Observation of spontaneous spin-splitting in the band structure of an n-type zinc-blende ferromagnetic semiconductor

TL;DR

This study reports the first observation of large spontaneous spin-splitting in the conduction band of n-type zinc-blende ferromagnetic semiconductor (In,Fe)As using tunnelling spectroscopy, challenging existing theoretical models.

Contribution

It provides experimental evidence of significant spin-splitting in n-type FMS and highlights the inadequacy of the mean-field Zener model to explain ferromagnetism and spin splitting in (In,Fe)As.

Findings

Observed spin-splitting energy of 31.7-50 meV in (In,Fe)As

Mean-field Zener model fails to predict Curie temperature accurately

Bias-dependent TAMR reveals magnetic anisotropy and impurity band overlap

Abstract

Large spin splitting in the conduction band (CB) and valence band (VB) of ferromagnetic semiconductors (FMSs), predicted by the influential mean-field Zener model[1,2] and assumed in many spintronic device proposals[3-8], has never been observed in the mainstream p-type Mn-doped FMSs[9-15]. Here using tunnelling spectroscopy in Esaki-diode structures, we report the observation of such a large spontaneous spin-splitting energy ({\Delta}E = 31.7 - 50 meV) in the CB bottom of n-type FMS (In,Fe)As, which is surprising considering the very weak s-d exchange interaction reported in several zinc-blende (ZB) type semiconductors[16,17]. The mean-field Zener model also fails to explain consistently the ferromagnetism and the spin splitting energy {\Delta}E of (In,Fe)As, because we found that the Curie temperature (TC) values calculated using the observed {\Delta}E are much lower than the experimental TC by a factor of 400. These results urge the need for a more sophisticated theory of FMSs. Furthermore, bias-dependent tunnelling anisotropic magnetoresistance (TAMR) reveals the magnetic anisotropy of each component of the (In,Fe)As band structure [CB, VB, and impurity band (IB)]. The results suggest that the energy range of IB overlaps with the CB bottom or VB top, which may be important to understand the strong s-d exchange interaction in (In,Fe)As[18,19].