Mn Sites Research Articles

Kinetics of Oxygen Catalyst Based on Pyrochlore Oxide Nano-Particles for Low Temperature Water Electrolysis

Alkaline water electrolysis is carried out at 60 to 80 oC using nickel-based electrode materials and highly concentrated KOH solutions, which make highly corrosive environment against the electrodes, the separator, the current collector, and the others of the cell. Reduction in operating temperature would be preferable to suppress corrosion, although it would also make the catalytic activity for OER and HER down, making the cell voltage up. Since oxygen evolution is known as a high overpotential reaction which is more than 400 mV with nickel-based oxides at 60 oC or more, and alternative materials with lower overpotential and higher stability for OER in AWE have been needed. This paper presents a novel oxygen catalyst nano-particles based on bismuth ruthenium oxide, in which ruthenium at the B site in the pyrochlore structure is partly replaced with manganese1).Mn-doped bismuth ruthenium oxide (MBRO) was prepared by precipitation of metal hydroxide precursor, which had been obtained by addition of NaOH solution into the metal salt solution containing Bi(III) and Ru(III) with Mn(II), followed by calcination at 600 oC. XRD, EDX, EXAFS, ICP-AES, and RBS were used for structural and compositional analyses of MBRO. The obtained oxide was added into distilled water and the solution was treated by ultrasonic vibration to make a suspension, which was then dropped on a titanium disk (4 mm diam.) and dried at RT for 24 h, resulting in oxide particles-coated titanium disk, in which no binders or ionomers were used in the process. The disk was mounted in a commercial rotating disk electrode and was used as the working electrode with a platinum counter electrode, an Hg/HgO reference electrode, and 0.1 mol/L KOH solution. Polarization curves were measured by LSV for OER, of which the results were analyzed to obtain the onset potential, Tafel slope, exchange current density. All measurements were performed at RT (25 oC).EDX and EXAFS studies of MBRO showed that Ru(IV) was replaced with Mn(IV) at the B-site of pyrochlore structure, and this doping made a single phase of pyrochlore structure oxide without sub-product up to 30 at% replacement . Linear sweep voltammetry revealed the overpotential of MBRO for OER was less than 200 mV and the Tafel slope analysis of the LSV curve gave 40 mV/dec or less. The onset potential and the Tafel slope were almost the same when Ru:Mn atomic ratio of MBRO was changed from 100:0 to 70:30 at%. Further analysis of the polarization curve indicated the exchange current density slightly decreased with increasing Mn ratio. These results suggest that Ru is the active site for OER but Mn is as well. The Tafel slope of OER catalysts reported in literatures is mostly 60 mV/dec or more, including more than 200 mV/dec of nickel-based oxides2,3). Therefore, MBRO is a hyper catalytic oxide for OER in alkaline media and can significantly reduce the overpotential of the anode in AWE even at ambient temperature.Ref.1) M. Morimitsu, Patent US.2022085387.A1.2) T. Shinagawa, A. T. Garcia-Esparza, K. Takanabe, Scientific Reports, Vol. 5, Article No.13801 (2015).3) S. Yagi, et al., Nature Communications, Vol. 6, Article No. 8249 (2015).

Refining Mn(III) Active Sites in MnO2-Based Self-Assembled Electrodes Via Oxygen Vacancy Induction to Enhance Oxygen Reaction Performance in Zinc-Air Batteries

