NMR characterization of Co sites in La+Co-doped Sr hexaferrites with enhanced magnetic anisotropy

Abstract

We investigated La+Co-doped ${\mathrm{BaFe}}_{12}{\mathrm{O}}_{19}$ and ${\mathrm{SrFe}}_{12}{\mathrm{O}}_{19},$ a recently discovered improved variant of the most popular hard magnetic M-type hexaferrites, with nuclear magnetic resonance at low temperature, and in magnetic fields up to 5 T. Two satellite lines were observed between 70 and 76 MHz in the zero-field powder spectra of ${}^{57}\mathrm{Fe}.$ Satellites in Co+Ti-doped samples at the same frequencies have been assigned in the literature to k-Fe with a $f1$-Co neighbor, and a-Fe with a $f1$-Co neighbor. We report observation of a ${}^{59}\mathrm{Co}$-resonance in this structure. The signal is due to ${\mathrm{Co}}^{2+}$ in a low-spin state. Together with the field dependence of the hyperfine field this gives a very strong indication that ${\mathrm{Co}}^{2+}$ substitutes in the presence of La for $f2$-Fe, in contrast to the behavior assumed in Co+Ti-doped samples.