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

Abstract

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 ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Fe}}_{12\ensuremath{-}y}{\mathrm{Co}}_{y}{\mathrm{O}}_{19}$ ($x,y\ensuremath{\le}0.4$), $^{57}\mathrm{Fe}$ 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 $4{f}_{1}$ and $4{f}_{2}$. 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 $4{f}_{1}$ 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 $4{f}_{2}$ sites.