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

What Can Electric Noise Spectroscopy Tell Us on the Physics of Perovskites?

Electric noise spectroscopy is a non-destructive and a very sensitive method for studying the dynamic behaviors of the charge carriers and the kinetic processes in several condensed matter systems, with no limitation on operating temperatures. This technique has been extensively used to investigate several perovskite compounds, manganese oxides (La1−xSrxMnO3, La0.7Ba0.3MnO3, and Pr0.7Ca0.3MnO3), and a double perovskite (Sr2FeMoO6), whose properties have recently attracted great attention. In this work are reported the results from a detailed electrical transport and noise characterizations for each of the above cited materials, and they are interpreted in terms of specific physical models, evidencing peculiar properties, such as quantum interference effects and charge density waves.

Spin reorientation in antiferromagnetic Dy2FeCoO6 double perovskite

We explored the electronic and magnetic properties of the lanthanide double perovskite Dy₂FeCoO₆ by combining magnetization, Raman and Mössbauer spectroscopy and neutron diffraction along with density functional theory (DFT) calculations. Our magnetization measurements revealed two magnetic phase transitions in Dy₂FeCoO₆. First, a paramagnetic to antiferromagnetic transition at T _N = 248 K and subsequently, a spin reorientation transition at T _SR = 86 K. In addition, a field-induced magnetic phase transition with a critical field of H _c ≈ 20 kOe is seen at 2 K. Neutron diffraction data suggested cation-disordered orthorhombic structure for Dy₂FeCoO₆ in Pnma space group which is supported by Raman scattering results. The magnetic structures ascertained through representational analysis indicate that at T _N, a paramagnetic state is transformed to Γ₅(Cx, Fy, Az) antiferromagnetic structure while, at T _SR, Fe/Co moments undergo a spin reorientation to Γ₃(Gx, Ay, Fz). The refined magnetic moment of (Fe/Co) is 1.47(4) μ _B at 7 K. The antiferromagnetic structure found experimentally is supported through the DFT calculations which predict an insulating electronic state in Dy₂FeCoO₆.

Insulator-to-half metal transition and enhancement of structural distortions in [Formula: see text] double perovskite oxide via hole-doping

Using density functional theory calculations, we found that recently high-pressure synthesized double perovskite oxide [Formula: see text] exhibits ferrimagnetic (FiM) Mott-insulating state having an energy band gap of 0.20 eV which confirms the experimental observations (Feng et al. in Inorg Chem 58:397-404, 2019). Strong antiferromagnetic superexchange interactions between high-energy half-filled [Formula: see text]-[Formula: see text] and low-energy partially filled [Formula: see text] orbitals, results in a FiM spin ordering. Besides, the effect of 3d transition metal (TM = Cr, Mn, and Fe) doping with 50% concentration at Ni sites on its electronic and magnetic properties is explored. It is established that smaller size cation-doping at the B site enhances the structural distortion, which further gives strength to the FiM ordering temperature. Interestingly, our results revealed that all TM-doped structures exhibit an electronic transition from Mott-insulating to a half-metallic state with effective integral spin moments. The admixture of Ir 5d orbitals in the spin-majority channel are mainly responsible for conductivity, while the spin minority channel remains an insulator. Surprisingly, a substantial reduction and enhancement of spin moment are found on non-equivalent Ir and oxygen ions, respectively. This leads the Ir ion in a mixed-valence state of [Formula: see text] and [Formula: see text] in all doped systems having configurations of [Formula: see text] ([Formula: see text]) and [Formula: see text] ([Formula: see text]), respectively. Hence, the present work proposes that doping engineering with suitable impurity elements could be an effective way to tailor the physical properties of the materials for their technological potential utilization in advanced spin devices.

∑3 Twin Boundaries in Gd2Ti2O7 Pyrochlore: Pathways for Oxygen Migration

Understanding the chemistry at twin boundaries (TB) is a well-recognized challenge, which could enable the capabilities to manipulate the functional properties in complex oxides. The study of this atomic imperfection becomes even more important, as the presence of twin boundaries has been widely observed in materials, regardless of the dimensionalities, due to the complexities in growth methods. In the present study, we provide atomic-scale insights into a ∑3(111̅) ⟨11̅0⟩ twin boundary present in pyrochlore-structured Gd₂Ti₂O₇ using atomic-resolution electron microscopy and atomistic modeling. The formation of the observed TB occurs along (111̅) with a 71° angle between two symmetrically arranged crystals. We observe distortions (∼3 to 5% strain) in the atomic structure at the TB with an increase in Gd-Gd (0.66 ± 0.03 nm) and Ti-Ti (0.65 ± 0.02 nm) bond lengths in the (11̅0) plane, as compared to 0.63 nm in the ordered structure. Using atomistic modeling, we further calculate the oxygen migration barrier for vacancy hopping at 48f-48f sites in the pyrochlore structure, which is the primary diffusion pathway for fast oxygen transport. The mean migration barrier is lowered by ∼25% to 0.9 eV at the TB as compared to 1.23 eV in the bulk, suggesting the ease in oxygen transport through the ∑3 twin boundaries. Overall, these results offer a critical understanding of the atomic arrangement at the twin boundaries in pyrochlores, leading to control of the interplay between defects and properties.

