【作者】 傅华华；

【导师】 姚凯伦；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2007， 博士

【摘要】 近些年来,分子固体磁学材料的研究成为交叉学科领域的前沿课题,其中对基于纯有机分子的磁性材料的合成和设计引起了人们的广泛关注,该类纯有机磁性材料中,最著名的是D. Shiomi等人实验上合成的基于纯有机S=1的双自由基分子和S=1/2的单自由基分子交替排列构成的复合物,其中分子间是反铁磁相互作用,而分子内是较强的铁磁作用.该类磁性材料具有较好的亚铁磁性,但磁转变温度极低,只有6 K作用,离实际应用尚有较远的距离,极需要对该类磁性材料的磁相互作用机理和量子相变规律作理论上的研究;并从理论的角度,提出提高该类磁性材料居里温度的方法和途径.对于该类磁性材料,我们在理论上将其抽象为自旋S=1/2的亚铁磁棱型链模型.首先,我们采取的哈密顿量为基于XY的模型,利用量子统计的格林函数理论,在Tyablikov的解耦近似框架下,对该亚铁磁棱型自旋链的磁学和热力学性质进行了研究.结果发现:亚铁磁棱型链系统的自旋波激发谱由三支构成,其中两支来源系统的铁磁激发,第三支来源系统反铁磁激发;磁化强度在低温下随外场的变化表现1/3的磁化平台,磁化强度在不同外场下随温度的变化表现丰富的亚铁磁行为;并且,当双自由基分子内铁磁作用小于分子间反铁磁作用时,亚铁磁系统的热容量随温度的变化呈现清晰的双峰结构;对该系统热容量的进一步研究发现,重复单元为三个S=1/2自旋的亚铁磁棱链的物理性质逐渐过渡到自旋S=1和S=1/2反铁磁作用交替排列的亚铁磁链.在基于一般的海森堡哈密顿量模型对亚铁磁棱型链的研究时,我们同样采用格林函数理论,并在Tyablikov的解耦框架下,对其磁学性质进行较系统的研究.结果发现:分子间的反铁磁作用对系统亚铁磁的形成起到重要的作用;并且,适当降低反铁磁作用空间对称性有利于提高系统磁性和居里温度;双自由基分和单自由基间的二聚化的形成有利于增强系统的磁性,并提高居里温度;同时,两个双自由基分子与其中间单自由基间三聚化的形成也利于增强系统的磁性,并提高了磁性系统的居里温度.在研究中我们发现,我们提出的棱型链模型内磁性作用全为反铁磁作用时,该理论模型变为一个最简单的阻挫自旋链模型.在前面理论工作的基础上,我们进一步研究了反铁磁阻挫棱型链的磁学和热力学性质.结果发现:阻挫棱型链的磁化强度在低温下随外场的变化也呈现1/3的磁化平台结构,并且磁化率在低温区出现清晰的双峰结构,低温区的峰来源于系统的阻挫效应,而较高温区的峰来源系统的反铁磁作用.我们的理论结果解释了阻挫化合物Cu3(CO3)2(OH)2在实验上发现的一些新颖的物理现象.同时对一般全对称的阻挫棱型链的研究发现:在无外磁场条件下,系统的热容量随阻挫强度的变化呈现丰富的双峰结构;但向系统加入较弱的外磁场后,发现热容量在中间温区又出现较小的峰,使得热容量随温度的变化曲线呈现清晰的三峰结构,当外场稍微增大,中间温区的峰消失,热容量又回到双峰结构.并由我们的理论结果认为中间温区的峰来源量子系统的场致磁有序.在最后,我们采取其他的数值方法,如量子转移矩阵方法和有限晶格的精确对角化方法,对S=1/2的棱型自旋链的磁学和热力学性质进行了研究,和前面格林函数方法计算得到的结果定性符合.更多还原

