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CaCu 3 Mn 2 Te 2 O 12 was synthesized using high-temperature and high-pressure conditions. The compound possesses an A- and B site ordered quadruple perovskite structure in Pn 3̅ symmetry with the charge combination of CaCu 3 2+ Mn 2 2+ Te 2 6+ O 12 . A ferrimagnetic phase transition originating from the antiferromagnetic interaction between A' site Cu 2+ and B site Mn 2+ ions is found to occur at T C ≈ 100 K. CaCu 3 Mn 2 Te 2 O 12 also shows insulating electric conductivity. Optical measurement demonstrates the energy bandgap to be about 1.9 eV, in agreement with the high B site degree of chemical order between Mn 2+ and Te 6+ . The first-principles theoretical calculations confirm the Cu 2+ (↓)-Mn 2+ (↑) ferrimagnetic coupling as well as the insulating nature with an up-spin direct bandgap. The current CaCu 3 Mn 2 Te 2 O 12 provides an intriguing example of an intrinsic ferrimagnetic insulator with promising applications in advanced spintronic devices.

α-CdFe 2 O 4 stabilizes its normal spinel structure due to the covalent Cd-O bond, in which all the connections between adjacent FeO 6 octahedral are edge-shared, forming a typical geometrically frustrated Fe 3+ magnetic lattice. As the high-pressure methods were utilized, the post-spinel phase β-CdFe 2 O 4 with a CaFe 2 O 4 -type structure was synthesized at 8 GPa and 1373 K. The new polymorph has an orthorhombic structure with the space group Pnma and an 11.5% higher density than that of its normal spinel polymorph (α-CdFe 2 O 4 ) synthesized at ambient conditions. The edge-shared FeO 6 octahedra form zigzag S = 5/2 spin ladders along the b -axis dominating its low-dimensional magnetic properties at high temperatures and a long-range antiferromagnetic ordering with a high Néel temperature of T N1 = 350 K. Further, the rearrangement of magnetic ordering was found to occur around T N2 = 265 K, below which the competition of two phases or several couplings induce complex antiferromagnetic behaviors.

The B-site Fe/Os ordered and disordered quadruple perovskite oxides CaCu 3 Fe 2 Os 2 O 12 were synthesized under different high-pressure and high-temperature conditions. The B-site ordered CaCu 3 Fe 2 Os 2 O 12 is a system with a very high ferrimagnetic ordering temperature of 580 K having the Cu 2+ (↑)Fe 3+ (↑)Os 5+ (↓) charge and spin arrangement. In comparison, the highly disordered CaCu 3 Fe 2 Os 2 O 12 has a reduced magnetic transition temperature of about 350 K. The Cu 2+ Fe 3+ Os 5+ charge combination remains the same without any sign of changes in the valence state of the constituent ions. Although the average net moments of each sublattice are reduced, the average ferrimagnetic spin arrangement is unaltered. The robustness of the basic magnetic properties of CaCu 3 Fe 2 Os 2 O 12 against site disorder may be taken as an indication of the tendency to maintain the short-range order of the atomic constituents.

Defect engineering is an important means of improving the electrochemical performance of the Co 3 O 4 electrocatalyst in the oxygen evolution reaction (OER). In this study, operando soft X-ray absorption spectroscopy (SXAS) is used to explore the electronic structure of Co 3 O 4 under OER for the first time. The defect-rich Co 3 O 4 (D-Co 3 O 4 ) has a Co 2.45+ state with Co 2+ at both octahedral (O h ) and tetrahedral (T d ) sites and Co 3+ at O h , whereas Co 3 O 4 has Co 2.6+ with Co 2+ and Co 3+ at T d and O h sites, respectively. SXAS reveals that upon increasing the voltage, the Co 2+ in D-Co 3 O 4 is converted to low-spin Co 3+ , some of which is further converted to low-spin Co 4+ ; most Co 2+ in Co 3 O 4 is converted to Co 3+ but rarely to Co 4+ . When the voltage is switched off, Co 4+ intermediates quickly disappear. These findings reveal Co(O h ) in D-Co 3 O 4 can be rapidly converted to active low-spin Co 4+ under operando conditions, which cannot be observed by ex situ XAS.

