F.G. Gandra

Nd2O3 doped low silica calcium aluminosilicate glasses: Thermomechanical properties

The effects of Nd2O3 doping on the thermal and mechanical properties of vacuum melted, low silica, calcium aluminosilicate glasses are presented. For the doped glasses, the vitrification limit was found to correspond to a maximum load of 5 wt % Nd2O3. The influence of the rare earth doping on the thermal diffusivity, thermal conductivity, and Vickers hardness was such that all these physical parameters decreased by roughly the same amount, namely 8%, between the undoped and the 5 wt % doped sample. The dependence of these parameters, as a function of the Nd2O3 doping, strongly supports the idea that the Nd3+ act as network modifiers.

The magnetic and magnetocaloric properties of Gd5Ge2Si2 compound under hydrostatic pressure

The Gd5Ge2Si2 compound presents a giant magnetocaloric effect with transition temperature at around 276 K and is a very good candidate for application as an active regenerator material in room temperature magnetic refrigerators. Recently it has been shown that pressure induces a colossal magnetocaloric effect in MnAs, a material that presents a giant magnetocaloric effect and a strong magnetoelastic coupling, as also happens with the Gd5Ge2Si2 compound. This motivated a search of the colossal effect in the Gd5Ge2Si2 compound. This work reports our measurements on the magnetic properties and the magnetocaloric effect of Gd5Ge2Si2 under hydrostatic pressures up to 9.2 kbar and as a function of temperature. Contrary to what happens with MnAs, pressure increases the Curie temperature of the compound, does not affect the saturation magnetization and decreases markedly its magnetocaloric effect.

Ambient pressure colossal magnetocaloric effect in Mn1−xCuxAs compounds

Magnetic refrigeration is a good alternative to gas compression technology due to higher efficiency and environmental concerns. Magnetocaloric materials must exhibit large adiabatic temperature variations and a large entropic effect. MnAs shows the colossal magnetocaloric effect under high pressures or with Fe doping. In this work the authors introduce a class of materials—Mn1−xCuxAs—revealing a peak colossal effect of −175J∕(Kkg) for a 5T field variation at 318K and ambient pressure.

Structure and properties of water free Nd2O3 doped low silica calcium aluminate glasses

Abstract In this work, Nd 2 O 3 doped low silica calcium aluminate glasses have been melted under vacuum conditions. Measurements of thermal diffusivity, density, optical absorption coefficient, glass transformation temperature ( T g ), crystallisation temperature ( T x ), dilatometric softening temperature ( T d ) and Vickers microhardness have been carried out. The results provided information about the samples structure and showed that for Nd 2 O 3 concentration up to 5 wt% there was no devitrification of the samples. The decrease in hardness and thermal diffusivity was about 8%, while the density increased about 8% with the increasing concentration of Nd 2 O 3 . The thermomechanical properties combined with their quantum efficiency indicate that this material is a candidate for an active medium for solid-state lasers.

Resonant microwave cavity response of amorphous ribbons

In this study we report microwave experiments on the amorphous ribbon Fe4.6Co70.4Si15B10 using the usual setup for magnetic resonance experiments at 9.4 GHz to observe the cavity response. Several samples cut from the same tape were annealed, either in the presence or in the absence of a magnetic field. For each sample the magnetization and magnetoimpedance (MI) curves were obtained, with MI ratios between 1% and 40%. The magnetic resonance spectrum consists of the usual ferromagnetic line and an anomalous low field cavity response for H<100 Oe. The shape of this low field signal depends on the thermal treatment and on the orientation of the external field. We believe that this signal is in part a ferromagnetic peak but it also carries information of the skin‐depth changes with a sweeping magnetic field.

The physical properties of Gd3Ru: A real candidate for a practical cryogenic refrigerator

The magnetization, the specific heat, and the magnetocaloric effect (MCE) for Gd3Ru are presented as function of temperature at different magnetic fields. The results show a maximum entropy change −ΔS= 30 J/kg K @ 5 T, which is the highest value for the R3M compounds. With a non-hysteretic transition of first order type at TC = 54 K, it presents a temperature change ΔTmax = 5.7 K around 59 K with a refrigerating cooling power of 700 J/kg and these results are comparable to values found for giant MCE materials. This compound is stable and able to operate at temperatures between 90 K and 40 K with a minimum −ΔS= 5 J/kg K. These figures were obtained by sweeping the magnetic field without using sample preparation routines. This methodology is appropriate to evaluate the MCE for the cycling process of a cryogenic magnetic refrigerator.

Study on the observation of Eu2+ and Eu3+ valence states in low silica calcium aluminosilicate glasses

The optical, magnetic and structural properties of Eu doped low silica calcium aluminosilicate glasses were investigated. The optical absorption coefficient presented two bands at 39,246 and 29,416 cm(-1), which were assigned respectively to the [Formula in text], and [Formula in text] transitions of Eu(2+). The fluorescence measured at 300 K on a sample doped with 0.5 wt% of Eu(2)O(3) exhibited a broad band centered at 17,350 cm(-1), which is attributed to the [Formula in text] transition of Eu(2+), whereas the additional peaks are due to the [Formula in text] transitions of Eu(3+). From magnetization and XANES data it was possible to evaluate the fractions of Eu(2+) and Eu(3+) for the sample doped with 0.5 and 5.0 wt% of Eu(2)O(3), the values of which were approximately 30 and 70%, respectively.

ESR of Er3+, Gd3+, Yb3+ and Dy3+ in the heavy-fermion compounds UPt3 and UBe13

Abstract Low temperature ( T>0.6K ) Electron Spin Resonance (ESR) experiments were carried out for the Heavy-Fermion compounds UBe 13 and UPt 3 doped with several Rare Earth ions. The measured Korringa rate for all Rare Earth were always within the same order of magnitude of a normal metal and no significant deviation from the linear behavior was observed. The experimental results lead to the conclusion that the coupling between the Uranium ion and the Rare Earth ion through the s-f electron interaction is very small, if any.

Anisotropic magnetocaloric effect in gadolinium thin films: Magnetization measurements and acoustic detection

In this letter, it is demonstrated the ability of the magnetoacoustic technique in detecting the magnetocaloric effect in gadolinium thin films (1.0 μm and 3.0 μm thick), which is not accessible through conventional temperature sensors because of the reduced mass of the samples. The method, which detects the direct effect of the sample temperature variation, proved to be sensitive to the anisotropy of the films, making possible for the investigation of the anisotropic magnetocaloric effect. Magnetization measurements were also carried out, and from these measurements both the adiabatic temperature and the isothermal entropy variations were calculated. The acoustically detected magnetocaloric effect shows very good agreement with these calculations.

Study of the magnetocaloric properties of the antiferromagnetic compounds RGa2 (R = Ce, Pr, Nd, Dy, Ho and Er).

Magnetocaloric properties of antiferromagnetic RGa(2) (R = Ce, Pr, Nd, Dy, Ho and Er) compounds have been reported. These systems present an antiferromagnetic transition below 15 K and a field induced metamagnetic transition from the antiferromagnetic to ferromagnetic state. Our results show that the character of the magnetic field induced transition along the series affects the magnetocaloric properties. For the compounds with R = Ho, Dy and Er both negative and positive magnetocaloric effect (MCE) were observed above μ(0)ΔH = 2 T where the rate between negative and positive MCE contributions depends on how the magnetic transitions occur in these compounds. The evaluated values of maximum magnetocaloric properties of RGa(2) compounds are similar to other potential magnetic refrigerant materials reported in the literature.