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Indium-substituted yttrium iron garnet samples, Y3Fe5−xInxO12 (x = 0–0.7, step 0.1), were prepared using a citrate solgel method. Synchrotron X-ray diffraction (SXRD) measurements combined with the Rietveld refinement technique were used to investigate the crystallization, structure parameters and lattice distortion in the samples. Magnetization and Curie temperature of the samples were measured with the SQUID and VSM equipments. The non-magnetic indium ions were found to reside at the octahedral sites, leading to an increase of the total magnetization and a decrease in the Curie temperature. Molecular field coefficients Naa, Ndd and Nad were determined by fitting the experimental thermomagnetization curves in the framework of a two-sublattice ferrimagnetic model. The stiffness constant of the spin waves, D, in these samples was calculated based on the exchange interaction constants, Jij, and its temperature dependence was derived for the sublattice magetizations. The calculated results for D(T) are in good agreement with the experimentally measured data and are significantly large around room temperature.

Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds embedded with a regular interval in a semiconductor InAs matrix, which resembles the crystal structure of Fe-based superconductors. Contrary to the case of Fe-based pnictides, these FeAs/InAs superlattices (SLs) exhibit ferromagnetism, whose Curie temperature (TC) increases rapidly with decreasing the InAs interval thickness tInAs (TC ∝ tInAs−3), and an extremely large magnetoresistance up to 500% that is tunable by a gate voltage. Our first principles calculations reveal the important role of disordered positions of Fe atoms in the establishment of ferromagnetism in these quasi-2D FeAs-based SLs. These unique features mark the FeAs/InAs SLs as promising structures for spintronic applications., In this manuscript, the authors grow very thin layers of FeAs in a matrix of InAs. The resulting superlattice displays ferromagnetism, with the Curie temperature varying depending on the layer thickness. These results further illustrate the wide array of intriguing ground states of materials with tetrahedral FeAs bonds

(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature $({T}_{\mathrm{C}})$ is above room temperature. However, the origin of ferromagnetism with high ${T}_{\mathrm{C}}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ including the $\mathrm{Fe}\text{\ensuremath{-}}3d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The $\mathrm{Fe}\text{\ensuremath{-}}3d$ resonant ARPES spectra demonstrate that the $\mathrm{Fe}\text{\ensuremath{-}}3d$ IB crosses the Fermi level $({E}_{\mathrm{F}})$ and hybridizes with the VB of GaSb. These observations indicate that the VB structure of $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ is consistent with that of the IB model which is based on double-exchange interaction between the localized $3d$ electrons of the magnetic impurities.

We demonstrate spin-valve magnetoresistance with a current-in-plane (CIP) configuration in (Ga,Fe)Sb / InAs (thickness $t_\mathrm{InAs}$ nm) / (Ga,Fe)Sb trilayer heterostructures, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature ($T_\mathrm{C}$). An MR curve with an open minor loop is clearly observed at 3.7 K in a sample with $t_\mathrm{InAs}$ = 3 nm, which originates from the parallel - antiparallel magnetization switching of the (Ga,Fe)Sb layers and spin-dependent scattering at the (Ga,Fe)Sb / InAs interfaces. The MR ratio increases (from 0.03 to 1.6%) with decreasing $t_\mathrm{InAs}$ (from 9 to 3 nm) due to the enhancement of the interface scattering. This is the first demonstration of the spin-valve effect in Fe-doped FMS heterostructures, paving the way for device applications of these high- $T_\mathrm{C}$ FMSs., Comment: 15 pages, 7 figures

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.

We report switching of magnetic anisotropy (MA) from in-plane to perpendicular with increasing the thickness $d$ of a (001)-oriented ferromagnetic-semiconductor (FMS) $(\mathrm{G}{\mathrm{a}}_{0.7},\mathrm{F}{\mathrm{e}}_{0.3})\mathrm{Sb}$ layer with a high Curie temperature (${T}_{\mathrm{C}}g320$ K), using ferromagnetic resonance at room temperature. We show that the total MA energy (${E}_{\ensuremath{\perp}}$) along the [001] direction changes its sign from positive (in-plane) to negative (perpendicular) with increasing $d$ above an effective critical value ${d}_{\mathrm{C}}^{*}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. We reveal that (Ga,Fe)Sb has twofold symmetry in the film plane. Meanwhile, in the plane perpendicular to the film including the in-plane [110] axis, the twofold symmetry with the easy magnetization axis along [110] changes to fourfold symmetry with the easy magnetization axis along $\ensuremath{\langle}001\ensuremath{\rangle}$ with increasing $d$. This peculiar behavior is different from that of (Ga,Mn)As, in which only the in-plane MA depends on the film thickness and has fourfold symmetry due to its dominant cubic anisotropy along the $\ensuremath{\langle}100\ensuremath{\rangle}$ axes. This work provides an important guide for controlling the easy magnetization axis of high-${T}_{\mathrm{C}}$ FMS (Ga,Fe)Sb for room-temperature device applications.

