Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

Electrochemistry in Magnetic Fields

Developing new strategies to advance the fundamental understanding of electrochemistry is crucial to mitigating multiple contemporary technological challenges. In this regard, magnetoelectrochemistry offers many strategic advantages in controlling and understanding electrochemical reactions that might be tricky to regulate in conventional electrochemical fields. However, the topic is highly interdisciplinary, combining concepts from electrochemistry, hydrodynamics, and magnetism with experimental outcomes that are sometimes unexpected. In this Review, we survey recent advances in using a magnetic field in different electrochemical applications organized by the effect of the generated forces on fundamental electrochemical principles and focus on how the magnetic field leads to the observed results. Finally, we discuss the challenges that remain to be addressed to establish robust applications capable of meeting present needs.

A new type of half-metallic fully compensated ferrimagnet

Half-metallic fully compensated ferrimagnets (HM-FCFMs) constitute a special class of half-metals exhibiting zero magnetization at zero temperature. While there have been a number of theoretical studies predicting the existence of such materials over the last 25 years, very few of those have been synthesized and observed that they exhibit expected properties. Herein, we demonstrate that a NiAs-type hexagonal-structured (CrFe)S compound could serve as an HM-FCFM material. It has a half-metallic nature of 100% spin-polarised Fermi surfaces and yet zero magnetisation at the ground state. The magnetisation shows linear behaviour as a function of the magnetic field at temperatures below the compensation temperature (~ 190 K). In addition, it shows a high magnetic coercivity of 3.8 T at 300 K. These magnetic features contribute to a significant development in the application of HM-FCFMs for spintronics devices.

Electronic Band Structures of the Possible Topological Insulator Pb2BiBrO6 and Pb2SeTeO6 Double Perovskite: An Ab Initio Study

Using the frameworks of density functional theory, we found a new class of three-dimensional (3D) topological insulators (TIs) in Pb2BiBrO6 and Pb2SeTeO6 double perovskites. Our ab initio theoretical calculations show that Pb2BiBrO6 and Pb2SeTeO6 are Z2 nontrivial, and their bandgaps are 0.390 eV and 0.181 eV, respectively. The topology comes from two mechanisms. Firstly, the band inversion occurs at Γ point in the absence of spin-orbit interactions and secondly, the bandgap is induced by the SOC. This results in a larger bandgap for this new class of topological insulators than conventional TI. In Pb2BiBrO6 double perovskites, our slab calculations confirm that the topology-protected surface metallic bands come from the BiBrO4 surface which means that one can build a transport device using Pb2BiBrO6 double perovskites with a PbO layer as an outmost protection layer. The mechanical stabilities such as bulk, shear, Young’s moduli, Poisson’s and Pugh’s ratio, longitudinal, transverse, and average sound velocity, together with Debye temperature are also studied. Our results show that these Pb2AA’O6 (A = Sb and Bi; A’ = Br and I) and Pb2SeTeO6 are mechanically stable.

Synthesis Attempt and Structural Studies of Novel A2CeWO6 Double Perovskites (A2+ = Ba, Ca) in and outside of Ambient Conditions

This comprehensive work showcases two novel, rock-salt-type minerals in the form of amphoteric cerium-tungstate double perovskite and ilmenite powders created via a high-temperature solid-state reaction in inert gases. The presented studies have fundamental meaning and will mainly focus on a detailed synthesis description of undoped structures, researching their possible polymorphism in various conditions and hinting at some nontrivial physicochemical properties like charge transfer for upcoming optical studies after eventual doping with selectively chosen rare-earth ions. The formerly mentioned, targeted A₂BB'X₆ group of compounds contains mainly divalent alkali cations in the form of ^XIIA = Ba²⁺, Ca²⁺ sharing, here, oxygen-arranged clusters (^IIX = O^2-) with purposely selected central ions from f-block ^VIB = Ce^4/3+ and d-block ^VIB' = W^4/5/6+ since together they often possess some exotic properties that could be tuned and implemented into futuristic equipment like sensors or energy converters. Techniques like powder XRD, XPS, XAS, EPR, Raman, and FTIR spectroscopies alongside DSC and TG were involved with an intent to thoroughly describe any possible changes within these materials. Mainly, to have a full prospect of any desirable or undesirable phenomena before diving into more complicated subjects like: energy or charge transfer in low temperatures; to reveal whether or not the huge angular tilting generates large enough dislocations within the material's unit cell to change its initial properties; or if temperature and pressure stimuli are responsible for any phase transitions and eventual, irreversible decomposition.

SrV0.3Fe0.7O2.8: A Vacancy-Ordered Fe-Based Perovskite Exhibiting Room-Temperature Magnetoresistance

We report room-temperature (RT) magnetoresistance (MR) in a novel Fe-based perovskite, SrV0.3Fe0.7O2.8. This compound contains ordered oxygen vacancies in every fifth primitive perovskite (111)p plane, leading to a layered structure consisting of triple-octahedral and double-tetrahedral layers. Along with the oxygen vacancies, the transition-metal ions are also ordered: the octahedral sites are occupied by 100% of Fe ions, while the tetrahedral sites are occupied by 25% of Fe ions and 75% of V ions. As a result, SrV0.3Fe0.7O2.8 forms a magnetically striped lattice in which the octahedral layers with 100% of magnetic Fe ions are separated by the diluted magnetic layer. The compound exhibits weak ferromagnetism and shows a large negative MR (-5% at 3 T) at RT, despite the small saturation moment (0.4 μB/Fe atom). Thus, this type of layered compound is promising for further large MR by an increase of magnetization through chemical substitution.

