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

本课题的目的是利用核磁共振从微观角度阐明典型纳米级分子磁体Mn_<12>Ac和V_<15>上的量子隧道效应和相关的自旋动力学。利用2000年和2001年购买的高频脉冲粉末放大器和He3核磁共振低温恒温器，我们构建了在极低温度下工作的高达800MHz的高频核磁共振仪器。在Mn_<12>Ac上，首次成功观测到^<55>Mn核磁共振，并确定了^<55>Mn在三个非等效Mn^<+4>和Mn^<+3>离子中的超精细相互作用，我们研究了以下几个方面：(1)在基态和第一激发态框架内，考虑自旋-声子相互作用下簇自旋塞曼能级的热涨落，分析了2.5K以下和0.5K以下的实验T_2和T_1。结果表明，1/T_2可以用强碰撞模型解释，而1/T_1可以通过标准摄动形式很好地理解。(2)通过对自旋回波信号强度的测量，发现团簇磁化恢复在初始短时间内表现为√<t> -行为，且每0.45T出现一次衰减，表明量子隧穿存在的证据。(3)在超低温下会发生雪崩效应。在V_<15>时，用热激活模型解释了质子1/T_1在不同电场值下的温度依赖性，该模型反映了由S=1/2和S=3/2基态构建的简单塞曼能级。

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.