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.
A detailed investigation of the electronic - and magneto - transport properties of Nd0.8Sr0.2MnO3 with the variation of grain size (down to 42 nm) is presented here. Interestingly, we observe that the ferromagnetic insulating state is suppressed and a metallic state is stabilized as the grain size of the sample is reduced. As a result, metal insulator transition is observed in this low doped manganite which is insulating in nature in its bulk form. Destabilization of polaronic order in the ferromagnetic insulating state due to enhanced surface disorder on grain size reduction has been attribut…
Experimentally Cr doping in the rutile phase of VO$_2$ is found to stabilize a charge ordered ferromagnetic insulating state in the doping range of 10\% to 20\%. In this work, we investigated its origin at 12.5\% Cr doping using a combination of ab-initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab-initio band structure onto a tight-binding Hamiltonian allows one to calculate the energy gain from different exchange pathways.…
Ferromagnetic insulator Pr0.8Ca0.2Mn1-yCoyO3 (0 <= y <= 0.7) thin films were epitaxially grown on (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (100) substrates by pulsed laser deposition. To probe the ferromagnetic insulator state of hole-doped manganites, the Co content dependences of the structural, magnetic, and transport properties were studied. Variation of lattice constant by the substitution of Co ions is well reproduced considering that divalent and trivalent Co ions substitute for Mn ions at the perovskite B-sites. For 0 <= y <= 0.3, the Curie temperature, saturation magnetization, and magnetoresi…
Using first-principles density functional calculations, we study the electronic and magnetic properties of ferromagnetic insulating double-perovskite compound La2NiMnO6, which has been reported to exhibit interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals existence of very soft infra-red active phonons that couple strongly with spins at the Ni and Mn sites through modification of the super-exchange interaction. We suggest that these modes are the origin for observed dielectric anomaly in La2NiMnO6.
This paper shows that the oxygen vacancies observed experimentally in thin films of LaCoO3 subject to tensile strain are thermodynamically stable according to ab initio calculations. By using DFT calculations, we show that oxygen vacancies on the order of 6 % forming chains perpendicular to the (001) direction are more stable than the stoichiometric solution. These lead to magnetic Co2+ ions surrounding the vacancies that couple ferromagnetically. The remaining Co3+ cations in an octahedral environment are non magnetic. The gap leading to a ferromagnetic insulating phase occurs naturally and w…
We report the observation of a large 1/f noise in the ferromagnetic insulating state (FMI) of a hole doped manganite single crystal of La0.80Ca0.20MnO3 which manifests hopping conductivity in presence of a Coulomb gap. The temperature dependent noise magnitude shows a deep within the FMI state, there is a sharp freeze out of the noise magnitude with temperature on cooling. As the material enters the FMI state, the noise becomes non-Gaussian as seen through probability density function and second spectra. It is proposed to arise from charge fluctuations in a correlated glassy phase of the polar…
We have studied the dielectric properties of the ferromagnetic spinel CdCr$_2$S$_4$ from first principles. Zone-center phonons and Born effective charges were calculated by frozen-phonon and Berry phase techniques within LSDA+U. We find that all infrared-active phonons are quite stable within the cubic space group. The calculated static dielectric constant agrees well with previous measurements. These results suggest that the recently observed anomalous dielectric behavior in CdCr$_2$S$_4$ is not due to the softening of a polar mode. We suggest further experiments to clarify this point.
