Change In Magnetic Moment Research Articles

Effects of transition metals (V, Cr, Mn, Fe and Ni) doping on magnetic properties of Co-based amorphous alloys

Amorphous alloys exhibit numerous interesting and abundant magnetic properties due to their disordered structural traits and have been extensively studied by researchers. In this work, the effects of a small amount of transition metals doping on the magnetic properties of Co80-xMxB20 (x = 0, 2, 4, 6, 8, 10; M = V, Cr, Mn, Fe, Ni) amorphous alloys were systematically investigated by utilizing first-principles molecular dynamics. The trend of the magnetic moment change in the simulated calculation conforms well to the experimental results. The doping of V atoms leads to the fastest rate of decrease in the magnetic moment of the surrounding Co atoms, and there is only an antiferromagnetic interaction between the V and Co atoms. When doped with Cr and Mn atoms, there is mutual competition between ferromagnetic and antiferromagnetic coupling in the amorphous alloy, Cr reduces the magnetic moment of the surrounding Co atoms, while the change in the magnetic moment of the Co atoms around Mn is small. The doping of Fe and Ni causes an increase in the magnetic moment of the surrounding Co atoms. The reason may be due to the charge transfer between atoms. The doping of Fe and Ni has a small effect on the average magnetic moment of the Co atoms in the amorphous alloy. The results provide necessary theoretical support for the development of the Co-based amorphous alloys with excellent magnetic characteristics.

Photoinduced Magnetization Switching in Organic Molecules Achieved Through Stabilization of the Triplet Excited States

AbstractOrganic photoresponsive materials, which change their physical properties upon irradiation with light, are attracting tremendous scientific attention, as they offer a flexible and sustainable platform for developing various optoelectronic devices. In this work, photoinduced magnetization switching in two carbazole‐functionalized organic molecules, (9H‐carbazol‐9‐yl)(4‐chlorophenyl)methanone (ClPhCz) and (9H‐carbazol‐9‐yl)(2,4‐dichlorophenyl)methanone (DClPhCz) is reported for the first time. It was found that stabilization of the triplet excited state in those molecules is long enough for a net change in magnetic moment to be detected with a SQUID magnetometer. The change in magnetic moment upon excitation with a blue light laser was larger in ClPhCz, remaining high upon repeating the photoirradiation. In contrast, DClPhCz exhibited a smaller magnetic response, which gradually decreased upon repeating the experiment.

Magnetic anisotropy of L1[formula omitted] FeNi (001), (010), and (111) ultrathin films: A first-principles study

In previous experiments, L10 FeNi thin films with different surfaces, including (001), (110), and (111), were produced and studied. Each surface defines a different alignment of the crystallographic tetragonal axis with respect to the film’s plane, resulting in different magnetic anisotropies. In this study, we use density functional theory calculations to examine three series of L10 FeNi films with surfaces (001), (010), and (111), and with thicknesses ranging from 0.5 to 3 nm (from 4 to 16 atomic monolayers). Our results show that films (001) have perpendicular magnetic anisotropy, while (010) favor in-plane magnetization, with a clear preference for the tetragonal axis [001]. We proposed calling this type of in-plane anisotropy fixed in-plane. A film with a surface (111) and a thickness of four atomic monolayers has a magnetization easy axis almost perpendicular to the plane of the film. As the thickness of the (111) film increases, the direction of magnetization rotates towards a tetragonal axis [001], positioned at an angle of about 45° to the plane of the film. Furthermore, the most significant changes in spin and orbital magnetic moments occur at a depth of about three near-surface atomic monolayers. Ultrathin L10 FeNi films with varying magnetic anisotropies may find applications in spintronic devices.

Magnetic Structure-Dependent Ultrafast Spin Relaxation in Magnet CrI3: A Time-Domain ab Initio Study.

