Here we report an unconventional magnetic and transport phenomenon in a layered cobalt oxide, NaxCoO2. Only for x = 0.75, a magnetic transition of the second order was clearly detected at Tm ∼ 22 K where an apparent specific-heat jump, an onset of extremely small spontaneous magnetization, and a kink in resistivity came in. Moreover large positive magnetoresistance effect was observed below Tm. These features of the transition strongly indicate the appearance of an unusual electronic state that may be attributed to the strongly-correlated electrons in Na0.75CoO2.

Electrical resistivity (ρ) and thermoelectric power (S) were measured for NaxCo2O4 samples with various Na contents (x). A highly precise control of Na nonstoichiometry was facilitated by optimizing the synthesis procedure. With increasing x, the absolute value of ρ monotonically decreased, while the value of S increased. For the x=1.5 sample, S reached a value as high as ∼120 μV/K (at 300 K).

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*This research represents a learning model of cooperation behavior for disaster prevention to study the basic relationship between disaster and social reaction. Multi-Agent system on virtual space, where disaster periodically happens and agents learn behavior strategy, is set up. Agents pursue for property selfishly or cooperate for disaster based on their own parameter of cooperation. Then some parametric studies on disaster characteristics and behavior rules were carried out. The results showed that cooperation emerges when disaster scale is moderate. If disaster scale is too large or too small, agents prefer short-term gain. Regardless of such condition, cooperation generates when agents restraint each other by penalty on uncooperative agent. Cooperation and penalty criteria increase as disaster scale gets large. This implies a point in common with some former studies in humanities and literature.Keywords : Multi-Agent, Disaster, Disaster, Cooperation, Social Dilemma, Catastrophic Risk -2143 -This research represents a learning model of cooperation behavior for disaster prevention to study the basic relationship between disaster and social reaction. Multi-Agent system on virtual space, where disaster periodically happens and agents learn behavior strategy, is set up. Agents pursue for property selfishly or cooperate for disaster based on their own parameter of cooperation. Then some parametric studies on disaster characteristics and behavior rules were carried out. The results showed that cooperation emerges when disaster scale is moderate. If disaster scale is too large or too small, agents prefer short-term gain. Regardless of such condition, cooperation generates when agents restraint each other by penalty on uncooperative agent. Cooperation and penalty criteria increase as disaster scale gets large. This implies a point in common with some former studies in humanities and literature.

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