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

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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.