Recent Advances in Perovskite Catalysts for Efficient Overall Water Splitting

Hydrogen is considered a promising clean energy vector with the features of high energy capacity and zero-carbon emission. Water splitting is an environment-friendly and effective route for producing high-purity hydrogen, which contains two important half-cell reactions, namely, the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). At the heart of water splitting is high-performance electrocatalysts that efficiently improve the rate and selectivity of key chemical reactions. Recently, perovskite oxides have emerged as promising candidates for efficient water splitting electrocatalysts owing to their low cost, high electrochemical stability, and compositional and structural flexibility allowing for the achievement of high intrinsic electrocatalytic activity. In this review, we summarize the present research progress in the design, development, and application of perovskite oxides for electrocatalytic water splitting. The emphasis is on the innovative synthesis strategies and a deeper understanding of structure–activity relationships through a combination of systematic characterization and theoretical research. Finally, the main challenges and prospects for the further development of more efficient electrocatalysts based on perovskite oxides are proposed. It is expected to give guidance for the development of novel non-noble metal catalysts in electrochemical water splitting.

First-Principles Study on Possible Half-Metallic Ferrimagnetism in Double Perovskites Pb2XX'O6 (X = Ti, Zr, Hf, V, Nb and Ta, X' = Tc, Ru, Os and Rh)

Pb-based double perovskite compounds with chemical formula Phey have abundant physical properties in the spintronic field. Among all the features, the spin interaction of half-metallic (HM) is regarded as an important performance measure because of its high potential in spintronic devices. In this research study, we calculate density of state (DOS) to investigate possible half-metal candidates by executing structural optimization based on the method of generalized gradient approximation (GGA) and strong correlation effect (GGA + U). Furthermore, following the earlier methods by calculating and comparing energy difference of various compounds with the four initial magnetic states: ferromagnetic, ferrimagnetic, antiferromagnetic and nonmagnetic, we can determine which magnetic state is more stable. Results indicate that there are 13 possible ferrimagnetic HM candidates in these combinations, including Pb₂NbTcO₆, Pb₂TaTcO₆, Pb₂TiRuO₆, Pb₂ZrRuO₆, Pb₂HfRuO₆, Pb₂VRuO₆, Pb₂NbRuO₆, Pb₂TadRuO₆, Pb₂ZrOsO₆, Pb₂HfOsO₆, Pb₂VOsO₆, Pb₂ZrRhO₆ and Pb₂HfRhO₆ under GGA and GGA + U schemes. The stability of analysis by analyzing the energy gap illustrates that all 13 possible candidates are half metals and ferrimagnetic states, so our studies could provide guidelines for scientists to fabricate new double perovskites in future.

Realization of a Half Metal with a Record-High Curie Temperature in Perovskite Oxides

Half metals, in which one spin channel is conducting while the other is insulating with an energy gap, are theoretically considered to comprise 100% spin-polarized conducting electrons, and thus have promising applications in high-efficiency magnetic sensors, computer memory, magnetic recording, and so on. However, for practical applications, a high Curie temperature combined with a wide spin energy gap and large magnetization is required. Realizing such a high-performance combination is a key challenge. Herein, a novel A- and B-site ordered quadruple perovskite oxide LaCu₃ Fe₂ Re₂ O₁₂ with the charge format of Cu²⁺ /Fe³⁺ /Re^4.5+ is reported. The strong Cu²⁺ (↑)Fe³⁺ (↑)Re^4.5+ (↓) spin interactions lead to a ferrimagnetic Curie temperature as high as 710 K, which is the reported record in perovskite-type half metals thus far. The saturated magnetic moment determined at 300 K is 7.0 μ_B f.u.^-1 and further increases to 8.0 μ_B f.u.^-1 at 2 K. First-principles calculations reveal a half-metallic nature with a spin-down conducting band while a spin-up insulating band with a large energy gap up to 2.27 eV. The currently unprecedented realization of record Curie temperature coupling with the wide energy gap and large moment in LaCu₃ Fe₂ Re₂ O₁₂ opens a way for potential applications in advanced spintronic devices at/above room temperature.

A Unique Mechanochemical Redox Reaction Yielding Nanostructured Double Perovskite Sr2FeMoO6 With an Extraordinarily High Degree of Anti-Site Disorder

Strontium ferromolybdate, Sr2FeMoO6, is an important member of the family of double perovskites with the possible technological applications in the field of spintronics and solid oxide fuel cells. Its preparation via a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr2FeMoO6 can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO3) in the form of nanostructured powders, without the need for solvents and/or calcination under controlled oxygen fugacity. The mechanically induced evolution of the Sr2FeMoO6 phase and the far-from-equilibrium structural state of the reaction product are systematically monitored with XRD and a variety of spectroscopic techniques including Raman spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe0 → Fe3+) with simultaneous reduction of Mo cations (Mo6+ → Mo5+), occuring during the mechanosynthesis of Sr2FeMoO6, is attributed to the mechanically triggered formation of tiny metallic iron nanoparticles in superparamagnetic state with a large reaction surface and a high oxidation affinity, whose steady presence in the reaction mixture of the milled educts initiates/promotes the swift redox reaction. High-resolution transmission electron microscopy observations reveal that the mechanosynthesized Sr2FeMoO6, even after its moderate thermal treatment at 923 K for 30 min in air, exhibits the nanostructured nature with the average particle size of 21(4) nm. At the short-range scale, the nanostructure of the as-prepared Sr2FeMoO6 is characterized by both, the strongly distorted geometry of the constituent FeO6 octahedra and the extraordinarily high degree of anti-site disorder. The degree of anti-site disorder ASD = 0.5, derived independently from the present experimental XRD, Mössbauer, and SQUID magnetization data, corresponds to the completely random distribution of Fe3+ and Mo5+ cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr2FeMoO6 nanoparticles exhibit superparamagnetism with the blocking temperature T B = 240 K and the deteriorated effective magnetic moment μ = 0.055 μ B per formula unit.

