Covalency and vibronic couplings make a nonmagnetic j=3/2 ion magnetic

Abstract For 4 d 1 and 5 d 1 spin–orbit-coupled electron configurations, the notion of nonmagnetic j =3/2 quartet ground state discussed in classical textbooks is at odds with the observed variety of magnetic properties. Here we throw fresh light on the electronic structure of 4 d 1 and 5 d 1 ions in molybdenum- and osmium-based double-perovskite systems and reveal different kinds of on-site many-body physics in the two families of compounds: although the sizable magnetic moments and g -factors measured experimentally are due to both metal d –ligand p hybridisation and dynamic Jahn–Teller interactions for 4 d electrons, it is essentially d − p covalency for the 5 d 1 configuration. These results highlight the subtle interplay of spin–orbit interactions, covalency and electron–lattice couplings as the major factor in deciding the nature of the magnetic ground states of 4 d and 5 d quantum materials. Cation charge imbalance in the double-perovskite structure is further shown to allow a fine tuning of the gap between the t 2 g and e g levels, an effect of much potential in the context of orbital engineering in oxide electronics.