The comprehensive search for multifunctional materials has resulted in the discovery of semiconductors and oxides showing ferromagnetic features persisting to room temperature. In this tutorial review the methods of synthesis of these materials, as well as the application of element-specific nano-analytic tools, particularly involving synchrotron radiation and electron microscopy, are described and shown to reveal the presence of nano-scale phase separations. Various means to control the aggregation of magnetic cations are discussed together with the mechanisms accounting for ferromagnetism of either condensed or diluted magnetic semiconductors. Finally, the question of whether high temperature ferromagnetism is possible in semiconductors not containing magnetic ions is touched upon.
A close link between the charge transfer electronic structures and polarity dependent high-TC ferromagnetism of TM2+:ZnO DMSs (TM2+ = 3d ions) is demonstrated. Trends in ferromagnetism across the 3d series of TM2+:ZnO DMSs predicted from their charge transfer energies reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.
Magnetic properties of the planar structure consisting of a ferromagnetic metal and the diluted magnetic semiconductor are considered (by the example of the structure Fe/Ga(Mn)As, experimentally studied in [F. Maccherozzi, e.a., Phys. Rev. Lett., 101, 267201 (2008)]). In the framework of the mean field theory, we demonstrate the presence of the significant amplification of the ferromagnetism, induced by the ferromagnetic metal in the near-interface semiconductor area, due to the indirect interaction of magnetic impurities. This results in the substantial expansion of the temperature range wher…
The origin of ferromagnetism in semimagnetic III-V materials is discussed. The indirect exchange interaction caused by virtual electron excitations from magnetic impurity level in the bandgap to the valence band can explain ferromagnetism in GaAs(Mn) no matter samples are degenerated or not. Formation of ferromagnetic clusters and percolation picture of phase transition describes well all available experimental data and allows to predict the Mn-composition dependence of transition temperature in wurtzite (Ga,In,Al)N epitaxial layers.
High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high temperature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V_N). Calculated results are in agreement with experimental observations and facts of real Cr-do…
The author reviews the present understanding of the hole-mediated ferromagnetism in magnetically doped semiconductors and oxides as well as the origin of high temperature ferromagnetism in materials containing no valence band holes. It is argued that in these systems spinodal decomposition into regions with a large and a small concentration of magnetic component takes place. This self-organized assembling of magnetic nanocrystals can be controlled by co-doping and growth conditions. Functionalities of these multicomponent systems are described together with prospects for their applications in …
Guided by the internal-reference rule and the known band offsets in III-V and II-VI diluted magnetic semiconductors, we discuss the feasibility of obtaining p-type conductivity, required for the carrier-induced ferromagnetism, as well as the cases for which the doping by shallow impurities may lead to the ferromagnetism driven by the double exchange. We consider the dependence of kinetic exchange on the p-d hybridization, on the electronic configurations of the magnetic ions, and on the energies of the charge transfer between the valence band of host materials and the magnetic ions. In the cas…
Creating magnetic semiconductors that work at warm circumstance is still a great challenge in the physical sciences. Here, we report the discovery of ferromagnetism in the metal-free perylene diimide semiconductor, whose Curie temperature is higher than 400 Kelvin. A solvothermal approach is used to reduce and dissolve the rigid-backbone perylene diimide crystallites, and radical anion aggregates were fabricated by the subsequent self-assembly and oxidation process. Magnetic measurements exhibit the ferromagnetic ordering with the saturated magnetization of 0.48 $μ_{\rm B}$ per molecule and th…
II-oxide and III-nitride semiconductors doped by nonmagnetic 2p light elements are investigated as potential dilute magnetic semiconductors (DMS). Based on our first-principle calculations, nitrogen doped ZnO, carbon doped ZnO, and carbon doped AlN are predicted to be ferromagnetic. The ferromagnetism of such DMS materials can be attributed to a p-d exchange-like p-p coupling interaction which is derived from the similar symmetry and wave function between the impurity (p-like t_2) and valence (p) states. We also propose a co-doping mechanism, using beryllium and nitrogen as dopants in ZnO, to …
The high TC ferromagnetism in (Ga,Mn)N were observed and almost all results are approximately similar to the experimental results in (Ga,Mn)As except the value of TC. Though all standard experiments on magnetism clearly support the results, the value is unexpectedly high. This work present and discuss the possibility of high TC ferromagnetism, after brief review of the experimental results. The key speculation to Bosonization method in three dimensions is resembled with the problems in Anderson localization.
