Local Structure around Mn in Ferromagnetic GaMnN Film Studied by X-Ray Absorption Fine Structure

Abstract

Recent observations from RHESSI have revealed that the number of non-thermal electrons in the coronal part of a flaring loop can exceed the number of electrons required to explain the hard X-ray-emitting footpoints of the same flaring loop. Such sources cannot, therefore, be interpreted on the basis of the standard collisional transport model, in which electrons stream along the loop while losing their energy through collisions with the ambient plasma; additional physical processes, to either trap or scatter the energetic electrons, are required. Motivated by this and other observations that suggest that high-energy electrons are confined to the coronal region of the source, we consider turbulent pitch-angle scattering of fast electrons off low-frequency magnetic fluctuations as a confinement mechanism, modeled as a spatial diffusion parallel to the mean magnetic field. In general, turbulent scattering leads to a reduction of the collisional stopping distance of non-thermal electrons along the loop, and hence to an enhancement of the coronal hard X-ray source relative to the footpoints. The variation of source size L with electron energy E becomes weaker than the quadratic behavior pertinent to collisional transport, with the slope of L(E) depending directly on the mean free path λ associated with the non-collisional scattering mechanism. Comparing the predictions of the model with observations, we find that λ ∼ (10 8 -10 9 ) cm for ∼30 keV, less than the length of a typical flaring loop and smaller than, or comparable to, the size of the electron acceleration region.

Advances in wide bandgap materials for semiconductor spintronics

“…slightly over 2 at.%) are shown in Fig. 18 [111,112]. The similarity of the experimental data with simulated curves for a sample containing this concentration of Mn substituted for Ga on substitutional lattice positions indicates that Mn is in fact soluble at these densities.…”

“…The ionic state of the substitutional Mn was found to be primarily Mn(II), so that these impurities act as acceptors when substituting for the Ga with valence three. However, when the electrical properties of these samples were measured, they were found to be resistive [111,112]. This result emphasizes how much more needs to be understood concerning the effects of compensation and unintentional doping of (Ga, Mn)N, since the EXAFS data indicated the samples should have shown very high p-type conductivity due to incorporation of Mn acceptors.…”

“…This result emphasizes how much more needs to be understood concerning the effects of compensation and unintentional doping of (Ga, Mn)N, since the EXAFS data indicated the samples should have shown very high p-type conductivity due to incorporation of Mn acceptors. The local structure and effective chemical valency of Mn in MBE-grown (Ga, Mn)N samples has been investigated by extended X-ray absorption fine structure [111,112]. It was concluded that most of the Mn was incorporated substitutionally on the Ga sublattice with effective valency close to þ2 for samples with $2 at.% Mn [111,112].…”

“…The local structure and effective chemical valency of Mn in MBE-grown (Ga, Mn)N samples has been investigated by extended X-ray absorption fine structure [111,112]. It was concluded that most of the Mn was incorporated substitutionally on the Ga sublattice with effective valency close to þ2 for samples with $2 at.% Mn [111,112]. There was also evidence that a fraction (from 1 to 36%, depending on growth condition) of the total Mn concentration could be present as small Mn clusters [111,112].…”

“…It was concluded that most of the Mn was incorporated substitutionally on the Ga sublattice with effective valency close to þ2 for samples with $2 at.% Mn [111,112]. There was also evidence that a fraction (from 1 to 36%, depending on growth condition) of the total Mn concentration could be present as small Mn clusters [111,112].…”