Franco, D. G., and C. Geibel. "Synthesis and study of the chiral magnetic system EuIr2P2." Physical Review B 104 (2021): 054416.
Abstract: Chiral materials, where no improper symmetry operations such as inversion are resent, are systems prone to the appearance of a skyrmion lattice. Recently it has been shown theoretically that not only ferromagnets (FMs) but also antiferromagnets (AFMs) can host such kind of phases. In this work we study a new candidate for AFM skyrmions, EuIr2P2, by means of magnetization and specific heat measurements on poly and single crystals. X-ray diffraction confirms a trigonal chiral crystal structure, where europium ions form helices along the c direction. In spite of predominantly FM interactions, Eu2+ ions order antiferromagnetically at
TN1=5 K in what seems to be an incommensurate amplitude-modulated magnetic state where the moments are oriented mainly along the c direction. A second magnetic transition takes place at TN2=2.9 K, involving the ordering of an in-plane component of the Eu moment likely resulting in an equal-moment structure. Specific heat data show a tail above TN1. Accordingly the magnetic entropy at
TN1 is strongly reduced in comparison to the expected Rln8 value. This evidences a significant amount of frustration. A simple analysis based on a Heisenberg model indicates that the observed properties imply the presence of several relevant interactions, with competing FM and AFM ones resulting in frustration. Thus
EuIr2P2 is a new interesting magnetic system, where chirality and frustration might result in unconventional magnetic textures.
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Haberkorn, H., M. Xu, W. R. Meier, J. Schmidt, S. Suárez, S. L. Bud’ko, and P. C. Canfield. "Substantial reduction of the anisotropy in the critical current densities Jcof Ni-doped CaKFe4As4single crystals by chemical and irradiation-induced disorder." Superconductor Science and Technology 34, no. 3 (2021): 035013.
Abstract: We demonstrate that the anisotropy in the critical current densities, Jc, of CaK(Fe1-xNix)4As4 (x = 0, 0.015, 0.025, and 0.030) single crystals reduces with increasing chemical and irradiation-induced disorder. The magnetic field dependences of Jc are analyzed by performing magnetization measurements with H applied parallel and perpendicular to the crystallographic c-axis. The results show that undoped crystals display large anisotropies in Jc due to an enhancement of the vortex pinning with H applied parallel to the crystallographic ab-planes. This anisotropy reduces substantially as Ni addition increases. Moreover, we found that random disorder introduced by proton irradiation enhances mainly the vortex pinning for H parallel to the c-axis. Consequently, using adequate fluencies, the vortex pinning at low temperatures in both undoped and doped samples becomes close to isotropic. These results make the CaKFe4As4 system promising for applications that require isotropic Jc under magnetic fields.
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Haberkorn, N., Y. Y. Zhang, Z. X. Bi, B. H. Park, L. Civale, and Q. X. Jia. "Effect of Co2N impurity on the superconducting properties of δ-MoN thin films grown by polymer assisted deposition." Materials Chemistry and Physics 259 (2021): 124184.
Abstract: We report the effect of Co2N impurity on the superconducting properties of δ-MoN thin films grown by polymer-assisted deposition on c-cut sapphire (Al2O3). The films show a superconducting transition temperature of 10.4 K and an upper critical field Hc2(0) perpendicular to the film surface around 3 T. The latter corresponds to a relatively large coherence length ξ, which enhances the two-dimensional limit when the magnetic field is applied parallel to the film surface. In comparison with pure δ-MoN films, the inclusion of Co2N impurity in the δ-MoN films could significantly modify the critical current density at the vortex-free state. The ability to tune the superconducting properties of metal-nitride superconductors by introducing chemically and structurally compatible impurity may find potential applications for superconducting single-photon detectors.
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Hofer, J. A., and N. Haberkorn. "Flux flow velocity instability and quasiparticle relaxation time in nanocrystalline β-W thin films." Thin Solid Films 730 (2021): 138690.
Abstract: We characterized the superconducting properties of a micropatterned 23 nm thick β-W film using electrical transport measurements in magnetic fields. The film is nanocrystalline and displays a smooth surface. The superconducting critical temperature is 4.6 K. The critical current densities display a fast drop with the magnetic field, indicating weak vortex pinning. From the analysis of the instability in the vortex motion at low magnetic fields in the flux-flow state, we observe vortex velocities up 800 m/s. An inelastic lifetime of quasiparticles around 0.6 ns is estimated at low temperatures. Together with high uniformity in thickness and simplicity in the fabrication process, the superconducting properties make β-W films promising for applications in fast single-photon detectors.
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Pérez, M., E. D. Martínez, J. Lipovetzky, J. Marín, M. Sofo Haro, F. Alcalde Bessia, M. Gómez Berisso, and J. J. Blostein. "High spatial resolution neutron detection technique based on Commercial Off-The-Shelf CMOS image sensors covered with NaGdF_4 nanoparticles." Journal of Instrumentation 16 (2021): P08008.
Abstract: We present a position-sensitive neutron detection technique based on a Commercial Off-The-Shelf CMOS image sensor (CIS) covered with nanoparticles of sodium gadolinium fluoride (NaGdF4). The synthesis procedure and characterization of the NaGdF4 nanoparticles are detailed, as well as the deposition method of the conversion layers over the surface of the chips. We also present a manufacture method of test patterns made with neutron-absorbing materials. These patterns were designed to evaluate the performance of the proposed technique. Analyzing the obtained neutron images we conclude that the intrinsic spatial resolution of the developed method is better than (15±6) μm, this upper bound for the spatial resolution is comparable with that obtained with the best neutron position-sensitive detectors available nowadays.