【Abstract】 In recent years, it is a pioneering research issue for the molecular solid magnetic materials. In particular, the synthesis and design of organic molecule-based magnetism have been attracting much attention from both experimental and theoretical points of view. Among these organic molecule-based magnetism, the most famous one is the molecule- based ferrimagnetism synthesized by D. Shiomi et al., which is composed of S=1 biradical and S=1/2 monoradical molecules alternating with antiferromagnetic interactions along the chain, and the S=1 site is composed of two S=1/2 spins by a finite ferromagnetic interaction. The organic molecular complex exhibits excellent ferrimagnetic order at low temperature, however, the magnetic transition temperature Tc of which is as low as about 6 K, which makes the organic magnetic material far from practical functionality. Such that, the magnetic coupling mechanism and the quantum magnetic transition of these organic ferrimagnetisms need to be studied in detail, and the new schemes to design and synthesize the organic molecule-based ferrimagneism with high Curie temperature should be put forward from a theoretical point of view.Based on the molecular magnetic materials synthesized experimentally, we have described above organic molecule-based magnetic materials as a spin-1/2 ferrimagnetic diamond-like chain. Firstly, we consider the diamond-like spin chain as a one-dimensional isotropic XY Heisenberg Hamiltonian model, and the magnetic properties and the thermodynamics of which have been studied by means of two-time Green’s function theory within Tyablikov’s decoupling approximation. The numerical results show that the spin-wave excitation spectra of the system are composed of three distinct branches of elementary excitations, the two of which, originate from the ferromagnetic interaction’excitations, and the third originates from the antiferromagnetic ones. The magnetic field dependence of the magnetization at low temperature displays as a 1/3 plateau with three critical fields, and the temperature dependence of the magnetization at various magnetic field shows rich ferrimagnetic behaviors. Meanwhile, when the intramolecular ferromagnetic interaction is less than the intermolecular antiferromagnetic ones, the temperature dependence of the specific heat of the system shows a distinct double-peak structure. With a deep research of the specific heat of the system, it is concluded that, with the further increasing of the intramolecule ferromagnetic interaction, the biradical having two S=1/2 spins becomes closer and closer to that of effective S=1 monomolecular for the thermodynamic properties.Secondly, we consider the diamond-like spin chain as a one-dimension general Heisenberg Hamiltonian model, the magnetic and thermodynamic properties of which are studied by means of Green’s function with combination to Tyablikov’s decoupling approximation. It can be found that the intermolecular antiferromagnetic interactions play an important role on the formation of the ferrimagnetic order along the spin chain. And the lower space symmetry of the antiferromagnetic interactions has contribution to the spontaneous magnetization and the increasing of the Curie temperature. Meanwhile, the formation of dimer between the biradical and the monoradical has contribution to the the spontaneous magnetization and the decreasing of the Curie temperature, meanwhile, the trimer among the molecules plays a simliar role. So that, to synthesis the organic molecule-based ferrimagnetism with high Tc, the space symmetry of magnetic interactions, the dimer, and the trimer along the spin chain should be taken into account.As far as the above-mentioned spin-1/2 diamond-like spin model, we find that the model can be viewed as one of the simplest one-dimensional quantum frustrated system, assuming that the exchange interactions along the chain are all anitferromagnetic ones, and has already received a substantial amount of theoretical attention. Based on the foregoing theoretical model and method, the magnetization, the magnetic susceptibility, and the specific heat of the frustrated spin system have been discussed. It is found that the magnetic field dependence of the magnetization of the frustrated diamond-like spin chain also displays as a 1/3 plateau, and the temperature dependence of the magnetic susceptibility shows a double-peak structure at low temperature, the peak at lower temperature origins from the effect of spin frustration, and the one located at higher temperature origin from the antiferromagnetic interactions. It should be pointed out that our theoretical results are employed to explain the measurements and unusual behaviors on the frustrated compound Cu3(CO3)2(OH)2 successfully. Meanwhile, the curves of the specific heat of the general symmetrical frustrated diamond-like spin chain shows a distinct double-peak structure due to the effect of frustration in the absence of the magnetic field, nevertheless, it is interesting that, when a weak and finite magnetic field is applied to the spin system, an additional little peak appears at the intermediate temperature, and the curves of the specific heat show a three-peak structure due to the external field-induced magnetic order.In the end, we have applied other numerical methods, such as the quantum transfer matrix method and the finite-lattice exact diagonalization method, to study the magnetic and thermodynamic properties of the spin-1/2 diamond-like spin chain, and the numerical results can be compared with ones of the Green’s function method.更多还原