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The topological insulator BiSb is promising for spin-orbit torque applications thanks to its high electrical conductivity and giant spin Hall effect. Previous works have reported a large spin Hall angle for BiSb deposited on top of a ferromagnetic layer (FM) with perpendicular magnetic anisotropy. However, since BiSb has large surface roughness, obtaining a large spin Hall angle in BiSb (bottom)/FM is more challenging than in FM/BiSb(top) structures, especially when the FM layer is very thin. Here, we investigate the role of an interfacial layer on the effective spin Hall angle in BiSb (bottom)/FM with in-plane magnetization deposited by magnetron sputtering on sapphire substrates. We showed that inserting an interfacial layer with optimized thickness helps improve the effective spin Hall angle. We achieved a relatively high effective spin Hall angle of 1.7 in BiSb (bottom)/Ru 1 nm/ Co 1 nm/ Pt 1 nm structure.

The Topological Insulator (TI) BiSb, grown with a (012) film texture using molecular beam epitaxy, and coupled with a FM layer has been shown to have a very high Spin Hall Angle (SHA) > 2 and high electrical conductivity (>10 5 Ohm -1 m -1 ) in thin film stacks 1 . This makes the TI material ideally suited for spin transfer devices requiring several orders of magnitude less power consumption to operate than other traditional devices 2 . However, the practical implementation of this TI material into devices using PVD requires you overcome many severe material challenges. The BiSb material is very soft, easily damaged in ion milling or by subsequent depositions, it has a very low melting point ~ 280C, and hence possesses a very large in-plane grain size as deposited at room temperature, ~6X the targeted thickness in devices. Thermal Migration of Sb out of the TI material is also a severe problem, which is exacerbated by film roughness and large grain sizes, and all of this must be solved while growing and maintaining a non-prismatic (012) texture of BiSb with low roughness, good interfacial TI properties, at reasonable operating temperatures > 180C in actual device stacks, non-epitaxially on SiOx or ceramic substrates. We discuss here how we addressed these challenges through appropriate selection of epitaxial and non-epitaxial materials for buffer layer(s) (bottom interface), and (top interface) interlayer(s), and by using a non-uniform thickness doping of the BiSb(X) material to stabilize and grow a BiSbX(012) textured layer with low roughness, good TI properties, at elevated temperatures in FM thin film stacks.

We investigate the crystal growth of BiSb thin films on various GaAs substrates with different crystal orientation and surface termination. We found the non-trivial growth mode of BiSb which depends on the crystal orientation/surface termination of GaAs. While BiSb(0 0 1) can be grown epitaxially on GaAs(1 1 1)A, only textured BiSb(0 1 2) can be grown on GaAs(1 1 1)B even with the help of an ultrathin Bi buffer layer, indicating the critical role of surface termination of the GaAs(1 1 1) substrates. Meanwhile, only textured BiSb(0 0 1) thin films grow on GaAs(0 0 1) instead of BiSb(0 1 2), indicating that symmetry matching plays a little role on the growth of BiSb on GaAs(0 0 1).

We have grown Fe δ-doped (In,Fe)Sb thin films by low-temperature molecular beam epitaxy for magnetic-field sensors with ultrahigh Hall sensitivity. Magnetic circular dichroism spectroscopy measurements indicate intrinsic ferromagnetism in the Fe δ-doped (In,Fe)Sb layers. Although the Fe δ-doped (In,Fe)Sb thin films have lower Curie temperature than the Fe uniformly-doped (ln,Fe)Sb with the same average Fe concentration, their anomalous Hall effect is much stronger. The Hall sensitivity of the Fe δ-doped (In1−〈x〉,Fe〈x〉)Sb sample with an average Fe concentration 〈x〉 = 17% reaches 1522 Ω/T at room temperature, which is much higher than those of Fe uniformly-doped (In,Fe)Sb, (Ga,Fe)Sb, and commercial InSb Hall sensors.