Rechargeable zinc-air batteries are considered as a promising candidate for next-generation electrochemical energy conversion devices due to their safety, low cost, and high theoretical capacity/ energy density. Over the past few decades, α-MnO2-based electrodes have demonstrated their excellent performance and competitiveness in oxygen catalytic reactions. (1) According to eg filling theory, Mn(III) (t2g3eg1) sites in [MnO6] octahedral unit deliver superior bifunctional activity compared to Mn(II) (t2g3eg2) and Mn(IV) (t2g3eg0) sites.(2) However, the presence of unpaired single electrons of the Mn(III) site leads to low thermodynamic stability. Introducing oxygen vacancies to promote the electron rearrangement of the [MnO6] unit is a promising approach for enhancing the durability of Mn(III) active sites. Herein, we construct an oxygen-vacancy-enhanced MnO2-based electrode via a self-assembly process (NiCo2O4-MnO2@Ni foam). The intrinsic oxygen catalytic activity of NiCo2O4-MnO2@Ni foam is significantly improved by forming abundant highly stable Mn(III) active sites, which accelerate the mass transfer ability. As expected, the NiCo2O4-MnO2@Ni foam shows good bifunctional activity with a low overpotential of 0.69 V. The zinc-air battery assembled by NiCo2O4-MnO2@Ni foam exhibits a high peak power density of 576 mW cm-2, surpassing that of the Pt/C-Ir/C electrode. Additionally, benefiting from its stable flower-shaped hierarchical structure, the self-assembled electrode demonstrates long-term charge-discharge cycling stability, which lasts over 800 hours. This work provides innovative insights into enhancing MnO2-based electrodes by oxygen vacancy introduction.References N. Xu, Q. Nie, L. Luo, C. Z. Yao, Q. Gong, Y. Liu, X. Zhou, and J. Qiao, ACS applied materials & interfaces (2018).Z. Yin, R. He, H. Xue, J.-M. Chen, Y. Wang, X. Ye, N. Xu, J. Qiao and H. Huang, Energy Materials (2022).

Structural Evolution of Manganese Prussian Blue Analogue in Aqueous ZnSO4 Electrolyte.

Among different Prussian Blue Analogues (PBAs), manganese hexacyanoferrate (MnHCF), with open framework and two abundant electroactive metal sites, exhibits high potential for the grid-scale aqueous rechargeable zinc-ion batteries (ARZIBs) application. Until now, the intercalation mechanism of Zn2+ into MnHCF has not been clearly illustrated. In this work, combining different synchrotron X-ray techniques, the structural and microscopic evolution of MnHCF in 3 m ZnSO4 electrolyte is comprehensively studied, and a thorough understanding of the intercalation/release dynamic, in terms of local and long-range domain, is provided. The elemental distribution and structural information of Fe, Mn, Zn inside MnHCF electrodes is obtained from the X-ray fluorescence (XRF) elemental maps and X-ray absorption spectroscopy (XAS). The in-depth analysis of extended X-ray absorption fine structure (EXAFS) signals confirm that the rearrangement of Mn site, evidencing the cleavage of the Mn─N bond with the formation of a Mn─O bond, in an octahedral environment. The phase transformation of MnHCF takes place exclusively during the 1st cycle, and a mixture of rhombohedral and cubic zinc hexacynoferrate (ZnHCF) phases are formed during the first charge process. Thereafter, the newly formed cubic ZnHCF phase becomes the only stable one, existing in the subsequent cycles and exhibiting excellent electrochemical stability.

Operando Soft X-ray Absorption of LaMn1-x Co x O3 Perovskites for CO Oxidation.

We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn1-x Co x O3 catalysts subjected to a mixture of CO and O2 at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn2+ to Mn3+ and Mn4+, while Co largely maintains a valence state of Co2+. In the case of LaCoO3, we identify high-spin and low-spin Co3+ species combined with Co2+. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.

Chemical pressure induced spin reorientation in the magnetic structure of Ca3Mn2−xSnxO7 ( x=0,0.03,0.05 )

The effect of partially substituting Tin (Sn) at the Manganese (Mn) site of Ca3Mn2O7 , viz. Ca3Mn2−xSnxO7 with x=0.03,0.05 , on its structural and magnetic properties have been investigated using synchrotron diffraction, neutron diffraction, and bulk magnetization measurements. It is observed that with a substitution of x=0.03 , the minor (≈8%) tetragonal ( I4/mmm ) structural phase that is present in the predominantly orthorhombic ( Cmc21 ) undoped Ca3Mn2O7 , completely disappears. The compounds order antiferromagnetically, the ordering temperature decreases with increasing Sn-content, indicating a weakening of the antiferromagnetic exchange interactions. Interestingly, in the ordered state, the spin magnetic moments which were aligned along the a-axis of the unit cell in the undoped compound, are observed to have reoriented with their major components lying in the b − c plane in the Sn-doped compounds. The above influence of Sn-doping is seen to be stemming from a significant modification of the octahedral rotation and tilt mode geometry in the unit cell, that is known to be responsible for driving ferroelectricity in these compounds.

