Probing spin dynamics and quantum relaxation inLiY0.998Ho0.002F4via

M. J. Graf

¹

,

A. Lascialfari

²

,

F. Borsa

³

et al.

Abstract

We report measurements of 19 F nuclear spin-lattice relaxation 1/T 1 as a function of temperature and external magnetic field in LiY 0.998 Ho 0.002 F 4 single crystal, a single-ion magnet exhibiting interesting quantum effects. The 19 F 1/T 1 is found to depend on the coupling with the diluted rare-earth (RE) moments. Depending on the temperature range, a fast spin diffusion regime or a diffusion limited regime is encountered. In both cases we find it possible to use the 19 F nucleus as a probe of the rare-earth spin dynamics. The results for 1/T 1 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 T 3 power law. At low temperature the field dependence of 1/T 1 shows peaks in correspondence to the critical magnetic fields for energy level crossings (LC). The results can be explained by inelastic scattering between the fluorine nuclear spins and the RE magnetic levels. A key result of this study is that the broadening of the levels at LC is found to be 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.2

19F nuclear spin relaxation and spin diffusion effects in the single-ion magnet LiYF4:Ho3+

“…The magnetic field dependence of the relaxation rate was studied at temperatures 1.7 -4.2 K in the range of magnetic fields from 0.1 to 0.2 T for different angles between the field and the c-axis (the data for θ ~ 30 0 were presented earlier in Ref. [10]). This magnetic field range includes four groups of anti-crossing points (N4-N7, see Fig.…”

“…Nuclear Magnetic Resonance (NMR) has been proven to be an effective probe of the electronic spin dynamics in different paramagnetic crystals [2,3] and single molecule magnets (SMM) [4][5][6][7][8][9], and it can thus be used also in [10]. The spin dynamics at the anti-crossings of the hyperfine sublevels of the ground electronic doublet of Ho 3+ ions produce striking effects at low temperatures, comparable to those observed in SMM.…”

“…The experimental data obtained earlier [10] are reanalyzed, and additional measurements have been performed to observe the predicted variation of the fluorine spin-lattice relaxation time at the anti-crossings of electron-nuclear sublevels of Ho 3+ ions with the magnetic field component transverse to the symmetry axis of a crystal. Analysis of the relaxation rates for magnetic fields far from and in between the crossing points allowed us to determine the parameters of nuclear spin-diffusion processes.…”

“…The spin dynamics at the anti-crossings of the hyperfine sublevels of the ground electronic doublet of Ho 3+ ions produce striking effects at low temperatures, comparable to those observed in SMM. Large peaks (dips) were observed in the magnetic field dependence of the ac-susceptibility [11] and the fluorine nuclear relaxation rate [10] at temperatures in the range 1.7-4 K. The ac-susceptibility results were subsequently described by a microscopic theory [12], while the NMR measurements were analyzed using a phenomenological model [4,5].…”

“…10 and carry out microscopic calculations describing the 19 F spin-lattice relaxation measurements, as was done for the ac-susceptibility in Ref.…”