Strain-driven half-metallicity in a ferri-magnetic Mott-insulator Lu2NiIrO6: a first-principles perspective

Half-metallic ferromagnetic/ferrimagnetic (FiM) materials are a matter of enormous interest due to their potential technological applications in solid-state electronic devices. In this way, strain plays an important role to tune or control the physical properties of the systems; therefore, the influence of both biaxial ([110]) and hydrostatic ([111]) strain on the electronic and magnetic properties of recently synthesized double perovskite oxide Lu2NiIrO6 is investigated using density-functional theory calculations. The unstrained system exhibits a FiM Mott-insulating (i.e., having an energy gap of 0.20 eV) ground state due to strong antiferromagnetic superexchange coupling between high-energy half-filled Ni2+-e2g↑ and low-energy partially filled Ir4+ t32g↑t22g↓ orbitals. Interestingly, a half-metallic FiM state is predicted under biaxial and hydrostatic compressive strains of -8% and -6%, respectively. The admixture of Ir 5d orbitals in the spin-majority channel is mainly responsible for the conductivity with small contributions from Ni 3d orbitals. In contrast, all the tensile strain systems show almost the same electronic behavior (Mott-insulating FiM states) as found in the case of the unstrained system. The magnetic moments of the Ni (Ir) ion slightly decrease and increase as a function of compressive and tensile strains due to shortening and lengthening of the Ni-O(Ir-O) bond lengths, respectively. Moreover, our calculations show that compressive strain enhances the structural distortions, which could help to increase the Curie temperature of the system.

Structure and magnetism of the Rh4+-containing perovskite oxides La0.5Sr0.5Mn0.5Rh0.5O3 and La0.5Sr0.5Fe0.5Rh0.5O3

Synchrotron X-ray powder diffraction data indicate that La0.5Sr0.5Mn0.5Rh0.5O3 and La0.5Sr0.5Fe0.5Rh0.5O3 adopt distorted perovskite structures (space group Pnma) with A-site and B-site cation disorder. A combination of XPS and 57Fe Mössbauer data indicate the transition metal cations in the two phases adopt Mn3+/Rh4+ and Fe3+/Rh4+ oxidation state combinations respectively. Transport data indicate both phases are insulating, with ρ vs. T dependences consistent with 3D variable-range hopping. Magnetisation data reveal that La0.5Sr0.5Mn0.5Rh0.5O3 adopts a ferromagnetic state below Tc ∼ 60 K, which is rationalized on the basis of coupling via a dynamic Jahn-Teller distortion mechanism. In contrast, magnetic data reveal La0.5Sr0.5Fe0.5Rh0.5O3 undergoes a transition to a spin-glass state at T ∼ 45 K, attributed to frustration between nearest-neighbour Fe-Rh and next-nearest-neighbour Fe-Fe couplings.

Robust half-metallicity and magnetic phase transition in Sr2CrReO6 via strain engineering

Using ab-initio calculations, the electronic and magnetic properties of double perovskite oxide \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Sr}}_2 {\text{CrReO}}_6$$\end{document}Sr2CrReO6 with two type of strains: biaxial (along the [110]-direction) and hydrostatic (along [111]-direction) are investigated. The ground state of the unstrained system is half-metallic ferrimagnetic, due to a strong antiferromagnetic (AFM) coupling between Cr and Re atoms within both (GGA and GGA+U) exchange-correlation potentials. It is demonstrated that the robustness of half-metallicity can be preserved under the influence of both biaxial and hydrostatic strains. Interestingly, a transition from ferri-to-ferromagnetic is established due to Re spin flipping to that of the Cr ion (i.e. Cr and Re spin becomes parallel) within the GGA+U method for both biaxial and hydrostatic tensile strains of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge +2\%$$\end{document}≥+2%. The strong confinement of orbitals due to tensile strain results in the decrease of electron hopping which further reduced the AFM coupling strength between Cr and Re atoms, this leads to a ferri-to-ferromagnetic transition. However, the GGA scheme holds the ferrimagnetic state with both kinds of strains. This work shows that tensile strain is a feasible way to optimize the magnetic properties of perovskite oxides, which are presumed to be beneficial for spintronic technology.

