Breakdown of Hund’s third rule for intrinsic magnetic moments

Detailed X-ray magnetic circular dichroism measurements as a function of temperature are reported in ErCo2 at the Co L2,3 edges. The application of magneto-optical sum rules allows for the determination of the Co orbital and spin magnetic moments both in the ferrimagnetic and paramagnetic phases. In agreement with the predictions of Hund’s third rule, Co orbital and spin moments are parallel through out the overall temperature range, except in a narrow temperature window within the paramagnetic phase in which, contrary to what is expected, they are overall coupled antiparallel. This is the first time such behavior is observed in an intrinsic magnetic moment. A qualitative consideration of the balance between the inter-atomic and intra-atomic interactions acting on the cobalt atoms explains the phenomenon.