Ferrofluids, composed of magnetic nanoparticles suspended in a non-magnetic carrier liquid, have attracted considerable attention since their discovery in the 1960s. Their combination of liquid and magnetic properties gives rise to complex behaviors and unique functionalities, enabling a wide range of technological applications. Among these is the ability of the magnetic material to be moved by and to absorb heat when exposed to an external magnetic field – a process that can occur through various dissipation mechanisms depending on the system. A detailed understanding of these mechanisms is crucial for tailoring materials to specific applications. We provide a comprehensive overview of the theoretical principles underlying different energy dissipation processes and propose a coherent framework for their interpretation. Particular attention is devoted to describing the frequency-dependent susceptibility, which is the key parameter to describe dissipation. We demonstrate that dissipation, predicted from magnetometry-based studies, matches well with direct, frequency-dependent calorimetric results, expanding the available frequency range of the characterization. The demonstrating measurements were carried out with a dilute ferrofluid containing magnetite nanoparticles of a mean diameter of 10.6 nm.

Exchange-coupled interfaces are pivotal in exploiting two-dimensional (2D) ferromagnetism. Due to the extraordinary correlations among charge, spin, orbital and lattice degrees of freedom, layered magnetic transition metal chalcogenides (TMCs) bode well for exotic topological phenomena. Here we report the realization of wafer-scale Cr 2 Te 3 down to monolayer (ML) on insulating SrTiO 3 (111) and/or Al 2 O 3 (001) substrates using molecular beam epitaxy. Robust ferromagnetism persists in the 2D limit. In particular, the Curie temperature T C of 2 ML Cr 2 Te 3 increases from 100 K to ∼120 K when proximitized to topological insulator (TI) (Bi,Sb) 2 Te 3 , with substantially boosted magnetization as observed via polarized neutron reflectometry. Our experiments and theory strongly indicate that the Bloembergen–Rowland interaction is likely universal underlying T C enhancement in TI-coupled magnetic heterostructures. The topological-surface-enhanced magnetism in 2D TMC enables further exchange coupling physics and quantum hybrid studies, including paving the way to realize interface-modulated topological electronics.

Recently, a new class of high-temperature superconductors, RNiO2 (where R represents rare-earth elements) with infinite-layer (IL) structure, has been identified. They possess the same structural framework as the renowned high-Tc cuprates but with nickel replacing copper as the central element. In this study, we successfully synthesized infinite-layer samples of PrNi0.9Al0.1O2+δ in the bulk polycrystalline form through topotactic reduction of the PrNi0.9Al0.1O3 orthorhombic perovskite, via treatment with CaH2. The incorporation of aluminum at the octahedral sites promotes the stabilization of bulk derivatives of the infinite-layer structure since unreduced [AlO6] octahedra keep the layers together and prevent their decomposition. The lack of superconductivity in bulk samples has been a subject of intense debate in recent literature. One major theoretical question concerns whether hydrogen becomes incorporated into the structure during the reduction from RNiO3 to RNiO2─as suggested by theory. Here, we present neutron powder diffraction data demonstrating that hydride ions indeed reside within the IL lattice in samples of stoichiometry PrNi0.9Al0.1O2.10H0.16. Additional crystallographic analyses were carried out using temperature-dependent synchrotron X-ray diffraction on both reduced and oxidized phases. Furthermore, spectroscopic analysis via XAS and magnetometry confirms the reduction of Ni3+ to the Ni+ oxidation state, aligning with the crystallochemical evidence.

We report the critical behavior of the FexTi2S4 (x = 0.24, 0.42) intercalated Heideite sulfides in the vicinity of their second-order magnetic phase transition. Several methods including the modified Arrott plot, Widom scaling law, and the critical isotherm analysis reliably yield critical exponents beta = 1.105/0.442, gamma = 0.913/1.042, and delta = 1.826/3.357, in agreement with the scaling hypothesis, although slightly deviating from the conventional theoretical values. Using the renormalization group theory analysis, we conclude that the exchange distance J(r) and the correlation length critical exponent nu indicate that increasing Fe content induces a transition from localized spin interactions, described by the mean-field model, to an itinerant-electron 3D Heisenberg-like model near T-C. These features reveal a coupling between short- and long-range magnetic interactions, which are responsible for the unconventional critical behavior in FexTi2S4. Our results provide valuable insights into the magnetic nature and offer tuning mechanisms in FexTi2S4 intercalated sulfides.

