Zirconium-based compounds, such as ZrO x N y , have been investigated as a cathode of polymer electrolyte fuel cells. ZrO x N y was prepared using radio-frequency ͑rf͒ magnetron sputtering under an Ar + O 2 + N 2 with heating the substrate. The solubility of ZrO x N y deposited at 500°C was less than 5 ϫ 10 −8 mol dm −3 in 0.1 mol dm −3 H 2 SO 4 at 30°C under atmospheric condition, indicating that ZrO x N y had a high chemical stability in acidic media. The effect of the heat-treatment during the film deposition on the catalytic activity for oxygen reduction reaction ͑ORR͒ and the properties of ZrO x N y has been examined. The ORR current density on ZrO x N y deposited at a substrate temperature of 800°C was ϳ30 times greater than that at 50°C. The dependence of both the ORR current density at 0.4 V and the apparent activation energy of the ORR at 0.4 V on the substrate temperature changed near 500°C. The crystalline structure of ZrO x N y also changed near 500°C. In addition, the catalytic activity for the ORR increased with the increasing crystallinity and decreasing ionization potential of the specimens. Therefore, the crystalline structure, the crystallinity, and the ionization potential might affect the catalytic activity for the ORR.
We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare earth intermetallic compound, Nd2Fe14B. The constrained Monte Carlo method is applied to a Nd2Fe14B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants K A m (m = 1, 2, 4) at finite temperatures. Subsequently, it is found that the temperature dependence of K A 1 deviates from the Callen-Callen law,3 , even above room temperature, TR ∼ 300 K, when the Fe (Nd) anisotropy terms are removed to leave only the Nd (Fe) anisotropy terms. This is because the exchange couplings between Nd moments and Fe spins are much smaller than those between Fe spins. It is also found that the exponent n in the external magnetic field Hext response of barrier height FB = F 0 B (1 − Hext/H0) n is less than 2 in the low-temperature region below TR, whereas n approaches 2 when T > TR, indicating the presence of Stoner-Wohlfarth-type magnetization rotation. This reflects the fact that the magnetic anisotropy is mainly governed by the K A 1 term in the T > TR region.
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