Breakdown of Hund's third rule for intrinsic magnetic moments

¹ Helmholtz-Zentrum Berlin für Materialien und Energie GmbH - Albert-Einstein-Str. 15, 12489 Berlin, Germany, EU
² Instituto de Ciencia de Materiales de Aragón, and Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza 50009 - Zaragoza, Spain, EU
³ Advanced Light Source, Lawrence Berkeley National Laboratory, University of California Berkeley, CA 94720, USA

^a Julia.Herrero@unizar.es

Received: 6 October 2010
Accepted: 18 December 2010

Abstract

Detailed X-ray magnetic circular dichroism measurements as a function of temperature are reported in ErCo₂ at the Co L_{2, 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 throughout 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 that such behavior is observed in an intrinsic magnetic moment. A qualitative consideration of the balance between the interatomic and intra-atomic interactions acting on the cobalt atoms explains the phenomenon.