Nature of magnetic impurity induced superparamagnetism and anomalous Hall effect in β-FeSi2 single crystals

The magnetic properties of β-FeSi2 single crystals were investigated using magnetization and anomalous Hall effect measurements. These measurements clearly show the appearance of superparamagnetism, the origin of which is attributed to defect-induced magnetic aggregates. Unlike previous reports, based on our combined analysis using magnetization and the anomalous Hall effect, we found that two kinds of aggregates, which are magnetically distinct, exist in our samples. Among the two types found, one gives an anomalous Hall term, which follows a side-jump mechanism while the other is a completely new kind, suggesting that magnetic properties of β-FeSi2 are very unconventional. The sizes of the aggregates were estimated from our experimental data.