Development of topological insulating phases in IV-VI compounds under dynamic lattice deformations is studied using first-principles methods. Unlike the static state of topological phases at equilibrium conditions, we show that non-trivial topological phases are induced in the compounds by the dynamic lattice deformations from selective phonon modes. Calculations of the time-reversal polarization show that the Z2 invariant of the compounds is flipped by the atomic displacements of selective phonon modes and that the compounds exhibit oscillating topological phases upon dynamic lattice deformat…
We observe a dependence of the damping of a confined mode of precessing ferromagnetic magnetization on the size of the mode. The micron-scale mode is created within an extended, unpatterned YIG film by means of the intense local dipolar field of a micromagnetic tip. We find that damping of the confined mode scales like the surface-to-volume ratio of the mode, indicating an interfacial damping effect (similar to spin pumping) due to the transfer of angular momentum from the confined mode to the spin sink of ferromagnetic material in the surrounding film. Though unexpected for insulating systems…
The magnetic behavior of insulating doped diluted magnetic semiconductors (DMS) is characterized by the interaction of large collective spins known as bound magnetic polarons. Experimental measurements of the susceptibility of these materials have suggested that the polaron-polaron interaction is ferromagnetic, in contrast to the antiferromagnetic carrier-carrier interactions that are characteristic of nonmagnetic semiconductors. To explain this behavior, a model has been developed in which polarons interact via both the standard direct carrier-carrier exchange interaction (due to virtual carr…
We measure electron localization in different materials by means of a ``localization tensor'', based on Berry phases and related quantities. We analyze its properties, and we actually compute such tensor from first principles for several tetrahedrally coordinated semiconductors. We discuss the trends in our calculated quantity, and we relate our findings to recent work by other authors. We also address the ``hermaphrodite orbitals'', which are localized (Wannier-like) in a given direction, and delocalized (Bloch-like) in the two orthogonal directions: our tensor is related to the optimal local…
The discovery of reversible 300 K ferromagnetic ordering in a diluted magnetic semiconductor is reported. Switching of room-temperature ferromagnetism between "on" and "off" states is achieved in Co2+:ZnO by lattice incorporation and removal of the native n-type defect, interstitial Zn. Spectroscopic and magnetic data implicate a double-exchange mechanism for ferromagnetism. These results demonstrate for the first time reversible room-temperature ferromagnetic ordering in a diluted magnetic semiconductor, and present new opportunities for integrating magnetism and conductivity in semiconductor…
We investigated spin injection by spin pumping from a spin-injector(NiFe) into a spin-sink to detect spin fluctuations in the spin-sink. By scanning the ordering-temperature of several magnetic transitions, we found that enhanced spin pumping due to spin fluctuations applies with several ordering states: ferromagnetic(Tb) and antiferromagnetic(NiO, NiFeOx, BiFeO3, exchange-biased and unbiased IrMn). Results also represent systematic experimental investigation supporting that the effect is independent of the metallic and insulating nature of the spin-sink, and is observed whether the spin curre…
We study the enhancement of the magnetic transition temperature $T_c$ due to incident light in ferromagnetic semiconductors such as EuS. The photoexcited carriers mediate an extra ferromagnetic interaction due to the coupling with the localized magnetic moments. The Hamiltonian consists of a Heisenberg model for the localized moments and an interaction term between the photoexcited carriers and the localized moments. The model predicts a small enhancement of the transition temperature in semi-quantitative agreement with the experiments.
The Gilbert damping in ferromagnetic semiconductors is theoretically investigated based on the $s$-$d$ model. In contrast to the situation in metals, all the spin-conserving scattering in ferromagnetic semiconductors supplies an additional spin relaxation channel due to the momentum dependent effective magnetic field of the spin-orbit coupling, thereby modifies the Gilbert damping. In the presence of a pure spin current, we predict a new contribution due to the interplay of the anisotropic spin-orbit coupling and a pure spin current.