The local atomic arrangement correlates to the short-range magnetic interactions mediated by electron hopping between Ni e g states, suggesting the occurrence of magnetoelastic coupling and magnetic frustration in Ni 0.8 Ti 2 S 4 .

Ferrofluids containing magnetic nanoparticles represent a special class of magnetic materials due to the added freedom of particle tumbling in the fluids. We studied this process, known as Brownian relaxation, and its effect on the magnetic properties of ferrofluids with controlled magnetite nanoparticle sizes. For small nanoparticles (below 10 nm diameter), the Néel process is expected to dominate the magnetic response, whereas for larger particles, Brownian relaxation becomes important. Temperature- and magnetic-field-dependent magnetization studies, differential scanning calorimetry, and AC susceptibility measurements were carried out for 6 and 13.5 nm diameter magnetite nanoparticles suspended in water. We identify clear fingerprints of Brownian relaxation for the sample of large-diameter nanoparticles as both magnetic and thermal hysteresis develop at the water freezing temperature, whereas the samples of small-diameter nanoparticles remain hysteresis-free down to the magnetic blocking temperature. This is supported by the temperature-dependent AC susceptibility measurements: above 273 K, the data show a low-frequency Debye peak, which is characteristic of Brownian relaxation. This peak vanishes below 273 K.

Thermoelectric materials offer a promising avenue for energy management, directly converting heat into electrical energy. Among them, AgSbTe(2 )has gained significant attention and continues to be a subject of research at further improving its thermoelectric performance and expanding its practical applications. This study focuses on Ag-deficient Ag(0.7)Sb(1.12)Te(2)and Ag0.7Sb1.12Te1.95Se0.05 materials, examining the impact of compositional engineering within the AgSbTe2 thermoelectric system. These materials have been rapidly synthesized using an arc-melting technique, resulting in the production of dense nanostructured pellets. Detailed analysis through scanning electron microscopy (SEM) reveals the presence of a layered nanostructure, which significantly influences the thermoelectric properties of these materials. Synchrotron X-ray diffraction reveals significant changes in the lattice parameters and atomic displacement parameters (ADPs) that suggest a weakening of bond order in the structure. The thermoelectric characterization highlights the enhanced power factor of Ag-deficient materials that, combined with the low glass-like thermal conductivity, results in a significant improvement in the figure of merit, achieving zT values of 1.25 in Ag(0.7)Sb(1.12)Te(2 )and 1.01 in Ag0.7Sb1.12Te1.95Se0.05 at 750 K.

Among thermoelectric materials, skutterudites are the most prominent candidates in the mid-temperature range applications. In the multiple-filled Sr0.2Yb0.2Co4Sb12 skutterudite, with Sr and Yb as fillers, we have enhanced the thermoelectric performance of CoSb3 through the reduction of lattice thermal conductivity and the optimization of carrier concentration and electrical conductivity. The high-pressure synthesis of the double-filled derivative promotes filling fraction fluctuation. This is observed by high angular resolution synchrotron X-ray diffraction, showing a phase segregation that corresponds to an inhomogeneous distribution of the filler atoms, located at the 2a positions of the cubic space group Im3̅. In addition, scanning transmission electron microscopy (STEM) combined with EELS spectroscopy clearly shows a segregation of Sr atoms from the surface of the grains, which is compatible with the synchrotron X-ray powder diffraction results. Mean square displacement parameters analysis results in Einstein temperatures of ∼94 and ∼67 K for Sr and Yb, respectively, and a Debye temperature of ∼250 K. The strong effect on resonant and disorder scattering yields a significantly lower lattice thermal conductivity of 2.5 W m-1 K-1 at 773 K. Still, good weighed-mobility values were obtained, with high filling fraction of the Yb and Sr elements. This drives a reduced electrical resistivity of 2.1 × 10-5 Ω m, which leads to a peak zT of 0.26 at 773 K. The analysis and results performed for the synthesized (Sr,Yb)-double filled CoSb3, shed light on skutterudites for potential waste-heat recovery applications.