Strontium Ferromolybdate-Based Magnetic Tunnel Junctions

Thin-film strontium ferromolybdate is a promising material for applications in room-temperature magnetic tunnel junction devices. These are spin-based, low-power-consuming alternatives to CMOS in non-volatile memories, comparators, analog-to-digital converters, and magnetic sensors. In this work, we consider the main tasks to be solved when creating such devices based on strontium ferromolybdate: (i) selecting an appropriate tunnel barrier material, (ii) determining the role of the interface roughness and its quantification, (iii) determining the influence of the interface dead layer, (iv) establishing appropriate models of the tunnel magnetoresistance, and (v) promoting the low-field magnetoresistance in (111)-oriented thin films. We demonstrate that (i) barrier materials with a lower effective electronegativity than strontium ferromolybdate are beneficial, (ii) diminution of the magnetic offset field (the latter caused by magnetic coupling) requires a wavy surface rather than solely a surface with small roughness, (iii) the interface dead-layer thickness is of the order of 10 nm, (iv) the tunnel magnetoresistance deteriorates due to spin-independent tunneling and magnetically disordered interface layers, and (v) antiphase boundaries along the growth direction promote the negative low-field magnetoresistance by reducing charge carrier scattering in the absence of the field.

Computational design of double transition metal MXenes with intrinsic magnetic properties

Two-dimensional transition metal carbides (MXenes) have great potential to achieve intrinsic magnetism due to their available chemical and structural diversity. In this work, by spin-polarized density functional theory calculations, we designed and comprehensively investigated 50 double transition metal (DTM) MXenes MCr₂CT_x (T = H, O, F, OH, or bare) based on the chemical formula of M₂C (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, W). We highlight that ferromagnetic half-metallicity, antiferromagnetic semiconduction, as well as antiferromagnetic half-metallicity have been achieved in the DTM MXenes. Herein, ferromagnetic half-metallic ScCr₂C₂, ScCr₂C₂H₂, ScCr₂C₂F₂, and YCr₂C₂H₂ are characterized with wide band gaps and high Curie temperatures. Very interestingly, the ScCr₂C₂-based magnetic tunnel junction presents a tunnel magnetoresistance ratio as high as 176 000%. In addition, the antiferromagnetic semiconducting TiCr₂C₂, ZrCr₂C₂, and ZrCr₂C₂(OH)₂, possessing moderate band gaps and high Néel temperatures, have been predicted. Especially, the Néel temperature of ZrCr₂C₂(OH)₂ can reach 425 K. Moreover, the Dirac cone-like band structure feature is highlighted in antiferromagnetic half-metallic ZrCr₂C₂H₂. Our study provides a new potential strategy for designing MXenes in spintronics.

Structural Characterization and Physical Properties of Double Perovskite La2FeReO6+δ Powders

The structural, optical, dielectric, and magnetic properties of double perovskite La₂FeReO_6+δ (LFRO) powders synthesized by solid-state reaction method under CO reduced atmosphere are reported on in this paper. Reitveld refinements on the XRD data revealed that the LFRO powders crystallized in an orthogonal structure (Pbnm space group) with column-like morphology. The molar ratios of La, Fe, and Re elements were close to 2:1:1. XPS spectra verified the mixed chemical states of Fe and Re ions, and two oxygen species in the LFRO powders. The LFRO ceramics exhibited a relaxor-like dielectric behavior, and the associated activation energy was 0.05 eV. Possible origins of the dielectric relaxation behavior are discussed based on the hopping of electrons among the hetero-valence ions at B-site, oxygen ion hopping through the vacant oxygen sites, and the jumping of electrons trapped in the shallower level created by oxygen vacancy. The LFRO powders display room temperature ferromagnetism with Curie temperature of 746 K. A Griffiths-like phase was observed in the LFRO powders with a Griffiths temperature of 758 K. The direct optical band gap of the LFRO powders was 2.30 eV, deduced from their absorption spectra, as confirmed by their green photoluminescence spectra with a strong peak around 556 nm.

Spin and valence variation in Cobalt doped Barium Strontium Titanate Ceramics

In the present decade, owing to half-metallic ferromagnetism, controlled 3d transition metal-doping based defect engineering in oxide perovskites brings considerable attention to the the pursuit of spintronics. We aim to...

Spontaneous magnetic merons in a half-metallic Mn2I3Br3 monolayer with easy-plane anisotropy

Spontaneous magnetic merons are found to exist in a wide magnetic field range (0–6 T) stabilized by the large in-plane magnetic anisotropy and strong Dzyaloshinskii–Moriya interaction in a half-metallic Mn2I3Br3 monolayer.