High-Pressure Synthesis of a B-site Co2+/Mn4+ Disordered Quadruple Perovskite LaMn3Co2Mn2O12

A new oxide, LaMn₃Co₂Mn₂O₁₂, was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA'₃B₄O₁₂-type quadruple perovskite structure with space group Im3̅. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn³⁺₃Co²⁺₂Mn⁴⁺₂O₁₂, where the La³⁺ and Mn³⁺ are 1:3 ordered respectively at the A and A' sites, whereas the Co²⁺ and Mn⁴⁺ are disorderly distributed at the B site. This is in sharp contrast to R₂Co²⁺Mn⁴⁺O₆ (R = La and rare earth) double perovskites, in which the Co²⁺ and Mn⁴⁺ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn₃Co₂Mn₂O₁₂ displays spin glassy magnetic properties due to the random Co²⁺ and Mn⁴⁺ distribution, as demonstrated by dc and ac magnetic susceptibility as well as specific heat measurements. Possible factors that affect the B-site degree of order in perovskite structures are discussed.

Broadband Electron Spin Resonance Study in a Sr2FeMoO6 Double Perovskite

We report broadband magnetic resonance in polycrystalline Sr₂FeMoO₆ measured over the wide temperature (T = 10-370 K) and frequency (f = 2-18 GHz) ranges. Sr₂FeMoO₆ was synthesized by the sol-gel method and found to be ferromagnetic below T _C = 325 K. A coplanar waveguide-based broadband spectrometer was used to record the broadband electron spin resonance (ESR) both in frequency sweep and field sweep modes. From the frequency sweep mode at fixed dc magnetic fields, we obtain the spectroscopic splitting factor g ∼ 2.02 for T ≥ T _C K, which confirms the 3+ ionic state of Fe in the material. The effective g value was found to decrease monotonically with decreasing temperature in the ferromagnetic regime. Resonance frequency decreases and the line width of the spectra increases as the temperature decreases below T _C. At room temperature (RT) and above, the line width (ΔH) of the ESR signal increases linearly with frequency, giving Gilbert damping constant α ∼0.032 ± 0.005 at RT. However, at lower temperatures, a minimum emerges in the ΔH vs frequency curve, and the minimum shifts to a higher frequency with decreasing temperature, confining the linear frequency regime to a narrow-frequency regime. Additional inhomogeneous broadening and low-field-loss terms are needed to describe the line width in the entire frequency range.

Systematic investigation of the magneto-electronic structure and optical properties of new halide double perovskites Cs2NaMCl6 (M = Mn, Co and Ni) by spin polarized calculations

A cohesive study using density functional theory simulations is performed to reveal and understand the structural stability, optoelectronic and magnetic properties of Cs₂NaMCl₆ (M = Mn, Co and Ni) halide double perovskites. The exchange-correlation potential, which is the only unknown parameter in the state-of-the-art formulism is determined through the well-known generalized gradient approximation and integration of the mBJ potential to it. The structural optimization, mechanical stability criteria and tolerance factor confirmed the stability of the double perovskites in a cubic structure with Fm3̄m symmetry. The elastic constants endorsed the mechanical stability and justify the brittle character of these double perovskites. The spin polarized electronic band profile and behaviour of the dielectric constant and absorption coefficient in the spin up and down channels revealed the presence of half-metallic nature in these materials. Moreover, herein, we have discussed the origin of the half-metallic gap and magnetism. The unpaired electrons in the crystal field splitted d-orbitals of the M-sited constituents are responsible for the half-metallic and magnetic character. The total magnetic moment was determined to be 4μ_B, 4μ_B and 1μ_B for the Mn-, Co- and Ni-based double perovskites, respectively, with main contributions solely coming from the transition metal atoms. The perfect spin polarization at the Fermi level suggests the application of double perovskites in spintronic technology.

The unpaired electrons in the crystal field splitted d-orbitals of the M-site constituents are responsible for the half metallicity and magnetic character of the halide double perovskites.

Robust intrinsic half-metallic ferromagnetism in stable 2D single-layer MnAsS4

Two-dimensional (2D) intrinsic half-metallic materials are of great interest to explore the exciting physics and applications of nanoscale spintronic devices, but no such materials have been experimentally realized. Using first-principles calculations based on density-functional theory, we predicted that single-layer MnAsS4was a 2D intrinsic ferromagnetic (FM) half-metal. The half-metallic spin gap for single-layer MnAsS4is about 1.46 eV, and it has a large spin splitting of about 0.49 eV in the conduction band. Monte Carlo simulations predicted the Curie temperature (Tc) was about 740 K. Moreover, within the biaxial strain ranging from -5% to 5%, the FM half-metallic properties remain unchanged. Its ground-state with 100% spin-polarization ratio at Fermi level may be a promising candidate material for 2D spintronic applications.