Motivated by recent measurements of electron paramagnetic resonance (EPR) spectra in modulation-doped CdMnTe quantum wells, [F.J. Teran {\it et al.}, Phys. Rev. Lett. {\bf 91}, 077201 (2003)], we develop a theory of collective spin excitations in quasi-two-dimensional diluted magnetic semiconductors (DMSs). Our theory explains the anomalously large Knight shift found in these experiments as a consequence of collective coupling between Mn-ion local moments and itinerant-electron spins. We use this theory to discuss the physics of ferromagnetism in (II,Mn)VI quantum wells, and to speculate on th…
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 (ΔTc/Tc = 55%). Our calculation using the mean-field Zener theory reveals an unexpectedly large s-d exchange interaction in (In,Fe)As. Our results establish an effective way to control the ferromagnetism in quantum heterostructures of n-type FMSs, as well as require reconsideration on t…
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…
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…
Considering a general situation where a semiconductor is doped by magnetic impurities leading to a carrier-induced ferromagnetic exchange coupling between the impurity moments, we show theoretically the possible generic existence of three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3) between two ferromagnetic phases, arises from a subtle competition between indirect exchange induced by thermally activated carriers in a…
The extensive experimental and computational search for multifunctional materials has resulted in the development of semiconductor and oxide systems, such as (Ga,Mn)N, (Zn,Cr)Te, and HfO2, which exhibit surprisingly stable ferromagnetic signatures despite having a small or nominally zero concentration of magnetic elements. Here, we show that the ferromagnetism of (Zn,Cr)Te, and the associated magnetooptical and magnetotransport functionalities, are dominated by the formation of Cr-rich (Zn,Cr)Te metallic nanocrystals embedded in the Cr-poor (Zn,Cr)Te matrix. Importantly, the formation of these…
We show that doped cubic iron pyrite, which is a diamagnetic semiconductor, becomes ferromagnetic when $p$-type doped. We furthermore find that this material can exhibit high spin polarization both for tunneling and transport devices. These results are based on first principles electronic structure and transport calculations. This illustrates the use of $p$-type doping without magnetic impurities as a strategy for obtaining ferromagnetic semiconducting behavior, with implications for spintronic applications that require both magnetic ordering and good mobility. This is a combination that has b…
Clear evidence of ferromagnetic behavior at temperatures >400 K as well as spin polarization of the charge carriers have been observed in ZnMnO thin films grown on Al2O3 and MgO substrates. The magnetic properties depended on the exact Mn concentration and the growth parameters. In well-characterized single-phase films, the magnetic moment is 4.8?B/Mn at 350 K, the highest moment yet reported for any Mn doped magnetic semiconductor. Anomalous Hall effect shows that the charge carriers (electrons) are spin polarized and participate in the observed ferromagnetic behavior.
