Non-Kramers Freezing and Unfreezing of Tunneling in the Biaxial Spin Model

TL;DR

This paper investigates the tunneling behavior in a biaxial spin model under magnetic field, revealing a new instanton type and a transition from oscillatory to monotonic tunneling splitting at a specific field, with implications for Fe8 nanomagnets.

Contribution

It introduces a novel spin instanton and uncovers a quantum interference transition in tunneling behavior under magnetic field in the biaxial spin model.

Findings

Discovery of a new spin instanton type.

Identification of a transition from oscillatory to monotonic tunneling splitting.

Predictions applicable to Fe8 molecular nanomagnets.

Abstract

The ground state tunnel splitting for the biaxial spin model in the magnetic field, H = -D S_{x}^2 + E S_{z}^2 - g \mu_B S_z H_z, has been investigated using an instanton approach. We find a new type of spin instanton and a new quantum interference phenomenon associated with it: at a certain field, H_2 = 2SE^{1/2}(D+E)^{1/2}/(g \mu_B), the dependence of the tunneling splitting on the field switches from oscillations to a monotonic growth. The predictions of the theory can be tested in Fe_8 molecular nanomagnets.