Extraordinary magnetic properties of double perovskite Eu2CoMnO6wide band gap semiconductor

Some novel magnetic behaviours in double perovskite Eu2CoMnO6(ECMO) have been reported. The x-ray photoemission spectroscopy study shows the presence of mixed valence states of transition metal ions. The UV-visible absorption spectroscopic study suggests that the ECMO has a direct wide band gap. A second-order magnetic phase transition as a sudden jump in the magnetization curve has been observed around 124.5 K. The large bifurcation between the zero field cooling and field cooling, suggests existence of strong spin frustration in the system. The inverse DC susceptibility confirms the presence of the Griffiths like phase. Sharp steps in magnetization have been observed in theM-Hcurve at 2 K, which vanishes on increasing temperature. The AC susceptibility study demonstrates the Hopkinson like effect as well as the presence of volume spin-glass-like behaviour. The temperature dependent Raman spectrum shows the presence of spin-phonon coupling.

High Curie temperature and carrier mobility of novel Fe, Co and Ni carbide MXenes

Two-dimensional (2D) magnets with room temperature ferromagnetism and semiconductors with moderate band gap and high carrier mobility are highly desired for applications in nanoscale electronics and spintronics. By performing the first-principles calculations, we investigate novel Fe, Co, Ni carbide based pristine (M₂C) and functionalized (M₂CT₂, T: F, O, OH) MXenes. Our calculations show that Fe₂C, Co₂C, Ni₂C, Fe₂CF₂, Fe₂CO₂, Fe₂C(OH)₂, Co₂CF₂, Co₂C(OH)₂ and Ni₂CF₂ are dynamically and mechanically stable. More importantly, Fe₂C, Co₂C, Fe₂CF₂ and Fe₂C(OH)₂ exhibit intrinsic ferromagnetism (magnetic moments 2-5μ_B per unit cell). Monte Carlo simulations suggest high Curie temperatures of 590 and 920 K for Fe₂C and Fe₂CF₂, respectively, at the HSE06 level owing to the large spin magnetic moments and strong ferromagnetic coupling. Based on the deformation potential theory, we predict high and anisotropic hole mobility (0.2-1.4 × 10⁴ cm² V^-1 s^-1) for semiconducting Fe₂CO₂ and Co₂C(OH)₂. Additionally, Ni₂CF₂ demonstrates highly anisotropic electron mobility together with a direct band gap. Our results further show the effectiveness of surface functionalization in modulating the electronic and magnetic properties and broadening the properties of MXenes to achieve long-range intrinsic ferromagnetism well above room temperature and high carrier mobility.

Confinement-Driven Ferroelectricity in a Two-Dimensional Hybrid Lead Iodide Perovskite

Organic-inorganic hybrid perovskites (OIHPs) hold a great potential for scientific and technological endeavors in the field of ferroelectrics, solar cells, and electroluminescent devices, because of their structural diversity, low cost of manufacture, and ease of fabrication. However, lead iodide perovskite ferroelectrics with narrow band gap have rarely been reported. Here, we present a new two-dimensional (2D) layered lead iodide perovskite ferroelectric, (4,4-DFHHA)₂PbI₄ (4,4-DFHHA = 4,4-difluorohexahydroazepine), with a spontaneous polarization (P_s) of 1.1 μC/cm² at room temperature, a direct bandgap of 2.32 eV, and a high Curie temperature T_c of 454 K (beyond that of BaTiO₃, 393 K). On the basis of the nonferroelectrics (HHA)I, (4-FHHA)I, and (4,4-DFHHA)I (HHA = hexahydroazepine, 4-FHHA = 4-fluorohexahydroazepine), we assembled them with PbI₂ to form lead iodide perovskites. Because the space between adjacent one-dimensional (1D) chains is relatively large and the confinement effect is not obvious, the cations are still in a disordered state, and 1D OIHPs (HHA)PbI₃ and (4-FHHA)PbI₃ are also nonferroelectrics at room temperature. In the confined environment of the 2D PbI₄^2- framework for (4,4-DFHHA)₂PbI₄, the 4,4-DFHHA cations become ordered, and their asymmetric distribution leads to the spontaneous polarization. This work offers an efficient strategy for enriching the family of lead iodide perovskite ferroelectrics through the confinement effect and should inspire further exploration of the interplay between ferroelectricity and photovoltaics.

Probing the Griffiths like phase, unconventional dual glassy states, giant exchange bias effects and its correlation with its electronic structure in Pr2-x Sr x CoMnO6