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Puig, J., N. R. Cejas Bolecek, J. Aragón Sánchez, M. I. Dolz, M. Konczykowski, and Y. Fasano. "Bridge in micron-sized Bi2Sr2CaCu2O8 sample act as converging lens for vortices." Physica C: Superconductivity and its Applications 590 (2021): 1353948.
Abstract: We report on direct imaging of vortex matter nucleated in micron-sized BiSrCaCuO superconducting samples that incidentally present a bridge structure. We find that when nucleating vortices in a field-cooling condition the deck of the bridge acts as a converging lens for vortices. By means of Bitter decoration images allowing us to quantify the enhancement of vortex-vortex interaction energy per unit length in the deck of the bridge, we are able to estimate that the deck is thinner than m. We show that the structural properties of vortex matter nucleated in micron-sized thin samples are not significantly affected by sample-thickness variations of the order of half a micron, an important information for type-II superconductors-based mesoscopic technological devices.
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Sutter, J. G., A. Sarmiento Chávez, S. Soria, M. Granada, L. Neñer, S. Bengió, P. Granell, F. Golmar, N. Haberkorn, A. G. Leyva et al. "Tuning the magneto-electrical properties of multiferroic multilayers through interface strain and disorder." Journal of Alloys and Compounds 859 (2021): 157820.
Abstract: Artificially engineered superlattices were designed and fabricated to induce different growth mechanisms and structural characteristics. DC sputtering was used to grow ferromagnetic (La0.8Ba0.2MnO3)/ferroelectric (Ba0.25Sr0.75TiO3 or BaTiO3) superlattices. We systematically modified the thickness of the ferromagnetic layer to analyze dimensional and structural disorder effects on the superlattices with different structural characteristics. The crystalline structure was characterized by X-ray diffraction and transmission electron microscopy. The magnetic and electronic properties were investigated by SQUID magnetometry and resistance measurements. The results show that both strain and structural disorder can significantly affect the physical properties of the systems. Ba0.25Sr0.75TiO3 based superlattices with a low thickness of the ferromagnetic layers (4 nm) present compressive strain that decreases the ferromagnetic transition temperature from 250 K corresponding to the unstressed samples to 230 K. In these samples, the localization energy of the charge carrier through the electron-phonon interaction decreases at low temperatures (∼100 meV). Ba0.25Sr0.75TiO3 based superlattices with thicknesses of the ferromagnetic layers higher than 12 nm present tensile strain that reduces the charge carrier localization energy at low temperatures (∼1 meV), increasing the ferromagnetic transition temperature (Tc∼265 K). Structural defects in BaTiO3 based superlattices have a stronger influence on the magnetic properties than on the transport properties. Nevertheless, disorder blocks the ferromagnetic transition for highly disordered samples (thickness of the ferromagnetic layer < 3 nm). These results help to further understand the role of strain and interface effects in the magnetic and transport properties of manganite based multiferroic systems.
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Sutter, J. G., A. Sarmiento Chavez, Soria, M. Granada, L. Neñer, S. Bengió, P. Granel, F. Golmar, H. Haberkorn, G. Leyva et al. "Stress and disorder effect in the physical properties of artificially engineered multiferroic superlattices." Materials Chemistry and Physics 271 (2021): 124910.
Abstract: La0·8Ba0·2MnO3, Ba0·25Sr0·75TiO3 and BaTiO3 superlattices were grown to study the influence of structural disorder on the physical properties of multiferroic multilayers. Controlling the lattice mismatch of the superlattices allowed growing structures with different growth mechanisms. The manganite layers in the samples were used as “sensor layers”, that respond to the structural changes in the superlattices, induced by changing the thickness and nature of the ferroelectric layers. Stress has a weak influence on the magnetic properties of these systems. Transport properties are characterized by a high temperature thermally activated regime and a low temperature variable hopping one. The strain and structural disorder in the samples increases the localization energy of the current carriers for both regimes. Important interface effects can be achieved controlling the strain and disorder in the interfaces, allowing tuning the metal-insulator transition temperature. These results help to further understand the role of interface effects in the development of manganite based ferromagnetic/ferroelectric multilayered systems.
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Tolosa, M. R., M. J. Jimenez, P. Pedrazzini, C. Arreguez, L. P. Acosta, A. Gomez, M. S. Granovsky, H. Brizuela, D. Arias, and N. Nieva. "Stability of the Fe23Zr6 phase in Zr alloys." Revista SAM 1 (2021): 1.
Abstract: Alloyed with iron (Fe), niobium (Nb) and tin (Sn), zirconium (Zr) is the main element in the Zirlo-type alloys, vastly used as structural elements and as containers of burnable elements in nuclear reactors. Although Zr is a major component in this type of alloys, it is most important to know the phase diagrams of their components as well as possible. The binary phase diagram of the Fe-Zr system has been studied for some time now by several authors. However, in the Fe-rich region the existence of the Fe23Zr6 compound, which was first described in 1962, remains controversial. To clarify the origin of this phase, the present work deals with the manufacture and prolonged heat treatments at different temperatures of alloys located in the Fe-rich region of the Fe-Nb-Zr, Fe-Sn-Zr and Fe-Zr phase diagram. The experiments have been performed with raw materials whose degree of purity was varied. The phases present were identified by using X-ray diffraction (XRD), semiquantitative microanalysis by using scanning electron microscopy analysis with energy dispersive spectrometry (SEM-EDS) and quantitative microanalysis by using electron microprobe with wavelength dispersive spectrometry (SEM-WDS). Finally, by using the results of characterization of heat-treated alloys for long annealing times at different temperatures, it is suggested that the presence of the Fe23Zr6 compound is an equilibrium phase of the Fe-Zr binary system.
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