Photomaganetization in diluted magnetic semiconductors

Starting from a many–body Hamiltonian for a system of photogenerated electrons and holes, spin-split by magnetic ions in diluted magnetic semiconductors, we derive, presumably for the first time, an expression for the photomagnetization as a function of the photon power, frequency, excitonic interaction and the magnetic ion concentration. Damping of nonequilibrium carriers and spin excitons is considered phenomenologically. Our results agree qualitatively with some of the systematics of the photomagnetization observed in Hg _1?x Mn _x Te.     

THz spectroscopy of diluted magnetic semiconductors and semiconductor quantum structures in ultrahigh magnetic fields

Magneto-absorption spectra in ferromagnetic semiconductor In_1−xMn_xAs films and self-organized PbSe/PbEuTe quantum dot superlattices have been studied in the terahertz range at very high magnetic fields up to 500 T. Both heavy hole (HH) and light hole (LH) cyclotron resonance (CR) have been observed in bulk In_1−xMn_xAs thin films with different Mn concentrations. The detailed Landau level calculation in terms of the effective mass approximation well explained the CR peak positions, line shapes and the dependence of the circular polarization of the incident light on the CR spectra. In InMnAs/GaSb heterostructures that have higher ferromagnetic transition temperature (T_c) than the bulk samples, the observed HH and LH cyclotron masses are larger than that in the bulk thin films. We found that the CR peak position and its line shape suddenly change in the vicinity of the ferromagnetic transition temperature, suggesting the change in the electronic structure due to the ferromagnetic transition. Electron CR in PbSe/PbEuTe quantum dots has been observed and it was found that the effective mass of the electrons is considerably modified by the quantum confinement potential and the lattice strain around the dots. A large wavelength dependence of the absorption intensity was observed due to the interference effect of the radiation inside the sample.     

Ⅱ—Ⅵ族稀磁半导体ZnSe／Zn1—xMnxSe超晶格中的应力效应

本文通过Ⅱ－Ⅵ族稀磁半导体超晶格ＺｎＳｅ／Ｚｎ１－ｘＭｎｘＳｅ的光致发光谱的测量，对其应力效应进行了讨论。样品的组分ｘ＝０．２，０．３，０．４，测量温度为Ｔ＝１１￣３００Ｋ。结果表明：由于应力效应，ＺｎＳｅ／Ｚｎ１－ｘＭｎｘＳｅ超晶格中的激子能量随ｘ值增加而发生红移。在相同组分下，不同阱、垒宽度比使应力的分布产生明显变化，从而影响超晶格中激子能量。实验与理论计算结果相一致。超晶格中光致发光峰随温度     

Ferromagnetic clusters induced by a nonmagnetic random disorder in diluted magnetic semiconductors

In this work, we analyze the nonmagnetic random disorder leading to a formation of ferromagnetic clusters in diluted magnetic semiconductors. The nonmagnetic random disorder arises from randomness in the host lattice. Including the disorder to the Kondo lattice model with random distribution of magnetic dopants, the ferromagnetic–paramagnetic transition in the system is investigated in the framework of dynamical mean-field theory. At a certain low temperature one finds a fraction of ferromagnetic sites transiting to the paramagnetic state. Enlarging the nonmagnetic random disorder strength, the paramagnetic regimes expand resulting in the formation of the ferromagnetic clusters.     

稀磁半导体的制备与性质

报道了稀磁半导体的制备、性质与实验研究进展，介绍了稀磁半导体的应用及发展前景．     

Spin-polarized structural, electronic and magnetic properties of diluted magnetic semiconductors Cd1−xMnxTe in zinc blende phase

We have investigated the structural, electronic and magnetic properties of the diluted magnetic semiconductor (DMS) Cd_1−xMn_xTe (for x=0.75 and 1.0) in the zinc blende (B3) phase by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the frame work of spin-polarized density functional theory (SP-DFT). The electronic exchange-correlation energy is described by generalized gradient approximation (GGA). We have calculated the lattice parameters, bulk modulii and the first pressure derivatives of the bulk modulii, spin-polarized band structures, and total and local densities of states. We estimated the spin-exchange splitting energies Δ_x(d) and Δ_x(pd) produced by the Mn3d states, and we found that the effective potential for the minority spin is more attractive than that of the majority spin. We determine the s-d exchange constant N₀α (conduction band) and p-d exchange constant N₀β (valence band) and these somewhat agree with a typical magneto-optical experiment. The value of calculated magnetic moment per Mn impurity atom is found to be 4.08 μ_B for Cd_0.25Mn_0.75Te and 4.09 μ_B for Cd_0.0Mn_1.0Te. Moreover, we found that p-d hybridization reduces the local magnetic moment of Mn from its free space charge value of 5.0 μ_B and produces small local magnetic moments on the nonmagnetic Cd and Te sites.     

