N-type Ferromagnetic Semiconductor Research Articles

Minority-spin impurity band in n -type (In,Fe)As: A materials perspective for ferromagnetic semiconductors

Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream p-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in n-type FMSs is theoretically non-trivial. Soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) is a powerful approach to examine the mechanism of carrier-induced ferromagnetism in FMSs. Here our SX-ARPES study on the prototypical n-type FMS (In,Fe)As reveals the entire band structure including the Fe-3d impurity bands (IBs) and the host InAs ones, and provides direct evidence for electron occupation of the InAs-derived conduction band (CB). A minority-spin Fe-3d IB is found to be located just below the conduction-band minimum (CBM). The IB is formed by the hybridization of the unoccupied Fe-3d states with the occupied CBM of InAs in a spin-dependent way, resulting in the large spin polarization of CB. The band structure with the IB is varied with band filling, which cannot be explained by the rigid-band picture, suggesting a unified picture for realization of carrier-induced ferromagnetism in FMS materials.

Transport and magnetic properties of co-doped ferromagnetic semiconductor (In,Fe,Mn)As

We report epitaxial growth and properties of n-type ferromagnetic semiconductor (FMS) (In,Fe)As (Fe 6%) co-doped with Mn at different concentrations x (x = 0.4%, 0.8%, 2%, 4%, 8%). The electron concentration n of (In0.94−x,Fe0.06,Mnx)As decreases as x increases, leading to the decrease of Curie temperature (TC) as expected for electron-induced ferromagnetism. However, ferromagnetism is still observed at n as low as ∼ 3.8 × 1017 cm−3, which is one order of magnitude lower than the typical threshold value of (In,Fe)As (nth ∼ 6 × 1018 cm−3). This result highlights the possibility of enhancing the ferromagnetism by co-doping different magnetic elements in FMSs.

Inhomogeneity-induced high temperature ferromagnetism in n-type ferromagnetic semiconductor (In,Fe)As grown on vicinal GaAs substrates

We systematically investigate the electrical and magnetic properties of n-type ferromagnetic semiconductor (In,Fe)As thin films grown on vicinal GaAs(001) substrates with different lattice-relaxation layers and Fe-doping techniques. We show that spinodal decomposition and ferromagnetism can be enhanced in the (In,Fe)As thin films grown on InAs lattice-relaxation layers under the nucleation growth mode, while they are suppressed in those grown on the GaSb/AlSb/AlAs (from top to bottom) lattice-relaxation layers under the step-flow growth mode. We demonstrate that room-temperature ferromagnetism and good electrical properties can be obtained at the same time by using the Fe-delta-doping technique in combination with the step-flow growth mode.

Observation of quantum size effect at the conduction band bottom of n-type ferromagnetic semiconductor (In,Fe)As thin films

We study the quantum size effect (QSE) in the spin-dependent electronic structure at the conduction band (CB) bottom (Γ point) of an n-type ferromagnetic semiconductor (In0.92,Fe0.08)As ultrathin films (thickness t = 8–14 nm), by performing magnetic circular dichroism (MCD) spectroscopy in an infra-red photon energy range (E = 0.62–1.8 eV). The t-dependence of MCD peaks are well explained by the optical transitions from the valence band top to the quantized levels at the CB bottom of (In,Fe)As obtained from self-consistent calculations. These findings provide new possibilities of spin-device applications utilizing QSE in nm-scale (In,Fe)As thin films.

Electrical tuning of the band alignment and magnetoconductance in an n-type ferromagnetic semiconductor (In,Fe)As-based spin-Esaki diode

We report a strong bias dependence of the magnetoconductance (MC) of a spin-Esaki diode composed of n+-type ferromagnetic semiconductor (FMS) (In,Fe)As and p+-type Be doped InAs grown on a p+-InAs (001) substrate by molecular beam epitaxy. When the bias voltage V is increased above 450 mV in the forward bias, we found that the MC, measured at 3.5 K under a magnetic field H of 1 T in the in-plane [110] direction, changes its sign from positive to negative and its magnitude rises rapidly from 0.5% at V < 450 mV to −7.4% at V = 650 mV. Furthermore, the MC magnitude decreases as cos2(θ) when rotating H from the in-plane [110] direction to the perpendicular [001] direction, where θ is the angle between H and the [110] axis. Using a two-fluid model, we explain both the magnitude and the anisotropy of the MC based on the evolution of the spin-Esaki diode's band profile with V. This analysis provides insights into the density of states and spin-polarization of the conduction band and the Fe-related impurity band in n-type FMS (In,Fe)As.