Two-dimensional magnet CrI3 is a promising candidate for spintronic devices. Using nonadiabatic molecular dynamics and noncollinear spin time-dependent density functional theory, we investigated hole spin relaxation in two-dimensional CrI3 and its dependence on magnetic configurations, impacted by spin-orbit and electron-phonon interactions. Driven by in-plane and out-of-plane iodine motions, the relaxation rates vary, extending from over half a picosecond in ferromagnetic systems to tens of femtoseconds in certain antiferromagnetic states due to significant spin fluctuations, associated with the nonadiabatic spin-flip in tuning to the adiabatic flip. Antiferromagnetic CrI3 with staggered layer magnetic order notably accelerates adiabatic spin-flip due to enhanced state degeneracy and additional phonon modes. Ferrimagnetic CrI3 shows a transitional behavior between ferromagnetic and antiferromagnetic types as the magnetic moment changes. These insights into the spin dynamics of CrI3 underscore its potential for rapid-response spintronic applications and advance our understanding of two-dimensional materials for spintronics.

Excellent catalytic activity for NO decomposition on Rhn supported on grain-boundary terminations of the MgO (0 0 1) surface

In the present study, we first propose the grain-boundary termination of the MgO (001) surface (referred as GB@MgO(001)) as the substrate to support a series of Rhn clusters to trap and dissociate NO. Using van der Waals density functional theory calculations, we have studied the interaction between the Rhn (n = 1 ∼ 8) cluster and the MgOΣ5(210)GB@MgO(001) and the properties of the adsorption and dissociation of NO on Rhn/GB@MgO(001) systems. The Rhn clusters can tightly bond to the GB@MgO(001), with interfacial interactions comparable to those at the Rhn/MgO-nanotube interface. Stable quasi-two-dimensional Rhn clusters provide more active sites to NO. Moreover, NO dissociation is activated, with all the complete reaction paths occur below the reference state, indicating the possibility of the NO decomposition. An extremely small energy barrier of 0.037 eV was obtained for NO dissociating on the square-Rh4/GB@MgO(001) system. Molecular dynamic (MD) calculations reveal that NO can be completely decomposed at 527 K without obvious structure distortions of the complex. The adsorption of NO yields a change in the total magnetic moment of the Rhn/GB@MgO(001) system, but it has a minimal impact on the adsorption energy of NO. Our results also show that the optimal Rhn clusters that are good at trapping and dissociating NO are not the most stable structures, but rather clusters of relatively low dimension that are bound to the substrate with a moderate strength. As a result, the modification and tuning of the substrate structure can effectively enrich the structure of Rh cluster catalyst and improve its physical properties so that it can potentially play a greater role in other redox reactions, which motivates us to continue designing various types of GB@MgO(001) for different purposes.

Research on the de-tumbling method of magnetic moment variation of two stars based on the tangential formation

High-speed space debris threatens the safety of satellites in orbit, making the efficient removal of this debris an increasingly unavoidable necessity. By exploring the non-contact de-tumbling method for high-speed rotating targets, a de-tumbling technique is proposed. This method relies on leveraging the differential magnetic moments of two satellites arranged in tangential formation. Two single-coil electromagnetic satellites form a uniform magnetic field at the rotating targets, forming a tangential collinear distribution. The de-tumbling method of the simultaneous change of the magnetic moment of two satellites and the change of magnetic moment of one satellite and the other satellite is explored, and the de-tumbling effect is compared through numerical simulation. The simulation results indicate that the de-tumbling effect of the latter method is superior. Additionally, there is minimal variation observed in the relative distance between servicing stars and between servicing stars and target stars throughout the de-tumbling process.

Phase coexistence and transitions between antiferromagnetic and ferromagnetic states in a synthetic antiferromagnet

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field ∼100 mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields ∼ 40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices whose operation is based on spin textures in perpendicularly magnetized SAFs. Published by the American Physical Society 2024

Exploring stable half-metallicity and magnetism of Cr-based double half-Heusler alloys and its high magnetoresistance ratio in magnetic tunnel junction

