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We perform detailed muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) measurements in the classic antiferromagnet ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and explain the spectra by considering dynamic population and dissociation of charge-neutral muon-polaron complexes. We show that charge-neutral muon states in ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, despite lacking the signatures typical of charge-neutral muonium centers in nonmagnetic materials, have a significant impact on the measured $\ensuremath{\mu}\mathrm{SR}$ frequencies and relaxation rates. Our identification of such polaronic muon centers in ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ suggests that isolated hydrogen (H) impurities form analogous complexes, and that H interstitials may be a source of charge carrier density in ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$.

Precise measurements of the muonium (Mu) hyperfine interaction versus temperature are reported in a silica aerogel and mesoporous silica SBA-15 using a fast-timing spectrometer to detect the precession frequencies of Mu in a magnetic field of 1.14 T. The observed signals are a sensitive monitor of dynamics associated with the binding and unbinding of Mu from the silica surface. Above 100 K the Mu is effectively off the surface. Significant differences are observed in the way the lines show motional narrowing, and are attributed to differences in sample morphology. At room temperature the effective mean free path appears longer in SBA-15, suggesting it may offer advantages over aerogel as a source of Mu in vacuum.

We report $\ensuremath{\beta}$-detected NMR of ion-implanted $^{8}\mathrm{Li}$ in a single crystal and thin film of the strongly correlated metal ${\mathrm{LaNiO}}_{3}$. Spin-lattice relaxation measurements reveal two distinct local environments, both metallic as evident from $T$-linear Korringa $1/{T}_{1}$ below $200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with slopes comparable to other metals. A small approximately temperature-independent Knight shift of $\ensuremath{\sim}74\phantom{\rule{0.16em}{0ex}}\mathrm{ppm}$ is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations. We find no evidence for a magnetic transition from 4 to $310\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The similarity of these features in the two very different samples indicates that they are intrinsic and unrelated to dilute oxygen vacancies. We attribute the two environments to two distinct but similar crystallographic $^{8}\mathrm{Li}$ sites and not to any form of phase inhomogeneity, but this is inconsistent with the conventional rhombohedral structure of ${\mathrm{LaNiO}}_{3}$, and also cannot be simply explained by the common alternative orthorhombic or monoclinic distortions.

We report measurements of the diffusion rate of isolated ion-implanted $^{8}{\mathrm{Li}}^{+}$ within $\ensuremath{\sim}120\text{ }\text{ }\mathrm{nm}$ of the surface of oriented single-crystal rutile ${\mathrm{TiO}}_{2}$ using a radiotracer technique. The $\ensuremath{\alpha}$ particles from the $^{8}\mathrm{Li}$ decay provide a sensitive monitor of the distance from the surface and how the depth profile of $^{8}\mathrm{Li}$ evolves with time. The main findings are that the implanted ${\mathrm{Li}}^{+}$ diffuses and traps at the (001) surface. The $T$ dependence of the diffusivity is described by a bi-Arrhenius expression with activation energies of 0.3341(21) eV above 200 K, whereas at lower temperatures it has a much smaller barrier of 0.0313(15) eV. We consider possible origins for the surface trapping, as well the nature of the low-$T$ barrier.

We report measurements on the high-temperature ionic and low-temperature electronic properties of the three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$ using ion-implanted $^{8}\mathrm{Li}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range $5--28\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$, the probes penetrate beyond the expected range of the topological surface state, but are still within $250\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ of the surface. At temperatures above $\ensuremath{\sim}150\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, spin-lattice relaxation measurements reveal isolated $^{8}\mathrm{Li}^{+}$ diffusion with an activation energy ${E}_{A}=0.185(8)\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and attempt frequency ${\ensuremath{\tau}}_{0}^{\ensuremath{-}1}=8\ifmmode\pm\else\textpm\fi{}3\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{s}}^{\ensuremath{-}1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-type relaxation mechanism with a field-dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freeze-out of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin.

By measuring the prototypical antiferromagnet α-Fe_{2}O_{3}, we show that it is possible to determine the static spin orientation and dynamic spin correlations within nanometers from an antiferromagnetic surface using the nuclear spin polarization of implanted ^{8}Li^{+} ions detected with β-NMR. Remarkably, the first-order Morin spin reorientation in single crystal α-Fe_{2}O_{3} occurs at the same temperature at all depths between 1 and 100 nm from the (110) surface; however, the implanted nuclear spin experiences an increased 1/T_{1} relaxation rate at shallow depths revealing soft-surface magnons. The surface-localized dynamics decay towards the bulk with a characteristic length of ε=11±1 nm, closely matching the finite-size thresholds of hematite nanostructures.

A $\ensuremath{\beta}$-NMR study was performed on a ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ crystal in the metallic state using a beam of spin-polarized $^{8}\mathrm{Li}^{+}$ implanted at a mean depth of 90 nm. The $^{8}\mathrm{Li}^{+}$ spin-lattice relaxation rate is strongly influenced by the onset of incommensurate spin fluctuations. The nuclear relaxation rate can be explained using previously published bulk $^{17}\mathrm{O}$ NMR and inelastic neutron spectroscopy measurements of the dynamic magnetic susceptibility to model the hyperfine coupling. A well-resolved quadrupolar-split NMR for $^{8}\mathrm{Li}^{+}$ implies a static stopping position in an interstitial site. The $^{8}\mathrm{Li}^{+}$ Knight shift is highly sensitive to the anisotropic static susceptibility.