Are superparamagnetic spins classical?

TL;DR

This paper investigates whether superparamagnetic spins can be accurately described as classical objects, revealing that conventional classical models fail to capture certain quantum effects even at large spin sizes.

Contribution

It demonstrates that the traditional classical approach to superparamagnetism does not fully account for quantum effects in large spins with anisotropy, especially under symmetry-breaking interactions.

Findings

Classical models do not reproduce microscopic results for large spins.

The prefactor in the escape rate is highly sensitive to symmetry-breaking interactions.

Quantum effects persist even as spin size approaches infinity.

Abstract

Effective giant spins of magnetic nanoparticles are considered classically in the conventional theory of superparamagnetism based on the Landau-Lifshitz-Langevin equation. However, microscopic calculations for a large spin with uniaxial anisotropy, coupled to the lattice via the simplest generic mechanism, show that the results of the conventional theory are not reproduced in the limit S ->\infty. In particular, the prefactor Gamma_0 in the Arrhenius escape rate over the barrier Gamma =Gamma_0 exp[-Delta U/(k_B T)] has an anomalously large sensitivity to symmetry-breaking interactions such as transverse field