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- ItemAnalysis of electronic properties frommagnetotransport measurements on Ba(Fe1-xNix)2As2 thin films(Basel : MDPI AG, 2020) Shipulin, I.; Richter, S.; Thomas, A.A.; Nielsch, K.; Hühne, R.; Martovitsky, V.We performed a detailed structural, magnetotransport, and superconducting analysis of thin epitaxial Ba(Fe1-xNix)2As2 films with Ni doping of x = 0.05 and 0.08, as prepared by pulsed laser deposition. X-ray diffraction studies demonstrate the high crystalline perfection of the films, which have a similar quality to single crystals. Furthermore, magnetotransport measurements of the films were performed in magnetic fields up to 9 T. The results we used to estimate the density of electronic states at the Fermi level, the coefficient of electronic heat capacity, and other electronic parameters for this compound, in their dependence on the dopant concentration within the framework of the Ginzburg-Landau-Abrikosov-Gorkov theory. The comparison of the determined parameters with measurement data on comparable Ba(Fe1-xNix)2As2 single crystals shows good agreement, which confirms the high quality of the obtained films.
- ItemPredicting the dominating factors during heat transfer in magnetocaloric composite wires(Amsterdam : Elsevier B.V., 2020) Krautz, M.; Beyer, L.; Funk, A.; Waske, A.; Weise, B.; Freudenberger, J.; Gottschall, T.Magnetocaloric composite wires have been studied by pulsed-field measurements up to μ0ΔH = 10 T with a typical rise time of 13 ms in order to evaluate the evolution of the adiabatic temperature change of the core, ΔTad, and to determine the effective temperature change at the surrounding steel jacket, ΔTeff, during the field pulse. An inverse thermal hysteresis is observed for ΔTad due to the delayed thermal transfer. By numerical simulations of application-relevant sinusoidal magnetic field profiles, it can be stated that for field-frequencies of up to two field cycles per second heat can be efficiently transferred from the core to the outside of the jacket. In addition, intense numerical simulations of the temperature change of the core and jacket were performed by varying different parameters, such as frequency, heat capacity, thermal conductivity and interface resistance in order to shed light on their impact on ΔTeff at the outside of the jacket in comparison to ΔTad provided by the core.
- ItemMagnetocaloric properties of multicomponent Laves phase compounds and their composites(Bristol : IOP Publ., 2021) Ćwik, J.; Koshkid’ko, Yu; Nenkov, K.; Kolchugina, N.Heat capacity measurements have been performed for multicomponent (Ho0.9Er0.1)1-xGdxCo2 compounds with x = 0.05, 0.1, and 0.15. The isothermal magnetic entropy change, ΔSmag, allowing the estimation of the magnetocaloric effect, was determined based on the heat capacity measurements in magnetic fields up to 2 T. A numerical method, with the magnetic entropy change of individual (Ho0.9Er0.1)1-xGdxCo2 compounds, was used to calculate the optimal molar composition of the constituents and the resulting change of the isothermal magnetic entropy of composite, ΔScomp. The results show that proposed composite can be considered as a refrigerant material in magnetic refrigerators performing an Ericsson cycle in a temperature range of 90-130 K.
- ItemShielding Effect on Flux Trapping in Pulsed-Field Magnetizing for Mg-B Bulk Magnet(Bristol : IOP Publ., 2021) Oka, T.; Yamanaka, K.; Sudo, K.; Dadiel, L.; Ogawa, J.; Yokoyama, K.; Häßler, W.; Noudem, J.; Berger, K.; Sakai, N.; Miryala, M.; Murakami, M.MgB2 superconducting bulk materials are characterized as simple and uniform metallic compounds, and capable of trapping field of non-distorted conical shapes. Although pulsed-field magnetization technique (PFM) is expected to be a cheap and an easy way to activate them, the heat generation due to the magnetic flux motion causes serious degradation of captured fields. The authors precisely estimated the flux trapping property of the bulk samples, found that the flux-shielding effect closely attributed to the sample dimensions. The magnetic field capturing of Ti-5.0wt% sample reached the highest value of 0.76 T. The applied field which reached the centre of the sample surface shifted from 1.0 T to 1.2 T with increasing sample thickness from 3.67 mm to 5.80 mm. This means that the shielding effect was enhanced with increasing the sample thickness. Moreover, Ti-addition affected the frequency of flux jump happenings. The occurrence of flux jumps was suppressed in 5.0wt%Ti-added sample. This means that the heat capacity of the compounds was promoted by Ti addition.
- ItemEntropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range(Basel : MDPI, 2018-8-30) Haas, Sebastian; Mosbacher, Mike; Senkov, Oleg N; Feuerbacher, Michael; Freudenberger, Jens; Gezgin, Senol; Völkl, Rainer; Glatzel, UweWe determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy.
