Occupation sites and valence states of Co dopants in (La, Co)-codoped M-type Sr ferrite: Fe57 and Co59 nuclear magnetic resonance studies

To specify preferential occupation sites of Co substituents and to clarify charge and spin states of Co ions in (La, Co)-cosubstituted hexagonal magnetoplumbite-type (M-type) Sr ferrite Sr1−xLaxFe12−yCoyO19 (x,y≤0.4), Fe57 and Co59 nuclear magnetic resonance (NMR) spectra are measured under zero and external magnetic fields using powdered and single crystalline specimens. For comparison, NMR investigations of nondoped and La- or Co-doped M-type Sr ferrites are also performed. Ferrimagnetic M-type Sr ferrite contains the following five crystallographic Fe sites: the majority spin sites 12k, 2a, and 2b, and the minority spin sites 4f1 and 4f2. Based on Fe57 and Co59 NMR, a plausible model of (La, Co)-codoped Sr ferrite is deduced. To a considerable degree, the charge compensation between La3+ and Co2+ works in the equal (La, Co)-codoped case, where more than half of the Co ions are considered to be present in the minority spin 4f1 sites at the center of the oxygen tetrahedra, with the S=3/2 state carrying a small orbital moment owing to spin-orbit interaction. The remaining small number of high-spin Co2+ (S=3/2, L=1) ions with unquenched orbital moments would be distributed to the other octahedral 12k, 2a, and 4f2 sites.