Organic-molecular magnets based on a metal-organic framework with chemically tuned electronic and magnetic properties have been attracting tremendous attention due to their promising applications in molecular magnetic sensors, magnetic particle medicines, molecular spintronics, etc. Here, we investigated the magnetic behavior of a heterojunction comprising a ferromagnetic nickel (Ni) film and an organic semiconductor (OSC) 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) layer. Through the magneto-optical Kerr effect (MOKE), a photoemission electron microscopy (PEEM), X-ray magnetic circular dichroism (XMCD), and X-ray photoelectron spectroscopy (XPS), we found that the adsorption of F4-TCNQ on Cu(100)/Ni not only reverses the in-plane magnetization direction originally exhibited by the Ni layer but also results in enhanced magnetic ordering. Furthermore, the cyano group (CN) in adsorbed F4-TCNQ was found spin-polarized along with conspicuous charge transfer with Ni. The density functional theory (DFT) calculations suggest that the experimentally found spin polarization originates from hybridization between the CN group's π orbitals and Ni's d band. These findings signify that the hybrid states at the organic-ferromagnet interface play a key role in tailoring the magnetic behavior of interfaces. For the case of the F4-TCNQ and Ni heterojunction reported here, interface coupling is an antiferromagnetic one.

Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Herein, we report a new class of catalyst, namely, UCoO 4 with alternative CoO 6 and 5f-related UO 6 octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f-O 2p-Co 3d network. UCoO 4 exhibits a low overpotential of 250 mV at 10 mA cm -2 , surpassing other unitary cobalt-based catalysts ever reported. X-ray absorption spectroscopy revealed that the Co 2+ ion in pristine UCoO 4 was converted to high-valence Co 3+/4+ , while U 6+ remained unchanged during the OER, indicating that only Co was the active site. Density functional theory calculations demonstrated that the OER activity of Co 3+/4+ was synergistically enhanced by the covalent bonding of U 6+ -5f in the U 5f-O 2p-Co 3d network. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement.

B-site Os-doped quadruple perovskite oxides LaCu 3 Fe 4- x Os x O 12 ( x = 1 and 2) were prepared under high-pressure and high-temperature conditions. Although parent compound LaCu 3 Fe 4 O 12 experiences Cu-Fe intermetallic charge transfer that changes the Cu 3+ /Fe 3+ charge combination to Cu 2+ /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, Os 6+ 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 Im 3̅ space group. It experiences a spin-glass transition around 480 K. With further Os substitution up to x = 2, the crystal symmetry changes to Pn 3̅, where Fe and Os are orderly distributed in a rocksalt-type fashion at the B site. Moreover, this composition shows a long-range Cu 2+ (↑)Fe 3+ (↑)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.

Epitaxial CoFe 2 O 4 (CFO)/CoO bilayers were fabricated by pulsed laser deposition on flexible muscovite mica substrate. Samples with different CFO thicknesses were employed to study the phenomenon of exchange bias involving strongly anisotropic ferromagnet. Magnetic measurements exhibited great enhancement in the features of exchange bias. Raman and X-ray absorption spectroscopies indicated that a new phase emerged within the CFO layer because of the cation charge redistribution in CFO layer under bending, which in turn gave rise to anomalous hysteresis loops exhibited in the bent bilayers. These results provide a fundamental understanding about the mechanisms of exchange bias prevailing in these bilayers and call attention to the implementation of spintronic devices using flexible heterostructures such as the present CFO/CoO bilayers.

