3d Impurities Research Articles

Magnetic properties of 3d impurities in GaAs

Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature T C in (GaMn)As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga (1− x − y )Mn x Cr y )As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.

Medium-ranged interactions of transition-metal (3d and 4d) impurity pairs in Al and atomic structures of Al-rich Al–transition-metal alloys

We give systematic ab initio calculations for the interaction energies (from first to eighth neighbors) of impurity pairs X–X (X = Sc–Zn, Y–Cd) in Al and discuss the micromechanism of the structural stability of Al-rich AlX alloys. The calculations are based on the generalized gradient approximation in the density-functional theory and employ the all-electron full-potential Korringa–Kohn–Rostoker Green's function method for point defects, which guarantees the correct embedding of the cluster of impurities and vacancies in an otherwise perfect crystal. We show: (1) the fundamental features of phase diagrams of these alloys, such as ordering and segregation, are understood by the sign of first-neighboring X–X interaction energies; (2) the structural stability of Al-rich AlX alloys such as L1 2 (Al 3Sc and Al 3Y), DO 22 (Al 3V and Al 3Nb), and Mackay icosahedron in AlMn quasicrystal, are understood qualitatively by using the medium-ranged and oscillating X–X interactions, due to the strong sp-d (Al–X) hybridization; (3) the strength and oscillating behavior of the medium-ranged X–X interaction energies are specified very well by the d-electron numbers of X impurities.

Ferromagnetism in epitaxial germanium and silicon layers supersaturated with managanese and iron impurities

The possibility of laser synthesis of diluted magnetic semiconductors based on germanium and silicon doped with manganese or iron up to 10–15 at % has been shown. According to data on the electronic levels of 3d atoms in semiconductors, Mn and Fe impurities are most preferable for realizing ferromagnetism in Ge and Si through the Ruderman-Kittel-Kasuya-Yosida mechanism. Epitaxial Ge and Si layers 50–110 nm in thickness were grown on gallium arsenide or sapphire single crystal substrates heated to 200–480°C. The content of a 3d impurity has been measured by x-ray spectroscopy. The ferromagnetism of layers and high magnetic and acceptor activities of Mn in Ge, as well as of Mn and Fe in Si, are manifested in the observation of the Kerr effect, anomalous Hall effect, high hole conductivity, and anisotropic ferromagnetic resonance at 77–500 K. According to the ferromagnetic resonance data, the Curie point of Ge:Mn and Si:Mn on a GaAs substrate and of Si:Fe on an Al2O3 substrate is no lower than 420, 500, and 77 K, respectively.

Practical rules for orbital-controlled ferromagnetism of 3d impurities in semiconductors

We distill from first-principles spin-polarized total-energy calculations some practical rules for predicting the magnetic state (ferromagnetic/antiferromagnetic/paramagnetic) of substitutional transition-metal impurity with different charge state in various host crystal groups IV, III-V, II-VI, I-III-VI2, and II-IV-V2 semiconductors. The basic mechanism is the stabilization of a ferromagnetic bond between two transition metals if the interacting orbitals are partially-occupied. These rules are then subjected to quantitative tests, which substantiate the mechanism of ferromagnetism in these systems. We discuss cases where current electronic structure calculations agree with these rules, and identify a few cases where conflicts exist. The effect of doping on transition-metal magnetic properties is also covered by these rules by considering the oxidation state changes due to doping. In addition, we systematically apply these rules to ideal substitutional impurities, contrasting our predictions with experiment. Discrepancies may be used to assess the role of various nonidealities such as presence of additional dopants, precipitates, clusters, or interstitial sites.

Suppression of ferromagnetism due to hole doping in Zn1−xCrxTe grown by molecular-beam epitaxy

Electric and magnetic properties were investigated on p-type Zn1−xCrxTe doped with nitrogen (N) as an acceptor. Thin films of p-Zn1−xCrxTe(x≦0.09) were grown by molecular-beam epitaxy with the supply of N2 gas excited by rf plasma. With the increase of Cr composition x at an almost fixed N concentration of the order of 1020cm−3, the temperature dependence of resistivity changed from metallic behavior to an insulating one, accompanied with a significant decrease of the hole concentration. The magnetization measurements revealed that ferromagnetic behaviors observed in undoped Zn1−xCrxTe were suppressed due to the nitrogen doping; with N concentrations of the order of 1020cm−3, hysteresis loops in the magnetization curve disappeared, the magnitude of magnetization decreased, and the ferromagnetic transition were not observed down to 2 K according to the Arrott plot analysis. These experimental findings are discussed on the basis of the ferromagnetic double exchange interaction which is considered to work on the Cr 3d impurity level formed in the band gap of ZnTe.