Emergent physical properties of perovskite-type oxides prepared under high pressure

The perovskite ABO₃ family demonstrates a wide variety of structural evolutions and physical properties and is arguably the most important family of complex oxides. Chemical substitutions of the A- and/or B-site and modulation of oxygen content can effectively regulate their electronic behaviors and multifunctional performances. In general, the BO₆ octahedron represents the main unit controlling the electronic and magnetic properties while the A-site ion is often not involved. However, a series of unconventional perovskite materials have been recently synthesized under high pressure, such as the s-d level controlled Pb-based perovskite family and quadruple perovskite oxides containing transition metal ions at the A-site. In these compounds, the intersite A-B correlations play an important role in electronic behaviors and further induce many emergent physical properties.

Rare-earth tuned magnetism and magnetocaloric effects in double perovskitesR2NiMnO6

We present a comprehensive experimental study of magnetization (2 <T< 300 K, 1 <H< 8 T) and magnetocaloric effect in double perovskite materialsR₂NiMnO₆withR= Pr, Nd, Sm, Gd, Tb, and Dy. While a paramagnetic to ferromagnetic transition, with T_Cin the range∼100-200K, is a common feature that can be attributed to the ordering of Mn⁴⁺and Ni²⁺magnetic moments, qualitatively distinct behavior depending on the choice ofRis observed at low temperatures. These low-temperature anomalies in magnetization are also manifest in the change in magnetic entropy, -ΔS_M, whose sign depends on the choice ofR. In order to understand these results, we present theoretical analysis based on mean-field approximation and Monte Carlo simulations on a minimal spin model. The model correctly captures the key features of the experimental observations.

Effect of Heat Treatment on Structural, Magnetic and Electrical Properties of La2FeMnO6

In this study, the effect of heat treatment on the structural, magnetic and electrical properties of La2FeMnO6 prepared via the sol-gel and sintering method were investigated. The heat-treatment conditions, i.e., the calcination temperature (1023 K and 1173 K), sintering temperature and time (1273 K for 1 and 3 h) were carried out. X-ray diffraction (XRD) revealed orthorhombic pnma (62) symmetry without any impurity phase for all samples. X-ray photoelectron spectroscopy confirmed the presence of Fe2+-Fe3+-Fe4+ and Mn3+-Mn4+ mixed states, and lanthanum and oxygen vacancies resulting in various magnetic exchange interactions. Furthermore, the magnetisation hysteresis showed enhanced hysteresis loops accompanied by an increase in magnetisation parameters with calcination temperature. The Raman phonon parameters induced a redshift in the phonon modes, alongside an increase in the intensity and compression of the linewidth, reflecting a decrease in lattice distortion, which was confirmed by XRD. The temperature-dependent conductivity showed that the conduction mechanism is dominated by p-type polaron hopping, and the lowest activation energy was approximately 0.237 ± 0.003 eV for the minimum heat-treatment conditions. These results show that varying heat-treatment conditions can significantly affect the structural, magnetic and electrical properties of the La2FeMnO6 system.

A3A'3Zn6Te4O24 (A = Na, A' = Rare Earth) Garnets: A-Site Ordered Noncentrosymmetric Structure, Photoluminescence, and Na-Ion Conductivity

A large number of oxides that adopt the centrosymmetric (CS) garnet-type structure (space group Ia3̅d) have been widely studied as promising magnetic and host materials. Hitherto, no noncentrosymmetric (NCS) garnet has been reported yet, and a strategy to NCS garnet design is therefore significant for expanding the application scope. Herein, for the series A₃A'₃Zn₆Te₄O₂₄ (A = Na, A' = La, Eu, Nd, Y, and Lu), we demonstrated that the structural symmetry evolution from CS Ia3̅d (A' = La) to NCS I4₁22 (A' = Eu, Nd, Y, and Lu) could be achieved due to the A-site cationic ordering-driven inversion symmetry breaking. Na₃A'₃Zn₆Te₄O₂₄ (A' = rare earth) are the first garnets that possess NCS structures with A-site cationic ordering. Diffuse reflectance spectra and theoretic calculations demonstrated that all these NCS garnets are indirect semiconductors. Moreover, their potential applications as host materials for red phosphors and Na-ion conductors were also investigated in detail, which firmly confirmed the NCS structure and A-site cationic ordering. Our findings have paved the way to design NCS or even polar garnets that show intriguing functional properties, such as ferroelectricity, multiferroicity, and second harmonic generation.

Boosting the surface oxygen activity for high performance Iron-based perovskite oxide

Surface oxygen activities always play an important role in various heterogeneous reaction processes. In this study, the surface oxygen activity of studied perovskite oxides is greatly enhanced after the composition and morphology are tuned. It is worth noting that the surface oxygen activity is enhanced correspondingly, accompanied by higher surface area, better reducibility, and superior low-temperature reactivity of studied catalysts. The sample introduced with nickel atom and nanorods structure possesses higher surface oxygen activity and vacancies with superior performance including T₁₀ at 221 °C and T₉₀ at 243 °C, nearly 90 °C elevations. Double perovskite oxides, especially with nanorods structure are verified to be composed of more surface active oxygen, which could be related to low-temperature redox ability and superior oxygen vacancies. Based on the DFT calculation, introducing nickel element is confirmed to be able to efficiently boost the generation of oxygen vacancies and adsorption of oxygen molecular, in accord with the analysis of characterization. To sum up, the strategy of introducing the nickel atom and nanorods structure could effectively tune the surface oxygen activity and generate more oxygen vacancies, which would be beneficial to the catalytic performance of toluene catalytic oxidation correspondingly.

