ac-response of magnetic moments of diluted Ho3+ ions

Low-temperature dynamics of mesoscopic magnetic moments of a 0.1% at Ho3+-doped LiYF4 single crystal is probed by ac-susceptibility measurements above T=1.75 K. In the temperature and frequency range studied (1.75 Kless than or equal toTless than or equal to5 K,10 Hzless than or equal tofless than or equal to1500 Hz), the results clearly show that both thermally activated quantum tunneling of a nearly isolated magnetic moment and spin-spin cross relaxations occur. This interpretation is strongly supported by the calculated electro-nuclear Zeeman diagram either of a single ion or of a pair of diluted ions. (C) 2003 American Institute of Physics.