Influence of disorder on the ferromagnetic phase transition in diluted (III,Mn)V semiconductors is investigated analytically. The regime of small disorder is addressed, and the enhancement of the critical temperature by disorder is found both in the mean field approximation and from the analysis of the zero temperature spin stiffness. Due to disorder, the spin wave fluctuations around the ferromagnetically ordered state acquire a finite mass. At large charge carrier band width, the spin wave mass squared becomes negative, signaling the breakdown of the ferromagnetic ground state and the onset …
In recent experiments, high Curie temperatures Tc above room temperature were reported in ferromagnetic semiconductors Fe-doped GaSb and InSb, while low Tc between 20 K to 90 K were observed in some other semiconductors with the same crystal structure, including Fe-doped InAs and Mn-doped GaSb, InSb, and InAs. Here we study systematically the origin of high temperature ferromagnetism in Fe, Mn, Cr-doped GaSb, InSb, and InAs magnetic semiconductors by combining the methods of density functional theory and quantum Monte Carlo. In the diluted impurity limit, the calculations show that the impurit…
(Ga,Mn)As is a paradigm diluted magnetic semiconductor which shows ferromagnetism induced by doped hole carriers. With a few controversial models emerged from numerous experimental and theoretical studies, the mechanism of the ferromagnetism in (Ga,Mn)As still remains a puzzling enigma. In this Letter, we use soft x-ray angle-resolved photoemission spectroscopy to positively identify the ferromagnetic Mn 3d-derived impurity band in (Ga,Mn)As. The band appears hybridized with the light-hole band of the host GaAs. These findings conclude the picture of the valence band structure of (Ga,Mn)As dis…
A mean field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account: (i) strong spin-orbit and kp couplings in the valence band; (ii) the effect of strain upon the hole density-of-states, and (iii) the influence of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that theory describes the values of the Curie temperatures observed in…
Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. Firs…
It is suggested that p-d hybridization contributes significantly to the hole binding energy E_b of Mn acceptors in III-V compounds, leading in an extreme case to the formation of Zhang-Rice-like small magnetic polarons. The model explains both strong increase of E_b and evolution of Mn spin-resonance spectrum with the magnitude of valence-band offsets. For such a structure of Mn impurity in III-V materials, possible models accounting for the recently determined Curie temperature of about 940 K in a compensated Ga(0.91)Mn(0.09)N are discussed.
We have synthesized ferromagnetic InMnP, a member of III-Mn-V ferromagnetic semiconductor family, by Mn ion implantation and pulsed laser annealing. Clear ferromagnetic hysteresis loops and a perpendicular magnetic anisotropy are observed up to a Curie temperature of 42 K. Large values of negative magnetoresistance and magnetic circular dichroism as well as anomalous Hall effect are further evidences of a ferromagnetic order in InMnP. An effort is made to understand the transport mechanism in InMnP using the theoretical models. We find that the valence band of InP does not merge with the impur…
Low-dimensional multiferroicity, though highly scarce in nature, has attracted great attention due to both fundamental and technological interests. Using first-principles density functional theory, we show that ferromagnetism and ferroelectricity can coexist in monolayer transition metal phosphorus chalcogenides (TMPCs) - CuMP$_2$X$_6$ (M=Cr, V; X=S, Se). These van der Waals layered materials represent a class of 2D multiferroic semiconductors that simultaneously possess ferroelectric and ferromagnetic orders. In these monolayer materials, Cu atoms spontaneously move away from the center atomi…
The (In,Fe)Sb layers with the Fe content up to 13 at. % have been grown on (001) GaAs substrates using the pulsed laser deposition. The TEM investigations show that the (In,Fe)Sb layers are epitaxial and free of the inclusions of a second phase. The observation of the hysteretic magnetoresistance curves at temperatures up to 300 K reveals that the Curie point is above room temperature. The resonant character of magnetic circular dichroism confirms the intrinsic ferromagnetism in the (In,Fe)Sb layers. We suggest that the ferromagnetism of the (In,Fe)Sb matrix is not carrier-mediated and apparen…
In a recent Letter, Berciu and Bhatt have presented a mean-field theory of ferromagnetism in III-V semiconductors doped with manganese, starting from an impurity band model. We show that this approach gives an unphysically broad impurity band and is thus not appropriate for (Ga,Mn)As containing 1-5% Mn. We also point out a microscopically unmotivated sign change in the overlap integrals in the Letter. Without this sign change, stable ferromagnetism is not obtained.
A ferromagnetic semiconductor, rutile (Ti,Co)O2, exhibits anomalous Hall effect at room temperature. Strong dependence of the anomalous Hall effect, as well as the magneto-optic response, on the carrier concentration suggests carrier induced ferromagnetism in this compound. Both ferromagnetic responses are caused by the charge carriers at the band edge of host semiconductor, indicating possibilities of spintronics devices operable at room temperature.
Diluted magnetic semiconductors (DMSs), in which magnetic elements are substituted for a small fraction of host elements in a semiconductor lattice, can become ferromagnetic when doped. In this article we discuss the physics of DMS ferromagnetism in systems with semiconductor heterojunctions. We focus on the mechanism that cause magnetic and magnetoresistive properties to depend on doping profiles, defect distributions, gate voltage, and other system parameters that can in principle be engineered to yield desired results.