A new series of double half-Heusler (DHH) alloy Cr2FeCoZ2 (Z = As, Ge, S, P) are investigated by using the non-equilibrium Green's function in combination with the first-principles calculations. The calculations on magnetism reveal that these Cr-based DHH alloys are ferrimagnets due to the antiparallel magnetic coupling between Cr and Fe (Co). The electronic structure calculations of the Cr-based DHH alloys reveal half-metallicity, and Cr2FeCoAs2 possesses the largest spin-down gap of 1.181 eV. Within the c/a ranges from 1.60 to 2.52, Cr2FeCoAs2 maintains its 100 % spin-polarization, and the changes in total and spin-resovled atomic magnetic moment are negligible, showing a stable half metallicity and magnetism against the tetragonal distortion. Futhermore, the transport properties are investigated by constructing the Cr2FeCoZ2/GaAs/Cr2FeCoZ2 magnetic tunnel junction (MTJ) model. By changing the magnetic arrangement of the left and right electrodes into antiparallel configuration, the capability of transportation in both spin channels is significantly limited. Conversely, by modulating the magnetic arrangement of the left and right electrodes into parallel configuration, the transportation of electrons in spin-up channel is exceptionally strong while that of the spin-down channel is severely impeded. The Cr2FeCoAs2/GaAs/Cr2FeCoAs2 MTJ owns the highest tunneling magnetoresistance (TMR) ratio of ∼7.42 × 108 %, indicating that Cr2FeCoAs2 is the most promising candidate for high performance spintronic devices.

The adsorption of CO and NO on (8,0) SWCNT decorated with transition metals: A DFT study as a possible gas sensor

The adsorption and sensing capabilities of CO and NO on pristine and metal-decorated (8,0) SWCNT were analyzed by DFT. Both gases cause a slight deformation of the nanotube curvature in the direction of molecular adsorption. The molecule's adsorption and sensing performance are enhanced with the transition metal (Sc, Cr, Fe, and Ni) decoration on SWCNT. Decorated nanotubes are suitable for high-temperature sensors. According to relative energies for NO, the operational range is 900–1200 K. For CO/Cr- and Sc-SWCNT between 100 and 200 K and close to 550 K in the cases of decoration with Fe and Ni. From the recovery time analysis, the metal-decorated materials exhibit better performance at high temperatures and low pressures for NO compared to CO. Changes in magnetic moment suggest that Cr-decorated SWCNT could serve as a magnetic sensor for both molecules.

Characteristics of interacting carbon-antisite-vacancies in 4H silicon carbide

Spin defects in semiconductors have demonstrated promising electronic structures for potential applications in quantum computing and sensing. Among various proposed quantum byte systems, spin defects in silicon carbide have attracted significant attention due to several advantages they offer over other options. In this study, we investigate carbon-antisite-vacancy defects in 4H silicon carbide through ab initio density functional theory calculations. With the HSE06 functionals, the ab initio computation can predict much more accurate electronic structures. However, the corresponding computational cost is high, especially for supercells consisting of several hundreds of atoms. In this investigation, the carbon-antisite-vacancy defect is studied by using a high-performance computing cluster, with a specific focus on supercells that encompass two such defects. The extension of a single carbon-antisite-vacancy defect is depicted by referring to the spin density distribution. Different defect types show similar spin density patterns. Based on the single defect characteristics, supercells with paired carbon-antisite-vacancy defects are created. It is found that the binding energy can reach 2 eV for overlapping defects. In the case of insignificant overlap of the corresponding single defects, the ground state magnetic moment is 4 µB, accompanied by a negligible binding energy. However, if there is a significant overlap of the spin density, the magnetic moment changes to 2 µB. These findings can serve as helpful references for the study of spin defects in 4H silicon carbide, particularly in the potential carbon-antisite-vacancy application research.

