Butterfly hysteresis loop at non-zero bias field in antiferromagnetic   molecular rings: cooling by adiabatic magnetization
  At low temperatures, the magnetization of the molecular ferric wheel NaFe$_6$ exhibits a step at a critical field $B_c$ due to a field-induced level-crossing. By means of high-field torque magnetometry we observed a hysteretic behavior at the level-crossing with a characteristic butterfly shape which is analyzed in terms of a dissipative two-level model. Several unusual features were found. The non-zero bias field of the level-crossing suggests the possibility of cooling by adiabatic magnetization. 
O. Waldmann, R. Koch, S. Schromm, P. M\"uller, I. Bernt, R. W.   Saalfrank