The purpose of this study is to synthesize vertical ferromagnetic/semiconducting heterojunction nanowires by combing the catalyst-free selective-area growth of InAs nanowires and the endotaxial nanoclustering of MnAs and to structurally and magnetically characterize them. MnAs penetrates the InAs nanowires to form nanoclusters. The surface migration length of manganese adatoms on the nanowires, which is estimated to be 600 nm at 580 °C, is a key to the successful fabrication of vertical MnAs/InAs heterojunction nanowires with atomically abrupt heterointerfaces.

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We report in detail on the growth time dependence of ferromagnetic MnAs nanoclusters in vertical MnAs/InAs heterojunction nanowires and the analysis of magnetization switching in a heterojunction nanowire. The size of MnAs nanoclusters increases with increasing growth time of MnAs nanoclusters. In addition, we observe the magnetization and magnetization switching in one of the MnAs nanoclusters with a relatively large height parallel to the c-axis, i.e., the magnetic hard axis, possibly owing to the magnetic shape anisotropy along the c-axis, after the application of external magnetic fields nearly parallel to the c-axis of MnAs nanoclusters. These results suggest that the growth time of MnAs nanoclusters is a key to controlling the size, shape, and even magnetization of MnAs nanoclusters in vertical MnAs/InAs heterojunction nanowires.

We report on structural and magnetic characterization results of lateral MnAs nanowires formed by selective-area metal–organic vapor phase epitaxy. The lateral MnAs nanowires observed by backscattered and secondary electron microscopies have a high aspect ratio (i.e., 2.6 µm long and 360 nm wide) and well-defined crystal facets. The external magnetic field dependence in magnetic force microscopy observations of the lateral nanowires reveals that single and multiple magnetic domains were formed in the lateral nanowires. This study suggests that magnetic domains are tuned by the structural control of lateral nanowires. However, no magnetization switching is observed under a relatively high external magnetic field of 5000 G, which should be enough for the magnetization switching of the lateral nanowires, possibly owing to the magnetization switching of the magnetic force microscope tip itself used in this study during observations.

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