55 Mn NMR in Mn 12 acetate: Hyperfine interaction and magnetic relaxation of the cluster
55 Mn NMR in Mn 12 acetate: Hyperfine interaction and magnetic relaxation of the cluster
复制标题
Mn 12 醋酸盐中的 55 Mn NMR：簇的超精细相互作用和磁弛豫
DOI：
10.1103/physrevb.65.224425
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发表时间：
2002
影响因子：
3.7
通讯作者：
N. Univ.
中科院分区：
文献类型：
--
作者：
T. Kubo;T. Goto;T. Koshiba;K. Takeda;Kunio Awaga Nara Univ. of Education;Kyoto Univ.;Hokkaido Univ.;N. Univ.
在1.4- 2.0K温度范围内，研究了Mn 12 Ac定向粉末晶体在零场和外加沿着c轴磁场下的55 Mn核磁共振谱.对I=5/2核的3种由5重四极分裂谱线组成的55 MnNMR谱线，解释为由一个Mn 4 +离子和两个晶体学上不等价的Mn 3 +离子产生.各向同性的超精细场的Mn 4 +离子表示大量的减少（26%）相比，与理论评估。在分析Mn 3 +离子的超精细场时，除了考虑费米接触项外，还考虑了各向异性偶极项的贡献。采用偶极项的8%的缩减因子，这是估计从磁矩确定的极化中子衍射实验，我们得到了15%的费米接触项的减少。我们认为，这种可观的量的减少因子是由于共价和锰离子之间通过氧离子的强交换相互作用。利用Mn 12团簇中12个锰离子的超精细耦合常数，确定了S = 10的亚铁磁基态的超精细相互作用总量最大为0.3cm- 1，其大小对应于由团簇自旋看到的核超精细场0.32kG。在2.0 K下，通过观察沿c轴沿着0.20-1.90 T的外场，研究了55 Mn自旋回波强度的恢复。团簇的磁化强度在短时间内呈现平方根t恢复。随着外场的增加，弛豫时间在每0.45 T处显著下降后减小，这是由于磁能级交叉处自旋态之间的声子辅助量子隧穿效应。
The 5 5 Mn NMR in oriented powder crystal of Mn 1 2 Ac has been investigated at 1.4-2.0 K in zero field and with external fields along the c axis. Three kinds of 5 5 Mn NMR composed of fivefold quadrupole-split lines for I=5/2 nuclei have been interpreted to arise from a Mn 4 + ion and two crystallographically inequivalent Mn 3 + ions. The isotropic hyperfine field in the Mn 4 + ion indicates a large amount of reduction (26%) as compared with the theoretical evaluation. In the analysis of the hyperfine field in Mn 3 + ions the anisotropic dipolar contribution has been taken into account in addition to the Fermi-contact term. Adopting the reduction factor 8% of the dipolar term, which is estimated from the magnetic moment determined by a polarized-neutron-diffraction experiment, we obtained a reduction of 15% for the Fermi-contact term. We suppose that such an appreciable amount of the reduction factor is due to the covalence and strong exchange interaction among manganese ions via the oxygen ions. By using the hyperfine coupling constants of 12 manganese ions in the Mn 1 2 cluster, the total hyperfine interaction of the ferrimagnetic ground state of S = 10 has been determined to amount to 0.3 cm - 1 in magnitude at most, the magnitude of which corresponds to the nuclear hyperfine field 0.32 kG seen by the cluster spin The relaxation of the cluster magnetization after reversal of the external field was investigated by observing the recovery of the 5 5 Mn spin-echo intensity in the fields of 0.20-1.90 T along the c axis at 2.0 K. The magnetization of the cluster exhibited the square-root t recovery in the short-time regime. With increasing external field, the relaxation time decreased following significant dips at every 0.45 T, which is due to the effects of phonon-assisted quantum tunneling between the spin states at magnetic level crossings.