Crystal, electronic structure, dc and ac magnetization properties of the hole substituted (Sr²⁺) and partially B-site disordered double perovskite Pr_2-x Sr _x CoMnO₆ system have been investigated. The XRD pattern analysis showed a systematic decrease in the lattice parameters owing to the enhanced oxidation states of the Co/Mn ions. The electronic structure study by XPS measurements suggested the presence of mixed valence states of the B-site ions (Co²⁺ /Co³⁺ and Mn³⁺ /Mn⁴⁺) with significant enhancement of the average oxidation states due to hole doping. The mere absence of electronic states near the Fermi level in the valence band (VB) spectra for both pure (x  =  0.0) and Sr doped (x  =  0.5) systems indicated the insulating nature of the samples. Sr substitution is observed to increase the spectral weight near the Fermi level suggesting for an enhanced conductivity of the hole doped system. The dc magnetization data divulged a Griffiths like phase above the long-range ordering temperature. A typical re-entrant spin glass like phase driven by the inherent anti-site disorder (ASD) has been recognized by ac susceptibility study for both the pure and doped systems. Most interestingly, the emergence of a new cluster glass like phase (immediately below the magnetic ordering temperature and above the spin-glass transition temperature) solely driven by the Sr substitution has been unravelled by ac magnetization dynamics study. Observation of these dual glassy states in a single system is scarce and hence placed the present system amongst the rare materials. The isothermal magnetization measurements further probed the exhibition of the giant exchange bias effect originated from the interfacial exchange interactions due to existence of low temperature antiferromagnetic clusters embedded in the glassy matrix.

Aqueous Chemical Solution Deposition of Functional Double Perovskite Epitaxial Thin Films

Double perovskite structure (A₂ BB'O₆ ) oxides exhibit a breadth of multifunctional properties with a huge potential range of applications in fields as diverse as spintronics, magneto-optic devices, or catalysis, and most of these applications require the use of thin films and heterostructures. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films combining high performance with high throughput and low cost. In addition, the physical properties of these materials are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Thus, promoting spontaneous cationic ordering has become a relevant issue. In this work, our recent achievements by using polymer-assisted deposition (PAD) of environmentally friendly, water-based solutions for the growth of epitaxial ferromagnetic insulating double perovskite La₂ CoMnO₆ and La₂ NiMnO₆ thin films on SrTiO₃ and LaAlO₃ single-crystal substrates are presented. It is shown that the particular crystallization and growth process conditions of PAD (very slow rate, close to thermodynamic equilibrium conditions) promote high crystallinity and quality of the films, as well as favors spontaneous B-site cationic ordering.

Carbamazepine degradation by heterogeneous activation of peroxymonosulfate with lanthanum cobaltite perovskite: Performance, mechanism and toxicity

The widely used carbamazepine (CBZ) is one of the most persistent pharmaceuticals and suffers insufficient removal efficiency by conventional wastewater treatment. A synthesized Co-based perovskite (LaCoO₃) was used to activate peroxymonosulfate (PMS) in order to degrade CBZ. Results showed that LaCoO₃ exhibited an excellent performance in PMS activation and CBZ degradation at neutral pH, with low cobalt leaching. The results of FT-IR and XPS verified the high structurally and chemically stability of LaCoO₃ in PMS activation. Electron spin resonance (ESR) analysis suggested the generation of radical species, such as sulfate radicals (SO₄^-) and hydroxyl radicals (OH). Radical quenching experiments further revealed the responsibility of SO₄^- as the dominant oxidant for CBZ oxidation. Ten products were detected via the oxidation of CBZ, with the olefinic double bond attacked by SO₄^- as the initial step. Hydroxylation, hydrolysis, cyclization and dehydration were involved along the transformation of CBZ. The toxicity of CBZ solution was significantly reduced after treating by PMS/LaCoO₃.

High-Throughput Computational Search for Half-Metallic Oxides

Half metals are a peculiar class of ferromagnets that have a metallic density of states at the Fermi level in one spin channel and simultaneous semiconducting or insulating properties in the opposite one. Even though they are very desirable for spintronics applications, identification of robust half-metallic materials is by no means an easy task. Because their unusual electronic structures emerge from subtleties in the hybridization of the orbitals, there is no simple rule which permits to select a priori suitable candidate materials. Here, we have conducted a high-throughput computational search for half-metallic compounds. The analysis of calculated electronic properties of thousands of materials from the inorganic crystal structure database allowed us to identify potential half metals. Remarkably, we have found over two-hundred strong half-metallic oxides; several of them have never been reported before. Considering the fact that oxides represent an important class of prospective spintronics materials, we have discussed them in further detail. In particular, they have been classified in different families based on the number of elements, structural formula, and distribution of density of states in the spin channels. We are convinced that such a framework can help to design rules for the exploration of a vaster chemical space and enable the discovery of novel half-metallic oxides with properties on demand.

Machine learning lattice constants for cubic perovskite A22+BB′O6 compounds

The GPR model (M2) is developed to elucidate the statistical relationship among ionic radii, electronegativities, oxidation states, and lattice constants for cubic A₂²⁺BB′O₆ perovskites. The model demonstrates a high degree of accuracy and stability.

Electronic, magnetic, optical and thermoelectric properties of Ca2Cr1−xNixOsO6 double perovskites

With the help of density functional theory calculations, we explored the recently synthesized double perovskite material Ca₂CrOsO₆ and found it to be a ferrimagnetic insulator with a band gap of ∼0.6 eV.

