Substitutional Mn Atoms Research Articles

Structural study of MnxSi1-x magnetic semiconductor thin films

The structure of MnxSi1-x magnetic semiconductor thin films prepared by molecular beam epitaxy(MBE) on Si(100) substrate at 600 ℃ has been studied by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) technique. The XRD results show that in the MnxSi1-x thin films with high Mn doping concentrations (x=0.08 and 0.17), only diffraction peaks of crystalline Mn4Si7 are observed. XANES results indicate that all the Mn K-edge XANES spectra of MnxSi1-x thin films with different Mn doping concentrations (x=0.007, 0.03, 0.08 and 0.17) show the similar feature. XANES calculation based on multiple-scattering theory further reveals that the experimental spectra for samples with different Mn doping concentrations are reproduced by the calculated Mn4Si7 spectrum. These results reveal that for the MnxSi1-x magnetic semiconductor thin films, Mn atoms mainly exist in the Si thin film substrate in the form of Mn4Si7 nanocrystalline grains, the substitutional or interstitial Mn atoms scarcely exist.

Random telegraph noise from magnetic nanoclusters in the ferromagnetic semiconductor(Ga,Mn)As

Measurements of the low frequency electrical noise in the ferromagnetic semiconductor (Ga,Mn)As reveal an enhanced integrated noise at low temperature. For moderate localization, we find a 1/f normalized power spectrum density over the entire range of temperatures studied (4.2K < T < 70K). However, for stronger localization and a high density of Mn interstitials, we observe Lorentzian noise spectra accompanied by random telegraph noise. Magnetic field dependence and annealing studies suggest that interstitial Mn defects couple with substitutional Mn atoms to form nanoscale magnetic clusters characterized by a net moment of about 20 Bohr magnetons whose fluctuations modulate hole transport.

Computational and experimental imaging of Mn defects on GaAs (110) cross-sectional surfaces

We present a combined experimental and computational study of the (110) cross-sectional surface of Mn $\delta$-doped GaAs samples. We focus our study on three different selected Mn defect configurations not previously studied in details, namely surface interstitial Mn, isolated and in pairs, and substitutional Mn atoms on cationic sites (Mn$_{\rm Ga}$) in the first subsurface layer. The sensitivity of the STM images to the specific local environment allows to distinguish between Mn interstitials with nearest neighbor As atoms (Int$_{\rm As}$) rather than Ga atoms (Int$_{\rm Ga}$), and to identify the fingerprint of peculiar satellite features around subsurface substitutional Mn. The simulated STM maps for Int$_{\rm As}$, both isolated and in pairs, and Mn$_{\rm Ga}$ in the first subsurface layer are consistent with some experimental images hitherto not fully characterized.

Mn incorporation in as-grown and annealed (Ga,Mn)As layers studied by x-ray diffraction and standing-wave fluorescence

A combination of high-resolution x-ray diffraction and a new technique of x-ray standing wave uorescence at grazing incidence is employed to study the structure of (Ga,Mn)As diluted magnetic semiconductor and its changes during post-growth annealing steps. We find that the film is formed by a uniform, single crystallographic phase epilayer covered by a thin surface layer with enhanced Mn concentration due to Mn atoms at random non-crystallographic positions. In the epilayer, Mn incorporated at interstitial position has a dominant effect on lattice expansion as compared to substitutional Mn. The expansion coeffcient of interstitial Mn estimated from our data is consistent with theory predictions. The concentration of interstitial Mn and the corresponding lattice expansion of the epilayer are reduced by annealing, accompanied by an increase of the density of randomly distributed Mn atoms in the disordered surface layer. Substitutional Mn atoms remain stable during the low-temperature annealing.