In this work, Gd-filled skutterudite GdxCo4Sb12 was prepared using one step method under high pressure in a piston-cylinder-based press at 3.5 GPa and moderate temperature of 800 °C. A detailed structural characterization was performed using synchrotron X-ray diffraction (SXRD), revealing a filling fraction of x = 0.033(2) and an average bond length of 3.3499(3) Å. The lattice thermal expansion accessed via temperature-dependent SXRD led to a precise determination of a Debye temperature of 322(3) K, from the fitting of the unit-cell volume expansion using the second order Grüneisen approximation. This parameter, when evaluated through the mean square displacements of Co and Sb, displayed a value of 265(2) K, meaning that the application of the harmonic Debye theory underestimates the Debye temperature in skutterudites. Regarding the Gd atom, its intrinsic disorder value was ~5× and ~25× higher than those of the Co and Sb, respectively, denoting that Gd has a strong rattling behavior with an Einstein temperature of θE = 67(2) K. As a result, an ultra-low thermal conductivity of 0.89 W/m·K at 773 K was obtained, leading to a thermoelectric efficiency zT of 0.5 at 673 K.

Among chalcogenide thermoelectric materials, SnTe is an excellent candidate for intermediate temperature applications, in replacement of toxic PbTe. We have prepared pure polycrystalline SnTe by arc melting, and investigated the structural evolution by temperature-dependent neutron powder diffraction (NPD) from room temperature up to 973 K. In this temperature range, the sample is cubic (space group Fm-3m) and shows considerably larger displacement parameters for Te than for Sn. The structural analysis allowed the determination of the Debye model parameters and provided information on the Sn–Te chemical bonds. SEM images show a conspicuous nanostructuration in layers below 30 nm thick, which contributes to the reduction of the thermal conductivity down to 2.5 W/m·K at 800 K. The SPS treatment seems to reduce the number of Sn vacancies, thus diminishing the carrier density and increasing the Seebeck coefficient, which reaches 60 μV K−1 at 700 K, as well as the weighted mobility, almost doubled compared with that of the as-grown sample.

Abstract Oxide‐based 2D electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face‐dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle‐resolved photoemission spectroscopy is used to study the electronic structure of the 2DEG formed at the (110) surface of KTaO3 after deposition of a thin Al layer. The experiments reveal a remarkable anisotropy in the orbital character of the electron‐like dispersive bands, which form a Fermi surface consisting of two elliptical contours with their major axes perpendicular to each other. The measured electronic structure is used to constrain the modeling parameters of self‐consistent tight‐binding slab calculations of the band structure. In these calculations, an anisotropic Rashba splitting is found with a value as large as 4 meV at the Fermi level along the [−110] crystallographic direction. This large unconventional and anisotropic Rashba splitting is rationalized based on the orbital angular momentum formulation. These findings provide insights into the interpretation of spin‐orbitronics experiments and help to constrain models for superconductivity in the KTO(110)‐2DEG system.

In RbSn2Br5 halide, prepared by mechano-chemistry, the 5s2 lone electron pairs of Sn2+ were located from NPD and SXRD data. Among the thermoelectric properties, a huge Seebeck coefficient and an extremely low thermal conductivity are determined.

Transition-metal chalcogenides with intercalated layered structures are interesting systems in material physics due to their attractive electronic and magnetic properties, with applications in the fields of magnetic refrigerators, catalysts, and thermoelectrics, among others. In this work, we studied in detail the structural, electronic, and magnetic properties of (Fe,Ti)-based sulfides with formula FexTi2S4 (x = 0.24, 0.32, and 0.42), prepared as polycrystalline materials under high-pressure conditions. They present a layered Heideite-type crystal structure, as assessed by synchrotron X-ray diffraction. A local structure analysis using Fe K-edge extended X-ray-absorption fine structure (EXAFS) data unveiled a conspicuous contraction of the main Fe-S bond in Fe0.24Ti2S4 at the vicinity of the magnetic transition 60-80 K. We suggest that this anomaly is related to magnetoelastic coupling effects. The EXAFS analysis allowed extraction of the Einstein temperatures (θE), i.e., the phonon contribution to the specific heat, for the two bond pairs Fe-S(1) [θE ≈318 K; 290 K (C/T)] and Fe-Ti(1) [θE ≈218 K; 190 K (C/T)]. In addition to the structural and local vibrational measurements, we probed the magnetic properties using magneto-calorimetry, magnetometry under applied pressure, magnetoresistance (MR), and Hall effect measurements. We observed the appearance of a broad peak in the specific heat around 120 K in the x = 0.42 compound that we associated with an antiferromagnetic ordering electronic transition. We found that the antiferromagnetic transition temperature is pressure and composition sensitive and reduces at 1.2 GPa by ∼12 and ∼3 K, for the members with x = 0.24 and x = 0.42, respectively. Similarly, the saturation magnetization in the ordered phase depends on both pressure and iron content, reducing its value by 50, 90, and 30% for x = 0.24, 0.32, and 0.42, respectively. We observed clear jumps in the magnetic hysteresis loops, MR, and anomalous Hall effect (AHE) below 2 K at fields around 2-4 T. We associated this observation with the metamagnetic transitions; from the Berry-curvature a decoupling parameter of SH = 0.12 V-1 is determined. Comparison of the results on the temperature-dependent magnetization, MR, and AHE elucidates a strong inelastic scattering contribution to the AHE at higher temperatures due to the cluster spin-glass phase.