We propose that localized defects in magnetic semiconductors act as deep impurities and can be described by the Anderson model. Within this model, hybridization of d-orbitals and p-orbitals gives rise to a non-RKKY indirect exchange mechanism, when the localized d-electrons are exchanged through both conduction and valence bands. For semiconductors with indirect band gap the non-RKKY part of exchange integral is antiferromagnetic, which suppresses ferromagnetism. In case of direct band gap, this exchange mechanism can, under certain conditions, lead to enhancement of ferromagnetism. The indire…
In1-xMnxAs diluted magnetic semiconductor (DMS) thin films have been grown using metalorganic vapor phase epitaxy (MOVPE). Tricarbonyl(methylcyclopentadienyl)manganese was used as the Mn source. Nominally single-phase, epitaxial films were achieved with Mn content as high as x=0.14 using growth temperatures Tg>475 C. For lower growth temperatures and higher Mn concentrations, nanometer scale MnAs precipitates were detected within the In1-xMnxAs matrix. Magnetic properties of the films were investigated using a superconducting quantum interference device (SQUID) magnetometer. Room-temperature f…
Magnetic oxide semiconductors, oxide semiconductors doped with transition metal elements, are one of the candidates for a high Curie temperature ferromagnetic semiconductor that is important to realize semiconductor spintronics at room temperature. We review in this paper recent progress of researches on various magnetic oxide semiconductors. The magnetization, magneto-optical effect, and magneto-transport such as anomalous Hall effect are examined from viewpoint of feasibility to evaluate the ferromagnetism. The ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.
Using first-principles total-energy methods, we investigate Jahn-Teller distortions in III-V dilute magnetic semiconductors, GaAs:Mn and GaN:Mn in the cubic zinc blende structure. The results for an isolated Mn impurity on a Ga site show that there is no appreciable effect in GaAs, whereas, in GaN there is a Jahn-Teller effect in which the symmetry around the impurity changes from T$_{d}$ to D$_{2d}$ or to C$_{2v}$. The large effect in GaN occurs because of the localized d$^4$ character, which is further enhanced by the distortion. The lower symmetry should be detectable experimentally in cubi…
Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been studied using i) a single-impurity based generalized RKKY approach which goes beyond linear response theory, and ii) a mean-field-plus-spin-fluctuation (MF+SF) approach within a (purely fermionic) Hubbard-model representation of the magnetic impurities, which incorporates dynamical effects associated with finite frequency spin correlations in the ordered state. Due to a competition between the magnitude of the carrier spin polarization and its oscillation length scale, the ferromagnetic spin coupling is found to be opt…
Recent progress in understanding and controlling spintronic properties of (Ga,Mn)As and related compounds is contrasted to diverging experimental and theoretical results concerning the origin of high temperature ferromagnetism discovered in an ample class of magnetically doped wide-band-gap semiconductors.
Applying the dynamical coherent potential approximation to a simple model, we have systematically studied the carrier states in $A_{1-x}$Mn$_xB$-type diluted magnetic semiconductors (DMS's). The model calculation was performed for three typical cases of DMS's: The cases with strong and moderate exchange interactions in the absence of nonmagnetic potentials, and the case with strong attractive nonmagnetic potentials in addition to moderate exchange interaction. When the exchange interaction is sufficiently strong, magnetic impurity bands split from the host band. Carriers in the magnetic impuri…
The influence of He+ ion irradiation on the transport and magnetic properties of epitaxial layers of a diluted magnetic semiconductor (DMS) (In,Fe)Sb, a two-phase (In,Fe)Sb composite and a nominally undoped InSb semiconductor has been investigated. In all layers, a conductivity type conversion from the initial n-type to the ptype has been found. The ion fluence at which the conversion occurs depends on the Fe concentration in the InSb matrix. Magnetotransport properties of the two-phase (In,Fe)Sb layer are strongly affected by ferromagnetic Fe inclusions. An influence of the number of electric…
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 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.