Unconventional magnetism in the high pressure ‘all transition metal’ double perovskite Mn2NiReO6

Mn₂NiReO₆ has an unusual rotation of Mn spins from 80 down to 42 K where a collapse in weak ferromagnetism evidences switching of the canted components.

The effect of orbital-lattice coupling on the electrical resistivity of YBaCuFeO5 investigated by X-ray absorption

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential [Formula: see text] orbital at high temperature (300-150 K) to the [Formula: see text] orbital at/below 125 K. The analysis of the Fe K-edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab-plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3d orbital with lattice distortion of a single crystal of YBCFO.

Density functional study of structural, electronic and magnetic properties of new half-metallic ferromagnetic double perovskite Sr2MnVO6

In this paper, a new half-metallic (HM) double perovskite compound is predicted with the simultaneous presence of ferromagnetism and polar distortion. The structural, electronic and magnetic properties of Sr₂MnVO₆ (SMVO) are calculated by density functional theory (DFT) with both generalized gradient approximation (GGA) and GGA  +  U approaches, where U is the on-site Coulomb interaction parameter. Different orderings of B (B') cationic sites in A₂BB'O₆ double perovskite structure are evaluated, including rocksalt, columnar and layered arrangements for cubic, monoclinic and tetragonal crystal structures. It is found that the most stable ordering is obtained when B and B' are placed in a layered type ordering for a tetragonal crystal structure with I4/m space group, which is confirmed by phonon calculations. The B-site ordering of the Mn³⁺ and V⁵⁺ ions in a layered configuration leads to ferromagnetically coupled magnetic moments of 4.17 µ _B at Mn site and 0.23 µ _B at V site. Finally, SMVO is found to be a half-metallic ferromagnetic (HM-FM) compound with a band gap of 0.65 eV in a spin down channel with off-centered displacement of V atoms in the octahedral cage (second order Jahn -Teller effect) which can cause ferroelectricity. Therefore, SMVO is predicted to be a polar HM material and a promising candidate for multiferroic property with potential application in spintronics.

Weak ferromagnetic insulator with huge coercivity in monoclinic double perovskite La2CuIrO6

Insulating ferromagnets with high T _C are required for many new magnetic devices. More complexity arises when strongly correlated 3d ions coexist with strongly spin-orbit coupled 5d ones in a double perovskite. Here, we perform the structural, magnetic, and density functional theory (DFT) study of such double perovskite La₂CuIrO₆. A new P2₁/n polymorph is found according to the comprehensive analysis of x-ray, Raman scattering and phonon spectrum. The magnetization reveals a weak ferromagnetic (FM) transition at T _C  =  62 K and short range FM order in higher temperature range. A huge coercivity is found as high as H _C ~ 11.96 kOe at 10 K, which, in combination with the negative trapped field, results in the magnetization reversal in the zero field cooling measurement. The first principle calculations confirm the observed FM state and suggest La₂CuIrO₆ of this polymorph is a Mott insulating ferromagnet assisted by the spin-orbit coupling.

Octahedral tilting and emergence of ferrimagnetism in cobalt-ruthenium based double perovskites

Rare earth based cobalt-ruthenium double perovskites A₂CoRuO₆ (A  =  La, Pr, Nd and Sm) were synthesized and investigated for their structural and magnetic properties. All the compounds crystallize in the monoclinic P2₁/n structure with the indication of antisite disorder between Co and Ru sites. While La compound is already reported to have an antiferromagnetic state below 27 K, the Pr, Nd and Sm systems are found to be ferrimagnetic below [Formula: see text], 55 and 78 K respectively. Field-dependent magnetization data indicate prominent hysteresis loop below [Formula: see text] in the samples containing magnetic rare-earth ions, however, magnetization does not saturate even at the highest applied fields. Our structural analysis indicates strong distortion in the Co-O-Ru bond angle, as La³⁺ is replaced by smaller rare-earth ions such as Pr³⁺ , Nd³⁺ , and Sm³⁺ . The observed ferrimagnetism is possibly associated with the enhanced antiferromagnetic superexchange interaction in the Co-O-Ru pathway due to bond bending. The Pr, Nd and Sm samples also show the small magnetocaloric effect with the Nd sample showing the highest value of magnitude  ∼3 J kg^-1K^-1 at 50 kOe. The change in entropy below 20 K is found to be positive in the Sm sample as compared to the negative value in the Nd counterpart.

Magnetocaloric effects from an interplay of magnetic sublattices in Nd2NiMnO6

We present a combined experimental and theoretical study to understand the magnetism and magnetocaloric behavior of the double perovskite Nd₂NiMnO₆. The magnetic susceptibility data confirms a ferromagnetic transition with [Formula: see text] K. An additional feature at T  =  25 K, indicative of antiferromagnetic correlations, is present. A positive magnetocaloric effect (MCE) near [Formula: see text] and a negative MCE around T  =  25 K is inferred from the temperature dependence of the change in magnetic entropy at low magnetic fields. The negative MCE peak is suppressed on the application of a magnetic field and can be made to switch to a conventional positive MCE upon increasing magnetic field. We understand and reproduce these features in Monte Carlo simulations of a phenomenological Heisenberg model for Nd₂NiMnO₆. The validity of the model is tested using density functional theory calculations. We argue that this simple understanding of the experimental observations in terms of two antiferromagnetically coupled sublattices allows these results to be useful across a broader class of magnetocaloric materials.