Selective degradation of pollutants in the Mn-NG/PMS system: The crucial role of surface reactive Mn(IV) species

The combination of transition metal oxides with carbon-based materials is a promising strategy for constructing efficient catalysts for peroxymonosulfate (PMS) activation to degrade aqueous organic contaminants. However, the complex heterostructure of the composites often hinders the identification of the actual reactive species responsible for pollutant degradation. In this study, we integrate Mn3O4 into N-doped graphite (NG) to elucidate the mechanisms involved in the Mn-NG composite mediated PMS activation process. The Mn-NG/PMS system shows excellent performance in oxidizing bisphenol A (BPA) over a wide pH range (3.4 to 9.4) and in the presence of co-existing substrates. Mn sites in Mn3O4 are responsible for the activation of PMS, and the NG matrix enhances the dispersion of Mn in the Mn-NG composite. Multiple experimental results indicate that free radicals (i.e., hydroxyl radicals (HO•) and sulfate radicals (SO4•−)) are not generated, and surface reactive Mn(IV) is proposed as the primary reactive species for BPA oxidation. This study provides new insights into understanding the role of Mn oxides in carbon-based composites in PMS activation for degradation of organic pollutants.

Electron Donor–Acceptor Activated Single Atomic Sites for Boosting Oxygen Reduction Reaction

AbstractFabricating efficient non‐platinum‐group‐metal catalysts for the oxygen reduction reaction (ORR) in proton‐exchange membrane fuel cells still remains a big challenge. This study creates a unique bamboo‐like architecture of Mn and Fe single atomic sites (SASs) anchored on core‐shell structure of nanopaticles@carbon nanotubes (MnFe SASs/NPs@CNTs) via a precursor route, where the Fe nanoparticles (NPs) (identified as γ‐Fe and Austenite) are confined into CNTs, such an architecture exhibits compelling ORR activity and durability in 0.1 m HClO4. Experiments and calculations both reveal that the electron donor–acceptor paradigm between Mn SASs and Fe NPs launches a lattice‐electron coupling mechanism not only increasing occupation of π‐antibonding orbital in Mn−*O intermediates but also rising Jahn–Teller effect, thereby destabilize the Mn−*O intermediate and eventually facilitate the potential‐limiting step from *O to *OH. Such a coupling activation of the ORR‐inert Mn SASs greatly improves ORR performances of MnFe SASs/NPs@CNTs.

Dilution of helical antiferromagnetic matrix of CaMn7O12 under nonmagnetic Ga[formula omitted] doping

The work reports dilution studies on antiferromagnetic CaMn7O12. Introduction of nonmagnetic ion in an antiferromagnetic matrix changes the magnetic properties in an interesting way. The study becomes significant due to the fact that remarkable increase in magnetization is observed as a result of substitution of a few of the magnetic Mn ions with the nonmagnetic Ga ion. For 5% doping of Ga, the magnetization value becomes nearly thrice of the un-doped CaMn7O12. A sequential decrease in antiferromagnetic ordering transition temperature is also observed with increasing Ga percentage in the parent compound. The solid solubility limit of Ga in the matrix is found to be nearly 5%, above which it start showing impurity peaks. Experimental results are well complemented with the Density functional theory (DFT) based calculations. Theoretical calculations are carried out on (CaMn7O12)3 unit cell with one Ga atom doped at different Mn sites resulting around 5% doping concentration. Besides identifying the most favourable doping site for Ga, results indicate a reduction of bulk band gap and significant increase in magnetization, both of which are in agreement with the experimental results.

