William Herbert F, Krishnamoorthy Aravind, Rands Lucy, Van Vliet Krystyn J, Yildiz Bilge
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.
Phys Chem Chem Phys. 2015 Apr 28;17(16):11036-41. doi: 10.1039/c4cp05389c.
Cation diffusion is an important rate-limiting process in the growth of pyrrhotite (Fe1-xS) in passivating films on steels exposed to sulfidic environments, and for proposed synthetic applications of Fe1-xS, for example single-phase magnetic switching devices. Above the Néel temperature TN of 315 °C, where Fe1-xS is paramagnetic and structurally disordered, iron self-diffusivity *DFe predictably follows a standard, established Arrhenius law with temperature. However, we report (57)Fe tracer diffusion measurements below TN, obtained using secondary ion mass spectrometry (SIMS), that demonstrate a 100-fold reduction in diffusion coefficient as compared to the extrapolated, paramagnetic Arrhenius trend at 150 °C. The results can be described by a magnetic diffusion anomaly, where the vacancy migration energy for the spontaneously-magnetized cation sublattice is increased by approximately 40% over the paramagnetic state. These constitute the first set of consistent diffusivity data obtained in magnetic pyrrhotite, allowing more accurate prediction of pyrrhotite growth rates and determination of magnetic properties for synthetic devices.
阳离子扩散是暴露于硫化环境的钢表面钝化膜中磁黄铁矿（Fe1-xS）生长过程中的一个重要限速过程，对于Fe1-xS的拟合成应用，例如单相磁开关器件而言也是如此。在315°C的奈尔温度TN以上，Fe1-xS呈顺磁性且结构无序，铁的自扩散系数DFe可预测地遵循标准的、既定的阿仑尼乌斯定律随温度变化。然而，我们报告了使用二次离子质谱（SIMS）获得的低于TN的（57）Fe示踪剂扩散测量结果，结果表明，与150°C时外推的顺磁性阿仑尼乌斯趋势相比，扩散系数降低了100倍。这些结果可用磁扩散异常来描述，即自发磁化的阳离子亚晶格的空位迁移能比顺磁态增加了约40%。这些构成了在磁性磁黄铁矿中获得的第一组一致的扩散率数据，有助于更准确地预测磁黄铁矿的生长速率，并确定合成器件的磁性。