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., 24 pages, 7 figures

The large spin Hall effect in topological insulators (TIs) is very attractive for ultralow-power spintronic devices. However, evaluation of the spin Hall angle and spin–orbit torque (SOT) of TIs is usually performed on high-quality single-crystalline TI thin films grown on dedicated III-V semiconductor substrates. Here, we report on room-temperature ultralow power SOT magnetization switching of a ferrimagnetic layer by non-epitaxial BiSb TI thin films deposited on Si/SiO2 substrates. We show that non-epitaxial BiSb thin films outperform heavy metals and other epitaxial TI thin films in terms of the effective spin Hall angle and switching current density by one to nearly two orders of magnitude. The critical SOT switching current density in BiSb is as low as 7 × 104 A/cm2 at room temperature. The robustness of BiSb against crystal defects demonstrate its potential applications to SOT-based spintronic devices.

Giant spin-orbit torque (SOT) from topological insulators (TIs) provides an energy efficient writing method for magnetic memory, which, however, is still premature for practical applications due to the challenge of the integration with magnetic tunnel junctions (MTJs). Here, we demonstrate a functional TI-MTJ device that could become the core element of the future energy-efficient spintronic devices, such as SOT-based magnetic random-access memory (SOT-MRAM). The state-of-the-art tunneling magnetoresistance (TMR) ratio of 102% and the ultralow switching current density of 1.2 × 105 A cm−2 have been simultaneously achieved in the TI-MTJ device at room temperature, laying down the foundation for TI-driven SOT-MRAM. The charge-spin conversion efficiency θSH in TIs is quantified by both the SOT-induced shift of the magnetic switching field (θSH = 1.59) and the SOT-induced ferromagnetic resonance (ST-FMR) (θSH = 1.02), which is one order of magnitude larger than that in conventional heavy metals. These results inspire a revolution of SOT-MRAM from classical to quantum materials, with great potential to further reduce the energy consumption., It remains challenging to integrate topological insulators (TI) with magnetic tunnel junctions (MTJ) for spintronics applications. Here, the authors achieve a large tunneling magnetoresistance ratio and a low switching current density in a TI-MTJ device at room temperature, very promising for TI-driven magnetic memory.

Topological insulators (TIs) are promising for efficient spin current sources in spin–orbit torque (SOT) magnetoresistive random access memory (MRAM). However, TIs are usually deposited by molecular beam epitaxy on single crystalline III–V semiconductor or sapphire substrates, which are not suitable for realistic applications. Here, we studied SOT characteristics in sputtered BiSb topological insulator—Pt/Co/Pt—MgO heterostructures deposited on oxidized Si substrates, where Pt/Co/Pt trilayers have a large perpendicular magnetic anisotropy field of 4.5 kOe. We show that the BiSb layer has a large effective spin Hall angle of θSHeff = 2.4 and a high electrical conductivity of σ = 1.0 × 105 Ω−1 m−1. The magnetization can be switched by a small current density of 2.3 × 106 A cm−2 at a pulse width of 100 µs, which is 1 or 2 orders of magnitudes smaller than those in heavy metals. Our work demonstrates the high efficiency and robustness of BiSb as a spin current source in realistic SOT-MRAM.

We investigated the spin-dependent transport properties of a lateral spin-valve device with a 600 nm-long GaAs channel and ferromagnetic MnGa electrodes with perpendicular magnetization. Its current-voltage characteristics show nonlinear behavior below 50 K, indicating that tunnel transport through the MnGa/GaAs Schottky barrier is dominant at low temperatures. We observed clear magnetoresistance (MR) ratio up to 12% at 4 K when applying a magnetic field perpendicular to the film plane. Furthermore, a large spin-dependent output voltage of 33 mV is obtained. These values are the highest in lateral ferromagnetic metal / semiconductor / ferromagnetic metal spin-valve devices reported so far.

The unidirectional magnetoresistance (UMR) is one of the most complex spin-dependent transport phenomena in ferromagnet/non-magnet bilayers, which involves spin injection and accumulation due to the spin Hall effect (SHE) or Rashba-Edelstein effect (REE), spin-dependent scattering, and magnon scattering at the interface or in the bulk of the ferromagnet. While UMR in metallic bilayers has been studied extensively in very recent years, its magnitude is as small as 10$^-$$^5$, which is too small for practical applications. Here, we demonstrate a giant UMR effect in a heterostructure of BiSb topological insulator -- GaMnAs ferromagnetic semiconductor. We obtained a large UMR ratio of 1.1%, and found that this giant UMR is governed not by the giant magnetoresistance (GMR)-like spin-dependent scattering, but by magnon emission/absorption and strong spin-disorder scattering in the GaMnAs layer. Our results provide new insight into the complex physics of UMR, as well as a strategy for enhancing its magnitude for device applications.