A new oxide, LaMn 3 Co 2 Mn 2 O 12 , was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA' 3 B 4 O 12 -type quadruple perovskite structure with space group Im 3̅. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn 3+ 3 Co 2+ 2 Mn 4+ 2 O 12 , where the La 3+ and Mn 3+ are 1:3 ordered respectively at the A and A' sites, whereas the Co 2+ and Mn 4+ are disorderly distributed at the B site. This is in sharp contrast to R 2 Co 2+ Mn 4+ O 6 (R = La and rare earth) double perovskites, in which the Co 2+ and Mn 4+ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn 3 Co 2 Mn 2 O 12 displays spin glassy magnetic properties due to the random Co 2+ and Mn 4+ 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.

The report on the superconductivity of the two-legged spin ladders BaFe 2 S 3 and BaFe 2 Se 3 has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba 9 Fe 3 Te 15 , including its synthesis and magnetic properties. The high-pressure synthesized Ba 9 Fe 3 Te 15 crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe 6 octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced one-dimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a T max at 190 K followed by a spin glass (SG) state with freezing at T f ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe 2+ , i.e., 0.84 μ B per Fe in Ba 9 Fe 3 Te 15 . The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba 9 Fe 3 Te 15 as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.

An ilmenite-like monoclinic phase of HgMnO 3 with space group P 2 1 / c was prepared using high-pressure and high-temperature methods at 18 GPa and 1473 K. The MnO 6 octahedra form a two-dimensional (2D) network in the bc plane, leading to a long-range antiferromagnetic ordering with a low Néel temperature of T N ∼ 32 K. As the synthesis pressure increases to 20 GPa, a new perovskite-like rhombohedral phase with space group R 3̅ c was found to occur. The rhombohedral phase exhibits a three-dimensional (3D) network for the MnO 6 octahedra, giving rise to an antiferromagnetic ordering at T N ∼ 60 K. X-ray absorption spectroscopy confirms the invariable Mn 4+ charge state in these two polymorphic phases, in agreement with the Curie-Weiss and bond valence sum analysis. HgMnO 3 provides an interesting example to study the magnetic properties from 2D to 3D by varying synthesis pressure.

A new 3d-5d hybridization oxide, CaCu 3 Mn 2 Os 2 O 12 (CCMOO), was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes to an A-site-ordered but B-site-disordered quadruple perovskite structure with a space group of Im 3̅ (No. 204). The charge states of the transition metals are determined to be Cu 2+ /Mn 3.5+ /Os 4.5+ by X-ray absorption spectroscopy. Although most B-site-disordered perovskites possess lower spin-ordering temperatures or even nonmagnetic transitions, the current CCMOO displays a long-range ferrimagnetic phase transition with a critical temperature as high as ∼280 K. Moreover, a large saturated magnetic moment is found to occur [7.8 μ B /formula units (f.u.) at 2 K]. X-ray magnetic circular dichroism shows a Cu 2+ (↑)Mn 3.5+ (↑)Os 4.5+ (↓) ferrimagnetic coupling. The corner-sharing Mn/OsO 6 octahedra with mixed Mn and Os charge states make the compound metallic in electrical transport, in agreement with a specific heat fitting at low temperature. This work provides a rare example with high spin-ordering temperature and a large magnetic moment in B-site-disordered 3d-5d hybridization perovskite oxides.

A new oxide NaCu 3 Fe 2 Os 2 O 12 is synthesized using high pressure and temperature conditions. The Rietveld structural analysis shows that the compound possesses both A- and B-site ordered quadruple perovskite structure in Pn3̅ symmetry. The valence states of transition metals are confirmed to be Cu 2+ /Fe 3+ /Os 5.5+ . The three transition metals all take part in magnetic interactions and generate strong Cu 2+ (↑)Fe 3+ (↑)Os 5.5+ (↓) ferrimagnetic superexchange interactions with a high Curie temperature about 380 K. Electrical transport measurements suggest its half-metallic properties. The first-principles theoretical calculations demonstrate that the compound has a spin-down conducting band and a spin-up insulating band with a wide energy gap.