Unraveling the nature of carrier-mediated ferromagnetism in diluted magnetic semiconductors

After more than a decade of intensive research in the field of diluted magnetic semiconductors (DMS), the nature and origin of ferromagnetism, especially in III–V compounds, is still controversial. Many questions and open issues are under intensive debates. Why after so many years of investigations, Mn-doped GaAs remains the candidate with the highest Curie temperature among the broad family of III–V materials doped with transition metal (TM) impurities? How can one understand that these temperatures are almost two orders of magnitude larger than that of hole-doped (Zn,Mn)Te or (Cd,Mn)Se? Is there any intrinsic limitation or is there any hope to reach room-temperature ferromagnetism in the dilute regime? How can one explain the proximity of (Ga,Mn)As to the metal–insulator transition and the change from Ruderman–Kittel–Kasuya–Yosida (RKKY) couplings in II–VI compounds to double-exchange type in (Ga,Mn)N? In spite of the great success of density functional theory-based studies to provide accurately the critical temperatures in various compounds, till very lately a theory that provides a coherent picture and understanding of the underlying physics was still missing. Recently, within a minimal model, it has been possible to show that among the physical parameters, the key one is the position of the TM acceptor level. By tuning the value of that parameter, one is able to explain quantitatively both magnetic and transport properties in a broad family of DMS. We will see that this minimal model explains in particular the RKKY nature of the exchange in (Zn,Mn)Te/(Cd,Mn)Te and the double exchange type in (Ga,Mn)N and simultaneously the reason why (Ga,Mn)As exhibits the highest critical temperature among both II–VI and III–V DMS's.     

Magneto-optical properties and exciton dynamics in diluted magnetic semiconductor nanostructures

Nanostructures of diluted magnetic semiconductors were fabricated to study novel magneto-optical properties that are derived from quantum confined band electrons interacting with magnetic ions. Quantum dots (QDs) of Cd_0.97Mn_0.03Se were grown by the self-organization on a ZnSe substrate layer. QDs of Zn_0.69Cd_0.23Mn_0.08Se and quantum wires (QWRs) of Cd_0.92Mn_0.08Se and Zn_0.69Cd_0.23Mn_0.08Se were fabricated by the electron beam lithography. A single quantum well (QW) of ZnTe/Zn_0.97Mn_0.03Te and double QWs of Cd_0.95Mn_0.05Te–CdTe were grown by molecular beam epitaxy. Magneto-optical properties and the formation and relaxation dynamics of excitons were investigated by ultrafast time-resolved photoluminescence (PL) spectroscopy. Excitons in these nanostructures were affected by the low-dimensional confinement effects and the interaction with magnetic ion spins. The exciton luminescence of the Cd_0.97Mn_0.03Se QDs shows the confined exciton energy due to the dot size of 4–6 nm and also shows marked increase of the exciton lifetime with increasing the magnetic field. The QDs of Zn_0.69Cd_0.23Mn_0.08Se fabricated by the electron beam lithography display narrow exciton PL spectrum due to the uniform shape of the dots. The exciton luminescence from the QWRs of Cd_0.92Mn_0.08Se and Zn_0.69Cd_0.23Mn_0.08Se shows the influence of the one-dimensional confinement effect for the exciton energy and the luminescence is linearly polarized parallel to the wire direction. The transient PL from the ZnTe/Zn_0.97Mn_0.03Te QWs displays, by the magnetic field, the level crossing of the exciton spin states of the nonmagnetic and magnetic layers and the spatial spin separation for the excitons. Cd_0.95Mn_0.05Te–CdTe double QWs show the injection of the spin polarized excitons from the magnetic well to the nonmagnetic QW.     