Surface segregation of transition metal impurities on the TiC(1 0 0) surface

The segregation energies of 3d (Sc–Cu), 4d (Y–Ag) and 5d (La–Au) transition metal impurities on the (1 0 0) surface of TiC have been obtained using first-principles electronic structure calculations. The results are in agreement with available experimental data and show that the difference in atomic size between the impurity and host species, as well as the difference in surface energies determines if the impurity will segregate towards the surface or not. The results indicate that the difference in size is the dominant factor for the trends in segregation of transition metal impurities towards the (1 0 0) surface of TiC.

First principles calculations of the local moments for 4d impurities in bcc and fcc iron

First-principles calculations of the local magnetic moments for 4d impurities in bcc and fcc iron hosts are performed. The molecular cluster discrete variational method (DVM) is used to obtain the electronic structure in spin-polarized calculations in the framework of the local approximation of the density-functional theory. An assessment of the effect of the cluster size on the calculated local moments is accomplished for the bcc matrix, for which the calculated results can be compared with available experimental data.

Size effects on the local magnetism and Kondo behavior of isolated Fe impurities in nanocrystalline metallic hosts

Local magnetism of 3d impurities in nonmagnetic metallic hosts is a subject of intense experimental and theoretical investigations. A key problem is the study of Kondo effect arising from antiferromagnetic exchange interaction between the impurity moment and host-conduction electrons leading to moment instability below a characteristic temperature TK. While extensive studies have been carried out for many d and f impurities in bulk metallic hosts 1,2 little information is available on the Kondo behavior of single magnetic impurities in nanocrystalline materials. Recently resistivity studies in thin films and nanowires of AusFed ,C usCrd, and CusFed alloys have revealed the suppression of Kondo interaction below a critical thickness and width. 3 The results, initially interpreted as being due to confinement of the Kondo screened electron cloud, were later shown to arise from size

Specific Features of the Absorption and Phase Velocity of Ultrasound near the Low-Temperature Phase Transition Induced by 3d Impurities in a ZnSe Crystal

This paper reports on the results of temperature investigations into the absorption and velocity of ultrasound in ZnSe: Ni and ZnSe: Cr crystals in the frequency range 33–268 MHz. The frequency dependence of the absorption at the maximum is analyzed, and the energy of the excited state of the Ni2+ ions is calculated. The dynamic contribution to the effective elastic modulus is determined, and the results obtained are used to construct the temperature dependences of the relaxed and unrelaxed elastic moduli.

Lattice Instability Induced by 3d Impurities in II–VI Compound Semiconductors

Nickel-impurity-induced transverse displacements of ions in a Zn1−x NixSe lattice (x = 0.0025) were detected. This type of displacement correlates with macroscopic distortions of a crystal associated with transverse ultrasonic waves that are propagated along the 〈110〉 direction. The shear instability is assumed to be due to the hybridization of the sp 3 bonds with the 3d states of the impurity centers.

Charge State of a Transition Metal Impurity in II–VI Semiconductors

The results of calculating the electronic structure of semiconductor compounds A II B VI: 3d(A = Zn; B = S, Se, Te; 3d = Sc-Cu) at a low content of 3d impurities are discussed. The excess charge of an impurity ion with respect to the charge of the zinc ion is determined for the whole series of 3d impurities. It is found that the excess charge gradually varies from +0.6|e| for the scandium impurity to −0.2|e| for the copper impurity. Photoionization of an impurity ion is simulated by adding a hole or an electron to the impurity center. The added charge is redistributed between the impurity ion and its nearest neighbors, thus decreasing or increasing the total excess charge of the impurity center by a magnitude of ∼ 0.2|e|.

Hole-Mediated Stabilization of Cubic GaN

We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.

Local force constants of transition metal dopants in a nickel host: Comparison to Mossbauer studies

We have used the x-ray absorption fine-structure technique to obtain temperature-dependent mean-squared relative displacements for a series of dopant atoms in a nickel host. We have studied the series Ti, V, Mn, Fe, Nb, Mo, Ru, Rh, and Pd doped into Ni, and have also obtained such data for pure Ni. The data, if interpreted in terms of the correlated Einstein model of Hung and Rehr, yield a ratio of a ~host-host! to ~host-impurity! effective force constant, where the effective force constant is due to a cluster of atoms. We have modified the method of Hung and Rehr so that we obtain a ratio of near-neighbor single spring constants, rather than effective spring constants. We find that the host to the 4d impurity force constant ratio decreases monotonically as one increases the dopant atomic number for the series Nb, Mo, Ru, and Rh, but after a minimum at Rh the ratio increases sharply for Pd. We have compared our data to Mossbauer results for Fe dopants in Ni, and find qualitative disagreement. In Mossbauer studies, the ratio of the Ni-Ni to Fe-Ni force constant is found to be extremely temperature dependent and less than one. We find the corresponding ratio, as interpreted in terms of x-ray absorption spectra and the correlated Einstein model, to be greater than one, a result that is supported by elastic constant measurements on NixFe(12x) alloys.