Recycled Cathodes in Rechargeable Aqueous Batteries as Ready-Made Electrodes for Oxygen Evolution Catalysis.

The development of a low-cost and efficient oxygen evolution reaction (OER) electrode is of critical importance for water electrolysis technologies. The general approach to achieving a high-efficiency OER electrode is to regulate catalytic material structures by synthetic control. Here we reported an orthogonal approach to obtaining the OER electrode without intentional design and synthesis, namely, recycling MnO2 cathodes from failed rechargeable aqueous batteries and investigating them as ready-made catalytic electrodes. The recycled MnO2 cathode showed very little Zn2+ storage capacity but surprisingly high OER activity with a low overpotential of 307 mV at 10 mA cm-2 and a small Tafel slope of 77.9 mV dec-1, comparable to the state-of-the-art RuO2 catalyst (310 mV, 86.9 mV dec-1). In situ electrochemical and theoretical studies jointly revealed that the accelerated OER kinetics of the recycled MnO2 electrode was attributed to the enlarged active surface area of MnO2 and optimized electronic structure of Mn sites. This work suggests failed battery cathodes as successful catalysis electrodes for sustainable energy development.

LiMn2O4/Graphite Cell Degradation Mechanisms Studying How Mn Deposition Accelerates Lithiated Graphite Reactivity with Electrolyte

In this work, pouch cells of spinel LiMn2O4 (LMO) / artificial graphite (AG) were tested under varying conditions to investigate degradation mechanisms. Mn dissolution from the positive electrode and deposition onto the graphite negative electrodes immediately after formation was found to be significantly suppressed by operating cells at −10 °C during the formation cycle and limiting the upper cutoff voltage. A formation cycle at an elevated temperature of 70 °C greatly increases the Mn deposition and gas generation from electrolyte reduction after just a single cycle and had long term effects at increasing lifetime gassing and Mn deposition. The cold formation advantage disappeared once all cells were cycled at 40 °C, with similarly terrible cycle life (60–200 cycles) regardless of formation conditions. Studying fully lithiated graphite (LiC6) extracted from LMO cells in an isothermal microcalorimeter (IMC), we found that the parasitic heat flow associated with electrolyte reduction and graphite delithiation increased for anodes with significant Mn deposition. Therefore, Mn deposition on the negative electrode causes cell failure by compromising the anode passivation, increasing lithiated graphite-electrolyte reactivity and thereby accelerating the lithium inventory loss due to electrolyte reduction on the Mn sites on the negative electrode.

Electronic structure and delithiation mechanism of vanadium and nickel doped Li2MnPO4F cathode material for lithium-ion batteries

This study calculates the energy band structure and density of states of Lithium manganese fluorophosphate (Li2MnPO4F, a lithium transition metal phosphate compounds) using the first-principles plane-wave pseudopotential approach within the density-functional theory. The model of Li2M0.5Mn0.5PO4F (M = V, Ni) with transition metal doped Mn sites is constructed by using the CASTEP module. The calculation findings indicate that the transition metal doping can regulate the energy band structure of the intrinsic system, and Li2MnPO4F makes the band gap decrease, and the volume increase with the Li ions of being deintercalated, and the electrons can be readily stimulated from the valence band to the conduction band. The findings indicate that Li2MnPO4F is a favorable cathode material for high-voltage lithium ion batteries (LIBs). The introduction of vanadium (V) and nickel (Ni) doping reduces the band gap, facilitating an easier excitation of electrons from the valence band to the conduction band. This study provides a theoretical study of new cathode materials for high performance LIBs.