Understanding complex multiple sublattice magnetism in double double perovskites

Understanding magnetism in multiple magnetic sublattice system, driven by the interplay of varied nature of magnetic exchanges, is on one hand challenging and on other hand intriguing. Motivated by the recent synthesis of AA[Formula: see text]BB[Formula: see text]O[Formula: see text] double double perovskites with multiple magnetic ions both at A- and B-sites, we investigate the mechanism of magnetic behavior in these interesting class of compounds. We find that the magnetism in such multiple sublattice compounds is governed by the interplay and delicate balance between two distinct mechanisms, (a) kinetic energy-driven multiple sublattice double exchange mechanism and (b) the conventional super-exchange mechanism. The derived spin Hamiltonian based on first-principles calculations is solved by classical Monte Carlo technique which reproduces the observed magnetic properties. Finally, the influence of off-stoichiometry, as in experimental samples, is discussed. Some of these double double perovskite compounds are found to possess large total magnetic moment and also are found to be half-metallic with reasonably high transition temperature, which raises the hope of future applications of these large magnetic moment half-metallic oxides in spintronics and memory devices.

Evolution of transition metal charge states in correlation with the structural and magnetic properties in disordered double perovskites Ca2-xLaxFeRuO6 (0.5 ≤ x ≤ 2)

A series of disordered Ca_1.5La_0.5FeRuO₆, CaLaFeRuO₆ and La₂FeRuO₆ double perovskites were prepared by the solid-state reaction method and investigated by neutron powder diffraction, X-ray absorption near-edge structure (XANES) analysis at the Ru-K edge, Mössbauer spectroscopy, DC magnetization and resistivity measurements. All compounds crystallize in the orthorhombic crystal structure with the space group Pbnm down to 3 K, showing a random distribution of Fe and Ru at the B site. Thermogravimetric analysis indicates oxygen deficiency in the Ca-rich and formal oxygen hyperstoichiometry in the La-rich members of the present series. While Mössbauer spectra verify the Fe³⁺ state for all compositions, the XANES study reveals a variable Ruⁿ⁺ oxidation state which decreases with increasing La content. The end member actually is a Ru³⁺/Ru⁴⁺ compound with possibly some cation vacancies. From magnetic susceptibility and neutron diffraction measurements, the presence of a G-type antiferromagnetic ordering was observed with a drastic increase in transition temperature from 275 K (Ca_1.5La_0.5FeRuO₆) to 570 K (La₂FeRuO₆). Mössbauer spectroscopy confirms the presence of long-range ordering but, due to local variations in the exchange interactions, the magnetic states are microscopically inhomogeneous. All the samples are variable range hopping semiconductors. A complex interplay between structural features, charge states, anion or cation defects, and atomic disorder determines the magnetic properties of the present disordered 3d/4d double perovskite series.

Structural, optical and dielectric properties of tellurium-based double perovskite Sr2Ni1-x Zn x TeO6

In this paper, Sr₂Ni_1−xZn_xTeO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) double perovskite compounds were synthesised by the conventional solid-state method, and the structural, optical and dielectric properties were investigated. The Rietveld refinement of X-ray diffraction data shows that all compounds were crystallised in monoclinic symmetry with the I2/m space group. Morphological scanning electron microscopy reported that the grain sizes decreased as the dopant increased. The UV-vis diffuse reflectance spectroscopy conducted for all samples found that the optical band gap energy, E_g, increased from 3.71 eV to 4.14 eV. The dielectric permittivity ε′ values increased for the highest Zn-doped composition, Sr₂Ni_0.2Zn_0.8TeO₆, being ∼1000 and ∼60 in the low- and high-frequency range, respectively. All samples exhibited low dielectric loss (tan δ ≤ 0.20) in the range of 10⁴–10⁵ Hz frequency. Impedance measurement revealed that grain resistance decreased with enhancement in Zn content in the Sr₂NiTeO₆ crystal lattice.

In this paper, Sr₂Ni_1−xZn_xTeO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) double perovskite compounds were synthesised by the conventional solid-state method, and the structural, optical and dielectric properties were investigated.

Structural stability and evolution of half-metallicity in Ba2CaMoO6: interplay of hole- and electron-doping

Half-metallic ferromagnetic materials have attracted a lot of attention due to their probable technological applications in spintronics. In this respect, doping plays a crucial role in tailoring or controlling the physical properties of the system. Herein, the impact of both hole and electron doping on the structural, electronic and magnetic properties of the recent high pressure synthesized non-magnetic insulator Ba₂CaMoO₆ double perovskite oxide are investigated by replacing one of the Mo ions with Nb and Tc. The structural and mechanical stability of the undoped/doped materials are analyzed by calculating the formation energies and stiffness tensors, respectively, which confirm the system's stability. Interestingly, our results revealed that Nb- and Tc-doped systems display an electronic transition from insulating to p- and n-type half-metallic ferromagnetic states, respectively. The most striking feature of the present study is that oxygen ions become spin-polarized, with a magnetic moment of ∼0.12 μ_B per atom, and are mainly responsible for conductivity in the Nb-doped system. However, the admixture of Tc 4d non-degenerate orbitals are primarily contributing to the metallicity in the Tc-doped structure, with a moment of ∼0.59 μ_B. It is also found that Nb and Tc ions remain in the 5+ and 7+ states with electronic configurations of t22g↑t22g↓e0g↑e0g↓ and t32g↑t22g↓e0g↑e0g↓, with spin states of S = 0 and S = 1/2 in the individual doped systems, respectively. Hence, the present work proposes that a doping strategy with a suitable candidate could be beneficial to tune the physical properties of the materials for their potential utilization in advanced spin-based devices.

