We present a systematic study of the crystal-field interactions in the LiRF 4 (R = Gd, Ho, Er, Tm, and Yb) family of rare-earth magnets. Using detailed inelastic neutron scattering measurements, we have been able to quantify the transition energies and wave functions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to line-shape broadening in LiTmF 4 and level splitting in LiYbF 4 . Furthermore, using high-resolution neutron scattering from LiHoF 4 , we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.

The ability to control the magnetization switching in nanoscale devices is a crucial step for the development of fast and reliable techniques to store and process information. Here we show that the switching dynamics can be controlled efficiently using a microwave field with slowly varying frequency (autoresonance). This technique allowed us to reduce the applied field by more than 30% compared to competing approaches, with no need to fine-tune the field parameters. For a linear chain of nanoparticles the effect is even more dramatic, as the dipolar interactions tend to cancel out the effect of the temperature. Simultaneous switching of all the magnetic moments can thus be efficiently triggered on a nanosecond timescale. * Electronic address: manfredi@unistra.fr