Selda Kılıç Çetin

Effect of Ca doping on thermally activated flux flow in the Y3Ba5Cu8O18 superconductor

Y3Ba5Cu8O18 (Y-358) and Y3Ba5Ca2Cu8O18 (YCa-358) compounds were prepared by the so-called sol–gel method. The effect of doping Ca atoms into the Y3Ba5Cu8O18 superconductor is studied by employing the electrical resistivity measurements at various magnetic fields. The possible degradation in microstructural and superconducting properties due to the addition of Ca was discussed. The resistivity as a function of temperature measurements shows that the depression in superconducting temperature is more pronounced for both samples. A systematic analysis of the magnetoresistivity of the Y-358 and YCa-358 compounds hasbeen carried out by using the thermally activated flux flow (TAFF) model. The TAFF activation energy, U, is field dependent and obeys the power law U =cH−α, where α increases while c decreases with the addition of Ca. Furthermore, U and the calculated upper critical field, Hc2, decrease with the addition of Ca.

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn, Gd and Mn/Gd) nanocrystals

Abstract We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the Ax:Zn1−xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd2O3 is observed. Due to the large mismatch of the ionic radii between Mn2+ and Gd3+ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn2+ and Gd3+ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.

Room temperature magnetocaloric effect in Pr1.75Sr1.25Mn2O7 double-layered perovskite manganite system

Abstract In this work, we have studied on double-layered perovskite (Ruddlesden–Popper) manganite structure in Pr1.75Sr1.25Mn2O7 synthesised by sol–gel method. The crystal structure of the double-layered perovskite is found as tetragonal from the X-ray diffraction analysis with I4/mmm space group. A high Curie temperature, TC = 305 K is observed from the temperature dependence of magnetisation measurement. The isothermal magnetisation curves showed that magnetic phase transition is second order due to the positive slope of the Arrott plots. Maximum magnetic entropy change (ΔSM) and adiabatic temperature change (ΔTad) values are calculated as 3.99 J kg−1 K−1 and 2.1 K under external magnetic field of 70 kOe, respectively. Since our double-layered perovskite manganite sample has desired TC value and relatively high ΔSM, it can be a potential candidate as a magnetocaloric material for room temperature magnetic cooling systems.