Oxide-based two-dimensional electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face-dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle-resolved photoemission spectroscopy is used to study the electronic structure of the 2DEG formed at the (110) surface of KTaO3 after deposition of a thin Al layer. Our experiments revealed a remarkable anisotropy in the orbital character of the electron-like dispersive bands, which form a Fermi surface consisting of two elliptical contours with their major axes perpendicular to each other. The measured electronic structure is used to constrain the modeling parameters of self-consistent tight-binding slab calculations of the band structure. In these calculations, an anisotropic Rashba splitting is found with a value as large as 4 meV at the Fermi level along the [-110] crystallographic direction. This large unconventional and anisotropic Rashba splitting is rationalized based on the orbital angular momentum formulation. These findings provide insights into the interpretation of spin-orbitronics experiments and help to constrain models for superconductivity in the KTO(110)-2DEG system.

Abstract Thermoelectric M x Co4Sb12 skutterudites are well-known to exhibit a reduced thermal conductivity thanks to the rattling effect of the M-filler at the large cages occurring in the framework, centered at the 2a sites of the I m 3 ‾ $Im\overline{3}$ space group. A novel Eu-filled skutterudite has been synthesized under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural refinement from high-angular resolution synchrotron X-ray diffraction (SXRD) patterns unveils an unusual position for Eu filler atoms. By difference Fourier synthesis they are found at 12d sites, conforming statistically occupied octahedra within the mentioned cages around 2a positions. The Debye temperature was estimated by averaging the isotropic displacements by the atomic masses, leading to θ D ${\theta }_{D}$ of 273(2) K. Oftedal plots concerning the y and z Sb fractional positions, the unit-cell parameter a and M filling fraction include the novel Eu specimen in the trend observed for other filled materials prepared under high-pressure, including rare-earths, alkali or alkali-earth elements, all accepted as rattlers in filled skutterudites. A total thermal conductivity (κ) of 0.82 W m−1 K−1 is measured at 773 K for Eu0.02(1)Co4Sb12, below that of other filled skutterudites, which is promoted by the enhanced phonon scattering of Eu located at 12d sites. FE-SEM images showed large, homogeneous grains, well compacted after the high-pressure synthesis.

The development of new magnetic refrigerants demands an effective investigation of materials with a large magnetocaloric effect in a wide temperature range. Herein, we report on the structural, magnetic, and magnetocaloric properties of the two-site disordered double perovskite GdSrCoFeO6 prepared by the modified solid-state synthesis method. Temperature-dependent synchrotron X-ray diffraction analysis revealed that GdSrCoFeO6 crystallizes in the orthorhombic phase (Pnma), with Gd3+/Sr2+ and Co2+/3+/Fe3+/4+ ions randomly distributed on the A-and B-sites, respectively. An observed lattice parameter anomaly around 60 K indicates the occurrence of the magnetoelastic coupling, which coincides with the presence of ferro/ferrimagnetic (FM/FiM) ordering below T-C approximate to 65 K from the magnetic measurements. These results match well with our first-principles calculation prediction of low-temperature magnetic (FM/FiM) and electronic (insulating/metal) transitions related to a combined effect of Co and Fe short-and long-range competitions, crossings of spin state at Co ions, and the hybridization degree between Gd-4f and Co-3d states. Additionally, a modified Arrott plot and Kouvel-Fisher analysis were used to establish the nature of the magnetic phase transition in GdSrCoFeO6, yielding the critical exponent beta = 1.46(6)/1.45(6), gamma = 1.48(5)/1.17(2), and delta = 2.01(3)/1.80(5), respectively. The specific heat analysis reveals two well-defined broad peaks (similar to 10 and similar to 70 K), which match well with a Schottky anomaly (Gd-4f ) and the magnetic transition of FM/FiM to paramagnetic order, respectively. The magnetocaloric effect (MCE) analysis reveals a maximum magnetic entropy change Delta SM m a x approximate to 13 J kg(-1) K-1 (at similar to 8 K) under a field of 0-7 T. These results evidence that the Schottky anomaly and the magnetoelastic coupling seem to be key factors for driving further enhancements to the MCE in GdSrCoFeO6, making it a possible candidate for cryogenic applications.

