Abstract We have measured the magnetic susceptibility and heat capacity of CuO nanoparticles (16 nm) from 2 K to 400 K using a Quantum Design Physical Properties Measurement System (PPMS). The magnetization curves, acquired at various field strengths from 0 Oe to 50 kOe, are similar to literature data of bulk and nanoscale CuO showing a minimum at about 150 K, an upturn as temperature approaches 0 K, and a broad maximum at high temperatures extending beyond 400 K. The heat capacity data between 200 K and 400 K show several broad peaks. The number of these peaks and the temperatures at which they occur differ significantly from the magnetic transitions known to exist in bulk CuO. To further investigate these transitions in nanoscale CuO, we performed temperature dependent x-ray diffraction (XRD) at temperatures from 90 K to 700 K from which lattice parameters as a function of temperature were derived using a Rietveld refinement. Although no phase transitions were observed in these data, changes in the slopes of the lattice parameters are apparent at the transition temperatures observed in the heat capacity and susceptibility measurements. The various transitions in the heat capacity data are attributed to competing ferromagnetic and antiferromagnetic interactions caused by structural properties that are unique to nanophase CuO, and the temperature range of multiferroicity in nanoscale CuO is shown to extend to temperatures higher than those observed for bulk CuO. The heat capacity and temperature dependent XRD measurements are the first of their kind to be reported for CuO nanoparticles.

中文翻译：

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纳米级 CuO 中多铁性的扩展温度区域

摘要 我们使用量子设计物理特性测量系统 (PPMS) 测量了 CuO 纳米粒子 (16 nm) 从 2 K 到 400 K 的磁化率和热容量。在 0 Oe 到 50 kOe 的各种场强下获得的磁化曲线与块状和纳米级 CuO 的文献数据相似，显示在约 150 K 处出现最小值，随着温度接近 0 K 出现上升，在高温下出现宽阔的最大值超过 400 K。200 K 和 400 K 之间的热容量数据显示出几个宽峰。这些峰的数量和它们出现的温度与已知存在于块状 CuO 中的磁转变有很大不同。为了进一步研究纳米级 CuO 中的这些转变，我们在 90 K 到 700 K 的温度下进行了温度相关的 X 射线衍射 (XRD)，从中使用 Rietveld 精修推导出作为温度函数的晶格参数。尽管在这些数据中没有观察到相变，但在热容量和磁化率测量中观察到的转变温度下，晶格参数斜率的变化是明显的。热容数据的各种转变归因于由纳米相 CuO 独有的结构特性引起的竞争性铁磁和反铁磁相互作用，并且纳米级 CuO 的多铁性温度范围扩展到比块体 CuO 观察到的温度更高的温度。