Superlattices are of great importance due to their potential as new materials genome to synthesize new functional materials. Thus, tuning of the ground state of superlattices is crucial to further control their physical properties. In this study, superlattices (SLs) consisting of alternating layers of SrRuO 3 (SRO) (5 nm) and La 0.5 Ca 0.5 MnO 3 (LCMO) (5 nm) are epitaxially grown on SrTiO 3 (STO) and LaAlO 3 (LAO) substrates with 10-unit-cell periods. A variation in the substrate-induced-strain for this choice of SLs triggers observation of remarkable properties, such as magnetic anisotropy and large magnetic hysteresis. The strain states experienced by LCMO and SRO in these SLs result in strong ferromagnetic interlayer coupling and weak antiferromagnetic interlayer coupling at low temperatures in SLs of LCMO-SRO/STO and a strong antiferromagnetic interlayer coupling in SLs of LCMO-SRO/LAO. Besides, a large magnetic hysteresis resulting from the predominant magnetic anisotropy of SRO together with the strength of magnetic coupling is observed in SLs of LCMO-SRO/LAO along the out-of-plane direction of the LAO substrate. These four different magnetic behaviors along four different directions of substrate orientations are interpreted in terms of preferential orbital occupation and competing magnetic exchange coupling together with magnetic anisotropy. This study demonstrates the subtleties in controlling the strength of magnetic coupling at the interface and stands as a model system to realize fascinating magnetic phenomena in layer-by-layer hetero-epitaxial oxide films., Competing Interests: The authors declare no competing financial interest.

Magnetic refrigeration, resulting from the magnetocaloric effect of a material around the magnetic phase-transition temperature, is a topic of great interest as it is considered to be an alternate energy solution to conventional vapor-compression refrigeration. The viability of a magnetic refrigeration system for magnetic cooling can be tested by exploiting materials in various forms, from bulk to nanostrucutres. In this study, magnetocaloric properties of self-assembled Mn3O4-La(0.7)Sr(0.3)MnO3 nanocomposites, with varying doping concentrations of Mn3O4 in the form of nanocrystals embedded in the La(0.7)Sr(0.3)MnO3 matrix, are investigated. The temperatures corresponding to the paramagnetic-to-ferromagnetic transitions are higher, and the values of change in magnetic entropy under a magnetic field of 2 T show an enhancement (highest being ∼130%) for the nanocomposites with low doping concentrations of Mn3O4, compared to that of pure La(0.7)Sr(0.3)MnO3 thin films. Relative cooling power remain close to those of La(0.7)Sr(0.3)MnO3. The enhanced magnetic phase-transition temperature and magnetocaloric effect are interpreted and evidenced in the framework of interfacial coupling between Mn3O4 and La(0.7)Sr(0.3)MnO3. This work demonstrates the potentiality of self-assembled nanostructures for magnetic cooling near room temperature under low magnetic fields.

Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captured significant attention in the past decade. Recent studies have been focused on the advance of synthesis mechanisms as well as the potential on device applications. In order to create further opportunities upon functional mesocrystals, we fabricated a self-assembled nanocomposite composed of magnetic CoFe2O4 mesocrystal in Sr-doped manganites. This combination exhibits intriguing structural and magnetic tunabilities. Furthermore, the antiferromagnetic coupling of the mesocrystal and matrix has induced an additional magnetic perturbation to spin-polarized electrons, resulting in a significantly enhanced magnetoresistance in the nanocomposite. Our work demonstrates a new thought toward the enhancement of intrinsic functionalities assisted by mesocrystals and advanced design of novel mesocrystal-embedded nanocomposites.

