Chemical disorder and Pb207 hyperfine fields in the magnetoelectric multiferroic Pb(Fe1/2Sb1/2)O3 and its solid solution with Pb(Fe1/2Nb1/2)O3

Abstract

We report on the results of magnetic susceptibility, electron paramagnetic resonance, and $^{207}\mathrm{Pb}$ nuclear magnetic resonance (NMR) studies of the magnetoelectric multiferroic $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{S}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ (PFS) ceramic, as well as its solid solution with $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ (PFN) of different degrees of the 1:1 ordering of magnetic $\mathrm{F}{\mathrm{e}}^{3+}$ and nonmagnetic $\mathrm{S}{\mathrm{b}}^{5+}$ ions. The ordering has been studied by x-ray diffraction (XRD) and NMR methods. In particular, two spectral lines, originating from the ordered and disordered regions, respectively, are resolved in the $^{207}\mathrm{Pb}$ NMR spectra. This demonstrates the presence of spatially heterogeneous ordering where ordered regions are embedded into a disordered matrix. Combining XRD and NMR data, we have determined both the long-range order parameter $s$ and the volume fraction of ordered regions ${s}^{\ensuremath{'}}$ for all investigated samples. The values vary in the range $s=0\ensuremath{-}0.93$ and ${s}^{\ensuremath{'}}=0\ensuremath{-}1$. We have found that the $^{207}\mathrm{Pb}$ Fermi contact interaction strongly depends on the disorder in the Fe/Sb positions: whereas it reaches 7.08 MHz in the ordered lattice, it is almost zero in the disordered environment. These results are further supported by the studies of PFS-PFN solid solutions. The analysis of experimental data in terms of density functional theory reveals a noticeably higher hybridization between Pb 6s and Fe 3d orbitals in the ordered case. The ordering of magnetic and nonmagnetic ions has a strong impact on the magnetic properties of PFS, leading to a transformation of the long-range ordered antiferromagnetic phase in chemically ordered samples to the spin glass state already in partially $(s=0.35)$ disordered specimens. In our opinion, the difference in the magnetic properties of PFN and PFS is related to the fact that PFN is completely disordered, in contrast to PFS, which is only partially disordered, with small ordered regions existing in the disordered matrix that prevent the percolation of the nearest-neighbor Fe\ensuremath{-}Fe exchange interaction across the lattice.