Bimetallic MOF derived Ni[sbnd]Mn phosphide for high-performance supercapacitor electrode material

Nickel‑manganese bimetallic phosphides are prepared by phosphidization of MOF-derived bimetallic hydroxides. The bimetallic phosphides exhibit crystalline Ni2P phase, show high specific surface area, and are composed of nanosheets. When supported on carbon cloth (CC) and used as the electrode material in 4 M KOH, the (Ni0.93Mn0.07)2P-18/CC shows a specific capacitance of 851.1C g−1 at 1 A g−1 and maintains 84.87 % capacity after 5000 cycles. The origin of the high specific capacity is investigated, and electron interaction between Mn and Ni sites, the high specific surface area, and the facile ion transport leads to the high specific capacity and stability. The constructed (Ni0.93Mn0.07)2P-18//AC asymmetric supercapacitor has an energy density of 59.83 Wh kg−1 at a power density of 750 W kg−1 and a capacity retention of 95.5 % after 5000 cycles.

Hydroxyl-aluminum pillared bentonite enhanced Mn(II) removal by chlorine oxidation

Al-PILC was used to catalyze the chlorine oxidation of Mn(II) in aqueous solution. The effects of various catalysts, catalyst dosage, chlorine dosage, pH value, temperature and organic content on the oxidation process were investigated. Results show that 1.5 mg/L chlorine can quickly oxidize Mn(II) from 0.5 mg/L to less than 0.04 mg/L with 10 mg/L Al-PILC. Using catalysts with higher porosity and higher SA, increase in chlorine concentration, increase in catalyst dosage, higher pH, and higher temperature can significantly enhance the rate of Mn(II) catalytic oxidation. The Mn(II) oxidation process includes the homogeneous oxidation, catalytic oxidation on the surface of the catalysts and self-catalytic oxidation produced by the newly produced MnOx. Al-PILC surface provides active sites for chlorine oxidation Mn(II) in the water, and also provides binding sites for the newly produced MnOx, which has higher catalytic activity and thus has an self-catalytic oxidation effect. The higher the porosity and SA of Al-PILC, the more catalytic oxidation active sites and loading sites, and the better the catalytic oxidation effect. The study promotes the understanding of chlorine catalyzed oxidation Mn(II) in aqueous solution, but also provide important guide to study newly efficient catalysts to oxidize Mn(II) with chlorine in aqueous solution.

Innovative Insights into Water-Oxidation Mechanism: Investigating Birnessite's Reaction with Cerium(IV) Ammonium Nitrate.

Developing Mn-based water-oxidation reaction (WOR) catalysts is key for renewable energy storage, utilizing Mn's abundance, cost-effectiveness, and natural role. Cerium(IV) ammonium nitrate (CAN) has been widely utilized as a sacrificial oxidant in the exploration of WOR catalysts. In this study, advanced techniques, such as X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and in situ electron paramagnetic resonance (EPR), to delve into the WOR facilitated by CAN and birnessite were employed. XANES analysis has demonstrated that the average oxidation states (AOSs) of Mn in birnessite, a birnessite/CAN mixture, and in the birnessite/CAN mixture postwater addition are 3.7, 3.8, and 3.9, respectively. In situ Raman spectroscopy performed in the presence of birnessite and CAN revealed a distinct peak at 784 cm-1, which is attributed to Mn(IV)═O. A shift of this peak to 769 cm-1 in H218O confirms its association with Mn(IV)═O. No change in this peak was observed in D2O, further supporting the notion that it is linked to Mn(IV)═O rather than Mn-OH (D). Furthermore, EPR spectroscopy shows the presence of Mn(IV). It is suggested that the WOR mechanism initiates with the oxidation of birnessite by CAN, which enhances the concentration of Mn(IV) sites in the birnessite structure. Under acidic conditions, birnessite, enriched in Mn(IV), facilitates oxygen evolution and subsequently transitions into a form with reduced Mn(IV) levels. This process highlights the critical function of the Mn (hydr)oxide structure, similar to its role in the water-oxidizing complex of Photosystem II, where it serves as charge storage for oxidizing equivalents from CAN, paving the way for a four-electron reaction that drives the WOR.