Ferromagnetic semiconductors (FMSs) with high Curie temperature (TC) are highly required for spintronics device applications. So far, however, the mainly studied FMSs, Mn-doped III-V FMSs, are only p-type and their TC values were much lower than 300 K. To search for new FMSs with high TC, most efforts have been concentrated on wide-gap materials [1], which yields no reliable and systematic result. We present a new class of FMSs with high TC, Fe-doped narrow-gap III-V FMSs. Because Fe atoms replace the cation (group-III) sites in the isoelectronic Fe3+ state, the carrier type can be controlled independently and thus both n-type and p-type FMSs are obtained. Using low-temperature molecular beam epitaxy, we have successfully grown zinc-blende-type thin-film crystals of both p-type FMSs [(Ga,Fe)Sb [2], (Al,Fe)Sb [3]] and n-type FMSs [(In,Fe)As [4], (In,Fe)Sb [5]]. The most notable feature in these Fe-based FMSs is that the TC value increases monotonically as the Fe content increases; and there is a tendency that TC is higher as the bandgap is narrower, which contradicts the prediction of the conventional mean-field Zener model. Intrinsic room-temperature ferromagnetism has been observed in (Ga1-x,Fex)Sb with x > 23% [2] and (In1-x,Fex)Sb with x > 16% [5]. These findings indicate that the Fe-doped III-V FMSs are promising for spintronics devices operating at room temperature. References [1] T. Dielt et al., Phys. Rev. B 63, 195205 (2001). [2] N. T. Tu et al., Appl. Phys. Lett. 108, 192401 (2016) [Featured Article]. [3] L. D. Anh et al., Appl. Phys. Let 107, 232405 (2015). [4] P. N. Hai et al., Appl. Phys. Lett. 101, 182403 (2012). [5] N. T. Tu et al., APEX 11 (6), 063005 (2018).

The electronic structure and the magnetism of the ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature ${T}_{\mathrm{C}}$ can exceed room temperature, were investigated by means of x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable $3{d}^{6}$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and nondispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe $3d$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe $3d$ states have a finite contribution to the density of states at the Fermi energy. These states, presumably consisting of majority-spin $p\text{\ensuremath{-}}d$ hybridized states or minority-spin $e$ states, would be responsible for the ferromagnetic order in this material.

Spin bipolar transistors (SBTs) with spin dependent output characteristics can be applied to non-volatile memory cells and reconfigurable logic circuits. III-V based carrier-induced ferromagnetic semiconductors (FMSs) are suitable materials for realizing SBTs, because they are compatible with non-magnetic III-V semiconductors with respect to the crystal structure and conductivity. In particular, Fe-based FMSs are promising materials for SBTs because both p-type and n-type FMSs are available and their ferromagnetism is maintained at room temperature. In this study, we fabricated spin bipolar transistors using a (Ga, Fe)Sb/(In, Fe)As p-n junction on top of a p-n-p AlGaAs/GaAs heterojunction bipolar transistor structure, grown by molecular beam epitaxy. We obtained the modulation of the collector current by the base voltage and the magnetoresistance (MR) effect. The bias dependence and temperature dependence of MR suggest that the observed MR signal is likely to originate from the spin-valve effect at the ferromagnetic (Ga, Fe)Sb/(In, Fe)As p-n junction. Our results indicate that Fe-based FMSs are promising for semiconductor spintronics devices.