Ferromagnetic semiconductors (FMSs), which show both ferromagnetic and semiconducting characteristics, are promising materials for future low-power spintronics devices. Among various FMSs, Fe-doped III-V FMSs are promising, because both p-type and n-type materials are available and the highest Curie temperature ( $T_{\mathrm{C}}$ ) exceeds room temperature. In this work, we demonstrate clear magnetoresistance due to the spin valve effect in ferromagnetic heterostructures containing high- $T_{\mathrm{C}}$ (Ga, Fe)Sb. The samples examined here consist of (Ga 0.75 , Fe 0.25 )Sb ( $40\ \text{nm},\ T_{\mathrm{C}} > 320\ \mathrm{K}$ ) / InAs (thickness $t=0,\ 3,\ 6,\ 9\ \text{nm}$ ) / (Ga 0.8 , Fe 0.2 )Sb ( $40\ \text{nm},\ T_{\mathrm{C}} > 320\ \mathrm{K}$ ) grown by low-temperature molecular-beam epitaxy. Clear MR of ∼2% with an open minor loop is observed at 3.7 K when $t =3\ \text{nm}$ , whose peaks (⋍ ±0.1 T) are consistent with the coercive forces of the (Ga, Fe)Sb layers obtained with superconducting quantum interference device (SQUID) magnetometry. We found that the GMR ratio increases (from 0.03 to 1.28%) with decreasing $t$ (from 9 to 3 nm), which is caused by the enhancement of spin-dependent scattering at the InAs/(Ga, Fe)Sb interfaces. This is the first demonstration of the spin-valve effect in Fe-doped FMS heterostructures, paving the way for device applications of high- $T_{\mathrm{C}}$ FMSs.

Ferromagnetic semiconductor (FMSs), which exhibits both magnetic and semiconducting properties, are promising for semiconductor-based spintronics devices. Magnetic anisotropy (MA) plays an important role in determining the magnetization direction of ferromagnetic materials. Here, we present the thickness-dependence control of MA in (Ga 0.7 , Fe 0.3 )Sb by growing samples with different thicknesses $d=20-55\ \text{nm}$ grown on AlSb by low-temperature molecular beam epitaxy. We used ferromagnetic resonance (FMR) to estimate the MA constants of the (Ga 0.7 , Fe 0.3 )Sb. We systematically measured the dependence of the FMR resonance field on the external magnetic-field direction and fitted a theoretical curve to obtain the MA fields of the (Ga, Fe)Sb thin films, and obtained the effective MA constants ( $K_{\text{eff}}$ ) of (Ga, Fe)Sb. From the sign of $K_{\text{eff}}$ , we found that MA changes from in-plane magnetic anisotropy (IMA) to perpendicular magnetic anisotropy (PMA) with increasing $d$ . To understand this, we separated the $K_{\text{eff}}$ into the volume contribution ( $K_{\mathrm{V}}$ ) and the interface contribution ( $K_{\mathrm{I}}$ ). We show that the IMA originates from the film interface due to the tensile strain when the films are thin, whereas $K_{\mathrm{V}}$ causes PMA in (Ga, Fe)Sb. The observation of FMR at room temperature and control of MA are important steps towards device applications of (Ga, Fe)Sb.

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 study the strain dependence of the magnetic anisotropy of room-temperature ferromagnetic semiconductor $(\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x},\mathrm{F}{\mathrm{e}}_{x})\mathrm{Sb}$ ($x=20%$) thin films epitaxially grown on different buffer layers, using ferromagnetic resonance measurements. We show that the magnetocrystalline anisotropy $({K}_{\mathrm{i}})$ in $(\mathrm{G}{\mathrm{a}}_{0.8},\mathrm{F}{\mathrm{e}}_{0.2})\mathrm{Sb}$ exhibits a dependence on the epitaxial strain and changes its sign from negative (in-plane magnetization easy axis) to positive (perpendicular magnetization easy axis), when the strain is changed from tensile to compressive. Meanwhile, the shape anisotropy (${K}_{\mathrm{sh}}$) is negative and dominant over ${K}_{\mathrm{i}}$. Therefore, the effective magnetic anisotropy $({K}_{\mathrm{eff}}={K}_{\mathrm{i}}+{K}_{\mathrm{sh}})$ is always negative, leading to the in-plane magnetic anisotropy in all the $(\mathrm{G}{\mathrm{a}}_{0.8},\mathrm{F}{\mathrm{e}}_{0.2})\mathrm{Sb}$ samples. This work demonstrates ferromagnetic resonance and strong shape anisotropy at room temperature in III-V ferromagnetic semiconductors. We also observed very high Curie temperature $({T}_{\mathrm{C}}\ensuremath{\gtrsim}400\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ in $p$-type (Ga,Fe)Sb, which is the highest ${T}_{\mathrm{C}}$ reported so far in III-V based ferromagnetic semiconductors.

