Probing spin dynamics and quantum relaxation in LiY_0.998Ho_0.002F₄ via ¹⁹F NMR

We report measurements of ¹⁹F nuclear spin-lattice relaxation 1∕T₁ as a function of temperature and external magnetic field in a LiY_0.998Ho_0.002F₄ single crystal, a single-ion magnet exhibiting interesting quantum effects. The ¹⁹F 1∕T₁ is found to depend on the coupling with the diluted rare-earth (RE) moments, making it an effective probe of the rare-earth spin dynamics. The results for 1∕T₁ show a behavior similar to that observed in molecular nanomagnets, a result which we attribute to the discreteness of the energy levels in both cases. At intermediate temperatures the lifetime broadening of the crystal field split RE magnetic levels follows a T³ power law. At low temperature the field dependence of 1∕T₁ shows peaks in correspondence to the critical magnetic fields for energy level crossings (LC). A key result of this study is that the broadening of the levels at LC is found to become extremely small at low temperatures, about 1.7 mT, a value which is comparable to the weak dipolar fields at the RE lattice positions. Thus, unlike the molecular magnets, decoherence effects are strongly suppressed, and it may be possible to measure directly the level repulsions at avoided level crossings.