Low-energy excitations in theS=12molecular nanomagnetK6[V15IVAs6O42(H2O)]∙8H2Ofrom proton NMR andμSR

Abstract

Zero- and longitudinal-field muon-spin-rotation $(\ensuremath{\mu}\mathrm{SR})$ and $^{1}\mathrm{H}$ NMR measurements on the $S=\frac{1}{2}$ molecular nanomagnet ${\mathrm{K}}_{6}[{\mathrm{V}}_{15}^{IV}{\mathrm{As}}_{6}{\mathrm{O}}_{42}({\mathrm{H}}_{2}\mathrm{O})]∙8{\mathrm{H}}_{2}\mathrm{O}$ are presented. In LF experiments, the muon asymmetry $P(t)$ was fitted by the sum of three different exponential components with fixed weights. The different muon relaxation rates ${\ensuremath{\lambda}}_{i}$ $(i=1,2,3)$ and the low-field $H=0.23\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ $^{1}\mathrm{H}$ NMR spin-lattice relaxation rate $1∕{T}_{1}$ show a similar behavior for $T>50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$: starting from room temperature they increase as the temperature is decreased. The increase of ${\ensuremath{\lambda}}_{i}$ and $1∕{T}_{1}$ can be attributed to the ``condensation'' of the system toward the lowest-lying energy levels. The gap $\ensuremath{\Delta}\ensuremath{\sim}550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ between the first and second $S=\frac{3}{2}$ excited states was determined experimentally. For $T<2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the muon relaxation rates ${\ensuremath{\lambda}}_{i}$ stay constant, independently of the field value $H\ensuremath{\leqslant}0.15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The behavior for $T<2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ strongly suggests that, at low $T$, the spin fluctuations are not thermally driven but rather originate from quasielastic intramolecular or intermolecular magnetic interactions. We suggest that the very-low-temperature relaxation rates could be driven by energy exchanges between two almost degenerate $S=\frac{1}{2}$ ground states and/or by quantum effects.