Manipulation of the quantum tunneling of nanomagnets by using time-dependent high magnetic fields

Quantum tunneling of magnetization of S = frac(1, 2) Cu^{2 +} ions doped in glass matrix has been investigated. An adiabatic condition has been achieved by using a fast sweeping magnetic field. The forbidden zero field quantum tunneling is observed even in the dilute limit, where the dipolar coupling is negligible. The tunneling gap is independent of the doping concentration and the value is evaluated to be 20 mK. As the origin of this small tunneling gap, the hyperfine coupling with the on-site nuclear spin (I = frac(3, 2)) is considered. It is found that the tunneling is allowed because the total spin is integer in this coupled system of S = frac(1, 2) and I = frac(3, 2).