Construction of the nuclear-magnetic-relaxation theory on low-dimensional magnets

提出了一种改进的自旋波理论，并将其应用于低维量子磁体。本文描述了一维铁磁体的双峰比热、铁磁链和团簇中的核自旋晶格弛豫以及haldane间隙反铁磁体的热行为。提到其他玻色子和费米子表示，我们证明了自旋波在低维中仍然有效。介观磁学是材料科学的热点之一，我们可以在分子磁体到体磁体的过程中观察到量子-经典交叉。因此，金属离子磁性团簇是有趣的，其中包括一系列分子环，它使我们能够系统地研究零维自旋动力学作为分子大小和自旋幅度的函数。核自旋-晶格弛豫时间Ti作为低频自旋动力学的有力探针，已经在含有铜、铬和铁离子的各种金属轮中被测量过。因此，考虑到晶体场各向异性和各向异性自旋-自旋相互作用以及海森堡相互作用，我们在微观上计算了铁磁和反铁磁环簇的1/ Ti。下图将我们的计算结果与铁磁环簇Cu6的实验结果进行了比较，其中我们强调了离散能级与Ti的寿命展宽以及不可忽略的各向异性效应是解释1/ Ti的特征峰与t的函数关系的关键因素。我们还计算了反铁磁环簇Cu8，并指出了Dzyaloshinsky-Moriya相互作用的可能意义。

A modified spin-wave theory is developed and applied to low-dimensional quantum magnets. Double-peaked specific heat for one-dimensional ferrimagnets, nuclear spin-lattice relaxation in ferrimagnetic chains and clusters, and thermal behavior of Haldane-gap antiferromagnets are described within the scheme. Mentioning other bosonic and fermionic representations as well, we demonstrate that spin waves are still effective in low dimensions.Mesoscopic magnetism is one of the hot topics in materials science, where we can observe a quantum-to-classical crossover on the way from molecular to bulk magnets. Metal-ion magnetic clusters are thus interesting and among others is a series of molecular rings, which allows us to systematically investigate zero-dimensional spin dynamics as a function of the molecule size and the spin magnitude. The nuclear spin-lattice relaxation time Ti works as a powerful probe to the low-frequency spin dynamics and has indeed been measured for various metallic wheels containing copper, chromium, and iron ions. Thus motivated, we microscopically calculate 1/ Ti for both ferromagnetic and antiferromagnetic ring clusters, taking account of crystal-field anisotropies and anisotropic spin-spin interactions as well as the Heisenberg interaction. The following figure compares our calculations with experiments] on the ferromagnetic ring cluster Cu6, where we stress the lifetime broadening of descrete energy levels in proportion to Ti and the nonnegligible anisotropy effect as the key factors to interpret the characteristic peaks of 1/ Ti as a function of T. We calculate the antiferromagnetic ring cluster Cu8 as well and point out possible significance of the Dzyaloshinsky-Moriya interaction.