Crossover between Thermally Assisted and Pure Quantum Tunneling in Molecular MagnetMn12-Acetate

Abstract

The crossover between thermally assisted and pure quantum tunneling has been studied in single crystals of high spin (S = 10) uniaxial molecular magnet Mn12 using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow ( approximately 0. 1 K) or broad ( approximately 1 K) temperature interval depending on the magnitude of the field transverse to the anisotropy axis.

Dislocation-induced spin tunneling inMn12acetate

“…Measuring temperature dependence of the resonance field H z for a given k gives the information on the level n that dominates tunneling at a given temperature. 11,12 Eq. (72) remains correct if the resonances for different n take place at different values of the field H z , provided that the field is being swept through all these resonances.…”

“…9,11,32 Suggested explanations include collective effects due to dipolar interaction between Mn 12 molecules 43 and fluctuating random noise. 48 The first requires some special initial conditions which are not satisfied in experiment 45,46,47 while the second requires certain assumptions about the spectrum of fluctuations.…”

“…7,8). Numerous experiments on Mn 12 performed since 1996 uncovered a number of other interesting phenomena, such as memory effects, 9 non-exponential relaxation, 9,10,11 and a peculiar crossover between thermal and quantum behavior 12,11 predicted by theory. 13,14,15,16,17,18,19,20,21,22,23,24 Uniaxial spin Hamiltonian, in the field parallel to the anisotropy axis, H = −DS 2 z − H z S z , has pairs of degenerate levels for H z = kD, k = 0, ±1, ±2, .…”

“…6 There exist two macroscopic experimental approaches to the study of spin tunneling in Mn 12 (see, e.g., Refs. 6,7,8,25,26,27,28,29,10,9,30,11,12,31,32 and references therein). Both clearly demonstrate the effect of resonant spin tunneling.…”

Calculations of the onset temperature for tunneling in multispin systems

“…Quantum tunneling in spin systems has been the topic of many theoretical [3] and experimental studies [4][5][6][7] over the past few decades. At high enough temperature, quantum effects are not significant and transitions are due to thermal activation described by the Arrhenius law.…”