There are two competing views of itinerant ferromagnetism, the first viewing ferromagnetism as resulting from the indirect coupling between local moments via the itinerant carrier dynamics, the so-called RKKY mechanism, while in the alternative picture, ferromagnetism results from the spin polarization of itinerant carriers by the strong atomic Hund interaction - the so-called double exchange (DE) scenario. Which view describes the ferromagnetism in diluted magnetic semiconductors, materials with promise for spintronic applications, is still unclear. Here, we describe the detailed physical res…
To clarify whether or not (Ga,Cr)As is an intrinsic diluted magnetic semiconductor, a systematic study of the magnetic circular dichroism (MCD) was carried out for a series of (Ga,Cr)As epilayers grown by the low-temperature molecular-beam epitaxy technique. The present work provides unambiguous evidence for the intrinsic ferromagnetism of the (Ga,Cr)As epilayers with all the necessary properties in accordance with that of a diluted magnetic semiconductor, especially the hysteresis characteristics, which is opened up in the magnetic field dependence of MCD.
We report on a comprehensive combined experimental and theoretical study of Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad agreement between theoretical expectations and measured data allows us to conclude that T_c in high-quality metallic samples increases linearly with the number of uncompensated local moments on Mn_Ga acceptors, with no sign of saturation. Room temperature ferromagnetism is expected for a 10% concentration of these local moments. Our magnetotransport and magnetization data are consistnent with the picture in which Mn impurities incorporated during…
We predict a new mechanism of enhancement of ferromagnetic phase transition temperature $T_c$ in uniaxially stressed diluted magnetic semiconductors (DMS) of p-type. Our prediction is based on comparative studies of both Heisenberg (inherent to undistorted DMS with cubic lattice) and Ising (which can be applied to strongly enough stressed DMS) models in a random field approximation permitting to take into account the spatial inhomogeneity of spin-spin interaction. Our calculations of phase diagrams show that area of parameters for existence of DMS-ferromagnetism in Ising model is much larger t…
The concept of low-voltage depletion and accumulation of electron charge in semiconductors, utilized in field-effect transistors (FETs), is one of the cornerstones of current information processing technologies. Spintronics which is based on manipulating the collective state of electron spins in a ferromagnet provides complementary technologies for reading magnetic bits or for the solid-state memories. The integration of these two distinct areas of microelectronics in one physical element, with a potentially major impact on the power consumption and scalability of future devices, requires to f…
We report on infrared (IR) optical experiments on Bi$_2$Te$_3$ and Mn-doped Bi$_2$Te$_3$ epitaxial thin films. In the latter film, dilute Mn doping (4.5\%) of the topologically nontrivial semiconductor host results in time-reversal-symmetry-breaking ferromagnetic order below $T_C$=15 K. Our spectroscopic study shows both materials share the Bi$_2$Te$_3$ crystal structure, as well as classification as bulk degenerate semiconductors. Hence the Fermi energy is located in the Bi$_2$Te$_3$ conduction band in both materials, and furthermore, there is no need to invoke topological surface states to d…
Ferromagnetic semiconductors discovered in two-dimensional (2D) materials open an avenue for highly integrated and multifunctional spintronics. The Curie temperature (TC) of existed 2D ferromagnetic semiconductors is extremely low and the modulation effect of their magnetism is limited compared with their 2D metallic counterparts. The interfacial effect was found to effectively manipulate the three-dimensional magnetism, providing a unique opportunity for tailoring the 2D magnetism. Here we demonstrate that the TC of 2D ferromagnetic semiconductor Cr2Ge2Te6 can be enhanced by 130% (from ~65 K …
We investigate the effect of quantum confinement on the ferromagnetism of diluted magnetic semiconductor Ga$_{1-x}$Mn$_x$As using a combination of tight-binding and density functional methods. We observe strong majority-spin Mn $d$-As $p$ hybridization, as well as half metallic behavior, down to sizes as small as 20 Åin diameter. Below this critical size, the doped holes are self-trapped by the Mn-sites, signalling both valence and electronic transitions. Our results imply that magnetically doped III-V nanoparticles will provide a medium for manipulating the electronic structure of dilute magn…
We present results of a numerical mean field treatment of interacting spins and carriers in doped diluted magnetic semiconductors, which takes into account the positional disorder present in these alloy systems. Disorder is found to enhance the ferromagnetic transition temperature significantly for metallic densities not too far from the metal-insulator transition. Concurrently, the ferromagnetic phase is found to have very unusual temperature dependence of the magnetization as well as specific heat as a result of disorder. Unusual spin and charge transport is implied.
Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, we report the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe2 monolayer synthesized by chemical vapor deposition. Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~105 at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm…
Electronic structure calculations using quantum Monte Carlo (QMC) methods conclusively show that pure CaB$_6$ is a narrow-gap semiconductor with an $X$-point gap $\sim 1.3$ eV. This should put to rest controversies as to whether the compound is a semimetal or a semiconductor, as also theories suggesting it to be an excitonic insulator. Our extensive Hartree-Fock (HF) and density functional theory (DFT) calculations support the view that the hexaboride ferromagnetism can be induced by contamination with magnetic element atoms like Fe and that presence of La is not essential for this. La impurit…
Diluted magnetic semiconductors (DMSs) based on group-IV materials are desirable for spintronic devices compatible with current silicon technology. In this work, amorphous Mn-doped SiGe thin films were first fabricated on Ge substrates by radio frequency magnetron sputtering and then crystallized by rapid thermal annealing (RTA). After the RTA, the samples became ferromagnetic (FM) semiconductors, in which the Curie temperature increased with increasing Mn doping concentration and reached 280 K with 5% Mn concentration. The data suggest that the ferromagnetism came from the hole-mediated proce…
Magnetic semiconductors have attracted interest because of the question of how a magnetic metal can be derived from a paramagnetic insulator. Here our approach is to carrier dope insulating FeSi and we show that the magnetic half-metal which emerges has unprecedented optical properties, unlike those of other low carrier density magnetic metals. All traces of the semiconducting gap of FeSi are obliterated and the material is unique in being less reflective in the ferromagnetic than in the paramagnetic state, corresponding to larger rather than smaller electron scattering in the ordered phase.
For the prototype diluted ferromagnetic semiconductor (Ga,Mn)As, there is a fundamental concern about the electronic states near the Fermi level, i.e., whether the Fermi level resides in a well-separated impurity band derived from Mn doping (impurity-band model) or in the valence band that is already merged with the Mn-derived impurity band (valence-band model). We investigate this question by carefully shifting the Fermi level by means of carrier compensation. We use helium-ion implantation, a standard industry technology, to precisely compensate the hole doping of GaAs-based diluted ferromag…
We review pitfalls in recent efforts to make a conventional semiconductor, namely ZnO, ferromagnetic by means of doping with transition metal ions. Since the solubility of those elements is rather low, formation of secondary phases and the creation of defects upon low temperature processing can lead to unwanted magnetic effects. Among others, ion implantation is a method of doping, which is highly suited for the investigation of those effects. By focussing mainly on Fe, Co or Ni implanted ZnO single crystals we show that there are manifold sources for ferromagnetism in this material which can …
Over the last decade the search for compounds combining the resources of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by continual demonstrations of remarkable low-temperature functionalities found for ferromagnetic structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample observations of ferromagnetic signatures at high temperatures in a number of non-metallic systems. In this paper, recent experimental and theoretical developments are reviewed emphasising that, from the one hand, they disentang…
We have synthesized Ga1-xMnxAs1-yPy and Ga1-xMnxP1-yNy by the combination of ion implantation and pulsed-laser melting. We find that the incorporation of isovalent impurities with smaller atomic radii leads to a realignment of the magnetic easy axis in Ga1-xMnxP1-yNy/GaP and Ga1-xMnxAs1-yPy/GaAs thin films from in-plane to out-of-plane. This tensile-strain-induced magnetic anisotropy is reminiscent of that observed in Ga1-xMnxAs grown on larger lattice constant (In,Ga)As buffer layers indicating that the role of strain in determining magnetic anisotropy is fundamental to III-Mn-V materials. In…