Magnetic properties of Mn-doped ZnO diluted magnetic semiconductors

A series of Mn-doped ZnO films have been prepared in different sputtering plasmas by using the inductively coupled plasma enhanced physical vapour deposition. The films show paramagnetic behaviour when they are deposited in an argon plasma. The Hall measurement indicates that ferromagnetism cannot be realized by increasing the electron concentration. However, the room-temperature ferromagnetism is obtained when the films are deposited in a mixed argon-nitrogen plasma. The first-principles calculations reveal that antiferromagnetic ordering is favoured in the case of the substitution of Mn^2＋ for Zn^2＋ without additional acceptor doping. The substitution of N for O （NO^-） is necessary to induce ferromagnetic couplings in the Zn-Mn-O system. The hybridization between N 2p and Mn 3d provides an empty orbit around the Fermi level. The hopping of Mn 3d electrons through the empty orbit can induce the ferromagnetic coupling. The ferromagnetism in the N-doped Zn-Mn-O system possibly originates from the charge transfer between Mn^2＋ and Mn^3＋ via NO^-, The key factor is the empty orbit provided by substituting N for O, rather than the conductivity type or the carrier concentration.     

Origins of ferromagnetism in transition metal doped diamond

Using first-principles calculations, we investigated the electronic and magnetic properties of Mn-doped, Fe-doped, and Co-doped diamond. It is found that the Mn-, Fe-, and Co-doped diamond are stabilized in ferromagnetic configurations. The origins of the magnetic ordering are explained successfully by the phenomenological band coupling model based on the p–d and d–d level repulsions between the dopant ions and host elements. According to Heisenberg model, high Curie temperature may be expected for Mn-, Fe-, and Co-doped diamond if there are no native defects or other impurities.     

Magnetic behavior of nanocrystalline powders of Co-doped ZnO diluted magnetic semiconductors synthesized by polymerizable precursor method

This paper reports the first synthesis of nanocrystalline powders of Co-doped ZnO (i.e. Zn_0.9Co_0.1O) diluted magnetic semiconductor by a polymerizable precursor method using nitrate salts of Zn and Co and a mixed solution of citric acid and ethylene glycol as a chelating agent and reaction medium, respectively. The polymeric precursors were characterized by TG-DTA to determine the thermal decomposition and crystallization temperature which was found to be at 723 K. The precursors were calcined at different temperatures of 773, 873, 973, and 1073 K for 1 h to obtain nanocrystalline powders. The morphology and crystalline size of the calcined particles were evaluated by SEM, TEM and Scherrer's equation. The average particle sizes calcined at 773, 873, 973, and 1073 K for 1 h were, respectively, 20, 60, 80, 150 nm, obtained from TEM. The XRD and Fourier transmission infrared (FT-IR) results indicated that the synthesized Zn_0.9Co_0.1O powders have the pure wurtzite structure without any significant change in the structure affected by Co substitution. Optical absorption measurements showed absorption bands indicating the presence of Co²⁺ in substitution of Zn²⁺. Room temperature magnetization results revealed a ferromagnetic behavior for the Zn_0.9Co_0.1O powders. Although the specific magnetization seemed to change with the particle size but there was no clear dependency since the largest magnetization was observed in the powders calcined at 873 K (60 nm). Instead, the specific magnetization appeared to show a trend of dependency on the lattice constant c of the wurtzite unit cell.     

Coherently photoinduced ferromagnetism in diluted magnetic semiconductors

Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-band-gap laser radiation. The mechanism for photoinduced ferromagnetism is coherence between conduction and valence bands induced by the light which leads to an optical exchange interaction. The ferromagnetic critical temperature T(C) depends both on the properties of the material and on the frequency and intensity of the laser and could be above 1K.     

Electronic structure of p-type (Ga,Fe)N diluted magnetic semiconductors

By ab-initio calculation we show that the (Ga,Fe)N ground state may be changed from anti-ferromagnetic to ferromagnetic by acceptor defect like Ga vacancies. The electronic structures are calculated by using the Korringa-Kohn-Rostoker (KKR) method combined with coherent potential approximation (CPA). We show that we can increase the magnetic moment of Fe in p-type GaN by oxygen co-doping. Mechanism of exchange interactions between magnetic ions in p-type (Ga,Fe)N is also studied. The effect of external magnetic field on the electronic structure of (Ga, Fe)N and p-type (Ga, Fe)N is investigated.     

