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

本项目的目的是用核磁共振技术从微观角度阐明典型的纳米分子磁体Mn_&lt；12&gt；Ac和V_&lt；15&gt；的量子隧道效应和相关的自旋动力学。利用2000年和2001年购买的高频脉冲功率放大器和HE3核磁共振低温恒温器，建立了工作在极低温度下的800 MHz高频核磁共振仪。在首次成功地观察到55&gt；mN的核磁共振并测定了三种不同的不等价的mN；+4&t；和+3&gt；(1)在基态和第一激发态的框架下，考虑了自旋-声子相互作用引起的团簇自旋塞曼能级的热涨落，分析了实验中的T_2和T_1在低于2.5K到0.5K的S场中的分布。结果表明，1/T_2可以用强碰撞模型来解释，而1/T_1可以用标准微扰形式来很好地理解。(2)通过对自旋回波信号强度的测量，我们发现团簇磁化强度的恢复在最初的短时间内呈现√&lt；t&gt；行为，并且在每0.45T处出现下降，这是量子隧穿的证据。(3)在超低温下，发生了雪崩效应.在V_t_t；15&gt；上，用反映由S=1/2和S=3/2基态构造的简单塞曼能级的热激活模型解释了不同场值下质子的1/T_1与温度的关系.

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.