Defect engineering of copper-doped mesoporous manganese dioxide nanoflowers for enhancing electrocatalytic detection of hydrogen peroxide

Birnessite-type δ-MnO2 nanoflowers doped with copper ions (Cu-MnO2) can enhance the catalytic performance of MnO2-based catalysts for efficient electrochemical sensing of H2O2. There are no significant changes in the surface morphology and lattice structure of bulk MnO2 after Cu doping via engineered cation exchange. The catalytic Cu species are effectively incorporated into the tunnel entrance of δ-MnO2, the Mn site in the MnO6 octahedron, and surface defects. The Cu doping enhances the electrical and ionic conductivities, and generates numerous Mn3+ defects, oxygen vacancies, and Cu+/Cu2+ redox sites. Thus, the Cu doping facilitates the transport and adsorption of more H2O2 molecules to the catalytic sites, significantly increasing the catalytic rate. In contrast, pristine MnO2 exhibits negligible catalytic performance for H2O2 reduction because of its slow catalytic kinetics and low electrical conductivity. The Cu-MnO2 catalyst exhibits higher sensitivity (105.7 μA·mM−1·cm−2), a lower detection limit (0.6 μM), and a broader linear range (0.005 ∼ 4.2 mM) compared to pristine δ-MnO2.

Realizing low thermal conductivity in Cr-doped nanostructured higher manganese silicide

Higher manganese silicides have evolved as an efficient thermoelectric material because of their exceptional thermoelectric performance at intermediate temperatures. In this work, we report the ultralow-thermal conductivity with improved thermoelectric properties of Mn1-xCrxSi1.8 (x = 0, 0.025, 0.05 and 0.1) prepared by induction melting followed by ball milling and uni-axial induction hot-pressing. Bragg diffraction patterns confirmed that the as-synthesized compound belongs to the Mn15Si26 phase along with the solid solubility of Cr in the HMS lattice. In addition, the backscattered electron micrographs revealed the precipitation of excess Si in the HMS matrix, originating from Si-rich stoichiometry. Furthermore, an elevation in electrical conductivity with increasing Cr content has been observed because of its acceptor dopant behaviour at the Mn site. On the contrary, the Seebeck coefficient decreased upon an increase in Cr content, and bulk MnSi1.8 has shown the highest Seebeck coefficient of 220 μV/K at 773 K. Impressively, nanostructuring along with the generated point defects as a result of Cr dopant facilitates an intensified phonon scattering at the nanostructured grain boundaries that results in a very low κtotal of 0.99 W/mK at 773 K for ball-milled Mn0.9Cr0.1Si1.8, leading to an elevated zT of 0.46 at 773 K.

Enhancing atomic ordering, magnetic and transport properties of Mn2VGa Heusler alloy thin films toward negatively spin-polarized charge injection

Magnetic materials with negative spin polarization have attracted attention for their potential to increase the design freedom of spintronic devices. This study investigated the effects of off-stoichiometry on the atomic ordering, microstructure, and magneto-transport properties in Mn2+xV1−xGa (x = −0.2, 0, +0.2, +0.4) Heusler alloy films, which are predicted to have large negative spin polarization derived from a pseudo band gap in the majority spin channel. The Mn2+xV1−xGa films epitaxially grown on MgO(001) substrates exhibits variations of B2 and L21 order with the Mn concentration. A high-quality L21 ordered film was achieved in the Mn-rich composition (x = +0.2) with B2 and L21 order parameters of 0.97 and 0.86, respectively, and a saturation magnetization of 1.4 μB/f.u., which agrees with the Slater-Pauling rule. Scanning transmission electron microscopy observations showed that B2 and L21 phases coexist in Mn-poor and stoichiometric films, while the L21 phase is dominant in the Mn-rich film with small amounts of Mn–V and Mn–Ga disorders, as revealed by laboratory and anomalous X-ray diffraction. Combined first-principles calculations and anisotropic magnetoresistance analysis confirms that the addition of excess Mn preserves the high spin polarization by suppressing the formation of detrimental antisites of V atoms occupying Mn sites. Therefore, the Mn-rich composition is promising for negatively spin-polarized charge injection in Mn2VGa-based spintronic applications.

