Low-field microwave response and electron paramagnetic resonance identification of valence states of manganese including octahedral Mn5+ in YAlO3 and CaYAlO4

Abstract

Microwave response near zero magnetic field has been observed in YAlO3 and CaYAlO4 crystals dilutely doped with Mn in concentration ranging from 0.005 to 0.2 at. %. The response is due to magnetic-field-dependent nonresonant microwave losses, which co-exist with normal electron paramagnetic resonance (EPR) absorption due to Mn2+, Mn4+, and Mn5+ ions in YAlO3 and CaYAlO4. The valence states of manganese have been identified using a dual mode EPR cavity that allowed spectrum recording with polarization of the magnetic vector B1 of the microwave perpendicular or parallel to the static magnetic field B0. The Mn2+ ion, electron spin S=52, and Mn4+, S=32 have been identified using normal B1⊥B0 polarization, while Mn5+, S=1, in unusual octahedral position, has been identified using B1∥B0 polarization. The low-field response did not depend on mutual orientation of the vectors B1 and B0 and had the opposite phase with respect to the paramagnetic absorption. This shows that Mn-doped YAlO3 and CaYAlO4 exhibit magnetically induced microwave absorption, which has a minimum at zero magnetic field and increases with the applied magnetic field. The response in Mn-doped YAlO3 increases with the introduction of Ca2+ ions. The existence of manganese in different valence states, particularly metastable Mn5+ in octahedral position, and the effect of calcium co-doping suggest the formation of relatively free charge carriers that can migrate in the vicinity of paramagnetic manganese ions. The spin-dependent mechanism of the charge migration is discussed to explain magnetoinduced microwave losses.