Abstract
Large spin-splitting in the conduction band and valence band of ferromagnetic semiconductors, predicted by the influential mean-field Zener model and assumed in many spintronic device proposals, has never been observed in the mainstream p-type Mn-doped ferromagnetic semiconductors. Here, using tunnelling spectroscopy in Esaki-diode structures, we report the observation of such a large spontaneous spin-splitting energy (31.7–50 meV) in the conduction band bottom of n-type ferromagnetic semiconductor (In,Fe)As, which is surprising considering the very weak s-d exchange interaction reported in several zinc-blende type semiconductors. The mean-field Zener model also fails to explain consistently the ferromagnetism and the spin-splitting energy of (In,Fe)As, because we found that the Curie temperature values calculated using the observed spin-splitting energies are much lower than the experimental ones by a factor of 400. These results urge the need for a more sophisticated theory of ferromagnetic semiconductors.
Highlights
Large spin-splitting in the conduction band and valence band of ferromagnetic semiconductors, predicted by the influential mean-field Zener model and assumed in many spintronic device proposals, has never been observed in the mainstream p-type Mn-doped ferromagnetic semiconductors
The most widely used theory of the ferromagnetism in ferromagnetic semiconductors (FMSs), ‘mean-field Zener model’[7,8], which assumes that the Fermi level lies in the conduction band (CB) or valence band (VB) of the host semiconductors, predicted that the s,p-d exchange interactions would induce spin-splitting in these bands, given by DE 1⁄4 ða or bÞN0xS; ð1Þ
Recent experimental results in (Ga,Mn)As, which is widely recognized as a ‘canonical’ FMS, have revealed that the Fermi level lies in the Mnrelated impurity band (IB)[10,11,12,13,14,15,16], whereas the CB and VB of the host material remain nearly non-magnetic[13]
Summary
Large spin-splitting in the conduction band and valence band of ferromagnetic semiconductors, predicted by the influential mean-field Zener model and assumed in many spintronic device proposals, has never been observed in the mainstream p-type Mn-doped ferromagnetic semiconductors. Small spontaneous spin-splitting DE (B 10 meV) in the VB has been observed only in p-type Mn-doped II–VI based FMS (Cd,Mn)Te quantum wells (QWs), which is much lesspractical owing to the very low Curie temperature (TCo4 K)[17] These results casted doubts on the validity of the mean-field Zener model as a universal model to describe and predict the magnetic properties of FMSs. The realisation of FMSs with spin-split host semiconductor’s band structure at higher temperature range, which will give a decisive impact on the development of semiconductorbased spintronic devices, remains elusive. Direct observation of the spin-dependent band structure of (In,Fe)As would give valuable insights into the ferromagnetism in this material and the physics of FMSs in general, as well as give an important basis for device applications using spin-dependent band engineering
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