Proximity-Induced Superconductivity in a Ferromagnetic Semiconductor (In,Fe)As

We observed supercurrent through an n-type ferromagnetic semiconductor (In,Fe)As. For the purpose we prepared S/F/S (S: superconductor, F: ferromagnet) junctions, which exhibited zero resistance up to 1 µm of junction width with critical current over 1 µA below 1 K. The long reach of the superconducting proximity effect suggests that the induced superconductivity is brought about by spin-triplet paring in (In,Fe)As.

Ab initio GGA+U investigations of the electronic properties and magnetic orderings in Mn, Gd doped ZB/WZ structural CdSe

In the paper, the electronic properties and magnetic orderings in the Mn/Gd doped zinc-blende (ZB) and wurtzite (WZ) structural CdSe are investigated extensively using first-principles calculations. We find that the infiltration of Mn ions in the ZB/WZ structural CdSe will lead to the antiferromagnetic ordering; while the infiltration of Gd ions will lead to the ferromagnetic ordering. Moreover, the antiferromagnetic/ferromagnetic coupling interactions between the magnetic Mn/Gd ions will become more and more strong with decreasing Mn-Mn/Gd-Gd distance. Particularly, the electronic properties in the Mn mono-doping case (spin-polarized) and in the Mn double-doping case (ferromagnetic) are almost same, and they tend to be the magnetic/ferromagnetic semiconductor via the comparison from the different calculations; as a whole, the Mn doped ZB/WZ structural CdSe system tends to present the antiferromagnetic semiconductor character. In addition, the band gap in the Mn doped ZB/WZ structural CdSe decreases with increasing Mn concentration using GGA+U calculations. In contrast to the case of the Mn doped ZB/WZ CdSe: (i) the Gd doped ZB/WZ CdSe in the mono-doping case displays the magnetic semiconductor character; but the Gd doped ZB/WZ structural CdSe in the double-doping case presents the n-type ferromagnetic semiconductor, and the robust ferromagnetism is obtained in the system irregardless of the crystal structure and the distance between the magnetic ions; (ii) the band gap in the Gd doped ZB/WZ structural CdSe increases with increasing Gd concentration, Burstein-Mott-effect might be major reason.

New p- and n-type ferromagnetic semiconductors: Cr-doped BaZn2As2

We find new ferromagnetic semiconductors, Cr-doped BaZn2As2, by employing a combined method of the density functional theory and the quantum Monte Carlo simulation. Due to a narrow band gap of 0.2 eV in host BaZn2As2 and the different hybridization of 3d orbitals of Cr impurity, the impurity bound states have been induced both below the top of valence band and above the bottom of conduction band. The long-range ferromagnetic coupling between Cr impurities is obtained with both p- and n-type carriers.

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

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.

Modulation of ferromagnetism in(In,Fe)Asquantum wells via electrically controlled deformation of the electron wave functions

We demonstrate electrical control of ferromagnetism in field-effect transistors with a trilayer quantum well (QW) channel containing an ultrathin n-type ferromagnetic semiconductor (In,Fe)As layer. A gate voltage is applied to control the electron wavefunctions {\phi}i in the QW, such that the overlap of {\phi}i and the (In,Fe)As layer is modified. The Curie temperature is largely changed by 42%, whereas the change in sheet carrier concentration is 2 - 3 orders of magnitude smaller than that of previous gating experiments. This result provides a new approach for versatile, low power, and ultrafast manipulation of magnetization.

Coexistence of high-T(c) ferromagnetism and n-type electrical conductivity in FeBi2Se4.

