Observation of large spin splitting in the conduction band of n-type ferromagnetic semiconductor (In,Fe)As*

Ferromagnetic semiconductors (FMSs) both with large spin-split conduction band (CB) and valence band (VB) and with high Curie temperature ($T_{C})$ are highly desired for spintronic devices, which is not yet realized so far. Here, we report the first observation of large spontaneous spin splitting ($\Delta E=$50 meV) in the CB of n-type FMS (In,Fe)As using tunneling spectroscopy in (In,Fe)As-based Esaki diodes. The device structure consists of 50 nm-thick n$^{+}$(In,Fe)As/5 nm-thick InAs/250 nm-thick p$^{+\, }$InAs:Be grown on a p$^{+}$InAs(001) substrate. At small forward bias voltages, electrons tunnel from the (In,Fe)As CB to the p$^{+}$InAs VB, thus the tunneling conductance d$I$/d$V$ probes the density of states of the (In,Fe)As CB. In the d$^{2}I$/d$V^{2}-V$ curves, we clearly observe double-valley features at low temperatures, which evolve into single-valley features at temperatures above $T_{C\, }$of the (In,Fe)As films. This is clear evidence of the spin splitting of the (In,Fe)As CB bottom. We found that the mean-field Zener model also fails to explain consistently the $T_{C}$ and $\Delta E$ of (In,Fe)As. [1] L. D. Anh et al., Nature Communications (2016), arXiv:1609.01379.