An axial mode magnetoelectric antenna: Radiation predictions via multiphysics modeling with experimental validations

This study investigates an axial extension mode magnetoelectric antenna designed for near-field communication in dielectric cluttered environments. The antenna configuration consists of two magnetostrictive Metglas-polymer composites bonded on opposite sides of a PZT-5A actuator, creating a dumbbell configuration. Operating at its 88 kHz mechanical resonance, the antenna emits electromagnetic radiation in the near field by applying an AC voltage to the piezoelectric material, generating an acoustic wave that propagates through the volume and induces oscillating magnetizations. The design uses a system of uncoupled models: an electrostatic finite element model to predict strain that feeds into a Landau–Lifshitz–Gilbert micromagnetic model to predict magnetic moment changes and, subsequently, a dipole model to forecast near-field radiation characteristics. Measurements were conducted on the antenna’s impedance, quality factor, mechanical resonance, transmitted magnetic signal strength, and radiation patterns, with variations in the bias magnetic field, frequency, and applied voltage. The results exhibit a strong correlation with model predictions, and the radiated signal strengths compare favorably with those of state-of-the-art pacemaker communication devices. Computational parametric studies using Galfenol and Terfenol-D suggest the potential for up to a three order of magnitude reduction in the antenna's volume, which is critical for implanted medical devices.

Interstitial-oxygen induced and magnetically driven HCP-to-FCC transformation in CoCrFeNiOx high-entropy alloy: a first-principles study

ABSTRACT We report the influences of oxygen interstitials and magnetisms on phase stability and structural transformation of CoCrFeNi high-entropy alloy (HEA) from first-principles calculations. It is found the formation of oxygen interstitials is energetically favourable to occur in face-centred cubic (FCC) CoCrFeNiOx HEA as compared with that in hexagonal close-packed (HCP) one, and at those octahedral sites neighbouring with more Cr or less Ni. Meanwhile, it is determined the HEA prefers FCC over HCP phases when the oxygen concentration exceeds 4.2 and 5.1 at.% with and without considering its magnetisms, respectively. The HCP-to-FCC structural transformation in CoCrFeNiOx HEA could be magnetically driven, accompanied by the significant changes in the atomic magnetic moments in the HEA, particularly with an oxygen interstitial concentration larger than 2.7 at.%. Furthermore, the HCP-to-FCC transformation under hydrostatic pressure in CoCrFeNi and CoCrFeNiOx HEAs is investigated from generalised stacking fault energies, and it is revealed that the synergy effects of oxygen interstitials and magnetisms could facilitate the transformation in CoCrFeNiOx HEA. The coupled interstitials-induced and magnetically driven structural transformation paves a new avenue for the application of HEAs.

Growth pattern and electronic and magnetic properties of Cr-doped silver clusters.

Growth pattern and electronic and magnetic properties of Agn Cr (n = 1-16) clusters have been investigated via density functional theory (DFT) combined with CALYPSO structure search method. The optimized geometry shows that the growth of the global minimum structures of Agn Cr clusters have obvious rule. when n > 12, silver atoms grow around an icosahedron which is almost unchanged in each structure. Analyses of electronic properties indicate that the doped Cr atom can only enhance the stability of larger silver clusters. Optical absorption and photoelectron spectra of Agn Cr isomers have been predicted and can be used for their structural identification. The icosahedral Ag12 Cr cluster with large energy level gap can be seen as a superatom. The adsorption capacity of Cr atom in Agn Cr cluster to CO is much higher than that of free Cr atom. The intensity of IR and Ramam spectra can be dramatically enhanced when CO is absorbed on Agn Cr cluster that Cr atom is encapsulated by Ag atoms. Moreover, the red shift of IR and Raman spectra of CO adsorbed on these clusters is also very small compared to free CO. Magnetism calculations show that the magnetic moment of Agn Cr clusters decreases linearly from n = 6 to 12 and increases linearly from n = 12 to 16. The total magnetic moment of Agn Cr cluster is mainly localized on the Cr atom. The change of magnetic moment of Cr atom is related to the charge transfer between Cr and Ag atoms.

Aging and passivation of magnetic properties in Co/Gd bilayers

Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging the gap between volatile information in the photonic domain and nonvolatile information in the magnetic domain, without the need for any intermediary electronic conversion. Specifically, these systems exhibit strong spin–orbit torque effects, fast domain wall motion, and single-pulse all-optical switching of the magnetization. An important open challenge to bring these materials to the brink of applications is to achieve long-term stability of their magnetic properties. In this work, we address the time-evolution of the magnetic moment and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with various capping layers. Over the course of three months, the net magnetic moment and compensation temperature change significantly, which we attribute to quenching of the Gd magnetization. We identify that intermixing of the capping layer and Gd is primarily responsible for this effect, which can be alleviated by choosing nitrides for capping as long as reduction of nitride to oxide is properly addressed. In short, this work provides an overview of the relevant aging effects that should be taken into account when designing synthetic ferrimagnets based on Co and Gd for spintronic applications.

