STUDY OF QUANTUM TUNNELING EFFECT IN NANOSCALE MAGNET BY NUCLEAR MAGNETIC RESONANCE

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The purpose of the present projects is to clarify the quantum tunneling effect and relevant spin dynamics from the microscopic view point using NMR on the typical nano-scale molecular magnets Mn_<12>Ac and V_<15>. Using high frequency pulsed powander amplifier and He3 NMR cryostat purchased in 2000 and 2001, we constructed high frequency NMR apparatus up to 800MHz operating at very low temperatures.On Mn_<12>Ac, after the first success in observation of ^<55>Mn NMR and determination of the hyperfine interactions of the ^<55>Mn in three nonequivalent Mn^<+4> and Mn^<+3> ions, we have studied the followings: (1) The experimental T_2 and T_1 below 2.5K and down to 0.5K in the field s up to 8T were analyzed by considering the thermal fluctuation of Zeeman levels of the cluster spin by spin -phonon interaction within the framework of the ground-state and the first excited state. It turns out that 1/T_2 is interpreted in terms of strong collision model, while 1/T_1 is well understood via standard perturbation formalism. (2) From the measurement of the spin-echo signal intensity, we found that the recovery of the cluster magnetization exhibits √<t> -behavior during the initial short time, and there appear dips at every 0.45T, which indicates the evidence for the quantum tunneling. (3) At ultra-low temperatures there happens an avalanche effect.On V_<15>, the temperature dependence of 1/T_1 of proton in the various values of the applied field has been interpreted in terms of thermal-activated model reflecting the simple Zeeman levels constructed from the ground state of S=1/2 and S=3/2.