We have studied the magnetization process of the new insulating ferromagnetic semiconductor (Al,Fe)Sb by means of x-ray magnetic circular dichroism. For an optimally doped sample with 10% Fe, a magnetization was found to rapidly increase at low magnetic fields and to saturate at high magnetic fields at room temperature, well above the Curie temperature of 40 K. We attribute this behavior to the existence of nanoscale Fe-rich ferromagnetic domains acting as superparamagnets. By fitting the magnetization curves using the Langevin function representing superparamagnetism plus the paramagnetic linear function, we estimated the average magnetic moment of the nanoscale ferromagnetic domain to be $300{\ensuremath{\mu}}_{\mathrm{B}}\ensuremath{-}400{\ensuremath{\mu}}_{\mathrm{B}}$ and the fraction of Fe atoms participating in the nanoscale ferromagnetism to be $\ensuremath{\sim}50%$. Such behavior was also reported for (In,Fe)As:Be and Ge:Fe and seems to be a universal characteristic of the Fe-doped ferromagnetic semiconductors. Further Fe doping up to 14% led to the weakening of the ferromagnetism, probably because antiferromagnetic superexchange interaction between nearest-neighbor Fe-Fe pairs becomes dominant.

We investigated the spin Hall effect (SHE) in amorphous YPt alloy. In amorphous YPt thin films with various resistivity and thickness, we observed strong sensitivity of the effective spin Hall angle to resistivity, which was found to be governed by the intrinsic mechanism with large intrinsic spin Hall conductivity of 700 ± 100 Ω−1 cm−1 and the extrinsic side-jump mechanism with opposite polarity. Despite the amorphous nature, the measured spin relaxation length in YPt is 0.9 nm, which is longer than that expected for crystallized Pt-based alloys with the same resistivity. Our results highlight the importance of the intrinsic mechanism to the SHE in amorphous alloys.

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.

We report on the growth and characterization of BiSb thin films deposited on sapphire substrates by sputtering deposition with Ar and Kr plasma. By optimizing the growth conditions, we are able to obtain quasi-single-crystal BiSb(001) thin films with equivalent twin crystals. The conductivity of BiSb at the studied thicknesses exceeds 105 Ω-1m-1, reaching a maximum of 1.8×105 Ω-1m-1 at 10 nm. From the temperature dependence of the electrical resistivity, we confirm the existence of metallic surface states. Our results demonstrate that it is possible to obtain sputtered BiSb thin films with quality approaching that of epitaxial BiSb grown by molecular beam epitaxy.

We report on epitaxial crystal growth and spin Hall effect in BiSb topological insulator thin films. We show that BiSb thin films with conductivity as high as σ ~ 2.5∗105 Ω-1m-1 can be epitaxially grown on GaAs(111)A substrate. Meanwhile, evaluation of spin-orbit-torque in Bi0.9Sb0.1/MnGa bi-layers reveals a colossal spin Hall angle of θSH ~ 52 and a spin Hall conductivity σSH ~ 1.3∗107 ℏ/2e Ω-1m-1 at room temperature. We demonstrate that BiSb thin films can generate a colossal spin-orbit field of 2.3 kOe/(MA/cm2) and a critical switching current density as low as 1.5 MA/cm2 in Bi0.9Sb0.1/MnGa bi-layers. BiSb is the best candidate for the first industrial application of topological insulators.

We observe the signature of zero-field ground-state skyrmions in BiSb topological insulator/MnGa bilayers by using the topological Hall effect (THE). We observe a large critical interfacial Dzyaloshinskii-Moriya-Interaction (DMI) energy (DCS = 5.0 pJ/m) at the BiSb/MnGa interface that can be tailored by controlling the annealing temperature of the MnGa template. The THE was observed at room temperature even under the absence of an external magnetic field, which gives strong evidence of the existence of thermodynamically stable skyrmions in the MnGa/BiSb bilayers. Our results give insight to the role of interfacial DMI tailored by suitable material choice and growth technique for the generation of stable skyrmions at room temperature.

Spin–orbit torque switching using the spin Hall effect in heavy metals and topological insulators has a great potential for ultralow power magnetoresistive random-access memory. To be competitive with conventional spin-transfer torque switching, a pure spin current source with a large spin Hall angle (θSH > 1) and high electrical conductivity (σ > 105 Ω−1 m−1) is required. Here we demonstrate such a pure spin current source: conductive topological insulator BiSb thin films with σ ≈ 2.5 × 105 Ω−1 m−1, θSH ≈ 52 and spin Hall conductivity σSH ≈ 1.3 × 107 $$\frac{\hbar }{{2e}}$$ Ω−1 m−1 at room temperature. We show that BiSb thin films can generate a very large spin–orbit field of 2.3 kOe MA–1 cm2 and a critical switching current density as low as 1.5 MA cm–2 in Bi0.9Sb0.1/MnGa bilayers, which underlines the potential of BiSb for industrial applications. A large spin–orbit torque, generated in a conductive topological insulator (TI) Bi0.9Sb0.1 is further employed to effectively switch the magnetization of MnGa in a BiSb/MnGa bilayer Hall-bar device at room temperature.