Oxide and Organic–Inorganic Halide Perovskites with Plasmonics for Optoelectronic and Energy Applications: A Contributive Review

The ascension of halide perovskites as outstanding materials for a wide variety of optoelectronic applications has been reported in recent years. They have shown significant potential for the next generation of photovoltaics in particular, with a power conversion efficiency of 25.6% already achieved. On the other hand, oxide perovskites have a longer history and are considered as key elements in many technological applications; they have been examined in depth and applied in various fields, owing to their exceptional variability in terms of compositions and structures, leading to a large set of unique physical and chemical properties. As of today, a sound correlation between these two important material families is still missing, and this contributive review aims to fill this gap. We report a detailed analysis of the main functions and properties of oxide and organic–inorganic halide perovskite, emphasizing existing relationships amongst the specific performance and the structures.

Two-Dimensional Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) Monolayers: Half-Metallic Ferromagnets with Tunable Magnetic Properties under Strain

Two-dimensional (2D) ferromagnetic materials with high spin polarization are highly desirable for spintronic devices. 2D Janus materials exhibit novel properties due to their broken symmetry. However, the electronic structure and magnetic properties of 2D Janus magnetic materials with high spin polarization are still unclear. Inspired by the successful synthesis of a ferromagnetic FeCl₂ monolayer and 2D Janus MoSSe and WSSe, we systematically study the electronic structure and magnetic properties of Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) monolayers. Based on the Goodenough-Kanamori-Anderson theory, the ferromagnetism stems from the superexchange interaction mediated by Fe-X/Y-Fe bonds. The band gaps of spin-up channels are large enough (>4 eV) to prevent spin flipping, which is beneficial for spintronic devices. Additionally, the sizable magnetocrystalline anisotropy energy (MAE) indicates that Janus FeXY monolayers are suitable for information storage. More importantly, the half-metallic character is still kept in Janus FeXY monolayers, and their magnetic properties are enhanced by the biaxial compressive strain. The MAE of FeClI and FeBrI increases by 1 order of magnitude, and the Curie temperature of FeXY monolayers enhances by 100%. These results provide an example of the 2D Janus half-metallic materials and enrich the 2D magnetic material library.

Nonequilibrium magnetic properties of the Sr2FeMoO6type double perovskite structure

The Glauber-type stochastic dynamic interpreted by the mean-field theory has been applied to investigate the dynamic magnetic properties of the Sr₂FeMoO₆type double perovskite structure under the time varying magnetic field. First, we used the Glauber dynamics to obtain the dynamic mean-field equations. The time varying average Fe and Mo magnetizations are examined to find the phase region of the system. The dynamic Fe and Mo magnetizations, hysteresis loop areas and correlations are calculated depending on the temperature in order to determine the nature of the first and second order phase transitions, as well as to get the dynamic transition points for different ratio of the system parameters. The dynamic phase diagrams (DPDs) are constructed in the plane of reduced temperature and external magnetic field amplitude (T,h). The DPDs contain paramagnetic (p), ferromagnetic (f), ferrimagnetic-1 (i₁), ferrimagnetic-2 (i₂) phases and six mixed regions, (f + p), (f + i₁), (f + i₂), (i₁+ p), (i₂+ p) and (i₁+ i₂). The DPDs also exhibit dynamic tricritical points and reentrant phenomena, which strongly depend on interaction parameters.

Double Double to Double Perovskite Transformations in Quaternary Manganese Oxides

Control of cation ordering in ABX₃ perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA'BB'O₆ double double perovskites, where both A and B sites have 1:1 cation order, to (A_0.5 A'_0.5 )₂ BB'O₆ double perovskites with fully disordered A/A' cations can be achieved under pressure in CaMnMnWO₆ and SmMnMnTaO₆ , enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO₆ to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.

Low-Dimensional Metal Halide Perovskite Crystal Materials: Structure Strategies and Luminescence Applications

Replacing methylammonium (MA+ ), formamidine (FA+ ), and/or cesium (Cs+ ) in 3D metal halide perovskites by larger organic cations have built a series of low-dimensional metal halide perovskites (LDMHPs) in which the inorganic metal halide octahedra arranging in the forms of 2D layers, 1D chains, and 0D points. These LDMHPs exhibit significantly different optoelectronic properties from 3D metal halide perovskites (MHPs) due to their unique quantum confinement effects and large exciton binding energies. In particular, LDMHPs often have excellent broadband luminescence from self-trapped excitons. Chemical composition, hydrogen bonding, and external factors (temperature and pressure etc.) determine structures and influence photoelectric properties of LDMHPs greatly, and especially it seems that there is no definite regulation to predict the structure and photoelectric properties when a random cation, metal, and halide is chosen to design a LDMHP. Therefore, this review discusses the construction strategies of the recent reported LDMHPs and their application progress in the luminescence field for a better understanding of these factors and a prospect for LDMHPs' development in the future.

