The experimental evidence for the contraction of volume of gold implanted with hydrogen at low doses is presented. The contraction of lattice upon the addition of other elements is very rare and extraordinary in the solid-state, not only for gold but also for many other solids. To explain the underlying physics, the pure kinetic theory of absorption is not adequate and the detailed interaction of hydrogen in the lattice needs to be clarified. Our analysis points to the importance of the formation of hydride bonds in a dynamic manner and explains why these bonds become weak at higher doses, leading to the inverse process of volume expansion frequently seen in metallic hydrogen containers.
The La-excess alloys La1+δ(Fe0.85Si0.15)13 (δ = 0.06 and 0.09) exhibit large magnetocaloric effect which has been attributed to the occurrence of itinerant-electron metamagnetic transition near the Curie temperature TC. The maximum entropy change −ΔSm was shown to be from 4.5 to 11.5 J/kg K for the applied field variation ΔH from 20 to 70 kOe, respectively. The estimated relative cooling power for ΔH = 70 kOe was 418 J/kg. The alloys show a typical NaZn13-type cubic structure, featuring a doping-induced magnetovolume effect with the increase in TC. Under the applied pressure up to 2 GPa, the TC as deduced from resistance measurements decreased linearly, ΔTC = 113 (for δ = 0.06) and 111 K (for δ = 0.09), together with a corresponding decrease of resistivity, Δρ = 6.1 μΩ m at room temperature for both samples. At a low pressure, the effect of spontaneous magnetostriction on TC caused by applying the pressure appeared to have a similar magnitude to that of the negative magnetovolume effect caused by La-excess doping. In comparison with other stoichiometric La(Fe1−xSix)13 compounds, the pressure in our case was shown to have a smaller influence on TC.
Magnetic phase transitions are important for the application of individual materials toward modern spintronics and sensing. Using high-resolution sound velocity and pyrocurrent measurements, we have obtained a detailed magnetic phase diagram of multiferroic Ca2CoSi2O7 for magnetic fields applied along the crystallographic [100]- and [001]-directions. The magneto-acoustic measurements were conducted for both longitudinal and transverse modes in both static and pulsed magnetic fields up to 17 and 68 T, respectively. Distinct anomalies with significant changes in sound velocity and sound attenuation were observed at the onset of the structural and magnetic phase transitions. Interestingly, results obtained for fields parallel to the [100]-direction reveal a hysteresis and step-like decrements in ultrasound indicating a transition into a different metastable structural distortion below TN and at magnetic fields between 4 and 10 T that has not been previously reported. We believe that this field-induced distorted structure phase may be the cause of the subsequent phase transition at 11 T. At low temperatures, a softening of the lattice within the ordered phases is also observed, which may be due to residual spin fluctuations.
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