Investigation of multi-mode spin-phonon coupling and local B-site disorder in Pr2CoFeO6 by Raman spectroscopy and correlation with its electronic structure by XPS and XAS studies

Electronic structure of Pr₂CoFeO₆ (at 300 K) was investigated by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy techniques. All three cations, i.e. Pr, Co and Fe were found to be trivalent in nature. XPS valance band analysis suggested the system to be insulating in nature. The analysis suggested that Co³⁺ ions exist in low spin state in the system. Moreover, Raman spectroscopy study indicated the random distribution of the B-site ions (Co/Fe) triggered by same charge states. In temperature-dependent Raman study, the relative heights of the two observed phonon modes exhibited anomalous behaviour near magnetic transition temperature T _N ~ 270 K, thus indicating towards interplay between spin and phonon degrees of freedom in the system. Furthermore, clear anomalous softening was observed below T _N which confirmed the existence of strong spin-phonon coupling occurring for at least two phonon modes of the system. The line width analysis of the phonon modes essentially ruled out the role of magnetostriction effect in the observed phonon anomaly. The investigation of the lattice parameter variation across T _N (obtained from the temperature-dependent neutron diffraction measurements) further confirmed the existence of the spin-phonon coupling.

Phase stability and electronic structure of iridium metal at the megabar range

The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions.

Half-Metallic Property Induced by Double Exchange Interaction in the Double Perovskite Bi2BB'O6 (B, B' = 3d Transitional Metal) via First-Principles Calculations

In this paper, we identify three possible candidate series of half-metals (HM) from Bi-based double perovskites Bi2BB'O6 (BB' = transition metal ions) through calculations utilizing the density functional theory (DFT) and full-structural optimization, in which the generalized gradient approximation (GGA) and the strong correlation effect (GGA + U) are considered. After observing the candidate materials under four types of magnetic states, i.e., ferromagnetic (FM), ferrimagnetic (FiM), antiferromagnetic (AF), and nonmagnetic (NM), we found eight promising candidates for half-metallic materials. Under the GGA scheme, there are three ferromagnetic-half-metal (FM-HM) materials, Bi2CrCoO6, Bi2CrNiO6 and Bi2FeNiO6, and three FiM-HM materials, Bi2FeZnO6, Bi2CrZnO6 and Bi2CoZnO6. With implementation of the Coulomb interaction correction (GGA + U), we find two stable half-metallic materials: Bi2CrNiO6 and Bi2CrZnO6. We determine that the stability of some of these materials are tied to the double exchange interaction, an indirect interaction within the higher powers of localized spin interaction among transition metals via oxygen ions. Found in half-metallic materials, and especially those in the ferromagnetic (FM) state, the double exchange interaction is recognized in the FM-HM materials Bi2CrCoO6 and Bi2FeNiO6.

Crossover from negative to positive magnetoresistance in Sr2CrWO6/Sr2Fe10/9Mo8/9O6 superlattices

Sr₂CrWO₆/Sr₂Fe_10/9Mo_8/9O₆ (SCWO/SFMO) superlattices with 4, 6, 7, 10 periods (abbreviated as S-1, S-2, S-3, and S-4) were prepared on (0 0 1) SrTiO₃ (STO) substrates by pulsed laser deposition. All superlattices show macroscopic ferromagnetic behavior, and the magnetization increases with increasing period. The S-1 superlattice demonstrates semiconductor-like temperature-dependent resistivity in the whole measuring temperature range and negative magnetoresistance of  -5.3% at 2 K with 2 T magnetic field, while the other superlattices illustrate metallic behaviors and increasing positive magnetoresistance of 223.1%, 275.4%, and 766.1% under the same conditions. This work not only provides a feasible way to tune the MR effect in magnetic perovskite oxides, but also may stimulate further work on artificially micro-structured thin films with designable magnetic properties.

Defect driven spin state transition and the existence of half-metallicity in CoO

We unveil the native defect induced high spin to low spin state transition in [Formula: see text] and half-metallicity in CoO. First principles calculations unravel that, defect density holds a key role in dictating the spin-state transition in [Formula: see text] ion in CoO, and introducing the half-metallicity. Charge transfer in the vicinity of vacancy plane favors the stabilization and coexistence of bivalent [Formula: see text] and trivalent [Formula: see text] ion in CoO. We propose that defect engineering could serve as a route to design the half metallicity in transition metal mono-oxides.