Sol-Gel Synthesis of the Double Perovskite Sr2FeMoO6 by Microwave Technique

The effect of microwave radiation on the hydrothermal synthesis of the double perovskite Sr₂FeMoO₆ has been studied based on a comparison of the particle size and structural characteristics of products from both methods. A temperature, pressure, and pH condition screening was performed, and the most representative results of these are herein presented and discussed. Radiation of microwaves in the hydrothermal synthesis method led to a decrease in crystallite size, which is an effect from the reaction temperature. The particle size ranged from 378 to 318 nm when pH was 4.5 and pressure was kept under 40 bars. According to X-ray diffraction (XRD) results coupled with the size-strain plot method, the product obtained by both synthesis methods (with and without microwave radiation) have similar crystal purity. The Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques showed that the morphology and the distribution of metal ions are uniform. The Curie temperature obtained by thermogravimetric analysis indicates that, in the presence of microwaves, the value was higher with respect to traditional synthesis from 335 K to 342.5 K. Consequently, microwave radiation enhances the diffusion and nucleation process of ionic precursors during the synthesis, which promotes a uniform heating in the reaction mixture leading to a reduction in the particle size, but keeping good crystallinity of the double perovskite. Precursor phases and the final purity of the Sr₂FeMoO₆ powder can be controlled via hydrothermal microwave heating on the first stages of the Sol-Gel method.

Magnetic and transport properties of the mixed 3d-5d-4fdouble perovskite Sm2CoIrO6

Iridium-based double perovskites having mixed 3d-5d-4fmagnetic sub-lattices are expected to exhibit exotic magnetic phenomenon. In this paper, we report a study of structural, magnetic and transport properties of the mixed 3d-5d-4fdouble perovskite Sm₂CoIrO₆(SMCO), which crystallizes in monoclinic structure with space groupP2₁/nand the crystal symmetry remains same throughout the measured temperature down to 15 K. High resolution synchrotron x-ray diffraction reveals an isostructural phase transition around 104 K. Magnetization measurements on polycrystalline samples indicate that SMCO orders ferrimagnetically atT_FiM= 104 K; while, a second transition is observed below 10 K due to the rare-earth (Sm³⁺) ordering. The ferrimagnetic transition is well-understood by Néel's two-sublattice model, which is primarily ascribed to antiferromagnetic coupling between Co²⁺and Ir⁴⁺sub-lattices. Electronic transport measurement shows the insulting behaviour of SMCO, which follows Mott variable-range hopping conduction mechanism. However, dielectric measurements as a function of temperature rules out the presence of magneto dielectric coupling in this compound.

Scrutinizing the stability and exploring the dependence of thermoelectric properties on band structure of 3d-3d metal-based double perovskites Ba2FeNiO6 and Ba2CoNiO6

Through the conventional DFT computation, we have designed new oxide double perovskites Ba2FeNiO6 and Ba2CoNiO6. The structural and thermodynamic stabilities are predicted by optimizing the crystal structure and evaluation of enthalpy of formation, respectively. Then by using the optimized lattice constant, we have explored the different physical properties. The GGA + mBJ electronic band-structure illustrates Ba2FeNiO6 is a half-metal with 100% spin polarization at the Fermi level. While Ba2CoNiO6 shows a ferromagnetic semiconducting nature. The change in the electronic structure when Fe is replaced by Co is explained with the help of the orbital diagram and exchange interaction. The eg-eg hybridization that happens via O-p states is strong because Fe-O-Ni and Co-O-Ni bond angles are strictly 180°. The narrow bandgaps in the semiconducting channels prompted us to analyze the applicability of these materials towards thermoelectric technology. Besides this, we have investigated the dependency of transport properties on electronic band structure. The semiconducting nature in Ba2CoNiO6 results in a significant ZT around 0.8 at room temperature makes it suitable for wasted-energy regeneration.

Metamagnetic Behavior in a Quadruple Perovskite Oxide

A cubic quadruple perovskite oxide CeMn₃Cr₄O₁₂ has been synthesized under high-pressure and high-temperature conditions of 8 GPa and 1273 K. The X-ray absorption spectroscopy reveals that the Ce ions are in a trivalent state, as represented by the ionic model of Ce³⁺Mn³⁺₃Cr³⁺₄O₁₂. The magnetic study demonstrates three independent antiferromagnetic transitions attributed to Ce (∼10 K), Mn (46 K), and Cr (133 K) ions. Furthermore, a magnetic field-induced antiferromagnetic-to-ferromagnetic (metamagnetic) transition of Ce³⁺ 4f moments is observed at low temperatures below 20 K, exhibiting a rare example of metamagnetism in the Ce³⁺-oxides. This finding represents that the 3d-electron magnetic sublattices play a role in the metamagnetism of 4f-electron magnetic moments, demonstrating a new aspect of the 3d-4f complex electron systems.

Os Doping Suppressed Cu-Fe Charge Transfer and Induced Structural and Magnetic Phase Transitions in LaCu3Fe4-xOsxO12 (x = 1 and 2)

B-site Os-doped quadruple perovskite oxides LaCu₃Fe_4-xOs_xO₁₂ (x = 1 and 2) were prepared under high-pressure and high-temperature conditions. Although parent compound LaCu₃Fe₄O₁₂ experiences Cu-Fe intermetallic charge transfer that changes the Cu³⁺/Fe³⁺ charge combination to Cu²⁺/Fe^3.75+ at 393 K, in the Os-doped samples, the Cu and Fe charge states are found to be constant 2+ and 3+, respectively, indicating the complete suppression of charge transfer. Correspondingly, Os⁶⁺ and mixed Os^4.5+ valence states are determined by X-ray absorption spectroscopy for x = 1 and x = 2 compositions, respectively. The x = 1 sample crystallizes in an Fe/Os disordered structure with the Im3̅ space group. It experiences a spin-glass transition around 480 K. With further Os substitution up to x = 2, the crystal symmetry changes to Pn3̅, where Fe and Os are orderly distributed in a rocksalt-type fashion at the B site. Moreover, this composition shows a long-range Cu²⁺(↑)Fe³⁺(↑)Os^4.5+(↓) ferrimagnetic ordering near 520 K. This work provides a rare example for 5d substitution-suppressed intermetallic charge transfer as well as induced structural and magnetic phase transitions with high spin ordering temperature.