Growth and characterization of Mn-doped cubic-GaN

We report on the growth and characterization of Mn-doped cubic-GaN films. The n-type carriers, which are resulting from the formation of nitrogen vacancies, were intentionally utilized. For a 3% Mn sample with a high carrier density (∼1×10 20 cm −3), we found substitutional Mn atoms on both the Ga-site and N-site, i.e., (Ga,Mn)N and Ga(N,Mn) exist together. A ferromagnetic behavior was observed in this sample at low temperature (∼5 K), although a 3% Mn semi-insulating sample in which Mn atoms substitute only the Ga-sites in the cubic-GaN lattice showed paramagnetism even at low temperature. The structural and magnetic properties of the relatively high Mn composition (⩾5%) samples were found to be governed by precipitate clusters of antiferromagnetic GaMn 3N and ferromagnetic Mn 4N.

Electronic, optical spectra and the distribution of Mn impurities in GaN and group-IV semiconductors

By use of the first-principles density-functional approach, pair impurity configurations, magnetic and optical properties of Mn-doped GaN, Ge and Si were calculated. From the total energies calculated for various possible distributions, substitutional Mn atoms in GaN were found to prefer the dimer configuration with the shortest Mn–Mn distances, while in Ge:Mn and Si:Mn the uniform distribution of Mn atoms is more stable. The most remarkable changes in optical spectra occur in the infrared region due to the optical interband transitions between impurity-induced states.

Clustering of Mn in (Ga,Mn)As

Clustering of (Ga,Mn)As is studied with the supercell density-functional calculations using the projector augmented-wave method. We find large binding energies for a single substitutional Mn atom to existing dimers and trimers giving rise to trimer and tetramer formation. For larger clusters this binding energy drops. In all cases studied, ferromagnetic alignment is found to be energetically more favourable than the antiferromagnetic one.

Determination of the local concentrations of Mn interstitials and antisite defects in GaMnAs.

We present a method for the determination of the local concentrations of interstitial and substitutional Mn atoms and As antisite defects in GaMnAs. The method relies on the sensitivity of the structure factors of weak reflections to the concentrations and locations of these minority constituents. High spatial resolution is obtained by combining structure factor measurement and x-ray analysis in a transmission electron microscope. We demonstrate the prevalence of interstitials with As nearest neighbors in as-grown layers.

Diluted magnetic Ga1-xMn xN alloys: a first-principles study

The utilization of the quantum properties of the electron spin wave function will allow the development of a new class of devices. The problem is still controversial and unsettled, even qualitatively, especially for concentrated spin systems such as 3d metals and their alloys. The variety of crystal structures of 3d metals makes difficult the direct comparison between the experimental results and the theoretical conclusions. On the account of this difficulty, substitutional alloys with the same crystal structure, especially face-centered cubic alloys, have been investigated extensively. In this work the properties of diluted Ga1-xMnxN (x = 0.0630; 0.0315) alloys are calculated in the zinc-blende phase, within the framework of the density functional theory, using the fullpotential linearized augmented plane wave (FLAPW) method and the local density approximation (LDA). The alloys are simulated using 32-atom and 64-atom large unit cells, containing one substitutional Mn atom for a Ga atom. The calculations are spin-polarized and we analyze band structures, density of states and total magnetizations. A half-metallic state is predicted at a0 ~ 4:45A. The majority-spin band has a rather sharp peak, characteristic of a narrow band, while the minority-spin has a gap. The total magnetization of the cell is 4.00mB which does not change with the Mn concentration. The valence band is ferromagnetically coupled with the Mn atoms, and the spin splitting is not linearly dependent on the Mn concentration.

