Low-temperature magnetic hysteresis in acetate single crystals

¹ Department of Physics, New York University 4 Washington Place, New York, NY 10003 USA
² Physics Department, City College of the City University of New York New York, NY 10031 USA
³ Department of Chemistry and Biochemistry, University of California at San Diego La Jolla, CA 92093 USA

Corresponding author: andy.kent@nyu.edu

Received: 28 July 1999
Accepted: 5 December 1999

Abstract

Precise magnetic-hysteresis measurements of small single crystals of Mn₁₂ acetate of spin 10 have been conducted down to 0.4 K using a high-sensitivity Hall magnetometer. At higher temperature (> 1.6 K) 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 higher-order uniaxial magnetic anisotropy (fourth order in S_z), 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.