Epitaxial growth and characterization of superconductor Al / ferromagnetic semiconductor (In,Fe)As heterostructures on InP substrates*

              Heterostructures of superconductor (SC) / n-type ferromagnetic semiconductor (FMS) (In,Fe)As are expected to realize topological SCs without external magnetic field because n-type FMS (In,Fe)As exhibits unique properties such as spontaneous spin-splitting [1], gate controllability of ferromagnetism [2] and spin-triplet superconductivity [3]. High-quality Al / (In,Fe)As heterostructures will be promising for superconducting spintronics devices.

              In this work, we have successfully grown Al / FMS (In,Fe)As heterostructures on InP (001) substrates by molecular beam epitaxy. The (In,Fe)As layers were grown on (In_1-y,Al_y)As (y = 0.10, 0.15, 0.19) buffer layers, and thus receive tunable in-plane compressive strain. Unlike the conventional electron-induced ferromagnetism, the (In,Fe)As layers with large strain show enhanced ferromagnetism while being highly resistive at low temperatures, possibly because the density of interstitial Fe atoms increases as the compressive strain increases. On the other hand, the top Al layer with single crystal domains exhibits a superconducting transition at around 1.2 K.

[1] L. D. Anh et al., Nature Comm. 7, 13810 (2016). [2] L. D. Anh et al, PRB 92, 161201(R) (2015). [3] T. Nakamura et al., PRL 122, 107001 (2019).