The recent development of MBE techniques for growth of III-V ferromagnetic semiconductors has created materials with exceptional promise in spintronics, i.e. electronics that exploit carrier spin polarization. Among the most carefully studied of these materials is (Ga,Mn)As, in which meticulous optimization of growth techniques has led to reproducible materials properties and ferromagnetic transition temperatures well above 150 K. We review progress in the understanding of this particular material and efforts to address ferromagnetic semiconductors as a class. We then discuss proposals for how…
We demonstrate the spin valve effect by ballistic transport in fully epitaxial MnAs ferromagnetic metal / GaAs semiconductor / GaAs:MnAs granular hybrid heterostructures. The GaAs:MnAs material contains ferromagnetic MnAs nanoparticles in a GaAs matrix, and acts as a spin injector and a spin detector. Although the barrier height of the GaAs/MnAs interface was found to be very small, relatively large magnetoresistance was observed. This result shows that by using ballistic transport, we can realize a large spin valve effect without inserting a high tunnel barrier at the ferromagnetic metal / se…
The rapidly developing field of ferromagnetism in diluted magnetic semiconductors, where a semiconductor host is magnetically doped by transition metal impurities to produce a ferromagnetic semiconductor (e.g. Ga_{1-x}Mn_xAs with x ~ 1-10 %), is discussed with the emphasis on elucidating the physical mechanisms underlying the magnetic properties. Recent key developments are summarized with critical discussions of the roles of disorder, localization, band structure, defects, and the choice of materials in producing good magnetic quality and high Curie temperature. The correlation between magnet…
Using density functional theory calculations we establish the hitherto unknown compound CrCTe$_3$ to be a stable anti-ferromagnetic semiconductor in the R$\bar{3}$ crystal structure with an indirect fundamental gap . Successive layers in the bulk compound are weakly bound by van der Waals forces so that individual layers can be easily exfoliated. A monolayer of CrCTe$_3$ is also an anti-ferromagnetic semiconductor. The monolayer is structurally stable over a large range of compressive and tensile strains, and the anti-ferromagnetic state is robust over this strain range. Band gap of the monola…
The methodology used to obtain the values of the spin-orbit couplings from the spin expectation values from perturbation theory was incorrect. As a result Figs. 2 and 3 are incorrect.
Two-dimensional chromium oxide fluoride CrOF sheet was studied based on density functional theory. The investigation indicates that the CrOF sheet is an intrinsic ferromagnetic semiconductor. The calculated low cleavage energy implies that the ferromagnetic semiconductor can be exfoliated from its bulk form. The corresponding Curie temperature is 150 K. In particular, the Curie temperature increases up to 410 K under hole doping and the CrOF sheet becomes a half-metal. The versatile electronic and magnetic properties indicate that the two-dimensional CrOF sheet can be a promising candidate for…
We report on the resonant tunneling effect and the increase of tunneling magnetoresistance (TMR) induced by it in ferromagnetic-semiconductor GaMnAs quantum-well heterostructures. The observed quantum levels of the GaMnAs quantum well were successfully explained by the valence-band kp model with the p-d exchange interaction. It was also found that the Fermi level of the electrode injecting carriers is important to observe resonant tunneling in this system.
Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, since ferromagnetism generally accompanies metallicity, FMIs are innately rare to find in nature. Here, novel room-temperature FMI films are epitaxially synthesized by deliberate control of the ratio of two B-site cations in the double perovskite Sr2FeReO6. In contrast to the known ferromagnetic metallic phase in stoichiometric Sr2FeReO6, a FMI state with a high Curie temperature (Tc~400 K) and a large saturation magnetization (MS~1.8 μB/f.u.) is fo…
An overview of recent studies on ferromagnetism in Cr- and Mn-based II-VI diluted magnetic semiconductors is presented emphasizing differences in underlying exchange mechanisms. Examples of manipulations with spin ordering by carrier density, dimensionality, light, and electric field are given.
We demonstrate experimentally the electrical ballistic electron spin injection from a ferromagnetic metal / tunnel barrier contact into a semiconductor III-V heterostructure. We introduce the Oblique Hanle Effect technique for reliable optical measurement of the degree of injected spin polarization. In a CoFe / Al2O3 / GaAs / (Al,Ga)As heterostructure we observed injected spin polarization in excess of 8 % at 80K.
We present a theory of interstitial Mn in Mn-doped ferromagnetic semiconductors. Using density-functional theory, we show that under the non-equilibrium conditions of growth, interstitial Mn is easily formed near the surface by a simple low-energy adsorption pathway. In GaAs, isolated interstitial Mn is an electron donor, each compensating two substitutional Mn acceptors. Within an impurity-band model, partial compensation promotes ferromagnetic order below the metal-insulator transition, with the highest Curie temperature occurring for 0.5 holes per substitutional Mn.