Since superconductivity was first reported in nickelate thin films, many studies have been published about this family of materials, and different hypotheses have been proposed for explaining the mechanisms and structural dependence. Here, we report the synthesis of anchored infinite-layer LaNi0.9Al0.1O2.1 and its hole-doped derivatives by topotactic reduction from La1_xSrxNi0.9Al0.1O3 rhombohedral perovskites. LaNiO2 derivatives constitute a new family of high -temperature superconductors, with the same structure as high-Tc cuprates but based on nickel, only showing superconductivity in thin films for now. We describe a strategy to stabilize LaNiO2 derivatives in bulk: the presence of Al at the octahedral sites helps to stabilize/anchor the infinite-layer structure. The reasons for the bulk being non -superconductors are hotly debated in the literature. An important question is whether there is some hydrogen incorporated into the structure during the reduction process from LaNiO3 to LaNiO2, predicted theoretically but not reported experimentally. Our neutron powder diffraction data show that, indeed, hydrogen occupies the centers of the Ni- O squares, and spectroscopic evidence from EELS and XAS suggests that Ni is reduced to the Ni+ oxidation state, consistent with the crystallochemical data.

The RNiO3 nickelate perovskites represent a promising class of materials for spintronics applications such as in modern communication devices. This is related to the large diversity of properties shown by these compounds. Indeed, depending on the external parameters, such as temperature, pressure, or R cation size, they can exhibit para- or antiferromagnetic behavior and can transform from an insulator to a metal. Among them, PrNiO3 is one of the most important members because it exhibits tunable electronic and magnetic properties. However, our understanding of how these properties can be modified through external parameters, such as pressure and temperature, remains under debate. In this work, we characterized the structural, electronic, and magnetic properties of PrNiO3 in the range of 0-21 GPa and 10-973 K. For this purpose, we performed synchrotron X-ray diffraction (SXRD), X-ray absorption near-edge structure spectroscopy (XANES), and magnetic measurements. We compared the experimental XANES data with ab initio calculations to extract information about the electronic structure. The diffraction data demonstrated a sharp transition at the bulk level between monoclinic and orthorhombic PrNiO3 (P2(1)/n -> Pbnm) at T-IM similar to 130 K, while XANES data, which probe the medium-range and electronic structure, showed progressive changes between 90 and 130 K. The latter is in accordance with our previous EXAFS data, attesting that this transition occurs around 130 K and that it is driven at the local and medium-range scales. Between 700 and 800 K at ambient pressure, the Pbnm phase transforms into a rhombohedral phase (R3c), in agreement with a previous laboratory X-ray diffraction study. Our XANES data and ab initio simulations across this transition indicate a significant raising of the orbital overlap between Ni 3d and O 2p, suggesting superior electronic properties of the rhombohedral phase compared to the orthorhombic structure. Under isothermal cold compression, we found that the Pbnm to R3c transition is characterized by a large coexisting domain between 5.8 and 12.2 GPa. Based on the present and literature data, we proposed an extended pressure- and temperature-phase diagram of PrNiO3 and provided first-order constraints on the interplay between external parameters and properties in nickelates.

The structural changes in PrNiO3 around the Ni atoms are studied by XAS. The spin–phonon coupling and the softening of the lattice vibrations are observed. EXAFS is suitable for exploring the coupling between the spins and phonons.