Understanding the magnetism-ductility trade-off in FeCoMn alloys: The role of the BCC-B2 transition and Mn occupancies

The magnetism-ductility contradictory relationship presents a significant challenge in the development of magnetic alloys. The impact of the BCC-B2 transition, along with Mn site occupancy, on magnetism and ductility have been investigated by using first-principles calculations. The calculations involved the evaluation of magnetic moments, density of states (DOS), phase stability and ductility of FeCoMn alloys. The results of binary alloys confirm the enhancement of magnetism due to the BCC-B2 transition. Furthermore, the ordering phase transition can strengthen the magnetic interaction between Fe and Mn atoms, which is associated with minimal variations in the density of states of Fe and Mn in the B2 structure. Regarding the ductility of FeCoMn alloys, two factors contribute to increased brittleness. Firstly, the increased covalent component in bonding, as a result of the strong hybridization between different elements, leads to an increased brittleness. Secondly, the increased Peierls stress provides a larger resistance to dislocation motion, which also contributes to the increased brittleness. Finally, the Pearson correlation coefficients and data analysis indicate that VEC, spin polarizations and Mn content provide major contributions to the contradictory relationship between magnetism and ductility.

Regulating interfacial exchange coupling in perpendicular magnetized Fe/DO22-Mn3Ga bilayer films

Manganese gallium alloy with DO22-type chemical ordering (DO22-Mn3Ga) thin film represents an intriguing candidate for fabricating the fundamental composition of magnetic devices. In this paper, we prepare the Fe/DO22-Mn3Ga composite films on thermal MgO (100) substrates with different annealing temperatures and report the interfacial exchange coupling between Fe and DO22-Mn3Ga bilayers. Fe layer thickness and annealing temperature are predominant in regulating the nature and strength of interfacial exchange coupling. It is found that there is a ferromagnetic coupling between Fe and DO22-Mn3Ga layers. Their exchange coupling strength can be regulated by changing the Fe soft magnetic layer thickness in a reasonable range. Additionally, the interface diffusion generated from heat treatment induces to change of the nature of exchange coupling. It is possible that Fe replaces Mn site in the DO22-Mn3Ga unit cell due to smaller atomic radius and higher electronegativity compared with Mn. Antiferromagnetic coupling from annealed Fe/DO22-Mn3Ga film with high perpendicular magnetic anisotropy is technologically important to research synthetic antiferromagnetic structure for spin-transfer-torque magnetoresistive random access memory. Our works is helpful for the exploration of interfacial science and development of magnetic devices.

Structural phase transition of G-type antiferromagnetic SrMnO3 under pressure

Perovskite SrMnO3 is a versatile material with applications in transistors, supercapacitors, and spintronic devices. This study employs the DFT+U calculations to investigate the electronic and magnetic properties of two polymorphs: 4H-SrMnO3 and 3C-SrMnO3 under isotropic pressure conditions up to 60 GPa. A novel approach based on density functional perturbation theory (DFPT) is employed to precisely determine the on-site Hubbard U value, yielding a computed value of 4.552 eV, consistent with experimental reports. Below P = 14 GPa, the G-AFM phase of 4H-SrMnO3 is the most favored stable, and transitioning to the G-AFM phase of 3C-SrMnO3 beyond 14 GPa. Notably, we observed significant changes in the band gaps, exhibiting an increasing trend under isotropic pressure, while the magnetic moments of the Mn sites exhibit a decreasing trend from ≈2.8 μ B to ≈2.3 μ B for P = 0 GPa and 60 GPa, respectively. The projected density of states (PDOS) reveals the p-d hybridization contributions between the 3d-Mn and 2p-O covalent bonds, along with d-d Mott Hubbard interactions within the localized 3d orbitals of neighboring Mn-Mn atoms. Furthermore, a decrease in the Mn1-O-Mn2 distance prompts enhanced charge transfer which was facilitated by ligand holes, leading to a noticeable decrease in the magnetic moment of the Mn site in the G-AFM phase of SrMnO3.