Green sulfur bacteria, which live in extremely low-light environments, use chlorosomes to harvest light. A chlorosome is the most efficient, and arguably the simplest, light-harvesting antenna complex, which contains hundreds of thousands of densely packed bacteriochlorophylls (BChls). To harvest light efficiently, BChls in a chlorosome form supramolecular aggregates; thus, it is of great interest to determine the organization of the BChls in a chlorosome. In this study, we conducted a (13)C solid-state nuclear magnetic resonance and Mg K-edge X-ray absorption analysis of chlorosomes from wild-type Chlorobaculum tepidum. The X-ray absorption results indicated that the coordination number of the Mg in the chlorosome must be >4, providing evidence that electrostatic interactions formed between the Mg of a BChl and the carbonyl group or the hydroxyl group of the neighboring BChl molecule. According to the intermolecular distance constraints obtained on the basis of (13)C homonuclear dipolar correlation spectroscopy, we determined that the molecular assembly of BChls is dimer-based and that the hydrogen bonds among the BChls are less extensive than commonly presumed because of the twist in the orientation of the BChl dimers. This paper also reports the first (13)C homonuclear correlation spectrum acquired for carotenoids and lipids-which are minor, but crucial, components of chlorosomes-extracted from wild-type Cba. tepidum.

This study investigates the effect of Fe(3+) on the electronic structure of nanocrystalline ceria. Systematic synchrotron X-ray absorption spectroscopy coupled with scanning transmission electron microscopy/electron energy loss spectroscopy was utilized. The oxygen vacancies can be engineered and their number varied with the degree of iron doping. Comparing the local electronic structure around Ce sites with that around Fe sites reveals two stages of defect engineering. The concentration of Ce(3+) and the distribution of defects differ between lower and higher degrees of doping. Charge is transferred between Ce and Fe when the doping level is less than 5%, but this effect is not significant at a doping level of over 5%. This transfer of charge is verified by energy loss spectroscopy. These Fe-modified ceria nanoparticles exhibit core-shell-like structures at low doping levels and this finding is consistent with the results of scanning transmission electron microscopy/electron energy loss spectroscopy. More Fe is distributed at the surface for doping levels less than 5%, whereas the homogeneity of Fe in the system increases for doping levels higher than 5%. X-ray magnetic circular dichroism spectroscopy reveals that Ce, rather than Fe, is responsible for the ferromagnetism. Interestingly, Ce(3+) is not essential for producing the ferromagnetism. The oxygen vacancies and the defect structure are suggested to be the main causes of the ferromagnetism. The charge transfer and defect structure Fe(3+)-Vo-Ce(3+) and Fe(3+)-Vo-Fe(3+) are critical for the magnetism, and the change in saturated magnetization can be understood as being caused by the competition between interactions that originate from magnetic polarons and from paired ions.

The complex metal oxide SrCo0.5Ru0.5O(3-δ) possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy. We have discovered that Co(3+) ion is purely high-spin at room temperature, which is unique for a Co(3+) in an octahedral oxygen surrounding. We attribute this to the crystal field interaction being weaker than the Hund's-rule exchange due to a relatively large mean Co-O distances of 1.98(2) Å, as obtained by EXAFS and X-ray diffraction experiments. A gradual high-to-low spin state transition is completed by applying high hydrostatic pressure of up to 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. Thereby is the much debated intermediate spin state of Co(3+) absent in this material. These results allow us to draw an energy diagram depicting relative stabilities of the high-, intermediate-, and low-spin states as functions of the metal-oxygen bond length for a Co(3+) ion in an octahedral coordination.

To realize the origin of efficient spin injection at organic-ferromagnetic contact in organic spintronics, we have implemented the formation of quasi-molecular magnet via surface restructuring of a strong organic acceptor, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ), in contact with ferromagnetic cobalt. Our results demonstrate a spin-polarized F4-TCNQ layer and a remarkably enhanced magnetic anisotropy of the Co film. The novel magnetic properties are contributed from strong magnetic coupling caused by the molecular restructuring that displays an angular anchoring conformation of CN and upwardly protruding fluorine atoms. We conclude that the π bonds of CN, instead of the lone-pair electrons of N atoms, contribute to the hybridization-induced magnetic coupling between CN and Co and generate a superior magnetic order on the surface.