Topochemical nitridation of Sr2FeMoO6

The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to cubic, Fm3[combining macron]m double perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange interactions determine ferromagnetic order with TC ≈ 100 K. Substitution of oxide by nitride induces bond asymmetries and local electronically driven structural distortions, which combined with Fermi level lowering restricts charge itinerancy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.

Connection between Unusual Lattice Thermal Expansion and Cooperative Jahn-Teller Effect in Double Perovskites LaPbMSbO6 (M = Mn, Co, Ni)

Lattice thermal expansion (LTE) has been investigated in double perovskites LaPbMSbO₆ (M = Mn, Co, Ni). Ordinary LTE behavior with good thermal stability is identified for the Mn sample, whereas unusual LTE with a preferably expanded interplanar distance of (040) is revealed for Co and Ni samples. Temperature-dependent X-ray diffraction patterns ( T-XRD), Raman spectra ( T-Raman), and specific heat capacities ( T- C_p) consistently indicate that a rare isostructural displacive phase transition (IDPT) with a second-order phase transition nature is predominant near the critical temperature. Refinements of neutron powder diffraction (NPD) and in situ T-XRD data present temperature-sensitive bond parameters which are relevant to planar oxygen O1. X-ray photoelectron spectra (XPS) further confirm the Jahn-Teller (J-T) activated Co²⁺ (HS) or Ni³⁺ (HS/LS) cations at the B-site sublattice. This unusual LTE behavior could be understood by the cooperative J-T effect contributed by a Pb²⁺ ion and Co²⁺/Ni³⁺ ion from A- and B-site sublattices, respectively. The importance of 6s(Pb)-2p(O)-3d(Co/Ni) extended orbital hybridization on affecting thermal expansion behavior is highlighted on the basis of temperature-induced phonon mode softening. This study presents a microscopic description of connection between anisotropic thermal expansion and a cooperative J-T effect, which inspired exploration of thermal-mechanical coupled functional materials based on LaPbMSbO₆ double perovskites.

Ferromagnetism above 1000 K in a highly cation-ordered double-perovskite insulator Sr3OsO6

Magnetic insulators have wide-ranging applications, including microwave devices, permanent magnets and future spintronic devices. However, the record Curie temperature (T_C), which determines the temperature range in which any ferri/ferromagnetic system remains stable, has stood still for over eight decades. Here we report that a highly B-site ordered cubic double-perovskite insulator, Sr₃OsO₆, has the highest T_C (of ~1060 K) among all insulators and oxides; also, this is the highest magnetic ordering temperature in any compound without 3d transition elements. The cubic B-site ordering is confirmed by atomic-resolution scanning transmission electron microscopy. The electronic structure calculations elucidate a ferromagnetic insulating state with J_eff = 3/2 driven by the large spin-orbit coupling of Os⁶⁺ 5d² orbitals. Moreover, the Sr₃OsO₆ films are epitaxially grown on SrTiO₃ substrates, suggesting that they are compatible with device fabrication processes and thus promising for spintronic applications.

Pursuing high Curie temperature magnetic insulators has been one of the extensively studied subjects due to their wide appeal for spintronic applications. Here the authors experimentally and theoretically demonstrate a record high Curie temperature over 1000 K in B-site ordered double-perovskite, Sr3OsO6.

Magneto-elastic coupling behavior of the double perovskite Ba2FeMoO6

Ba₂FeMoO₆ has been reported to exhibit a ferromagnetic to a paramagnetic transition with a Curie temperature above room temperature, and thus it is a potential material for spintronics. These exciting properties are related, at least in part, to combined structural and magnetic instabilities. A conventional analysis of lattice parameter data from the present study and literature in terms of spontaneous strain shows that the magnetic ordering is accompanied by a significant volume strain, which implies a strong coupling behavior of magneto-elastic properties in Ba₂FeMoO₆. In addition, to address the correlation between anti-site defects (ASD) and magnetic properties, we have carried out a comparative study of six cubic Ba₂FeMoO₆ polycrystals with different degrees of ASD. The correlations among the Curie temperature, low-temperature saturation magnetism, and ASD are discussed in detail using a combination of diffraction and magnetic measurements. This systematic study, especially the strain analysis, of Ba₂FeMoO₆ will facilitate its potential applications in the field of spin electronics and thin film engineering.

Sr2Fe1+xRe1−xO6 double perovskites: magnetoresistance and (magneto)thermopower

Polycrystalline Sr₂Fe_1+xRe_1−xO₆ samples have been synthesized, structurally characterized by X-ray powder diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and measurements of their magnetotransport properties were performed.

Phase transitions in Ca2−xBaxNdSbO6 complex perovskites: a combined crystallographic and vibrational investigation

Phase transitions in Ca_2−xBa_xNdSbO₆ complex perovskites resulting from altered Ca/Ba compositions swap the local conformational environment of Nd³⁺.