215 research outputs found
Element specific characterization of heterogeneous magnetism in (Ga,Fe)N films
We employ x-ray spectroscopy to characterize the distribution and magnetism
of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor
phase epitaxy. Furthermore, photoelectron microscopy gives direct evidence for
the aggregation of Fe ions, leading to the formation of Fe-rich nanoregions
adjacent to the samples surface. A sizable x-ray magnetic circular dichroism
(XMCD) signal at the Fe L-edges in remanence and at moderate magnetic fields at
300 K links the high temperature ferromagnetism with the Fe(3d) states. The
XMCD response at the N K-edge highlights that the N(2p) states carry
considerable spin polarization. We conclude that FeN{\delta} nanocrystals, with
\delta > 0.25, stabilize the ferromagnetic response of the films.Comment: 4 pages, 3 figures, 1 tabl
Out-of-Plane Magnetic Anisotropy in Ordered Ensembles of FeN Nanocrystals Embedded in GaN
Phase-separated semiconductors containing magnetic nanostructures are
relevant systems for the realization of high-density recording media. Here, the
controlled strain engineering of GaFeN layers with FeN embedded
nanocrystals (NCs) \textit{via} AlGaN buffers with different Al
concentration \% is presented. Through the addition of Al
to the buffer, the formation of predominantly prolate-shaped
-FeN NCs takes place. Already at an Al concentration
\,\,5\% the structural properties---phase, shape,
orientation---as well as the spatial distribution of the embedded NCs are
modified in comparison to those grown on a GaN buffer. Although the magnetic
easy axis of the cubic '-GaFeN nanocrystals in the layer on
the buffer lies in-plane, the easy axis of the
-FeN NCs in all samples with AlGaN buffers
coincides with the growth direction, leading to a sizeable
out-of-plane magnetic anisotropy and opening wide perspectives for
perpendicular recording based on nitride-based magnetic nanocrystals.Comment: 29 pages, 10 figures, submitte
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
Genetic differentiation in the Agave deserti (Agavaceae) complex of the Sonoran desert
The Agave deserti complex, comprising A. deserti, A. cerulata and A. subsimplex, represents a group of species and subspecies with a near allopatric distribution and clear differences in morphology. Genetic differentiation and taxonomic status with respect to spatial distribution of 14 populations of the complex were analyzed in an effort to understand the evolution and speciation process within the genus. Allelic frequencies, levels of genetic variation, expected heterozygosity (H S ), proportion of polymorphic loci (P), and genetic differentiation (y and Nei's genetic distance) were estimated using 41 putative RAPD loci. All three species show high levels of genetic variation (H S ¼ 0.12-0.29, P ¼ 63.4-95.1), and low genetic differentiation between populations and species (y populations ¼ 0.1470.02 (SE); G st ¼ 0.1170.02). Accordingly, gene flow among populations was estimated as high by three different methods (N m ¼ 2.91-6.14). Nei's genetic distances between the three species were low compared to the values obtained from other Agavaceae, and there was no clear correlation with taxonomic divisions. In a UPGMA analysis, A. subsimplex and A. cerulata formed exclusive monospecific clusters, whereas the A. deserti populations appear in more than one cluster together with other species. The results were consistent with a pattern of genetic isolation by distance
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