Low temperature magnetic hysteresis in Mn$_{12}$ acetate single crystals

TL;DR

This study investigates magnetic hysteresis in Mn12 acetate crystals at very low temperatures, revealing a possible transition from thermally assisted to pure quantum tunnelling.

Contribution

It provides experimental evidence for a potential first-order transition between different quantum tunnelling regimes in Mn12 acetate.

Findings

Steps in magnetization shift to higher fields as temperature decreases.

Abrupt shift in step positions at very low temperatures.

Possible first evidence of a first-order transition in quantum tunnelling.

Abstract

Precise magnetic hysteresis measurements of small single crystals of Mn$_{12}$ acetate of spin 10 have been conducted down to 0.4 K using a high sensitivity Hall magnetometer. At higher temperature (>1.6K) step-like changes in magnetization are observed at regularly spaced magnetic field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a second order uniaxial magnetic anisotropy, first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.