Evidence for resonant tunneling of magnetization inMn12sacetate complex

Abstract

We have measured the dc magnetization at low temperatures of tetragonal crystals of Mn 12 acetate complex ͓Mn 12 O 12 ͑CH 3 COO͒ 16 ͑H 2 O͒ 4 ͔, a material composed of a large ͑Avogadro's͒ number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy ͑ϳ60 K͒ gives rise to a doubly degenerate ground state corresponding to spin projections of Ϯ10 along the easy axis ͑c axis͒; hysteretic behavior is found below a blocking temperature T b ϳ3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results. ͓S0163-1829͑97͒00709-1͔

Time-dependent heat capacity ofMn12clusters

“…Very recent dynamic hysteresis and magnetization relaxation dc and ac experiments demonstrate the existence of resonant tunneling between these quantized levels in the macroscopically distinct anisotropy wells. [3][4][5][6][7][8] Theoretical studies [9][10][11][12][13] agree with experimental results. It has also been theoretically calculated 9 and experimentally tested 14 that resonant spin tunneling between the levels in the two wells is greatly enhanced by increasing the transversal component of the magnetic field, the latter due to the increased degree of mixing between the unperturbed energy levels.…”

“…Both processes are governed by Boltzmann's characteristic time ϰexp⑀/T, where ⑀ 1 is 12 K, which is the energy gap between the lowest-lying levels. The levels involved in the second process are those at the top of the barrier [3][4][5][6][7] and therefore ⑀ 2 is about 40 K. The great difference in the values of the characteristic time of the two processes allows us to assume that the time dependence is mostly determined by the tunneling process. The former process considers two separated potential wells with an infinite barrier whose temperature has increased from T to T ϩ⌬T, so that we have a local thermalization of the levels in each well.…”

Theory of quantum tunneling of the magnetization in magnetic particles

“…4 Furthermore the resonant tunneling may be thermally assisted. [9][10][11][12][13][14] As we will show, several features observed in experiment such as the dependence of the steps on the sweep rate of the applied field and the absence of steps in half of the hysterisis curve, follow quite naturally from the Landau-Zener picture.…”

“…[9][10][11][12] These steps are characteristic of the resonant tunneling of the magnetization. 4 Furthermore the resonant tunneling may be thermally assisted.…”

Effects of the environment on nonadiabatic magnetization process in uniaxial molecular magnets at very low temperatures

“…For such small systems the discreteness of energy level plays an important role and staircase structures of the response of the magnetization to a sweeping magnetic field have been observed. [1][2][3][4][5][6] The staircase is explained as a quantum-mechanical transition at the avoided level crossing points, where levels of the Hamiltonian become almost degenerate, and form repulsive structures as shown in Fig. 1, which has been called resonant tunneling.…”