Magnetic phase transitions in the iron-doped Pr0.7Ca0.3Mn1−y FeyO3 manganites at high pressures

The atomic and magnetic structures of the iron-doped Pr0.7Ca0.3Mn1−yFeyO3 manganites (y=0, 0.1) have been studied at high pressures of up to 4 GPa in the temperature range 16–300 K. At normal pressure, Pr0.7Ca0.3MnO3 undergoes a phase transition from the paramagnetic to an antiferromagnetic (AFM) state of the pseudo-CE type and Pr0.7Ca0.3Mn0.9Fe0.1O3 undergoes a phase transition from the paramagnetic to the ferromagnetic state at low temperatures. Partial substitution of Mn atoms by Fe brings about a noticeable decrease in the average magnetic moment per atom. A new A-type AFM state was observed to form in Pr0.7Ca0.3MnO3 at a pressure P≈2.2 GPa and in Pr0.7Ca0.3Mn0.9Fe0.1O3 at 2.7 GPa. This phenomenon may originate from the anisotropy in the compressibility, which causes uniaxial contraction of the oxygen octahedra MnO6 in the structure and provides favorable conditions for the formation of an A-type AFM state. The structural parameters obtained were used to calculate the pressure dependence of bandwidth in the compounds under study.

Electronic structure and magnetism of diluted magnetic semiconductors and derivatives

Abstract We present the electronic structure and magnetic properties of substitutional and interstitial Mn atoms in a GaAs host. Interstitial Mn atoms carry lower magnetic moments compared to the substitutional Mn atoms and couple antiferromagnetically with them. Also, we present calculations of Curie temperatures using both mean field approximation and Monte Carlo simulations. In another study, we present results of binary compounds consisting of 3d elements (Ti–Ni) and group V elements (P, As and Sb) in the zinc-blende structure. We demonstrate that compounds of V, Cr and Mn show half metallic behavior for appropriate lattice constants. Of the different compounds studied, the Cr based systems exhibit the strongest interatomic exchange interactions, and are hence predicted to have the highest critical temperatures.

Magnetization of ultrathin (Ga,Mn)As layers

Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin $({\mathrm{Ga}}_{0.95}{\mathrm{Mn}}_{0.05})\mathrm{As}$ layers. The thinner layers (below 250 \AA{}) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 \AA{}) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers.

Multivalent substitution in a quasi-binary Ga1−x (II-Mn-IV)xAs solid solution

A process of the multivalent substitution of Mn atoms for cations in Ga1−xMnxAs (a base material) is proposed. According to this approach, the two-cation ternary compound with a chalcopyrite structure is used as a component forming a solid solution with GaAs. Based on the successful prediction and growth of high-temperature chalcopyrite ferromagnets, it is expected that the content of Mn in Ga1−xMnxAs can be increased. Quasi-binary (GaAs)1−x(ZnGeAs2)x:Mn and (GaAs1−x(CdSiAs2)x:Mn solid solutions are considered as candidate materials possessing higher Curie temperatures as compared to that of Ga1−xMnxAs.

Structural, magnetic and magnetotransport properties ofLa0.7Pb0.3Mn0.9TM0.1O3 (TM = Fe, Co, Ni) CMRperovskites

Neutron-diffraction, magnetic and transport measurements have been used in orderto study the structural and magnetic changes of the magnetoresistiveLa0.7Pb0.3Mn0.9TM0.1O3 (TM = Fe, Co, Ni) perovskite-likecompounds. Samples were prepared by the sol-gel low-temperature method. In allcases the exact stoichiometry is slightly different from the nominal oneprobably due to the presence of cation vacancies. All the phases exhibitferromagnetic behaviour and magnetoresistance. No appreciable differences in thestructure, as the TM3+ ion changes, have been observed. The magnetic momentand Curie temperature continuously decrease (with respect to the undopedcomposition) with the substitution of Mn atoms by Ni, Co or Fe, respectively.The magnetoresistance (MR = ΔR/R(0)), under 0 and 6 T applied fields,has been measured reaching a magnitude of about 65% in the case of the 10% Fedoped sample. The obtained results can be interpreted in terms of ferro- orantiferromagnetic exchange competitions between Mn3+-Mn4+ andTM3+-Mn4+ pairs.

III-V diluted magnetic semiconductor: Substitutional doping of Mn in InAs.

