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 ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_{12-y}{\mathrm{Co}}_y{\mathrm{O}}_{19}$ ($x,y\le 0.4$), ${}^{57}\mathrm{Fe}$ and ${}^{59}\mathrm{Co}$ 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 $4f_1$ and $4f_2$. Based on ${}^{57}\mathrm{Fe}$ and ${}^{59}\mathrm{Co}$ NMR, a plausible model of (La, Co)-codoped Sr ferrite is deduced. To a considerable degree, the charge compensation between ${\mathrm{La}}^{3+}$ and ${\mathrm{Co}}^{2+}$ 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 $4f_1$ 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 ${\mathrm{Co}}^{2+}$ ($S=3/2$, $L=1)$ ions with unquenched orbital moments would be distributed to the other octahedral $12k$, $2a$, and $4f_2$ sites.

• Received 10 November 2017
• Revised 19 June 2018

DOI:https://doi.org/10.1103/PhysRevB.98.064403