Structural, magnetic, and magnetocaloric properties of the multiferroic host double perovskite compound Pr2FeCrO6

We have investigated the crystal structure and the nature of the magnetic ground state of the polycrystalline compound Pr2FeCrO6 (PFCO) through X-ray diffraction (XRD), magnetization, and magnetocaloric effect studies. Analysis of the XRD pattern reveals that the PFCO compound exhibits a B-site disordered orthorhombic crystal structure. The random distribution of Fe3+ and Cr3+ magnetic sublattices at the B-sites of the crystallographic unit cell helps to generate several fascinating magnetic properties. The compound exhibits three distinct anomalies in both the temperature dependence of the magnetization and the magnetic entropy change (-ΔS) curves, namely, (i) a G-type canted antiferromagnetic (AFM) ordering of the transition metal ions (TN1), (ii) a progressive spin reorientation (SR) transition (TSR), and (iii) an AFM ordering of Pr3+ sublattices at very low temperature (TN2). Surprisingly, a novel "diamagnetism-like" behavior appears in the low-temperature region for low applied field values. Moreover, we have also constructed the thermal evolution of the magnetic crystal structures in different transition regions with the help of irreducible representations of the crystal symmetry. Overall, our study of B-site disordered PFCO may help to encourage basic fundamental and applied research on disordered rare-earth and transition metal-based perovskite systems due to their interesting magnetic properties over a broad temperature range.

Intrinsic ferromagnetic semiconductors in rhombohedral RMnO3 (R = Sc, Y, and Lu) with high critical temperature and large ferroelectric polarization

Ferromagnetic (FM) semiconductors have been recognized as the cornerstone for next-generation highly functional spintronic devices. However, the development in practical applications of FM semiconductors is limited by their low Curie temperatures (T _C). Here, on the basis of model analysis, we find that the FM super-exchange couplings in the d ⁵ - d ³ system can be significantly strengthened by reducing the virtual exchange gap (G _ex) between occupied and empty e _g orbitals. By first-principle calculations, we predict robust ferromagnetism in three rhombohedral RMnO₃ (R = Sc, Y, and Lu) compounds with the T _C that is as high as ∼1510 K (YMnO₃). The oxygen breathing motions open a band gap and create an unusual Mn²⁺/Mn⁴⁺ charge ordering of the Mn-d electrons, which play an important role in altering the G _ex. Interestingly, the rhombohedral RMnO₃ compounds are also ferroelectric (FE) with a large spontaneous polarization approaching that of LiNbO₃. These results not only deepen the understandings of magnetic couplings in d ⁵ - d ³ system, but also provide a way to design room-temperature FM-FE multiferroics.

Odd-Parity Spin-Triplet Superconductivity in Centrosymmetric Antiferromagnetic Metals

We propose a route to achieve odd-parity spin-triplet (OPST) superconductivity in metallic collinear antiferromagnets with inversion symmetry. Owing to the existence of hidden antiunitary symmetry, which we call the effective time-reversal symmetry (eTRS), the Fermi surfaces of ordinary antiferromagnetic metals are generally spin degenerate, and spin-singlet pairing is favored. However, by introducing a local inversion symmetry breaking perturbation that also breaks the eTRS, we can lift the degeneracy to obtain spin-polarized Fermi surfaces. In the weak-coupling limit, the spin-polarized Fermi surfaces constrain the electrons to form spin-triplet Cooper pairs with odd parity. Interestingly, all the odd-parity superconducting ground states we obtained host nontrivial band topologies manifested as chiral topological superconductors, second-order topological superconductors, and nodal superconductors. We propose that double perovskite oxides with collinear antiferromagnetic or ferrimagnetic ordering, such as SrLaVMoO_{6}, are promising candidate systems where our theoretical ideas can be applied to.

Ligand dependent magnetism of theJeff= 3/2 Mott insulator Cs2MX6(M= Ta, Nb,X= Br, Cl)

Magnetic and structural properties of double perovskite type bromides Cs₂MBr₆(M= Ta, Nb), where Ta⁴⁺(5d¹) and Nb⁴⁺(4d¹) ions form the face centered cubic lattice, are investigated and compared with chlorides Cs₂MCl₆. Cs₂TaBr₆exhibits the effective magnetic moment of 0.24μ_B, which is much smaller than the spin only value of ad¹ion, 1.73μ_B, and comparable to 0.25μ_Bin Cs₂TaCl₆. On the other hand, the effective magnetic moment of Cs₂NbBr₆is 0.7μ_Band is substantially smaller than 1.0μ_Bin Cs₂NbCl₆. On cooling, successive structural and magnetic phase transitions accompanying the release of electronic entropy approximatelyRln 4 in total as expected for theJ_eff= 3/2 state are observed. The type of the ligand changes the temperature dependence of magnetic susceptibility at low temperature as well as its magnitude. The role of the ligands on the magnetism ofJ_eff= 3/2 Mott insulators are discussed in the light of metal-ligand hybridization and the electron-lattice coupling.