Magnetic quantum tunneling in the half-integer high spin molecule CrNi6: a muSR study

In high spin molecules metal ions are coupled by ferro or antiferromagnetic short range interactions so that their magnetic moments are parallel or antiparallel to each other at temperatures (T) much smaller than the coupling constant J. This leads to a high spin (S) value in the ground state. In the crystal lattice, the molecules are well separated from each other and the active part of the Hamiltonian at k_B*T << J is H=-D*S_z^2, so that up and down spins states have identical energies, and S_z can escape from one state to the other via either over-the-barrier motion, or tunneling. So far, the escape rate Gamma in HSM was found to have smooth T dependence throughout the cooling process from DS^2 << k_B*T to k_B*T << D*S^2. In this case, the transition from the over-the-barrier motion to tunneling, is referred to as second-order transition. In the present work, we use the muon spin relaxation technique to show evidence for temperature-independent tunneling at k_B*T << D*S^2, in zero applied magnetic field (ZF), in the HSM CrNi6. Moreover, the crossover from the classical to quantum regime is sharp, and appears to be consistent with a first order transition of the escape rate. In a first order transition, over-the-barrier motion is abruptly replaced by tunneling between ground states, as the temperature is lowered down to the barrier height.