Ce substituted NiCo[formula omitted] microspheres and nanoflakes: Comparison on magnetic features

NiCo2−xCexO4 (0.0 ≤ x ≤ 0.08) microspheres (MCs) and nanoflakes (NFs) were synthesized hydrothermally. X-ray diffraction analysis confirmed the cubic microstructure of all the samples. The microsphere and nanoflake morphologies of the samples have been confirmed by SEM, along with EDX, TEM and HR-TEM. The magnetic properties of the NiCo2−xCexO4 (0.0 ≤ x ≤ 0.08) MCs and NFs were investigated via magnetization measurements using a vibrating sample magnetometer. Room-temperature magnetization measurements revealed weak ferromagnetic behavior for both MC and NF samples. Low temperature magnetization measurements showed strong ferromagnetic behavior for the different samples. The magnetic parameters for both samples were found to differ substantially at a given temperature, which is accredited to the differences in the morphology of the MC-like and NF-like samples. Room-temperature magnetization measurements revealed antiferromagnetic or paramagnetic behavior for MC samples and weak ferromagnetic character for NF samples. It was also found that the magnetization magnitude decreased with increasing cerium dopant level, when compared to the host product. The changes in magnetization are linked to the effect of introduced cerium ions and their magnetic states, which could generate structural/morphological changes, changes in the size and surface lattice, redistribution of cations among the crystal lattices, changes in magnetic moment, etc.

A Structurally Flexible Halide Solid Electrolyte with High Ionic Conductivity and Air Processability

AbstractIn this work, a structurally revivable, chloride‐ion conducting solid electrolyte (SE), CsSn0.9In0.067Cl3, with a high ionic conductivity of 3.45 × 10−4 S cm−1 at 25 °C is investigated. The impedance spectroscopy, density functional theory, solid‐state 35Cl NMR, and electron paramagnetic resonance studies collectively reveal that the high Cl− ionic mobility originates in the flexibility of the structural building blocks, Sn/InCl6 octahedra. The vacancy‐dominated Cl− ion diffusion encompasses co‐ordinated Sn/In(Cl) site displacements that depend on the exact stoichiometry, and are accompanied by changes in the local magnetic moments. Owing to these promising properties, the suitability of the CsSn0.9In0.067Cl3, as an electrolyte is demonstrated by designing all‐solid‐state batteries, with different anodes and cathodes. The comparative investigation of interphases with Li, Li–In, Mg, and Ca anodes reveals different levels of reactivity and interphase formation. The CsSn0.9In0.067Cl3 demonstrates an excellent humidity tolerance (up to 50% relative humidity) in ambient air, maintaining high structural integrity without compromises in ionic conductivity, which stands in contrast to commercial halide‐based lithium conductors. The discovery of a halide perovskite conductor, with air processability and structure revival ability paves the way for the development of advanced air processable SEs, for next‐generation batteries.

Microstructure and Raman spectroscopy analysis of LiNiZn ferrite ceramics sintered by spark plasma method

