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▲ [19p-W241-3] Observation of spontaneous spin-splitting in the band structure of n-type ferromagnetic semiconductor (In,Fe)As
Keywords:spontaneous spin-split,spin Esaki diodes,ferromagnetic semiconductors
The band structure of ferromagnetic semiconductors (FMSs) has been under active debate since their discovery. Ideally, it is expected that the s,p-d exchange interactions between carriers and localized magnetic moments would induce spin-splitting in the conduction band (CB) and valence band (VB) of FMSs. If it is the case, we can easily design the material properties by using well-established band engineering of semiconductors. Spontaneous spin-splitting in the VB due to ferromagnetism has been clearly observed only in Mn-based II-VI FMS (Cd,Mn)Te quantum wells (QWs), but the ferromagnetic order disappeared above 4K1. In Mn-based III-V FMSs such as (Ga,Mn)As, although higher Curie temperature (TC) was obtained, the hole carriers are in the Mn-related impurity band (IB), while the CB and VB of the host materials remain nearly nonmagnetic2,3. Therefore, FMSs with both large spin-split CB and VB and high TC are highly desired.
Here, we report on the observation of such band structure in n-type FMS (In,Fe)As, using tunneling spectroscopy in (In,Fe)As-based spin Esaki diodes. The device structure from the surface is 50 nm-thick n-type (In,Fe)As (with or without Be)/5 nm-thick InAs/250 nm-thick InAs:Be (Be concentration 5×1018 cm-3)/p+ type InAs (001) substrate. Two different p+n+ junction diodes, A and B, were prepared with the (In,Fe)As layers differing in the Fe concentration (6% and 8%, respectively), electron density (by co-doping Be donors at 5×1019 cm-3 in the (In,Fe)As layer in device B), and consequently TC (45 and 65 K, respectively). When the bias voltage V is small, electrons tunnel from the (In,Fe)As CB bottom to the p-InAs VB top, thus the tunneling conductance dI/dV directly probes the density of states of the (In,Fe)As CB. At 3.5 K, the dI2/dV2-V curves show double-valley features, which evolve into single-valley features at temperatures above TC of the (In,Fe)As films. These double-valley features correspond to the spontaneous spin splitting at the (In,Fe)As CB bottom at low temperatures. The spin splitting energy ΔE depends on the Fe concentration, temperature, and external magnetic field. Furthermore, the magnetoresistance of the diodes depends on the direction of the (In,Fe)As magnetization in the film plane, that is, tunneling anisotropic magnetoresistance (TAMR). At different bias voltages, we observed different anisotropy components of the (In,Fe)As band structure distinguished by their TAMR symmetry. The result indicates that the Fe-related IB lies very close to the CB bottom and VB top of (In,Fe)As4,5.
Refs: [1]Boukari et al., PRL 88, 207204 (2002). [2]Ohya et al., Nat. Phys 7, 342 (2011). [3]Ohya et al. PRB 86, 094418 (2012). [4]Hai et al., APL 101, 252410 (2012). [5]Anh et al., APL 104, 042404 (2014).
Here, we report on the observation of such band structure in n-type FMS (In,Fe)As, using tunneling spectroscopy in (In,Fe)As-based spin Esaki diodes. The device structure from the surface is 50 nm-thick n-type (In,Fe)As (with or without Be)/5 nm-thick InAs/250 nm-thick InAs:Be (Be concentration 5×1018 cm-3)/p+ type InAs (001) substrate. Two different p+n+ junction diodes, A and B, were prepared with the (In,Fe)As layers differing in the Fe concentration (6% and 8%, respectively), electron density (by co-doping Be donors at 5×1019 cm-3 in the (In,Fe)As layer in device B), and consequently TC (45 and 65 K, respectively). When the bias voltage V is small, electrons tunnel from the (In,Fe)As CB bottom to the p-InAs VB top, thus the tunneling conductance dI/dV directly probes the density of states of the (In,Fe)As CB. At 3.5 K, the dI2/dV2-V curves show double-valley features, which evolve into single-valley features at temperatures above TC of the (In,Fe)As films. These double-valley features correspond to the spontaneous spin splitting at the (In,Fe)As CB bottom at low temperatures. The spin splitting energy ΔE depends on the Fe concentration, temperature, and external magnetic field. Furthermore, the magnetoresistance of the diodes depends on the direction of the (In,Fe)As magnetization in the film plane, that is, tunneling anisotropic magnetoresistance (TAMR). At different bias voltages, we observed different anisotropy components of the (In,Fe)As band structure distinguished by their TAMR symmetry. The result indicates that the Fe-related IB lies very close to the CB bottom and VB top of (In,Fe)As4,5.
Refs: [1]Boukari et al., PRL 88, 207204 (2002). [2]Ohya et al., Nat. Phys 7, 342 (2011). [3]Ohya et al. PRB 86, 094418 (2012). [4]Hai et al., APL 101, 252410 (2012). [5]Anh et al., APL 104, 042404 (2014).