Local structures around Mn in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$As films grown by molecular-beam epitaxy have been studied by using Mn K-edge extended x-ray-absorption fine-structure (EXAFS) technique. Substitution of Mn atoms for the In sites is found in samples either grown at low substrate temperatures (near 200 \ifmmode^\circ\else\textdegree\fi{}C) or with a low Mn concentration (about 1 at. %). This result represents a significant extension of an earlier EXAFS study and serves as direct experimental evidence for III-V diluted magnetic semiconductors obtained by substitutional doping of Mn impurities in InAs. \textcopyright{} 1996 The American Physical Society.

The effect of Mn on the magnetic properties of YFe2

The fully self-consistent discrete variational method within the local-spin-density framework has been employed to obtain the electronic structure and magnetic moments of the Mn-containing cubic Laves phase pseudobinary compound of YFe2. The calculated results show that the substitution of Mn atoms on Fe sites weakens the ferromagnetism of this compound. The magnetic moments on distinct atomic sites were derived from the calculation. We found that when the Fe atom has one Mn atom in its nearest neighbor sites, its magnetic moment decreases rapidly, while the Mn moment is 0.71μB, parallel to the Fe moment. Based upon our results, the various experimental data have been satisfactorily explained.

The electronic structure and magnetic behaviour in the Mn-containing cubic Laves-phase pseudobinary compound of YFe2

The fully self-consistent discrete variational method within the local-spin-density framework has been employed to obtain the electronic structure and magnetic moments of the Mn-containing cubic Laves-phase pseudobinary compound of YFe2. The embedded-cluster model was used to simulate the local environments around Fe in this compound. The calculated results show that the substitution of Mn atoms on Fe sites weakens the ferromagnetism of this compound. With the change of the local environments around Fe. We find that when Fe has one or two Mn atoms in its nearest-neighbour sites, its magnetic moment decreases rapidly, while the Mn moment is 071 mu B, parallel to the Fe moment; when Fe has four or more than four Mn atoms in its nearest-neighbour sites, the moments of these Mn atoms are reversed and antiparallel to the Fe moments and meanwhile the Fe moments increase. In addition, calculations for the cluster with an Mn atom occupying a Y site indicates that such an Mn atom is in a high-spin state. Based upon our results, the various experimental data have been satisfactorily explained.

Effect of pressure on the Néel temperature of YMn 2: Resistivity of Y(Mn 1− xCu x) 2

Resistivity measurements have been carried out for Y(Mn 1− x Cu x ) 2 compounds with values of x of 0, 0.04, 0.07, 0.10, 0.20 and 0.30 in the temperature range from 4.2 up to 300 K at zero pressure and for YMn 2 under a pressure of 2 kbar. T N has been found to decrease rapidly by applying external pressure as well as by applying a chemical pressure caused by a partial substitution of Mn atoms by Cu atoms.

Ferromagnetism and heavy electron behavior in URu 2− xM xSi 2(M = Re, Tc and Mn)

The ferromagnetic state in URu 2−xRe xSi 2 has been confirmed with low field magnetization measurements and the Curie temperature found to be pressure independent up to 6.4 kbar. Substitution of Mn atoms for Ru in URu 2Si 2 also leads to a ferromagnetic transition with a small saturation moment appearing at x≳0.2. The ferromagnetism in URu 2− x M x Si 2 (M = Re, Tc or Mn) is apparently due to a sharp feature in the density of states of a band of strongly correlated electrons.

On the multiple-scattering-cluster model of the transition metal atoms isolated in rare gas matrices

Spin-polarized multiple-scattering Xα calculations have been performed for Mn Ar12 and Mn Ar6 clusters in order to describe the substitutional and interstitial Mn atom trapped in a crystalline Ar matrix. The calculated spin densities are used to interpret the experimental ESR hyperfine parameters for both the neutral Mn atom and its positive ion. Our results for the isolated neutral atom are in good agreement with the experiment, however, they are completely different in the case of the Mn+ ion. Some explanations for this discrepancy are considered and a critical discussion of the capabilities and limitations of the multiple-scattering-cluster model applied to impurities in van der Waals solids is also given.