This study uses density functional calculations to challenge the idea that N-doped SrO exhibits ferromagnetism, showing instead that it is paramagnetic due to stable N impurity pairing and weak exchange interactions.

Contribution

It provides a first-principles analysis revealing the stability of N impurity pairs and their magnetic behavior, contradicting previous assumptions of ferromagnetism in N-doped SrO.

Findings

Substitutional N impurities are unstable in SrO.

N impurity pairs form stable (N_{sub}-N_{int})^{2-} dimers.

These dimers behave like charged N_2 molecules with spin=1 but weak exchange interactions.

Abstract

N-doped SrO seems to be one of the model systems for d^0 magnetism, in which magnetism (or ideally, ferromagnetism) was ascribed to the localized N 2p spins mediated by delocalized O 2p holes. Here we offer a different view, using density functional calculations. We find that N-doped SrO with solely substitutional N impurities as widely assumed in the literature is unstable, and instead that a pairing state of substitutional and interstitial N impurities is significantly more stable and has a much lower formation energy than the former by 6.7 eV. The stable (N_{sub}-N_{int})^{2-} dimers behave like a charged (N_2)^{2-} molecule and have each a molecular spin=1. However, their spin-polarized molecular levels lie well inside the wide band gap of SrO and thus the exchange interaction is negligibly weak. As a consequence, N-doped SrO could not be ferromagnetic but paramagnetic.