46 research outputs found
The Mn site in Mn-doped Ga-As nanowires: an EXAFS study
We present an EXAFS study of the Mn atomic environment in Mn-doped GaAs
nanowires. Mn doping has been obtained either via the diffusion of the Mn used
as seed for the nanowire growth or by providing Mn during the growth of
Au-induced wires. As a general finding, we observe that Mn forms chemical bonds
with As but is not incorporated in a substitutional site. In Mn-induced GaAs
wires, Mn is mostly found bonded to As in a rather disordered environment and
with a stretched bond length, reminiscent of that exhibited by MnAs phases. In
Au-seeded nanowires, along with stretched Mn-As coordination we have found the
presence of Mn in a Mn-Au intermetallic compound.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Semiconductor Science and Technology. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The definitive
publisher-authenticated version is available online at
doi:10.1088/0268-1242/27/8/08500
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
Spin-polarized electronic structure of the core-shell ZnO/ZnO:Mn nanowires probed by x-ray absorption and emission spectroscopy
The combination of x-ray spectroscopy methods complemented with theoretical
analysis unravels the coexistence of paramagnetic and antiferromagnetic phases
in the Zn_0.9Mn_0.1O shell deposited onto array of wurtzite ZnO nanowires. The
shell is crystalline with orientation toward the ZnO growth axis, as
demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than
90% of Mn atoms substituted Zn in the shell while fraction of secondary phases
was below 10%. The value of manganese spin magnetic moment was estimated from
the Mn K{\beta} X-ray emission spectroscopy to be 4.3{\mu}B which is close to
the theoretical value for substitutional Mn_Zn. However the analysis of L_2,3
x-ray magnetic circular dichroism data showed paramagnetic behaviour with
saturated spin magnetic moment value of 1.95{\mu}B as determined directly from
the spin sum rule. After quantitative analysis employing atomic multiplet
simulations such difference was explained by a coexistence of paramagnetic
phase and local antiferromagnetic coupling of Mn magnetic moments. Finally,
spin-polarized electron density of states was probed by the spin-resolved Mn
K-edge XANES spectroscopy and consequently analyzed by band structure
calculations.Comment: Supplementary information available at
http://www.rsc.org/suppdata/ja/c3/c3ja50153a/c3ja50153a.pdf J. Anal. At.
Spectrom., 201
X-ray absorption and emission spectroscopy study of Mn and Co valence and spin states in TbM n1-x C ox O3
The valence and spin state evolution of Mn and Co on TbMn1-xCoxO3 series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and Kß x-ray emission spectroscopy (XES). Our results show the change from Mn3+ to Mn4+ both high-spin (HS) together with the evolution from Co2+ HS to Co3+ low-spin (LS) with increasing x. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the Kß XES data. In this paper, we determine that Co enters into the transition metal sublattice of TbMnO3 as a divalent ion in HS state, destabilizing the Mn long-range magnetic order since very low doping compositions (x=0.1). Samples in the intermediate composition range (0.4=x=0.6) adopt the crystal structure of a double perovskite with long-range ferromagnetic ordering which is due to Mn4+-O-Co2+ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for x=0.7 due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of Mn4+ HS and a fluctuating valence state Co2+ HS/Co3+ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behavior in these samples
Solid and Aqueous Speciation of Yttrium in Passive Remediation Systems of Acid Mine Drainage
International audienceYttrium belongs to the rare earth elements (REEs) together with lanthanides and scandium. REEs are commonly used in modern technologies, and their limited supply has made it necessary to look for new alternative resources. Acid mine drainage (AMD) is a potential resource since it is moderately enriched in REEs. In fact, in passive remediation systems, which are implemented to minimize the environmental impacts of AMD, REEs are mainly retained in basaluminite, an aluminum hydroxysulfate precipitate. In this study, the solid and liquid speciation and the local structure of yttrium are studied in high-sulfate aqueous solutions, basaluminite standards, and samples from remediation columns using synchrotron-based techniques and molecular modeling. Pair distribution function (PDF) analyses and ab initio molecular dynamics density functional theory models of the yttrium sulfate solution show that the YSO4+ ion pair forms a monodentate inner-sphere complex. Extended X-ray absorption fine structure (EXAFS) and PDF analyses show that Y is retained by basaluminite, forming a monodentate inner-sphere surface complex on the aluminum hydroxide surface. EXAFS of the column samples shows that more than 72% of their signal is represented by the signal of basaluminite with which YSO4+ forms an inner-sphere complex. The atomic view of the REE configuration in AMD environments could facilitate a deeper research of REE recovery from waste generated in AMD remediation systems
Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N
The sign, magnitude, and range of the exchange couplings between pairs of Mn
ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have
been grown by metalorganic vapor phase epitaxy and characterized by
secondary-ion mass spectroscopy; high-resolution transmission electron
microscopy with capabilities allowing for chemical analysis, including the
annular dark-field mode and electron energy loss spectroscopy; high-resolution
and synchrotron x-ray diffraction; synchrotron extended x-ray absorption
fine-structure; synchrotron x-ray absorption near-edge structure; infra-red
optics and electron spin resonance. The results of high resolution magnetic
measurements and their quantitative interpretation have allowed to verify a
series of ab initio predictions on the possibility of ferromagnetism in dilute
magnetic insulators and to demonstrate that the interaction changes from
ferromagnetic to antiferromagnetic when the charge state of the Mn ions is
reduced from 3+ to 2+.Comment: 12 pages, 14 figures; This version contains the detailed
characterization of the crystal structure as well as of the Mn distribution
and charge stat
Structural and paramagnetic properties of dilute Ga1-xMnxN
Systematic investigations of the structural and magnetic properties of single
crystal (Ga,Mn)N films grown by metal organic vapor phase epitaxy are
presented. High resolution transmission electron microscopy, synchrotron x-ray
diffraction, and extended x-ray absorption fine structure studies do not reveal
any crystallographic phase separation and indicate that Mn occupies
Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic
properties as a function of temperature, magnetic field and its orientation
with respect to the c-axis of the wurtzite structure can be quantitatively
described by the paramagnetic theory of an ensemble of non-interacting
Mn ions in the relevant crystal field, a conclusion consistent with the
x-ray absorption near edge structure analysis. A negligible contribution of Mn
in the 2+ charge state points to a low concentration of residual donors in the
studied films. Studies on modulation doped p-type (Ga,Mn)N/(Ga,Al)N:Mg
heterostructures do not reproduce the high temperature robust ferromagnetism
reported recently for this system.Comment: 15 pages, 14 figure
Oxide phase characterization in simulated high burn-up UO2 fuels in the early stages of a nuclear severe accident
International audienc