We successfully dope the magnetically silent double perovskite semiconductor Sr2GaSbO6 to induce ferromagnetism and tune its band gap, with Ga3+ partially substituted by the magnetic trivalent cation Mn3+, in a rigid cation ordering with Sb5+. Our new ferromagnetic semiconducting Sr2Ga1-xMnxSbO6 double perovskite, which crystallizes in tetragonal symmetry (space group I4/m) and has tunable ferromagnetic ordering temperature and band gap, suggests that magnetic ion doping of double perovskites is a productive avenue towards obtaining materials for application in next-generation oxide-based spin…
We investigate the spin-dependent tunneling properties in a three-terminal III-V-based ferromagnetic-semiconductor heterostructure with a 2.5-nm-thick GaMnAs quantum well (QW) and double barriers. We successfully control the quantum levels and modulate the spin-dependent current with varying the voltage of the electrode connected to the GaMnAs QW. Our results will open up a new possibility for realizing three-terminal spin resonant-tunneling devices.
We have grown (InyGa1-y)1-xMnxAs ferromagnetic semiconductor layers with Mn composition x up to 0.13 on InP substrates by molecular beam epitaxy. Near the lattice-matched composition, i.e., y ~ 0.53, the Curie temperature increases linearly with the ferromagnetically effective Mn composition x_eff, following the empirical equation T_C=1300x_eff. We obtained Curie temperatures above 100 K when x is relatively high (x>0.1; x_eff>0.08) and the hole concentration is in the order of 10^19 cm^-3.
Bulk magnetization in Pr_{0.6}Ca_{0.4}MnO_3 thin films with tensile (SrTiO_3) and compressive (LaAlO_3) substrate-induced strain is compared to the magnetooptical Kerr effect (MOKE) measurements. In the absence of an external magnetic field, in both films, a stable ferromagnetic insulating majority phase coexists with an antiferromagnetic insulating phase below ~120K. MOKE measurements indicate that at 5K a metastable ferromagnetic metallic (FM) phase is formed at the surface of the stretched film in a magnetic field below 1.1T already, while in the bulk the FM phase starts to form in the fiel…
Room temperature ferromagnetic semiconductor rutile Ti1-xCoxO2-δ(101) epitaxial thin films were grown on r-sapphire substrates by a dc sputtering method. Ferromagnetic magnetization, magnetic circular dichroism, and anomalous Hall effect were clearly observed at room temperature in sputter-grown films for the first time. The magnetization value is nearly as large as 3μB/Co that is consistent with the high spin state Co2+ in this compound recently established by spectroscopic methods. Consequently, its originally large magneto-optical response is further enhanced.
We investigated the origin of ferromagnetism in epitaxially grown Mn-doped Ge thin films. Using low-temperature molecular beam epitaxy, Mn-doped Ge films were successfully grown without precipitation of ferromagnetic Ge-Mn intermetallic compounds, such as Mn5Ge3. Magnetic circular dichroism measurements revealed that the epitaxially grown Mn-doped Ge films exhibited clear ferromagnetic behavior, but the Zeeman splitting observed at the critical points was not induced by the s,p-d exchange interactions. High-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy an…
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.
We report dynamic control of magnetization precession by light alone. A ferromagnetic (Ga,Mn)As epilayer was used for experiments. Amplitude of precession was modulated to a large extent by tuning the time interval between two successive optical pump pulses which induced torques on magnetization through a non-thermal process. Nonlinear effect in precession motion was also discussed.
We predict that a novel bias-voltage assisted magnetization reversal process will occur in Mn doped II-VI semiconductor quantum wells or heterojunctions with carrier induced ferromagnetism. The effect is due to strong exchange-coupling induced subband mixing that leads to electrically tunable hysteresis loops. Our model calculations are based on the mean-field theory of carrier induced ferromagnetism in Mn-doped quantum wells and on a semi-phenomenological description of the host II-VI semiconductor valence bands.
Diluted ferromagnetic III-V semiconductors typically show a high degree of compensation. Compensation is connected to the presence of comparable densities of charged defects of either sign. This naturally leads to the development of strong correlations between defect positions during growth and annealing. We show that these correlations are required to understand the experimentally observed transport and magnetic properties as well as the persistence of the energy gap upon doping with magnetic ions.