Optical and magnetic microstructures are directly fabricated inside a film of cerium-substituted yttrium iron garnet (Ce:YIG) by using a direct femtosecond laser writing. The laser irradiation induces local changes in both optical and magnetic properties of Ce:YIG. A quantitative phase imaging technique proves that the refractive index is increased by 0.015 ± 0.001, which is about 0.7% of Ce:YIG refrative index 2.2 at 632.8 nm. The magneto-optical polar Kerr effect measurement shows that the magnetization property is changed from hard to soft, decreasing the coercivity. Our proposed technique paves the way toward the development of novel three-dimensional magneto-optical devices for optical communication and photonic applications.

We present high-temperature ferromagnetism in heavily Fe-doped ferromagnetic semiconductor (FMS) (In1−x ,Fe x )Sb (x = 20%–35%) thin films grown by low temperature MBE. The Curie temperature (T C) of (In1−x ,Fe x )Sb reaches 385 K at x = 35%, which is significantly higher than room temperature and the highest value so far reported in III–V based FMSs. Moreover, large coercive force (H C = 160 Oe) and large remanent magnetization (M r/M S = 71%) have been observed at low temperature (10 K) for an (In1−x ,Fe x )Sb thin film with x = 35%, indicating that the FMS (In1−x ,Fe x )Sb is very promising for spintronics devices.

We show high-temperature ferromagnetism in heavily Fe-doped ferromagnetic semiconductor (Ga 1−x ,Fe x )Sb (x = 23% and 25%) thin films grown by low-temperature molecular beam epitaxy (LT-MBE). Our crystal structure analysis by scanning transmission electron microscopy (STEM) indicates that the (Ga 1−x ,Fe x )Sb thin films maintain the zinc-blende crystal structure at x = 25%. The intrinsic ferromagnetism was confirmed by magnetic circular dichroism (MCD) spectroscopy and anomalous Hall effect (AHE) measurements. The Curie temperature reaches 300 K and 340 K for x = 23% and 25%, respectively, which are the highest values reported so far in intrinsic III–V ferromagnetic semiconductors.

Over the past two decades, intensive studies on various ferromagnetic semiconductor (FMS) materials have failed to realize reliable FMSs that have a high Curie temperature (T C > 300 K), good compatibility with semiconductor electronics, and characteristics superior to those of their nonmagnetic host semiconductors. Here, we demonstrate a new n-type Fe-doped narrow-gap III–V FMS, (In1− x ,Fe x )Sb. Its T C is unexpectedly high, reaching ~335 K at a modest Fe concentration (x) of 16%. The anomalous Hall effect and magnetic circular dichroism (MCD) spectroscopy indicate that the high-temperature ferromagnetism in (In,Fe)Sb thin films is intrinsic and originates from the zinc-blende (In,Fe)Sb alloy semiconductor.

Transverse magneto-thermoelectric effects were studied in an (In,Fe)Sb ferromagnetic semiconductor thin film under an in-plane magnetic field. We find that the thermal voltage is governed by the planar Nernst effect. We show that the magnetic field intensity dependence, magnetic field direction dependence, and temperature dependence of the transverse Seebeck coefficient can be explained by assuming a Mott relation between the in-plane magneto-transport and magneto-thermoelectric phenomena in (In,Fe)Sb.

