Spin tunneling in a swept magnetic field

Department of Physics and Astronomy, Tufts University - Medford, MA 02155, USA Laboratoire de Magnétisme, Louis Néel, CNRS - BP 166, 38042-Grenoble, France

Received: 24 September 1996
Accepted: 25 May 1997

Abstract

We present a theory whose goal is to account for the recent exciting hysteresis experiments on the spin cluster crystal -acetate that indicate the occurrence of spin tunneling. We have found the tunneling probability P for the tunneling of spin from one spin state into another at a spin level crossing in response to a swept applied magnetic field H. It is shown that P is a function of only one parameter, the dimensionless sweep rate ρ given by , where is the rate at which the two energy levels approach each other as the field is swept (being proportional to the sweep rate ). is the minimum energy gap at the anti-crossing, being the tunneling rate thereat. Numerical integration leads to . We find that, contrary to widespread belief, the experiments cannot be understood in terms of tunneling of individual spin clusters in the presence of a static applied transverse field. Rather, the required transverse field must have dynamic nuclear spins and other dynamic spin clusters as its source.