The discovery of n-type ferromagnetic semiconductors (n-FMSs) exhibiting high electrical conductivity and Curie temperature (Tc) above 300 K would dramatically improve semiconductor spintronics and pave the way for the fabrication of spin-based semiconducting devices. However, the realization of high-Tc n-FMSs and p-FMSs in conventional high-symmetry semiconductors has proven extremely difficult due to the strongly coupled and interacting magnetic and semiconducting sublattices. Here we show that decoupling the two functional sublattices in the low-symmetry semiconductor FeBi2Se4 enables unprecedented coexistence of high n-type electrical conduction and ferromagnetism with Tc ≈ 450 K. The structure of FeBi2Se4 consists of well-ordered magnetic sublattices built of [FenSe4n+2]∞ single-chain edge-sharing octahedra, coherently embedded within the three-dimensional Bi-rich semiconducting framework. Magnetotransport data reveal a negative magnetoresistance, indicating spin-polarization of itinerant conducting electrons. These findings demonstrate that decoupling magnetic and semiconducting sublattices allows access to high-Tc n- and p-FMSs as well as helps unveil the mechanism of carrier-mediated ferromagnetism in spintronic materials.

Recent progress in III-V based ferromagnetic semiconductors: Band structure, Fermi level, and tunneling transport

Spin-based electronics or spintronics is an emerging field, in which we try to utilize spin degrees of freedom as well as charge transport in materials and devices. While metal-based spin-devices, such as magnetic-field sensors and magnetoresistive random access memory using giant magnetoresistance and tunneling magnetoresistance, are already put to practical use, semiconductor-based spintronics has greater potential for expansion because of good compatibility with existing semiconductor technology. Many semiconductor-based spintronics devices with useful functionalities have been proposed and explored so far. To realize those devices and functionalities, we definitely need appropriate materials which have both the properties of semiconductors and ferromagnets. Ferromagnetic semiconductors (FMSs), which are alloy semiconductors containing magnetic atoms such as Mn and Fe, are one of the most promising classes of materials for this purpose and thus have been intensively studied for the past two decades. Here, we review the recent progress in the studies of the most prototypical III-V based FMS, p-type (GaMn)As and its heterostructures with focus on tunneling transport, Fermi level, and bandstructure. Furthermore, we cover the properties of a new n-type FMS, (In,Fe)As, which shows electron-induced ferromagnetism. These FMS materials having zinc-blende crystal structure show excellent compatibility with well-developed III-V heterostructures and devices.

Control of ferromagnetism by manipulating the carrier wavefunction in ferromagnetic semiconductor (In,Fe)As quantum wells

We demonstrated the control of ferromagnetism in a surface quantum well containing a 5 nm-thick n-type ferromagnetic semiconductor (FMS) (In,Fe)As layer sandwiched between two InAs layers, by manipulating the carrier wavefunction. The Curie temperature (TC) of the (In,Fe)As layer was effectively changed by up to 12 K (ΔTC/TC = 55%). Our calculation using the mean-field Zener theory reveals an unexpectedly large s-d exchange interaction in (In,Fe)As. Our results establish an effective way to control the ferromagnetism in quantum heterostructures of n-type FMSs, as well as require reconsideration on the current understanding of the s-d exchange interaction in narrow gap FMSs.

Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow an n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps to understand the mechanism of carrier-mediated ferromagnetism in FMSs.

Ti-doped AlN potential n-type ferromagnetic semiconductor: Density functional calculations

The magnetism and the electronic structures of Ti-doped AlN are studied by the first-principles calculations. It is found that Ti prefers to substitute Al site in AlN host, and this doped configuration favors the ferromagnetic ground state. The magnetic moment of the supercell containing single Ti Al is 1.00 μ B. Electronic structures suggest magnetic moments mainly come from the doped Ti atom, Ti-doped AlN is n-type ferromagnetic semiconductor and the ferromagnetism can be explained by double-exchange mechanism. For O and Ti co-doped AlN, calculations show O N defect would induce magnetic moments increasing and change Ti-doped AlN from semiconductor to metal.