X‐ray, photo‐ESR and optical spectroscopy studies of ZnSxSe1−x solid solutions doped with Co

AbstractExperimental studies of X‐ray photoelectron and Co Lα X‐ray emission spectra of the ZnS:Co semiconductor were carried out. It was established that Co ions are in a Co2+ configuration and that the Co 3d impurity states are localized above the top of the valence band by 1.0 ± 0.2 eV. It was found that the covalency reduction factor for Co2+ defect changes through the alloy from 0.62 in ZnSe lattice to 0.59 in ZnS. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-density-wave glass state in Cr-based alloys with 3d impurities

Abstract A spin-density-wave (SDW) glass state has been found in Cr alloys. Magnetization measurements in ZFC and FC states have revealed two types of magnetic irreversibility (MI). The first one is caused by a “local SDW”. Depending on the electron concentration of impurity a temperature of divergence of M ZFC ( T ) and M FC ( T ), T i , can be either below or above T N . The second type of MI is due to the deformation of the soliton wall of the SDW domains and can be realized only in Cr alloys with an incommensurate SDW structure just below T N .

Hyperfine Interaction Studies on Y, Zr, Nb, Mo, Rh, In and Xe in Co

Nuclear magnetic resonance on oriented nuclei and modulated adiabatic fast passage on oriented nuclei measurements were performed on several 4d and 5sp impurities in polycrystalline Co(fcc) foils and Co(hcp) single crystals. The hyperfine fields of Y and Zr in Co(fcc), the hyperfine fields of Y, Zr, Nb, Mo, Rh, In and Xe in Co(hcp), the electric field gradients of Zr, Nb and In in Co(hcp), and the nuclear spin-lattice relaxations of Zr, Nb, Rh and In in Co(hcp) were determined. The dependence of the hyperfine fields and electric field gradients in Co(hcp) on the angle between the magnetization and the c axis was investigated in most cases. The magnetic-field dependence of the spin-lattice relaxation was studied for Nb, Rh and In in Co(hcp), applying the magnetic field perpendicular to the c axis. The known hyperfine interaction parameters of the 4d and 5sp impurities in Co(fcc) and Co(hcp) are summarized. The new results provide a more detailed picture of the hyperfine interaction in Co.

Calculated hyperfine fields of 3d and 4d impurities in Co HCP

We present first-principles calculations of the hyperfine fields and magnetic moments of 3d and 4d impurities in cobalt HCP host. The first-principles discrete variational method in the self-consistent charge approximation is employed to obtain the electronic structure in spin-polarized calculations within the framework of the local approximation of the density-functional theory. Good agreement with experimental results is obtained for the calculated hyperfine magnetic fields. In the 3d series, the Cr atom has the largest value for the magnetic moment after Co, and is antiferromagnetically coupled with the surrounding host atoms. For the 4d impurities, the largest moment belongs to Rh, which is coupled in parallely to the host atomic moments.

Electronic structure of cubic silicon carbide with substitutional 3d impurities at Si and C sites

The full-potential linear muffin-tin orbital method is used to calculate the electronic structure and cohesive energies for cubic silicon carbide doped with transition 3d metal impurities (Me=Ti, V, Cr, Mn, Fe, Co, Ni), substituting Si or C at the corresponding sublattice of the atomic matrix. It is established that all 3d impurities mainly occupy silicon sites. For the Ti → Si substitution, dopant impurity levels are located in the conduction band of SiC, whereas doping silicon carbide with other 3d impurities gives rise to additional donor or acceptor levels in the band gap. For 3C-SiC the effect of impurities on the lattice parameter (with the substitutions Me → Si) and on the impurity local magnetic moments (with the substitutions Me → Si, C) is studied.

A Simple Model of 3d Impurities in Cubic Silicon Carbide

A Simple Model of 3d Impurities in Cubic Silicon Carbide

Theoretical Investigation on Induced Magnetic Moment by Impurity Potential

We have theoretically examined whether magnetic moment of 3d impurity such as Fe or Co can be induced or not in magnetic dilute alloy when atoms with high electronegativity such as halogen or oxygen ones are doped into nonmagnetic matrix. Based on the impurity Anderson model, we have analyzed the magnetic state in magnetic dilute alloys, considering that the effect of doping atoms can be regarded as the impurity potential. Using the mean field approximation, we evaluate the green function for d electrons. Through the self-consistent calculation with respect to the number of d electrons with spin ↑ or ↓, it is disclosed that, although the initial state is non-magnetic, a magnetic moment can be induced by an impurity potential caused by doping atoms. In particular, when doped atoms have large electronegativity and/or there exists a strong Coulomb repulsive interaction between conduction electrons and doped atoms, magnetic moment is found to be easily induced from non-magnetic state.