By studying the electrical control of the magnetic properties of ferromagnetic semiconductors (FMSs), we can understand many fundamental aspects of carrier-induced ferromagnetism and explore the possibilities of device applications. Previous experiments on the electrical control of ferromagnetism in Mn-doped FMSs were limited to very low temperatures due to their low Curie temperature (TC). Here, we demonstrate electrical control ferromagnetism at high temperature (210 K) in an electric double layer transistor with an n-type high-TC FMS (In0.89,Fe0.11)Sb thin film channel. A liquid electrolyte is used instead of a conventional solid gate to obtain a large change (40%) of the electron density in the (In0.89,Fe0.11)Sb channel. By applying a small gate voltage (0 → +5 V), TC of the (In,Fe)Sb thin film can be changed by 7 K, indicating that the magnetization as well as ferromagnetic phase transition in (In,Fe)Sb can be controlled at high temperature by the gate electric field despite a small change of electron concentration Δn = 2.2 × 1017 cm−3. Our result paves a way for realizing semiconductor spintronic devices operating at room temperature with low power consumption.

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

We investigated the spin-valve effect in nano-scale silicon (Si)-based spin-valve devices using a Fe/MgO/Ge spin injector/detector deposited on Si by molecular beam epitaxy. For a device with a 20 nm Si channel, we observed clear magnetoresistance up to 3% at low temperature when a magnetic field was applied in the film plane along the Si channel transport direction. A large spin-dependent output voltage of 20 mV was observed at a bias voltage of 0.9 V at 15 K, which is among the highest values in lateral spin-valve devices reported so far. Furthermore, we observed that the sign of the spin-valve effect is reversed at low temperatures, suggesting the possibility of a spin-blockade effect of defect states in the MgO/Ge tunneling barrier.

We developed a 1.5-µm-band, transverse-magnetic-mode waveguide isolator based on the nonreciprocal-loss phenomenon. The device consists of an optical waveguide with an InGaAs/InGaAlAs multiple-quantum-well amplifier and a ferromagnetic MnAs layer attached to the waveguide. The combination of the optical waveguide and the magnetized MnAs layer produces a magneto-optic effect called the nonreciprocal loss–a phenomenon where the propagation loss of light is larger in backward than in forward propagation. We present the guiding principle of the design of the structure of the device. A prototype device with a MBE-grown epitaxial MnAs layer showed a nonreciprocity of 9.8 dB/mm at a wavelength of 1.54 µm. Our device can be monolithically integrated with waveguide-based optical devices on an InP substrate.

We systematically investigate the crystal structure, magneto-optical properties, magnetization, and magnetotransport properties of a new ferromagnetic semiconductor $(\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x},\mathrm{F}{\mathrm{e}}_{x})\mathrm{Sb}$ grown by low-temperature molecular beam epitaxy. Crystal structure analyses by x-ray diffraction, scanning transmission microscopy, and transmission electron diffraction indicate that the $(\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x},\mathrm{F}{\mathrm{e}}_{x})\mathrm{Sb}$ thin films maintain the zinc-blende crystal structure up to $x=20%$. We carried out the characterizations of the magnetic properties of the $(\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x},\mathrm{F}{\mathrm{e}}_{x})\mathrm{Sb}$ thin films by various methods, including magnetic circular dichroism spectroscopy, anomalous Hall effect, and superconducting quantum interference device magnetometry, and found that $(\mathrm{Ga},\mathrm{Fe})\mathrm{Sb}$ is an intrinsic ferromagnetic semiconductor without any second-phase precipitations. The Curie temperature ${T}_{\mathrm{C}}$ of $(\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x},\mathrm{F}{\mathrm{e}}_{x})\mathrm{Sb}$ depends on $x$ and hole concentration, as in the case of hole-induced ferromagnetism. The highest ${T}_{\mathrm{C}}$ reaches 230 K at $x=20%$, which is the highest value so far reported in III-V based ferromagnetic semiconductors.

