Quasiparticle scattering in superconductors

We compare results of high-resolution magnetic flux (susceptibility) measurements in very weak magnetic field, performed of three classes of superconductors. They reveal astonishing details of the transition to the superconducting state. Although Pb behaves also on this scale according to BCS predictions, La is more reminiscent of HTS materials, which exhibit anomalous features. We suggest that known structure peculiarities are due to a strong electron-lattice instability and lead to a resonance electron back scattering

The preparation route and final form of V-MXenes override the effect of the O/F ratio on their magnetic properties

This work was supported by OP VVV “Excellent Research Teams” project no. CZ.02.1.01/0.0/0.0/15_003/0000417 – CUCAM. P. E. would like to also acknowledge the Czech Science Foundation for the ExPro project (19-27551X). Computational resources and low-temperature infrastructure were supplied by the projects “e-Infrastruktura CZ” (e-INFRA CZ LM2018140) and MGML (LM2023065) supported by the Ministry of Education, Youth and Sports of the Czech Republic.Transition metal carbides and nitrides (MXenes) show a high potential for electrochemical energy storage in batteries and supercapacitors and for electrocatalysis. Their excellent electronic and magnetic characteristics have been highlighted in several theoretical studies. However, experimental research on MXenes is yet to confirm their predicted properties as candidates for controllable magnetic 2D materials. Here, we report our theoretical and experimental study of V2CTx MXenes (T = O, OH, F), providing key insights into their magnetism. Based on our density functional theory (DFT) analysis, we predicted ferromagnetic (FM) and antiferromagnetic (AFM) states of V2CTx, which are determined by the O/F ratio of surface functional groups. Accordingly, we prepared V2CTx MXenes in the form of multilayered powders and thin films with different O/F ratios. No experimental evidence of FM or AFM properties was found in any material. Nevertheless, powders and films with almost identical chemical compositions (in terms of O/F ratio) displayed different magnetic properties, whereas films with disparate chemical compositions revealed a similar magnetic character. Therefore, the preparation route and form of the final V2CTx material override the effect of the O/F ratio, which is often overestimated in theoretical studies. Moreover, these findings underscore the importance of preparing MXene materials to experimentally confirm their theoretically predicted properties.Peer reviewe

Magnetism and Magnetic Structure of NdCr2Si2C

International audienceA NdCr2Si2C polycrystal has been prepared and characterized by heat-capacity and magnetization, X-ray and neutron powder diffraction measurements. Ferromagnetism below TC = 21 K with Nd magnetic moments aligned parallel to the c-axis has been confirmed. We also studied the electronic structure of NdCr2Si2C by first-principles calculations, which predict the ordered magnetic moment both on the Nd and Cr sites. The magnetization and neutron diffraction data, however, do not allow resolving the question of existence of the Cr moment unambiguously

Thermal Traits of MNPs under High-Frequency Magnetic Fields: Disentangling the Effect of Size and Coating

We investigated the heating abilities of magnetic nanoparticles (MNPs) in a high-frequency magnetic field (MF) as a function of surface coating and size. The cobalt ferrite MNPs were obtained by a hydrothermal method in a water–oleic acid–ethanol system, yielding MNPs with mean diameter of about 5 nm, functionalized with the oleic acid. By applying another cycle of hydrothermal synthesis, we obtained MNPs with about one nm larger diameter. In the next step, the oleic acid was exchanged for 11-maleimidoundecanoic acid or 11-(furfurylureido)undecanoic acid. For the heating experiments, all samples were dispersed in the same solvent (dichloroethane) in the same concentration and the heating performance was studied in a broad interval of MF frequencies (346–782 kHz). The obtained results enabled us to disentangle the impact of the hydrodynamic, structural, and magnetic parameters on the overall heating capabilities. We also demonstrated that the specific power absorption does not show a monotonous trend within the series in the investigated interval of temperatures, pointing to temperature-dependent competition of the Brownian and Néel contributions in heat release

Self-Limitations of Heat Release in Coupled Core-Shell Spinel Ferrite Nanoparticles: Frequency, Time, and Temperature Dependencies.

We explored a series of highly uniform magnetic nanoparticles (MNPs) with a core-shell nanoarchitecture prepared by an efficient solvothermal approach. In our study, we focused on the water dispersion of MNPs based on two different CoFe2O4 core sizes and the chemical nature of the shell (MnFe2O4 and spinel iron oxide). We performed an uncommon systematic investigation of the time and temperature evolution of the adiabatic heat release at different frequencies of the alternating magnetic field (AMF). Our systematic study elucidates the nontrivial variations in the heating efficiency of core-shell MNPs concerning their structural, magnetic, and morphological properties. In addition, we identified anomalies in the temperature and frequency dependencies of the specific power absorption (SPA). We conclude that after the initial heating phase, the heat release is governed by the competition of the Brown and Néel mechanism. In addition, we demonstrated that a rational parameter sufficiently mirroring the heating ability is the mean magnetic moment per MNP. Our study, thus, paves the road to fine control of the AMF-induced heating by MNPs with fine-tuned structural, chemical, and magnetic parameters. Importantly, we claim that the nontrivial variations of the SPA with the temperature must be considered, e.g., in the emerging concept of MF-assisted catalysis, where the temperature profile influences the undergoing chemical reactions

