F. Terki

Emerging properties and applications of spin crossover nanomaterials

In the past few years we have witnessed the spectacular development of nano-materials of spin crossover complexes of 3d4–3d7 transition metal ions, including thin films, nanoparticles, nanopatterns and nanoscale assemblies. These developments were primarily motivated by fundamental questions concerning size reduction effects on the bistability properties. On the other hand, spin crossover nano-objects require new characterization techniques (scanning probe microscopy, micro-magnetometry, etc.), revealing unknown details of the SCO mechanism at the nanoscale and giving access also to properties, which have been largely unexplored until now. In this Highlight, we review these recent experimental developments and discuss perspectives for societal applications as well.

Postgrowth annealing of (Ga,Mn) As under As capping: An alternative way to increase TC

In situ postgrowth annealing of (Ga,Mn)As layers under As capping is adequate for achieving high Curie temperatures (TC) in a similar way as ex situ annealing in air or in N2 atmosphere practiced earlier. Thus, the first efforts give an increase of TC from 68 to 145 K after 2 h annealing at 180 °C. These data, in combination with lattice parameter determinations and photoemission results, show that the As capping acts as an efficient sink for diffusing Mn interstitials.

Low-frequency dynamics in an optical strong glass: vibrational and relaxational contributions

A detailed Raman and Brillouin scattering investigation on a strong glass over a wide range of temperature across the glass transition temperature is presented. The aim of the study is to determine whether the low-frequency vibrational behaviour is correlated with the elastic properties. A careful comparison of several models is carried out in order to determine the reliability of theoretical predictions devoted to the understanding of the dynamics of glass-forming systems. A clear correlation between the position of the boson peak and the hypersonic velocity in both polarizations is demonstrated; in particular, the connection is in better accordance if one considers the transverse velocity. On the other hand, the quasielastic Raman scattering has been analysed in terms of excitations directly coupled to the acoustic phonons. The correlation between the quasielastic Raman intensity and the hypersonic attenuation predicted for the low-temperature regime has been found to hold over a broad range below and above . We found a strong correlation between the Raman features (at ) and the Brillouin data (at ), suggesting that the same mechanism is responsible for the vibrational dynamics within the range. Moreover, we emphasize that the softening of the vibrational modes responsible for the boson peak occurs significantly below , while an increase of the quasielastic intensity sets in, indicating some similarity between the origin of the boson peak and the origin of the growth of the quasielastic line.

Elastic properties of xerogels and sintered aerogels probed by inelastic light scattering

Abstract Elastic properties of silica xerogels and sintered aerogels of densities from ρ =0.5 g/cm 3 to 2.2 g/cm 3 were investigated by Brillouin light scattering. Sound velocities and acoustic attenuation at hypersonic frequencies was measured as a function of density. The elastic modulus C follows a scaling power law C = ρ α with α =3.3±0.1 and confirms previous data at low frequencies. For xerogels, the comparison between as prepared and oxidized samples after heat treatments, shows an increase of the elastic constant greater than 50% without significant change of the α exponent. This α value is close to that observed for densified aerogels emphasizing that this exponent seems to be independent on the sample treatment and preparation. Therefore, the fractal behavior of the material is not essential to explain power-law dependence. The acoustic attenuation has also been determined as a function of the density. A relaxational process coupled to acoustic vibrations disappears after heat treatment. This effect is also related to large inelastic light contribution in the very low-frequency part of the Raman spectra.

Low frequency dynamics of silica xerogel porous system

Abstract Coherent inelastic neutron and light (Raman and Brillouin) scattering measurements are repbrted on silica xerogels over a wide range of densities and temperatures. Raman scattering results show the presence of a boson peak at temperatures where the quasielastic contribution is negligible. The boson peak of xerogels is at a frequency around 35cm−1 and has a shape similar to that of melted silica. At lower frequencies (7–16cm−1) another bump is visible in the Raman scattering, believed to be connected to the presence of the pores and its frequency connected to their mean sizes. This important result seems to indicate that the disorder introduced by the presence of the pores does not influence very much the density of states in the frequency range of the boson peak. A comparison is made between the Q dependence of the elastic and inelastic structure factors as a function of sample density and temperature. The absence of a peak in the inelastic structure factor for the Q value corresponding to the fi...

Experimental determination of the phonon density of states in filled skutterudites: evidence for a localized mode of the filling atom

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

Low frequency light scattering in silica xerogels

New Brillouin and Raman scattering measurements on silica xerogels in a wide range of densities are compared among each other and with data in melt quenched vitreous silica, in the 10-750 K temperature region. In the low frequency part of the Raman spectra, the boson peak characteristic of glasses is observed for all studied densities at low and high temperature when the quasielastic contribution is negligible. The results provide strong evidence of the presence of water in the pores of the silica skeleton.

Antisymmetric magnetoresistance anomalies and magnetic domain structure in GaMnAs/InGaAs layers

Antisymmetric magneto-resistance anomalies generated by a reversal of the magnetization are studied in a number of GaMnAs/InGaAs layers with out-of-plane (easy axis) magnetization. The anomalies occur independent of the magnetic field orientation. This shows, that once a magnetic domain with reversed magnetization is nucleated, simply the presence of the domain wall between the longitudinal contacts is sufficient to give rise to the anomaly. Very different shapes for magneto-resistance anomaly can be observed experimentally depending upon the sample. They reflect the various magnetic domain structures present inside the layers during the magnetization reversal process.

Planar Hall ring sensor for ultra-low magnetic moment sensing

The field sensitivity of a planar Hall effect (PHE) micro-ring type biosensor has been investigated as a function of magnetizing angle of the sensor material, for the sensing of low magnetic moment superparamagnetic labels. The field sensitivity is maximal at a magnetizing angle of α = 20°. At this optimized magnetizing angle, the field sensitivity of a PHE sensor is about 3.6 times higher than that measured at the conventional configuration, α = 90°. This optimization enables the PHE-ring sensor to detect superparamagnetic biolabels with ultra-low magnetic moments down to 4 × 10−13 emu.

Low-temperature anomalies of photoinduced second harmonic generation in skutterudites

Photoinduced second harmonic generation (PISHG) was found in skutterudite compounds of CeFe4Sb12 and Ce0.7Fe3.5Ni0.5 Sb12. Measurements versus temperature, pump–probe delaying time and external magnetic field were performed. The studied compounds belong to moderate heavy fermion compounds (HFC) in the ground state. The PISHG signals appear at 6.8 and 4.9 K for CeFe4Sb12 and Ce0.7Fe3.5Ni0.5Sb12, respectively. We suspect that these signals are due to anharmonic electron–phonon interactions creating a charge density non-centrosymmetry. The observed effects are caused either by a possible phase transition or by drastic changes in the electron structure of the HFC with decreasing temperature.