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

We demonstrate visible-light electroluminescence due to d-d transitions in GaAs:Mn based light emitting diodes (LEDs) [1][2]. We prepared p+n junctions with a p+GaAs:Mn layer. At a reverse bias voltage (-3 to -6V), holes are injected from the n-type layer to the depletion layer and accelerated by the intense electric field, and excite the d electrons of Mn in the p+GaAs:Mn layer by impact excitations. We observe visible-light emission E1 = 1.89eV and E2 = 2.16eV, which are exactly the same as the 4T1 -> 6A1 and 4A2 -> 4 T1 transition energy of Mn. Furthermore, by utilizing optical transitions between the p-d hybridized orbitals of Mn atoms doped in Si, we demonstrate Si-based LEDs that continuously emit reddish-yellow visible light at room temperature. The Mn p-d hybrid states are excited by hot holes that are accelerated in the depletion layers of reverse biased Si pn junctions. Above a threshold reverse bias voltage of about -4V, our LEDs show strong visible light emission with two peaks at E1 = 1.75eV and E2 = 2.30eV, corresponding to optical transitions from the t-a (spin-down anti-bonding) states to the e- (spin-down non-bonding) states, and from the e- to the t+a (spin-up anti-bonding) states. The internal quantum efficiency of the E1 and E2 transitions is 3-4 orders of magnitude higher than that of the indirect band-gap transition [3]. [1] P. N. Hai, et al., APL 104, 122409 (2014). [2] P. N. Hai, et al., JAP 116, 113905 (2014). [3] P. N. Hai, et al., submitted.

We investigate the crystal structure, transport and magnetic properties of Fe-doped ferromagnetic semiconductor (Al1-x,Fex)Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al1-x,Fex)Sb thin films with x lower or equal to 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al1-x,Fex)Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (TC) of 40 K. In the (Al1-x,Fex)Sb thin film with x = 14%, a sudden drop of mobility and TC was observed, which may be due to microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices., Manuscript and Supplemental Material, all 18 pages

In order to investigate the mechanism of ferromagnetic ordering in the new $n$-type magnetic semiconductor (In,Fe)As codoped with Be, we have performed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of ferromagnetic and paramagnetic samples. The spectral line shapes suggest that the ferromagnetism is intrinsic, originating from Fe atoms incorporated into the zinc-blende-type InAs lattice. The magnetization curves of Fe measured by XMCD were well reproduced by the superposition of a Langevin function representing superparamagnetic (SPM) behavior of nanoscale ferromagnetic domains and a $T$-linear function representing Curie-Weiss paramagnetism even much above the Curie temperatures. The data at 20 K showed a deviation from the Langevin behavior, suggesting a gradual establishment of macroscopic ferromagnetism on lowering temperature. The existence of nanoscale ferromagnetic domains indicated by the SPM behavior suggests spatial fluctuations of Fe concentration on the nanoscale.

We report on the crystal growth as well as the structural and magnetic properties of Bi0.8Sb0.2 topological insulator (TI)/MnxGa1-x bi-layers grown on GaAs(111)A substrates by molecular beam epitaxy. By optimizing the growth conditions and Mn composition, we were able to grow MnxGa1-x thin films on Bi0.8Sb0.2 with the crystallographic orientation of Bi0.8Sb0.2(001)[1 1 ¯ 0]//MnGa (001)[100]. Using magnetic circular dichroism (MCD) spectroscopy, we detected both the L10 phase ( x 0.6) of MnxGa1-x. For 0.50 ≤ x ≤ 0.55, we obtained ferromagnetic L10-MnGa thin films with clear perpendicular magnetic anisotropy, which were confirmed by MCD hysteresis, anomalous Hall effect as well as superconducting quantum interference device measurements. Our results show that the BiSb/MnxGa1-x bi-layer system is promising for perpendicular magnetization switching using the giant spin Hall effect in TIs.

We grew and characterized Bi1-xSbx thin films on GaAs(111)A substrates by molecular beam epitaxy. By optimizing the growth condition, we were able to grow Bi1-xSbx thin films epitaxially with the Sb concentration ranging from 0% to 100% and the epitaxial orientation of Bi1-xSbx(001)//GaAs(111). The conductivity of Bi1-xSbx exceeds 105 Ω−1 m−1 and approaches those of bulk values for thick enough thin films, which are higher than those of other Bi-based topological insulators by at least an order of magnitude. From the temperature dependence of their electrical conductivity, we confirmed the existence of metallic surface states of Bi1-xSbx inside and outside of the topological insulating region. Our results demonstrate the potential of Bi1-xSbx as a spin Hall material with high conductivity and possibly large spin Hall angle for spintronic applications.

