L.P. Cardoso

Magnetic properties and magnetocaloric effect of the HoAgGa compound

Magnetic properties and magnetocaloric effect (MCE) of the HoAgGa compound are investigated by magnetization and heat capacity measurements. A giant reversible MCE was observed around TC = 7.2 K. The maximum values of magnetic entropy change and adiabatic temperature are found to be 16 J kg−1 K−1 and 6 K, respectively, with a refrigerant capacity value of 262 J kg−1 for field change of 5 T. These magnetocaloric parameters also remain large for a wide range of temperature above TC. The large MCE as well as no hysteresis loss make HoAgGa an attractive candidate for low temperature magnetic refrigerant.

High pressure studies on bis (L-histidinate) nickel (II) monohydrate

Raman spectra of bis(l-histidinate)nickel(II) monohydrate crystal were obtained for pressures up to 9.5GPa. Our results show the disappearance of some of the Raman modes and the appearance of other modes. These modifications evidence that the sample undergoes phase transitions at around 0.8 and 3.2GPa. The role played by the Ni ions and hydrogen bonds in the dynamics of the phase transitions is discussed. Under decompression, down to atmospheric pressure, the original Raman spectra are recovered, showing that both phase transitions are fully reversible.

Low-temperature Raman spectra of the 2-(α-methylbenzylamino)-5-nitropyridine crystal

The polar organic 2-(α-methylbenzylamino)-5-nitropyridine crystal (MBANP) has been studied by Raman spectroscopy at low temperatures (from 300 to 10K). The effect of temperature change on the vibrational spectrum is discussed with the aid of DFT calculations. The behavior of the Raman spectra indicates that MBANP molecules present a different conformation at low temperatures associated with the rotation of the phenyl and pyridine rings. Temperature-dependent X-ray measurements and differential scanning calorimetry (DSC) analysis were utilized as complementary techniques to investigate the structural stability of MBANP crystal.

CRYSTALLITE SIZE DEPENDENCE ON MAGNETOCALORIC EFFECT OF GdAl2 INTERMETALLIC COMPOUND

In this work, the magnetic, magnetocaloric and structural properties of rare earth based GdAl2 intermetallic compound on nanometric scale were studied, focusing on the crystallite size dependence of the magnetocaloric effect. The nanoparticles were obtained in a mechanical milling process in which the alloys, previously synthetized in an arc furnace under argon atmosphere, were crushed inside a planetary high energy ball system also in an argon atmosphere at different milling times. The nanoparticles structure were analyzed by X-ray diffraction, as well as lattice strain and crystallite size values. Rietveld refinement method analysis of the experimental data has confirmed the existence of a single cubic phase for the GdAl2 series, as expected for a Laves phase compound. As the milling time increases, one has observed the crystallite size decreases down to 19 nm the minimum value. The lattice values increases due to atomic disorder induced by the mechanical milling process. Magnetic properties are also strongly crystallite size dependent, evidenced by the temperature interval enlargement of the second order magnetic transition as the particle size decreases. A table-like behavior with improved RCP (Relative Cooling Power) was obtained for larger crystallite size samples, while smaller crystallite size ones exhibited an increased |DSMmáx| at cryogenic temperatures, associated with the superparamagnetism. Therefore, the magnetocaloric effect on the GdAl2 nanostructured compounds was optimized in comparison to the bulk sample. CRYSTALLITE SIZE DEPENDENCE ON MAGNETOCALORIC EFFECT OF GdAl2 INTERMETALLIC COMPOUND

InGaP/GaAs(001) structural characterization by means of synchrotron radiation Renninger scan

We studied the initial state of VLS-growth of InAs nanorods onto GaAs[111]B via Au catalyst at growth temperature of 450°C where the growth was truncated after 20s, 40s, 60s and 300s, respectively. The whole samples have been analyzed at room temperature using out-of plane high-resolution x-ray diffraction in home laboratory and in-plane depth-resolved x-ray grazing-incidence diffraction (GID) with synchrotron radiation. Selected NRs have been inspected by TEM as well. Samples with growth time of 20s and 40s did not show InAs NRs. Instead we identified several Indium rich Au phases at the surface and a strongly enhanced diffuse scattering in vicinity of the GaAs peaks. InAs NRs appeared after 60s growth time crystallized in wurzite phase. At same time small In1-xGaxAs components with composition ranging between x=0.05 and 0.22 were observed grown with zinc-blende structure accompanied with large diffuse scattering close to the GaAs. Depth-resolved GID and HR-TEM showed that the In1-xGaxAs components are located at the basement of NRs. Additionally we found alloy clusters buried under truncated NRs. Our data suggest a model where InAs NRs grow out of an Indium enriched Au-Ga phase. However, inclusion of indium into Au-Ga (Te=339°C) first decreases the melting point of Au-GaIn eutecticum, but for higher Indium content the eutectic melting temperature increases up to 454°C valid for the Au-In system. For a growth temperature between 350°C and 460°C Au alloy droplets remain liquid since the gallium concentration exceeds about 30 to 0 at%, respectively. Therefore Au droplets with low gallium content become solid and cannot act as catalyst anymore. Hence, stable NR growth can be established only if the metallic alloy droplet contains a sufficient concentration of gallium.