M. Morales

Microstructure and optical properties of Pr3+-doped hafnium silicate films

In this study, we report on the evolution of the microstructure and photoluminescence properties of Pr3+-doped hafnium silicate thin films as a function of annealing temperature (TA). The composition and microstructure of the films were characterized by means of Rutherford backscattering spectrometry, spectroscopic ellipsometry, Fourier transform infrared absorption, and X-ray diffraction, while the emission properties have been studied by means of photoluminescence (PL) and PL excitation (PLE) spectroscopies. It was observed that a post-annealing treatment favors the phase separation in hafnium silicate matrix being more evident at 950°C. The HfO2 phase demonstrates a pronounced crystallization in tetragonal phase upon 950°C annealing. Pr3+ emission appeared at TA = 950°C, and the highest efficiency of Pr3+ ion emission was detected upon a thermal treatment at 1,000°C. Analysis of the PLE spectra reveals an efficient energy transfer from matrix defects towards Pr3+ ions. It is considered that oxygen vacancies act as effective Pr3+ sensitizer. Finally, a PL study of undoped HfO2 and HfSiOx matrices is performed to evidence the energy transfer.

Resistivity of RMn12−xFex alloys

The electrical resistivity of RMn12−xFex (R=Er,Ho) polycrystalline system with x=0, 4, 5, and 6 was measured over the temperature range of 4 to 300 K. The resistivity increases sharply near the Neel temperature TN for x=4, 5, and 6 which, according to existing theories, can be ascribed to the formation of superzone gaps. Then, the resistivity increase should be proportional to the staggered magnetization m′ below TN, whereas we find experimentally that it is quadratic in m′. Thermal smearing of the gaps can lead to such a dependence. There is no evidence for superzone effects in x=0 compounds. A combination of spin disorder, phonon, and impurity scattering gives a good account for the experimental data.

Comparison of the magnetic properties of ErMn12 − xFex series with their related hydrides and carbides

Abstract The magnetic properties of ErMn12 − xFex (0 ⩽ x ⩽ 8) and their related hydrides and carbides have been studied by magnetisation measurements and neutron diffraction. Modifications in the magnetic correlations are found depending on the nature of both the 3d metal and the inserted element.

Impact of hydrogen and carbon insertion on the fundamental characteristics of ErMn12−xFex compounds

Abstract The structural and magnetic properties of ErMn 12− x Fe x X y compounds (X=H,xa0C) have been studied by neutron diffraction experiments. After hydrogenation and carburisation, thanks to the extension of the 3d–3d distances, a subsequent increase of the 3d–3d exchange forces leads to markedly higher ferromagnetic ordering temperatures. Important modifications of the magnetocrystalline anisotropy of the starting compounds are observed to directly depend on the nature of the inserted element.

Transition metal distribution in the ErMn12−xFex (0 ⩽ x ⩽ 8) compounds

Abstract High-resolution neutron diffraction patterns have been collected at room temperature using ILL facilities for various ErMn 12− x Fe x compounds. Samples of composition x = 0, 2, 4, 6, 8 have been analysed in terms of the relative distribution of the transition metal (Mn or Fe) on the three available 8i, 8j and 8f sites of the structure type (SG: I4/mmm). Refinements of the crystal structures show that Mn atoms accommodate preferentially the 8i site contrarily to Fe atoms that fill majority the 8f ones. This observation is consistent with the criterial of relative atom size.

Characterisation by EPR analysis of the magnetic phase diagram of RFe12-xMxXy compounds (R=rare earth metal, M=transition metal, X=H, C)

Abstract The compounds of formula RFe 12− x M x with R=rare earth, M= d -metal and their interstitial derivatives, generally exhibit ferro- or ferrimagnetic properties. Preliminary EPR analyses performed on ErFe 12− x Mn x X y ( x =5, 4, 3; X=H, C) and NdFe 10.7 V 1.3 C y ( y =0, 1) are reported. A direct correlation between the different types of electron resonance traces and the bulk magnetic properties is established, clearly revealing the ferromagnetic and the paramagnetic domains of the magnetic diagram of phases. No spin reorientation transition (SRT) was evidenced for any of the studied compounds.

Transmission electron microscopy and XRD investigations of InAlN/GaN thin heterostructures for HEMT applications

InAlN/GaN layers grown by metalorganic vapor phase epitaxy have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that the surface morphology and the crystallinity of the InAlN layers critically depend on the In composition. AFM analysis points out that step flow growth may not be easily attained in this system: when the InAlN or AlN interlayer thickness is increased, the growth mode becomes three dimensional. However, the formed islands are hundred of nanometers apart, and were not observed in the transmission electron microscope. From the TEM investigation, it is pointed out that v-shaped defects are present at the surface of some layers and they may be as deep as the whole ternary alloy, they are connected to threading dislocations originated from the underlying template layers. As expected the measured residual strain is minimum when the ternary layer composition is close to 16-17% In.

Comparative study of structural damage under irradiation in SiC nanostructured and conventional ceramics

In the context of research on new materials for next generation nuclear reactors, it becomes more and more interesting to know what can be the advantages of nano-structured materials for such applications. In this study, we performed irradiation experiments on micro-structured and nano-structured {beta}-SiC samples, with 95 MeV Xe and 4 MeV Au ions. The structure of the samples was characterized before and after irradiation by grazing incidence X-ray diffraction and Raman spectroscopy. The results showed the occurrence of a synergy between electronic and nuclear energy loss in both samples with 95 MeV Xe ions, while the nano-structured pellet was found to have a better resistance to the irradiation with 4 MeV Au ions. (authors)