A major challenge for the next generation of spintronics devices is the implementation of ferromagnetic-semiconductor thin films as spin injectors and detectors. Spin-polarised carrier injection cannot be accomplished efficiently from metals, and coupled with the rarity of intrinsic ferromagnetic semiconductors this has driven intensive study of diluted magnetic semiconductors. Chief among these is the doped III-V compound (Ga,Mn)As. These materials suffer from a number of drawbacks; they (i) require magnetic-ion doping well above the solubility limit, and (ii) must be hole doped to above the …
A new group-IV ferromagnetic semiconductor, Ge1-xFex, was successfully grown by low-temperature molecular beam epitaxy (LT-MBE) without precipitation of ferromagnetic Ge-Fe intermetallic compounds. The ferromagnetism of Ge1-xFex films was investigated by magnetic circular dichroism (MCD). In particular, the influence of the Fe content (FFe/FGe =1 - 10%) and growth temperature (100, 200OC) on the ferromagnetism was carefully studied. The MCD measurements revealed that the band structure of the Ge1-xFex films was identical with that of bulk Ge, and that the large spin splitting of the band struc…
Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 um^2 device revealed that magnetization switching occurs at low critical current densities of 1.1 - 2.2 x 10^5 A/cm^2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.
A magnetic tunnel junctions composed of room temperature ferromagnetic semiconductor rutile Ti1-xCoxO2-d and ferromagnetic metal Fe0.1Co0.9 separated by AlOx barrier showed positive tunneling magnetoresistance (TMR) with a ratio of ~11 % at 15 K, indicating that Ti1-xCoxO2-d can be used as a spin injection electrode. The TMR decreased with increasing temperature and vanished above 180 K. TMR action at high temperature is likely prohibited by the inelastic tunneling conduction due to the low quality of the amorphous barrier layer and/or the junction interface.
We demonstrate the manipulation of the Curie temperature of buried layers of the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the effect of annealing. Patterning the GaAs-capped ferromagnetic layers into nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As layers and thus allows the removal of Mn interstitials using annealing. This leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples. For a fixed annealing time, the enhancement of the Curie temperature is larger for narrower nanowires.
We present high-temperature ferromagnetism and large magnetic anisotropy in heavily Fe-doped n-type ferromagnetic semiconductor (In1-x,Fex)Sb (x = 20 - 35%) thin films grown by low-temperature molecular beam epitaxy. The (In1-x,Fex)Sb thin films with x = 20 - 35% maintain the zinc-blende crystal and band structure with single-phase ferromagnetism. The Curie temperature (TC) of (In1-x,Fex)Sb reaches 390 K at x = 35%, which is significantly higher than room temperature and the highest value so far reported in III-V based ferromagnetic semiconductors. Moreover, large coercive force (HC = 160 Oe) …
We propose a new mechanism of indirect exchange interaction, which can be responsible for the ferromagnetic ordering in Mn-doped semiconductors (like GaMnAs) at low carrier concentration. The mechanism is based on the interplay of the hybridization of band states (conduction or valence) with localized impurity (donor or acceptor) states and the direct exchange interaction between localized spins and the band states. The indirect exchange coupling between two impurities occurs when the wavefunctions of the corresponding localized donor (acceptor) states overlap. This coupling is independent of …
Graphene has been predicted to develop a magnetic moment by proximity effect when placed on a ferromagnetic film, a promise that could open exciting possibilities in the fields of spintronics and magnetic data recording. In this work, we study in detail the interplay between the magnetoresistance of graphene and the magnetization of an underlying ferromagnetic insulating film. A clear correlation between both magnitudes is observed but we find, through a careful modelling of the magnetization and the weak localization measurements, that such correspondence can be explained by the effects of th…
We report single-color, time resolved magneto-optical measurements in ferromagnetic semiconductor (Ga,Mn)As. We demonstrate coherent optical control of the magnetization precession by applying two successive ultrashort laser pulses. The magnetic field and temperature dependent experiments reveal the collective Mn-moment nature of the oscillatory part of the time-dependent Kerr rotation, as well as contributions to the magneto-optical signal that are not connected with the magnetization dynamics.
