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

核磁共振研究纳米级磁体中的量子隧道效应

• 批准号: 11440106
• 负责人: GOTO Takao
• 金额: $ 9.41万
• 依托单位: KYOTO UNIVERSITY
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (B)
• 财政年份: 1999
• 资助国家: 日本
• 起止时间: 1999 至 2001
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-11440106/
• 关键词: nano-scale molecular magnet; Mn_<12>-acetate; ^<55>Mn NMR; nuclear magnetic resonance; quantum tunneling; V_<15>; ナノスケール磁性体; ^<55>Mn NMR; V15; 核スピン-格子緩和時間; スピン-スピン緩和時間; 量子トンネル現象; 核スピン・格子緩和時間; スピン・スピン緩和時間

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.

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。

项目成果

Seiji Miyashita, et al.: "Quantum Dynamics and Dissipation, Effects in the Resonant Tunneling of Molecular Magnets"Progress of Theoretical Physics. 145. 370-379 (2002)

Seiji Miyashita 等人：“量子动力学和耗散，分子磁体共振隧道效应”理论物理进展。

T.Goto,T.Kubo,T.Koshiba,Y.Fujii,A.Oyamada,J.Arai,T.Takeda,K.Awaga: "^<55>Mn NMR and Relaxation Studies on Mm_<12>-acetate"physica B. (2000)

T.Goto、T.Kubo、T.Koshiba、Y.Fujii、A.Oyamada、J.Arai、T.Takeda、K.Awaga：“^<55>Mn NMR 和 Mm_<12>-乙酸盐的弛豫研究”

Takao Goto, Takaji Kubo, et al.: "^<55>Mn NMR and relaxation studies on Mn_<12>-acetats"Physica B. 284-288. 1227-1228 (2000)

Takao Goto、Takaji Kubo等人：“Mn_ 12 -乙酸盐的^ 55 Mn NMR和弛豫研究”Physica B.284-288。

Takeshi Koshiba, Takao Goto, Takeji Kubo, Kunio Awaga: "Nuclear Magnetic Relaxation in Molecular Cluster Magnet M_<12>-ac"Progress of Theoretical Physics, Supple. 145. 394-398 (2002)

Takeshi Koshiba、Takao Goto、Takeji Kubo、Kunio Awaga：“分子簇磁体 M_<12>-ac 中的核磁弛豫”理论物理进展，软文。

I. Chiorescu, W. Wernsdorfer, A. Muller, S. Miyashita, B. Barbara: "Adiabatic Landau-Zener-Stuckelberg transition with or without dissipation in the low-spin molecular system V_<15>"Phys. Rev. B. 67. 020402 (2003)

I. Chiorescu、W. Wensdorfer、A. Muller、S. Miyashita、B. Barbara：“在低自旋分子系统 V_<15> 中，有或没有耗散的绝热 Landau-Zener-Stuckelberg 转变”Phys。