Insights into size-dominant magnetic microwave absorption properties of CoNi microflowers via off-axis electron holography.

In this study, CoNi flower-like hierarchical microstructures with different sizes were obtained via a one-step solvothermal method by simply adjusting the concentration of precursors and surfactant. The obtained CoNi microflowers possess uniform and tunable size, good monodispersity, and remarkable magnetic microwave absorption properties as well as electron holography phase images. Characterization results have demonstrated the dependency of properties of CoNi microflowers on their morphologies and sizes. The microflowers exhibit different stray magnetic fields that might be determined by whether the pristine nanoflakes on the flowers' surface was parallel or perpendicular to grid plane. And as the size of microflowers increased, the coercive force (Hc) value decreased while saturation magnetization (Ms) value gradually increased, and it can be also observed that the values of Ms and Hc at 5 K are higher than those at 300 K. In addition, the blocking temperature decreased when size increased. Typically, the 2.5 μm CoNi microflowers achieve the maximum reflection loss (RL) value of -28.5 dB at 6.8 GHz with a thickness of 2 mm, while on the other hand, the 0.6 μm flowers achieved a broader absorption bandwidth below -10 dB of 6.5 GHz. Therefore, it is believable that the CoNi flowers with different sizes and hierarchical structures in this work have great potential for high performance magnetic microwave absorption applications.