Dispersible cobalt chromite nanoparticles: facile synthesis and size driven collapse of magnetism

Multiferotické oxidy mají enormní aplikační potenciál díky vzájemně vázané magnetické a dielektrické odezvě samostatného materiálu. Mezi nimi je jedinečný kobalt chromit se spin-indukovanou elektrickou polarizací uzamčenou ke směru magnetizace a směru šíření vektoru spirálové magnetické fáze. Povaha základního stavu magnetické struktury vede ke komplexnímu velikostně vázanému magnetickému chování, které se zhroutí při dosažení kritické velikosti částic. V naší práci jsme se zaměřili na přípravu samostatných kobalt chromitových nanočástic (NPs). Provedli jsme hydrotermální rozklad kobaltovo (II) / Cr (III), oleátů v systému voda / ethanol bez nutnosti dodatečné tepelné úpravy, který vedl ke stabilním kobalt chromovým nanočásticim o průměru 3,0(1)-4.2(1) nm a log normální rozdělení velikosti 12-16%. Velikost částic k dosažení kritické meze byla laděna reakční teplotou dosahující typicky 240 +/- 10 ° C. Připravené NP jsou potaženy kovalentně vázanou kyselinou olejovou a mohou být snadno dispergovány v nepolárních rozpouštědlech, což je činí vynikajícími kandidáty pro různé povrchové modifikace. NP byly studovány pomocí velkého množství technik: rentgenové práškové difrakce, transmisní elektronové mikroskopie, maloúhlového rentgenového rozptylu a vibrační spektroskopie. Vliv velikosti NP na magnetické vlastnosti byl také zkoumán v závislosti na teplotě a velikosti magnetického pole, měřena střídavá susceptibilita a difuzní rozptyl neutronů. Byl pozorován nástup kolektivního skelného stavu v důsledku kolapsu kuželového magnetického uspořádání na dlouhé vzdálenosti. Rovnoměrně rozložené kobalt chromité NP potažené kyselinou olejovou s velikostí 3-4 nm jsou vynikající prototypy pro studium velikostního účinku na magnetické (a feroické) vlastnosti materiálu a mohou být podrobeny rozmanité povrchové funkcionalizaci požadované pro jejich vložení do inteligentních nanostruktur a nanokompozitů.Multiferoic oxides have enormous application potential thanks to the mutually coupled magnetic and dielectric response embedded in a standalone material. Among them, the unique case is the cobalt chromite with spin-induced electric polarization locked to the magnetization direction and propagation vector of the spiral magnetic phase. The nature of the ground state magnetic structure gives rise to complex size dependent magnetic behaviour, which collapses when reaching a critical particle size. In our work, we focused on preparation of standalone cobalt chromite nanoparticles (NPs). We introduced hydrothermal decomposition of cobalt(II)/chromium(III) oleates in a water/ethanol system without need of additional thermal treatment, which lead to stable cobalt chromite nanoparticles with diameter of 3.0(1)-4.2(1) nm and log normal size distribution of 12-16%. The size at the edge of the critical limit was tuned by the reaction temperature reaching typically 240 +/- 10 degrees C. The as-prepared NPs are coated with covalently bonded oleic acid and can be easily dispersed in non-polar solvents, which makes them excellent candidates for custom surface modifications. The NPs were studied using a large number of characterization techniques: powder X-ray diffraction, transmission electron microscopy, small-angle Xray scattering, and vibrational spectroscopies. The impact of the size effect on the magnetic properties was also investigated by temperature and magnetic field dependent magnetization, a.c. susceptibility and diffuse neutron scattering. The onset of the collective glassy state due to the collapse of long range conical magnetic order was observed. The uniform cobalt chromite NPs coated with oleic acid with size of 3-4 nm are excellent prototypes for studying the size effect on magnetic (and feroic) materials, and can be subjected to manifold surface functionalization required for their embedding in smart nanostructures and nanocomposites

Size dependence of the surface spin disorder and surface anisotropy constant in ferrite nanoparticles

The magnetic properties of nanoscale magnets are greatly influenced by surface anisotropy. So far, its quantification is based on the examination of the blocking temperature shift within a series of nanoparticles of varying sizes. In this scenario, the surface anisotropy is assumed to be a particle size-independent quantity. However, there is no solid experimental proof to support this simplified picture. On the contrary, our work unravels the size-dependent magnetic morphology and surface anisotropy in highly uniform magnetic nanoparticles using small-angle polarized neutron scattering. We observed that the surface anisotropy constant does not depend on the nanoparticle's size in the range of 3-9 nm. Furthermore, our results demonstrate that the surface spins are less prone to polarization with increasing nanoparticle size. Our study thus proves the size dependence of the surface spin disorder and the surface anisotropy constant in fine nanomagnets. These findings open new routes in materials based on a controlled surface spin disorder, which is essential for future applications of nanomagnets in biomedicine and magnonics