Magnetic and Transport Properties of FerromagneticSemiconductor GaDyN Thin Film

Magnetic properties and temperature dependence of electrical transportproperties of rare-earth-metal Dy-doped GaN thin film areexperimentally studied with a superconducting quantum interferencedevice magnetometer and van der Pauw method. It was found that thisthin nitride film has both semiconductor properties and ferromagnetismfrom 10 K to room temperature. The dopant-band (conducting band due todoping) electron conduction dominates the transport properties of thisfilm at low temperatures. These results indicate that Dy-doped GaN isan n-type ferromagnetic semiconductor at room temperature.

Electrical spin injection from an n-type ferromagnetic semiconductor into a III–V device heterostructure

The use of carrier spin in semiconductors is a promising route towards new device functionality and performance. Ferromagnetic semiconductors (FMSs) are promising materials in this effort. An n-type FMS that can be epitaxially grown on a common device substrate is especially attractive. Here, we report electrical injection of spin-polarized electrons from an n-type FMS, CdCr(2)Se(4), into an AlGaAs/GaAs-based light-emitting diode structure. An analysis of the electroluminescence polarization based on quantum selection rules provides a direct measure of the sign and magnitude of the injected electron spin polarization. The sign reflects minority rather than majority spin injection, consistent with our density-functional-theory calculations of the CdCr(2)Se(4) conduction-band edge. This approach confirms the exchange-split band structure and spin-polarized carrier population of an FMS, and demonstrates a litmus test for these FMS hallmarks that discriminates against spurious contributions from magnetic precipitates.

(Eu,Gd)Te - MBE Growth and Characterization

Monocrystalline thin layers of (Eu,Gd)Te, n-type ferromagnetic semiconductor, were grown by molecular beam epitaxy technique on BaF2 (111) substrates. Reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization proved epitaxial mode of growth and high crystal quality of the layers. Magnetic susceptibility and magnetic resonance measurements showed that in (Eu,Gd)Te layers ferromagnetic transition takes place at about 13 K. Electrical characterization carried out by the Hall effect and resistivity measurements revealed very high electron concentration of 10 cm−3 and sharp maximum of resistivity at transition temperature.

Epitaxial growth of an n-type ferromagnetic semiconductor CdCr2Se4 on GaAs(001) and GaP(001)

We report the epitaxial growth of CdCr2Se4, an n-type ferromagnetic semiconductor, on both GaAs and GaP(001) substrates, and describe the structural, magnetic, and electronic properties. Magnetometry data confirm ferromagnetic order with a Curie temperature of 130 K, as in the bulk material. The magnetization exhibits hysteretic behavior with significant remanence, and an in-plane easy axis with a coercive field of ∼125 Oe. Temperature-dependent transport data show that the films are semiconducting in character and n type as grown, with room-temperature carrier concentrations of n∼1×1018 cm−3.

The Faraday effect in magnetic semiconductors (abstract)

In n-type ferromagnetic semiconductors, both bound electrons and free carriers make contributions to the Faraday effect. The spin-orbital interaction of bound electrons, and the spin-spin exchange interaction between bound electrons and between bound electrons and free electrons exist. Thus, the Faraday rotation θ has been deduced as follows: θ=(eμ0/mc)∑∞l=1blλ2lL (He+ν′M)+(eμ0/mc) ∑∞n=1(an/λ2n) L[He+(ν+ν′)M] =θS+θF, where L is the thickness, He the external field, M the magnetization, ν′M the effective field related to the exchange interaction between bound electrons and free electrons, νM the effective field related to the spin-orbital interaction of bound electrons and so on, an and bl the constants related to the concentration N1 of free electrons and the concentration N2 of bound electrons, respectively. While λ>λ0, the wavelength at θ=0, or N1>N0∼1022/m3, θS>θF. Conversely, θS<θF. When θS≪θF, θ∼(a1/λ2+a2/λ4+...). The temperature behavior of θ mainly depends on that of M. θ tends to saturation with M. When θS≫θF, θ∼(b1λ2+b2λ4+⋅⋅⋅). θ is directly proportional to approximately He. The calculations are confirmed by the experimental results in CdCr2S4, etc.