Room-temperature ferromagnetism of the amorphous Cu-doped ZnO thin films

Amorphous copper-doped ZnO thin films (ZnO:Cu) prepared on glass substrates by the radio-frequency magnetron co-sputtering have been investigated. Magnetic measurements indicated that the amorphous ZnO:Cu thin films were ferromagnetic at room temperature and the saturation magnetization was much higher than that of the polycrystalline films. X-ray diffraction results showed there was no Cu₂O phase in amorphous ZnO:Cu films, which might be the reason for the high magnetic moment of the films. On the other hand, the high saturation magnetization of the amorphous ZnO:Cu films could also attribute to that there was no limit of solid solubility of Cu in amorphous ZnO solvent. The X-ray photoelectron spectroscopy study of the amorphous ZnO:Cu thin films reveal that copper was in Cu²⁺ chemical state.     

First-principles study of structural, electronic and magnetic properties in Cd1−xFexS diluted magnetic semiconductors

In this paper, we report theoretical investigations of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Cd_1−xFe_xS with x=0.25, 0.5 and 0.75 in zinc blende (B3) phase using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the density functional theory and the generalized gradient approximation. The analysis of band structures, density of states, total energy, exchange interactions and magnetic moments reveals that both the alloys may exhibit a half-metallic ferromagnetism character. The value of calculated magnetic moment per Fe impurity atom is found to be 4 μ_B. Moreover, we found that p-d hybridization reduces the local magnetic moment of Fe from its free space charge value of 4 μ_B and produces small local magnetic moments on Cd and S sites.     

Synthesis and structural,magnetic characterization of Zn(Mn)O diluted magnetic semiconductor

The bulk samples with nominal composition Zn_1−x Mn_x O [x = 5% and 7%] were synthesized at 930 °C by Standard Solid State Reaction method. The structural analysis reveals the single phase nature. The Topography study indicates the distribution of the particles. Magnetic property was affirmed by Vibrating Sample Magnetometer, Zn_1−x Mn_x O (with x = 5%), low concentration of dopant shows good ferromagnetism compared to high concentration in Zn_1−x Mn_x O (with x = 7%).     

H-impurity induced high-temperature ferromagnetism in Co-doped ZnO

Through first-principles total-energy calculations, the effect of H-impurity on the magnetic properties of Co-doped ZnO is studied. Instead of an antibonding location, a bond-centered location of Co-O is the most stable location for isolated H in Co-doped ZnO with a strong bond with oxygen which results in the Co neighbor displaced from the host site to form a Co dimer with the other Co. At the most stable position, due to the strong hybridization between the H-impurity states and the Co 3d-t_2g minority spin states at the Fermi level in the gap, H-impurity can mediate a strong short-ranged and long-ranged ferromagnetic spin-spin interaction between neighboring Co atoms. Results based on first-principles total-energy calculations show that H-impurity is a very effective agent that can make Co-doped ZnO process high-temperature ferromagnetism.     

Effects of disorder on ferromagnetism in diluted magnetic semiconductors

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. Within our mean-field approximation, disorder enhances the ferromagnetic transition temperature 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.     

Novel phase transitional properties on specific heat of diluted magnetic semiconductor

Three types of phase transitions in diluted magnetic semiconductor, first-order, second-order and mixed-order, are found in theory. Especially the mixed-type transition shows two-steps transition and novel specific heat property. Specific heat properties disclose a possible meta ferromagnetic phase confirmed by the experimental qualitative result.     

Plasma-assisted molecular beam epitaxy of wurtzite GaMnN displaying ferromagnetism assessed by means of X-ray magnetic circular dichroism

We report on the optimization of the growth conditions of wurtzite GaMnN grown by plasma-assisted molecular beam epitaxy in order to obtain intrinsic ferromagnetic behavior. By growing with a Ga/N ratio lower than unity and by introduction of periodic growth interruptions, we succeed in synthesizing single-phase GaMnN epilayers containing up to 6.3 at.% of Mn. The structural quality of the GaMnN epilayer and the absence of secondary phases are demonstrated by means of X-ray diffraction experiments and X-ray linear dichroism measurements performed at the Mn and Ga K-edges. The intrinsic ferromagnetism for 6.3 at.% of Mn in our GaMnN epilayer is demonstrated by means of magnetization measurements in a 5 T Quantum Design superconducting quantum interference device (SQUID) and X-ray magnetic circular dichroism investigations performed at the K-edge of Mn. The Curie temperature thus determined is equal to 8 K and a spontaneous magnetic moment of 2.4μ_B per Mn atom is found at 2 K.