- ItemMagnetoelastic coupling and ferromagnetic-type in-gap spin excitations in multiferroic α-Cu2V2O7(Bristol : Institute of Physics Publishing, 2018) Wang, L.; Werner, J.; Ottmann, A.; Weis, R.; Abdel-Hafiez, M.; Sannigrahi, J.; Majumdar, S.; Koo, C.; Klingeler, R.We investigate magnetoelectric coupling and low-energy magnetic excitations in multiferroic α-Cu2V2O7 by detailed thermal expansion, magnetostriction, specific heat and magnetization measurements in magnetic fields up to 15 T and by high-field/high-frequency electron spin resonance studies. Our data show negative thermal expansion in the temperature range ≤200 K under study. Well-developed anomalies associated with the onset of multiferroic order (canted antiferromagnetism with a significant magnetic moment and ferroelectricity) imply pronounced coupling to the structure. We detect anomalous entropy changes in the temperature regime up to ∼80 K which significantly exceed the spin entropy. Failure of Grüneisen scaling further confirms that several dominant ordering phenomena are concomitantly driving the multiferroic order. By applying external magnetic fields, anomalies in the thermal expansion and in the magnetization are separated. Noteworthy, the data clearly imply the development of a canted magnetic moment at temperatures above the structural anomaly. Low-field magnetostriction supports the scenario of exchange-striction driven multiferroicity. We observe low-energy magnetic excitations well below the antiferromagnetic gap, i.e., a ferromagnetic-type resonance branch associated with the canted magnetic moment arising from Dzyaloshinsii-Moriya (DM) interactions. The anisotropy parameter meV indicates a sizeable ratio of DM- and isotropic magnetic exchange.
- ItemSpin-glass state and reversed magnetic anisotropy induced by Cr doping in the Kitaev magnet α-RuCl3(College Park, MD : American Physical Society, 2019) Bastien, G.; Roslova, M.; Haghighi, M.H.; Mehlawat, K.; Hunger, J.; Isaeva, A.; Doert, T.; Vojta, M.; Büchner, B.; Wolter, A.U.B.Magnetic properties of the substitution series Ru1-xCrxCl3 were investigated to determine the evolution from the anisotropic Kitaev magnet α-RuCl3 with Jeff=1/2 magnetic Ru3+ ions to the isotropic Heisenberg magnet CrCl3 with S=3/2 magnetic Cr3+ ions. Magnetization measurements on single crystals revealed a reversal of the magnetic anisotropy under doping, which we argue to arise from the competition between anisotropic Kitaev and off-diagonal interactions on the Ru-Ru links and approximately isotropic Cr-Ru and isotropic Cr-Cr interactions. In addition, combined magnetization, ac susceptibility, and specific-heat measurements clearly show the destabilization of the long-range magnetic order of α-RuCl3 in favor of a spin-glass state of Ru1-xCrxCl3 for a low doping of x≤0.1. The corresponding freezing temperature as a function of Cr content shows a broad maximum around x ≤ 0.45.
- ItemRedox chemistry in the pigment eumelanin as a function of temperature using broadband dielectric spectroscopy(Cambridge : Royal Society of Chemistry, 2019) Motovilov, K.A.; Grinenko, V.; Savinov, M.; Gagkaeva, Z.V.; Kadyrov, L.S.; Pronin, A.A.; Bedran, Z.V.; Zhukova, E.S.; Mostert, A.B.; Gorshunov, B.P.Conductive biomolecular systems are investigated for their promise of new technologies. One biomolecular material that has garnered interest for device applications is eumelanin. Its unusual properties have led to its incorporation in a wide set of platforms including transistor devices and batteries. Much of eumelanin's conductive properties are due to a solid state redox comproportionation reaction. However, most of the work that has been done to demonstrate the role of the redox chemistry in eumelanin has been via control of eumelanin's hydration content with scant attention given to temperature dependent behavior. Here we demonstrate for the first time consistency between hydration and temperature effects for the comproportionation conductivity model utilizing dielectric spectroscopy, heat capacity measurements, frequency scaling phenomena and recognizing that activation energies in the range of ∼0.5 eV correspond to proton dissociation events. Our results demonstrate that biomolecular conductivity models should account for temperature and hydration effects coherently.
- ItemInvestigating the magnetic and magnetocaloric behaviors of LiSm(PO3)4(London : RSC Publishing, 2023) Tran, T.A.; Petrov, Dimitar N.; Phan, T.L.; Tu, B. D.; Nhat, H.N.; Tran, H.C.; Weise, B.; Cwik, J.; Koshkid'ko, Yu S.; Manh, T.V.; Hoang, T.P.; Dang, N.T.We report a detailed study on the magnetic behaviors and magnetocaloric (MC) effect of a single crystal of lithium samarium tetraphosphate, LiSm(PO3)4. The analyses of temperature-dependent magnetization data have revealed magnetic ordering established with decreasing temperature below Tp, where Tp is the minimum of a dM/dT vs. T curve and varies as a linear function of the applied field H. The Curie temperature has been extrapolated from Tp(H) data, as H → 0, to be about 0.51 K. The establishment of magnetic-ordering causes a substantial change in the heat capacity Cp. Above Tp, the crystal exhibits paramagnetic behavior. Using the Curie-Weiss (CW) law and Arrott plots, we have found the crystal to have a CW temperature θCW ≈ −36 K, and short-range magnetic order associated with a coexistence of antiferromagnetic and ferromagnetic interactions ascribed to the couplings of magnetic dipoles and octupoles at the Γ7 and Γ8 states. An assessment of the MC effect has shown increases in value of the absolute magnetic-entropy change (|ΔSm|) and adiabatic-temperature change (ΔTad) when lowering the temperature to 2 K, and increasing the magnetic-field H magnitude. Around 2 K, the maximum value of |ΔSm| is about 3.6 J kg−1 K−1 for the field H = 50 kOe, and ΔTad is about 5.8 K for H = 20 kOe, with the relative cooling power (RCP) of ∼82.5 J kg−1. In spite of a low MC effect in comparison to Li(Gd,Tb,Ho)(PO3)4, the absence of magnetic hysteresis reflects that LiSm(PO3)4 is also a candidate for low-temperature MC applications below 25 K.