Doped Mn Atoms Research Articles

Hydrogen interstitials-mediated ferromagnetism in MnxGe1−x magnetic semiconductors

Hydrogen interstitials-mediated ferromagnetism in MnxGe1-x magnetic semiconductors was studied by first-principles calculations. It was found that the H:1s state in Mn–H–Mn complexes can strongly hybridize with the valence states of Mn and change the spin polarization of Mn atoms. Although the doped Mn atoms tend to form the nearest-neighbor Mn atomic pairs with antiferromagnetic coupling in Mn-doped Ge without H, the asymmetrical configurations of Mn–H–Mn complexes show the ferromagnetic (FM) ground state in Mn-doped Ge with H interstitials. Therefore, Mn-doped Ge with H interstitials is predicted to be an FM semiconductor with higher Curie temperature and larger magnetization than Mn-doped Ge without H.

Synthesis and characterization of Zn1−xMnxO nanowires

Mn doped ZnO nanowires (NWs) were fabricated by a one-step vapor-solid process at 500°C. The doped Mn exists in the wurtzite lattice as substitutional atom without forming secondary phases. X-ray absorption near-edge structure reveals that the doped Mn atoms occupy the Zn sites, and they lead to an expansion in lattice constants. The I-V characteristic of a single Zn1−xMnxO NW shows a typical Ohmic contact with gold electrodes. The as-received NWs could be suitable for studying spintronics in one-dimensional diluted magnetic semiconductors.

Effect of Growth Temperature and Mn Incorporation on GaN:Mn Thin Films Grown by Plasma-Assisted MOCVD

In this paper, the growth of GaN:Mn thin films by plasma-assisted metalorganic chemical vapor deposition (PAMOCVD) method is reported. The method used in this study, utilizes a microwave cavity as a cracking cell to produce nitrogen radicals, which in turn reduce the growth temperature. Trimethylgallium (TMGa), nitrogen (N2) and cyclopentadienyl manganese tricarbonyl (CpMnT) were used as a source of Ga, N and Mn, respectively, while hydrogen gas was used as a carrier gas for both TMGa and CpMnT. The effect of growth temperature and Mn incorporation on structural properties and surface morphology of GaN:Mn films are presented. The growth of GaN:Mn thin films were conducted at varied growth temperature in range of 625 o C to 700 o C and the Mn/Ga molar fraction in the range of 0.2 to 0.5. Energy dispersive of X-ray (EDX) and X-ray diffraction (XRD) methods were used to analyze atomic composition and crystal structure of the grown films, respectively. The surface morphology was then characterized using both atomic force microscopy (AFM) and scanning electron microscopy (SEM) images. A systematic XRD analysis reveal that maximum Mn incorporation that still produces single phase GaN:Mn (0002) is 6.4 % and 3.2 % for the film grown at 650 o C and 700 o C, respectively. The lattice constant and full width at half maximum (FWHM) of the single phase films depend on the Mn concentration. The decrease in lattice constant accompanied by the increase in FWHM is due to incorporation of substitutional Mn on the Ga sub-lattice. The maximum values of doped Mn atoms incorporated in the wurtzite structure of GaN:Mn as substitutional atoms on Ga sub-lattice are 2.0 % and 2.5 % at 650 o C and 700 o C, respectively. AFM and SEM images show that the film grown at lower growth temperature and Mn concentration has a better surface than that of film grown at higher growth temperature and Mn concentration.

Neutral and charged embedded clusters of Mn in doped GaN from first principles

Based on extensive density-functional theory calculations, the spatial distribution and magnetic coupling of Mn atoms in Mn:GaN have been reinvestigated by doping up to five Mn atoms in large supercells, where the formation energies and the electronic structure for both the neutral and charged valence states are studied. The doped Mn atoms have a strong tendency to form substitutional Mn-N-Mn bonded embedded clusters with short-range magnetic interactions, where the long-range wurtzite structure is maintained. While for neutral pair doping the coupling is ferromagnetic regardless of the distance and orientation of the Mn atoms, the negatively charged states tend to weaken the parallel coupling. Significantly, for larger (than pair) cluster configurations for both neutral and all the energetically favorable charged states, states containing antiparallel coupling are always favored. Thus, we argue that the ``giant cluster moment'' in Mn:GaN, as proposed by Rao and Jena based on study of free clusters [Phys. Rev. Lett. 89, 185504 (2002)] and calculated by Sandratskii et al. [Phys. Rev. B 71, 045210 (2005)], is not applicable for ``larger'' $(>2)$ Mn-cluster-doped GaN. The size of the supercells employed and the atomic relaxation are found crucial for an accurate description, and are partially responsible for these discrepancies. Also important is that the electrical conductivity of Mn:GaN depends sensitively on the valence states, where the negatively charged state ${\mathrm{Mn}}^{2+}({d}^{5})$ exhibits highly insulating character as observed in experiments. The formation of embedded clustering leads to a strong local structural distortion and a significant spin polarization on neighboring N atoms due to hybridization of $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}3d$ and $\mathrm{N}\phantom{\rule{0.2em}{0ex}}2p$ orbitals. Our results highlight the intrinsically complex nature of the spatial distribution and magnetism in transition metal doped dilute magnetic semiconductors, and can rationalize some hitherto puzzling experimental observations, notably the low saturation moments, the contracted lattice constants, and the highly insulating behavior.

Enhancement of magnetic properties in (Ga,Mn)N nanowires due to N2 plasma treatment

The local structure of Mn impurities in ferromagnetic (Ga,Mn)N nanowires was investigated using extended x-ray absorption fine structure (EXAFS). By N2 plasma treatment, the nitrogen concentration increased and the ferromagnetic signal also increased. The EXAFS results showed that the Mn concentration occupying Ga sites increased and interstitially doped Mn atoms significantly decreased due to the plasma treatment. As a result, the N vacancies reduced and crystalline quality improved, resulting in the enhancement of ferromagnetic properties.

Dilute magnetic III–V semiconductor spintronics materials: A first-principles approach

Group-III nitride semiconductors, such as GaN, when doped with 3d transition metals such as Mn or Cr show ferromagnetism with high Curie temperature. Such dilute magnetic semiconductors (DMS) with larger band gaps and smaller lattice constants compared to GaAs based DMS, are potential candidates for room temperature spintronics devices. We have investigated the magnetic coupling between doped Mn (or Cr) atoms in clusters as well as crystals of GaN from first principles using molecular orbital theory for (GaN) x TM 2 clusters and TB-LMTO band calculations for wurtzite structured (Ga 14TM 2)N 16 supercells. Our calculations reveal that the coupling between TM-impurity atoms is ferromagnetic with a bulk magnetic moment of ∼3.5 μ B per Mn atom and ∼2.7 μ B per Cr atom.

Ferromagnetism in Mn-doped GaN: From clusters to crystals

The magnetic coupling between doped Mn atoms in clusters as well as crystals of GaN has been studied from first principles using molecular orbital theory for clusters and linearized muffin tin orbitals-tight binding formulation (LMTO-TB) for crystals. The calculations, based on density functional theory and generalized gradient approximation for exchange and correlation, reveal the coupling to be ferromagnetic with a magnetic moment ranging from 2.0 to 4.0 Bohr magnetons per Mn atom depending on its environment. Mn atoms also tend to cluster and bind more strongly to N atoms than to Ga atoms. The significant binding of Mn to GaN clusters further indicates that it may be possible to increase the Mn concentration in GaN by using a porous substrate that offers substantial interior surface sites.