Non-linear response of single-molecule magnets: field-tuned quantum-to-classical crossovers

TL;DR

This paper investigates how single-molecule magnets exhibit quantum-to-classical crossover behaviors in their nonlinear magnetic response under varying magnetic fields, combining experiments with theoretical modeling.

Contribution

It extends previous studies by experimentally exploring finite dc fields and analyzing quantum nonlinear responses using heuristic and quantum master equation models.

Findings

Quantum nonlinear susceptibility differs from classical predictions near zero field.

Bias fields can induce quantum nonlinear responses at resonant fields.

Classical-like behavior is recovered between resonant fields.

Abstract

Quantum nanomagnets can show a field dependence of the relaxation time very different from their classical counterparts, due to resonant tunneling via excited states (near the anisotropy barrier top). The relaxation time then shows minima at the resonant fields H_{n}=n D at which the levels at both sides of the barrier become degenerate (D is the anisotropy constant). We showed that in Mn12, near zero field, this yields a contribution to the nonlinear susceptibility that makes it qualitatively different from the classical curves [Phys. Rev. B 72, 224433 (2005)]. Here we extend the experimental study to finite dc fields showing how the bias can trigger the system to display those quantum nonlinear responses, near the resonant fields, while recovering an classical-like behaviour for fields between them. The analysis of the experiments is done with heuristic expressions derived from simple balance equations and calculations with a Pauli-type quantum master equation.