We report the structural and magnetic properties of the Fe-doped GaSb quantum dots (QDs) (nominal Fe concentration x = 4.7%–16.6%) grown on GaAs (001) substrates by molecular beam epitaxy. The QDs with nanometer-scale dimensions consist of two areas with different crystal structures, a zinc-blende GaAsSb wetting layer and a new phase of FeGaSb alloy that has a simple cubic lattice. The size and distribution of the QDs depend on the Fe concentration, as revealed by atomic force microscopy. Magnetic force microscopy measurements at zero applied magnetic field show the presence of ferromagnetism in the QDs at room temperature with an easy axis in the 1¯10 direction, which is consistent with magnetometry measurements. The Curie temperature in these QDs is very high (>400 K), which is promising for spintronic applications at room temperature.

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.

By using magnetic circular dichroism (MCD) spectroscopy with photon energy in both visible (1.5–5 eV) and infrared light regions (0.6–1.7 eV), we systematically investigate the band structure of p-type ferromagnetic semiconductor (Ga1−x,Fex)Sb with various Fe concentrations x = 2%–20% grown by low-temperature molecular beam epitaxy. We observed two peaks in the infrared MCD spectra that can be explained by the optical transitions related to the Fermi level (EF) located in an Fe-related impurity band (IB) in the bandgap. As x increases, the energy shifts of the two peaks suggest that the Fe-related IB extends into the bandgap and EF rises correspondingly. Furthermore, the mobility of hole carriers in these (Ga,Fe)Sb thin films estimated by Hall measurements is very low (0.2–2 cm2/Vs), which is consistent with our conclusion that the hole carriers and EF reside in the IB rather than in the valence band. Our results provide insights into the band structure of p-type ferromagnetic semiconductors (Ga,Fe)Sb with high Curie temperature, which is promising for the realization of spintronic devices operating at room temperature.

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.

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 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 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

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

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.

A p-type ferromagnetic semiconductor (Ga1−x,Fex)Sb (x = 3.9%–13.7%) has been grown by low-temperature molecular beam epitaxy (MBE) on GaAs(001) substrates. Reflection high energy electron diffraction patterns during the MBE growth and X-ray diffraction spectra indicate that (Ga,Fe)Sb layers have the zinc-blende crystal structure without any other crystallographic phase of precipitates. Magnetic circular dichroism (MCD) spectroscopy characterizations indicate that (Ga,Fe)Sb has the zinc-blende band structure with spin-splitting induced by s,p-d exchange interactions. The magnetic field dependence of the MCD intensity and anomalous Hall resistance of (Ga,Fe)Sb show clear hysteresis, demonstrating the presence of ferromagnetic order. The Curie temperature (TC) increases with increasing x and reaches 140 K at x = 13.7%. The crystal structure analyses, magneto-transport, and magneto-optical properties indicate that (Ga,Fe)Sb is an intrinsic ferromagnetic semiconductor.

We systematically investigated the influence of strain on the electronic structure and ferromagnetism of (In,Fe)As thin films. It is found that while the shift of the critical point energies of compressive-strained (In,Fe)As layers grown on (In1−y,Gay)As (y = 0.05, 0.1) buffer layers can be explained by the hydrostatic deformation effect (HDE) alone, those of tensile-strained (In,Fe)As layers grown on (Ga1−z,Alz)Sb (z = 0, 0.5, 1) buffer layers can be explained by the combination of HDE and the quantum confinement effect (QCE). The Curie temperature TC of the (In,Fe)As layers strongly depends on the strain, and shows a maximum for the (In,Fe)As layer grown on a GaSb buffer layer. The strain dependence of TC can be explained by the s-d exchange mechanism taking into account HDE and QCE.

We demonstrated the control of ferromagnetism in a surface quantum well containing a 5-nm-thick n-type ferromagnetic semiconductor (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 ({\Delta}Tc/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., Comment: 28 pages icluding both main text(15 pages, 4 figures) and supplementary information (13 pages)