A new n-type magnetic semiconductor (In1− x ,Co x )As (x = 3–18%) has been successfully grown by low-temperature molecular beam epitaxy (LT-MBE) on GaAs(001) substrates. Reflection high energy electron diffraction (RHEED) patterns during the MBE growth and transmission electron microscopy (TEM) images indicate that (In,Co)As layers have zinc-blende crystal structure with a small fraction of embedded CoAs nanoclusters. The electron concentration of the (In,Co)As layers can be changed in the range of 1.9 × 1018–2.4 × 1019 cm−3 by changing the Co concentration. The metal–insulator transition (MIT) is observed at x = 5%. Large negative magnetoresistance (up to −17.5% at 0.95 T) is observed at low temperature and can be attributed to spin-disorder scattering in the (In,Co)As matrix.

We investigated the spin transport in nano-scale silicon (Si)-based spin-valve devices with Fe electrodes, MgO/Ge tunnel barriers, and a 20 nm-long Si channel. We observed a clear spin-valve effect when a magnetic field was applied in the film plane along and perpendicular to the Si channel transport direction. Systematic investigations of the bias voltage dependence, temperature dependence, and magnetic-field direction dependence of the magnetoresistance indicate that the observed spin-valve effect is governed by the spin transport through the nano-scale Si channel. The spin-valve effect remains observable up to 200 K. For the device with MgO/Ge tunnel barriers, with a bias voltage of 1.7 V at 50 K, the spin-dependent output voltage is 13 mV, which is among the highest values reported so far.

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]., Comment: main manuscript: page 1-22, supplementary materials: page 23-36

The electronic structure of doped Mn in (Ga,Mn)As is studied by resonant inelastic X-ray scattering (RIXS). From configuration-interaction cluster-model calculations, the line shapes of the Mn $L_3$ RIXS spectra can be explained by $d$-$d$ excitations from the Mn$^{3+}$ ground state, dominated by charge-transferred states, rather than a Mn$^{2+}$ ground state. Unlike archetypical $d$-$d$ excitation, the peak widths are broader than the experimental energy resolution. We attribute the broadening to a finite lifetime of the $d$-$d$ excitations, which decay rapidly to electron-hole pairs in the host valence and conduction bands through hybridization of the Mn $3d$ orbital with the ligand band., 5 pages, 3 figures

Introducing spin degrees of freedom into the present semiconductor electronics is a very important issue for realizing novel devices which will be needed in the future information technology. For fabricating such devices, it is necessary to exploit and fabricate semiconductor-based magnetic materials. III-V-based ferromagnetic semiconductors and MnAs/III-V hybrid nanostructures are hopeful candidates and model systems for future spintronic devices [1, 2].

We show high-temperature ferromagnetism in heavily Fe-doped ferromagnetic semiconductor (Ga1−x,Fex)Sb (x = 23% and 25%) thin films grown by low-temperature molecular beam epitaxy. Magnetic circular dichroism spectroscopy and anomalous Hall effect measurements indicate intrinsic ferromagnetism of these samples. The Curie temperature reaches 300 K and 340 K for x = 23% and 25%, respectively, which are the highest values reported so far in intrinsic III-V ferromagnetic semiconductors.

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow a 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 novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs., arXiv admin note: substantial text overlap with arXiv:1106.0561

The valence-band structure and the Fermi level (${E}_{\mathrm{F}}$) position of ferromagnetic-semiconductor GaMnAs are quantitatively investigated by electrically detecting the resonant tunneling levels of a GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant level from the heavy-hole first state is clearly observed in the metallic GaMnAs QW, indicating that holes have a high coherency and that ${E}_{\mathrm{F}}$ exists in the band gap. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated in these double-barrier heterostructures.

We investigate the spin-dependent transport of GaMnAs-based magnetic tunnel junctions (MTJs) containing a paramagnetic AlMnAs barrier with various thicknesses. The barrier height of AlMnAs with respect to the Fermi level of GaMnAs is estimated to be 110 meV. We observe tunneling magnetoresistance (TMR) ratios up to 175% (at 2.6 K), which is higher than those of the GaMnAs-based MTJs with other barrier materials in the same temperature region. These high TMR ratios can be mainly attributed to the relatively high crystal quality of AlMnAs and the suppression of the tunneling probability near at the in-plane wave-vector k||=0., Comment: 12 pages, 3 figures, accepted for publication in Appl. Phys. Lett