Magnetization measurements in La0.5Ca0.5MnO3 manganite show that the high-temperature long-range ferromagnetic-metallic phase transforms to antiferromagnetic-insulating phase, although a fraction of ferromagnetic-metallic phase undergoes glass-like kinetic arrest and coexists at low temperature with the equilibrium antiferromagnetic-insulating phase. We show here through resistivity measurements that the residual arrested ferromagnetic-metallic fraction can be converted to the equilibrium antiferromagnetic-insulating phase by successive annealing at higher temperatures, possibly through hetero…
A narrow-gap ferromagnetic In(1-x)Mn(x)Sb semiconductor alloy was successfully grown by low-temperature molecular beam epitaxy on CdTe/GaAs hybrid substrates. Ferromagnetic order in In(1-x)Mn(x)Sb was unambiguously established by the observation of clear hysteresis loops both in direct magnetization measurements and in the anomalous Hall effect, with Curie temperatures T_C ranging up to 8.5 K. The observed values of T_C agree well with the existing models of carrier-induced ferromagnetism.
We carefully investigated the ferromagnetic coupling in the as-grown and annealed ferromagnetic semiconductor GaMnAs/AlGaMnAs bilayer devices. We observed that the magnetic interaction between the two layers strongly affects the magnetoresistance of the GaMnAs layer with applying out of plane magnetic field. After low temperature annealing, the magnetic easy axis of the AlGaMnAs layer switches from out of plane into in-plane and the interlayer coupling efficiency is reduced from up to 0.6 to less than 0.4. However, the magnetic coupling penetration depth for the annealed device is twice that o…
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 x 10^18 c…
Magneto-optical properties of ZnO:Co and ZnO:Ni films were measured. Magnetization measurements show that some of the films are paramagnetic and others are ferromagnetic. Magnetic circular dichroism clarified that Zn1-xCoxO and Zn1-xNixO included in samples are paramagnetic diluted magnetic semiconductors. Ferromagnetic precipitations seem to be responsible for the observed ferromagnetic behaviors. Criteria to judge the ferromagnetic DMS are also discussed.
Spin injection using ferromagnetic semiconductors at room temperature is a building block for the realization of spin-functional semiconductor devices. Nevertheless, this has been very challenging due to the lack of reliable room-temperature ferromagnetism in well-known group IV and III-V based semiconductors. Here, we demonstrate room-temperature spin injection by using spin pumping in a (Ga,Fe)Sb / BiSb heterostructure, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature (TC) and BiSb is a topological insulator (TI). Despite the very small magnetization of (Ga,…
The resistivity, temperature, and magnetic field dependence of the anomalous Hall effect in a series of metallic Ga1-xMnxAs thin films with 0.015=<x=<0.08 is presented. A quadratic dependence of the anomalous Hall resistance on the resistivity is observed, with a magnitude which is in agreement with Berry phase theories of the anomalous Hall effect in dilute magnetic semiconductors.
We report the annealing-induced enhancement of ferromagnetism and nano-particle formation in group-IV-based ferromagnetic-semiconductor GeFe. We successfully increase the Curie temperature of the Ge0.895Fe0.105 film up to ~220 K while keeping a single ferromagnetic phase when the annealing temperature is lower than 500°C. In contrast, when annealed at 600°C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high Curie temperature nearly up to room temperature, are formed in the film. Our results show that annealing is quite effective to improve the magnetic proper…
We describe a systematic study of the exchange coupling between a magnetically hard metallic ferromagnet (MnAs) and a magnetically soft ferromagnetic semiconductor (GaMnAs) in bilayer and trilayer heterostructures. An exchange spring model of MnAs/GaMnAs bilayers accounts for the variation of the exchange bias field with layer thickness and composition. We also present evidence for hole-mediated interlayer exchange coupling in MnAs/p-GaAs/\GaMnAs trilayers and study the dependence of the exchange bias field on the thickness of the spacer layer.
Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. We show that the uniaxial $A^{1+2x}$Ti$^{4+}$$_{1-x}$O$_3$ (ATO), $A=$Ni$^{2+}$,Co$^{2+}$ and $0.6<x \leq 1$, thin films are electrically insulating ferromagnets already at room-temperature. The octahedra network of the ATO and ilmenite structures are similar yet different octahedra-filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra c…
We demonstrate the exchange coupling of a ferromagnetic semiconductor (Ga1-xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet (T_B = 48 +- 2 K) and the Curie temperature of the ferromagnet (T_C = 55.1 +- 0.2 K) are comparable. The resulting exchange bias manifests itself as a clear shift in the magnetization hysteresis loop when the bilayer is cooled in the presence of an applied magnetic field and an enhancement of the coercive field.
