Magnetic Ordering and Nonlinear Response of Complex Systems Related to C_<60>

本文主要用非线性磁化率（x_2）测量方法研究了几种含晶体团簇的复杂磁体和超导体。讨论了复杂系统在非均质边界处特征相互作用的影响。首先，设计并构建了一套基于新型低温恒温器的测量系统，以观察在低频和弱场幅值的恶劣实验条件下的非线性磁化率。使用该系统，我们已经能够检查高次谐波（高达9阶）以及每种复杂材料的磁响应的基本原理。对于CoCl_2-GIC，当激励幅值(h)大于0.6Oe时，交流磁化各谐波分量在较高临界温度附近呈现正峰值异常。这个事实证实了x_2在临界温度下呈负散度。同时也证明了当h小于0.3Oe时，x_2异常剖面变为反对称异常是一种固有现象。对于陶瓷超导体YBa_2Cu_4O_8，在足够低于晶型T_c的临界温度下，x_2表现出明显的峰值异常。推测这一异常是d波超导晶粒在陶瓷中相序转变受挫的反映。对于CaO-SiO_2-0.5Fe_2O_3玻璃磁体，沉积的铁磁团簇与其周围的非晶态边界区之间存在磁相互作用。这种相互作用导致ESR场的急剧下降和直流磁化率的居里-魏斯定律。此外，尽管多次尝试合成IBr- c_ <60>，但使用不同的IBr分压掺杂得到的化合物均未发现自发磁化现象。

Several kinds of complex magnets and superconductors containing crystallographic clusters have been studied chiefly by nonlinear magnetic susceptibility (x_2) measurement. Effects of characteristic interaction at the heterogeneous boundaries have been discussed for the complex systems.First of all, a measurement system with a new cryostat apparatus was designed and constructed in order to observe nonlinear susceptibility under severe experimental conditions with both 1ow frequency and weak field amplitude. Using the system, we have been able to examine higher harmonics (up to 9'th order) as well as fundamental of the magnetic responses for each complex materia1.For CoCl_2-GIC,every harmonic component of the AC magnetization shows positive peak anomaly near the upper critical temperature, when the excitation amplitude (h) is larger than 0.6Oe. This fact confirms that x_2 shows negative divergence at the critica1 temperature. It is a1so proved to be an intrinsic phenomenon that the profile of the x_2 anomaly changes into anti-symmetric one when h becomes smaller than 0.3Oe.For ceramic superconductor YBa_2Cu_4O_8, x_2 shows sharp peak anomaly at a certain critical temperature enough below T_c of the crystalline specimens. This anomaly is speculated to be a reflection of frustrated phase ordering transition of the d-wave superconducting grains in the ceramics.For glassy magnet CaO-SiO_2-0.5Fe_2O_3, magnetic interaction between deposited ferrimagnetic clusters and amorphous boundary region around them is suggested. The interaction causes drastic descending shift of the ESR field as well as Curie-Weiss law of the DC susceptibility.In addition, despite several attempts to synthesize IBr-C_<60>, no spontaneous magnetization has been found in any compounds obtained by using various partial pressure of IBr for the doping.