In this study, the effects of different parameters (sintering temperature and holding time) of spark plasma sintering (SPS) on the microstructure and magnetic properties of LiNiZn ferrite ceramics were systematically investigated, and the samples sintered by SPS (SPS samples) were compared with the samples sintered by traditional sintering (TS samples). X-ray diffraction analysis confirmed that all the samples were ferrites with spinel structure. The microstructure of the samples was analyzed by scanning electron microscopy. Compared with TS samples, SPS samples exhibited finer and more uniform grains and denser structure. At the sintering temperature of 1000 °C, SPS samples exhibited higher saturation magnetization (Ms, 79.7 emu⋅g−1) and lower ferromagnetic resonance linewidth (ΔH, 231 Oe) than TS samples (Ms: 69.3 emu⋅g−1, ΔH: 298 Oe). The phase structure and cation distribution of the samples were analyzed by Raman spectroscopy. Herein, it was speculated that the instantaneous high temperature generated by spark plasma redistributed the cations in the crystal, and the distribution mode was stabilized by rapid cooling. Based on the change of magnetic moment of ferrite molecules caused by cation distribution, a method of judging the specific Ms by fitting analysis of Lorentz curve of A1g Raman signal of ferrite ceramics was proposed. The SPS samples showed more compact and uniform microstructure and more reasonable cation distribution than TS samples, and these characteristics were mainly responsible for the remarkable improvement of magnetic properties. The results indicate that SPS method can effectively alleviate the contradiction between Ms and ΔH, and provide an alternative pathway to prepare high-performance soft magnetic ferrite.

Robust Weak Antilocalization Effect Up to ∼120 K in the van der Waals Crystal Fe5-xGeTe2 with Near-Room-Temperature Ferromagnetism.

The van der Waals Fe5-xGeTe2 is a 3d ferromagnetic metal with a high Curie temperature of 275 K. We report herein the observation of an exceptional weak antilocalization (WAL) effect that can persist up to 120 K in an Fe5-xGeTe2 nanoflake, indicating the dual nature with both itinerant and localized magnetism of 3d electrons. The WAL behavior is characterized by the magnetoconductance peak around zero magnetic field and is supported by the calculated localized nondispersive flat band around the Fermi level. The peak to dip crossover starting around 60 K in magnetoconductance is visible, which could be ascribed to temperature-induced changes in Fe magnetic moments and the coupled electronic band structure as revealed by angle-resolved photoemission spectroscopy and first-principles calculations. Our findings would be instructive for understanding the magnetic exchanges in transition metal magnets as well as for the design of next-generation room-temperature spintronic devices.

Lattice constants and magnetism of L10-ordered FePt under high pressure

We studied the relationship between the lattice constant and magnetism of L10-ordered FePt under high pressure by means of first-principles calculations and synchrotron x-ray measurements. Based on our calculations, we found that the c/a ratio shows a local maximum at ∼20 GPa and that the Pt magnetic moment first remains almost unchanged and is sharply suppressed at ∼60 GPa. As for the c/a, we experimentally verified the local maximum at ∼20 GPa by powder x-ray diffraction. We also measured the x-ray magnetic circular dichroism at the Pt L edge up to ∼20 GPa. Any significant change in the Pt magnetic moment was not observed in agreement with the calculations. These results, thus, indicate that magnetic states, where the magnetization of Fe decreased and that of Pt did not change, can be created in L10-ordered FePt by lattice deformation under high pressure.

Initiating High-Voltage Multielectron Reactions in NASICON Cathodes for Aqueous Zinc/Sodium Batteries

Sodium superionic conductor (NASICON) is a class of compounds with robust polyanionic frameworks and high thermal stability, which are regarded as prospective cathodes candidates for secondary batteries. However, NASICON cathodes typically have low discharge plateaus and low practical capacities in aqueous electrolytes. Here, Na 3 V 1.75 Fe 0.25 (PO 4 ) 2 F 3 is investigated as a cathode material for the aqueous zinc/sodium batteries. While the addition of F helps with the improvement of NASICON structural stability, the low-cost Fe substitution has a positive impact on the capacity increment, reaction voltage increases, and cycling stability improvement. Because the Fe 3+ substitution could induce a change in the spin magnetic moments of the 3d orbitals of the VO 4 F 2 and FeO 4 F 2 octahedra, the 2-electron reaction of V is activated, which are V 4+ /V 3+ and V 5+ /V 4+ redox couples. As a result, the novel Na 3 V 1.75 Fe 0.25 (PO 4 ) 2 F 3 cathode delivers a high operating voltage of 1.7 V, a high energy density of 209 W·h·kg −1 and stable lifespan (83.5% capacity retention after 6,000 cycles at 1 A·g −1 ) in the aqueous zinc/sodium batteries. This research demonstrates the practicality of activating multielectron reactions to optimize the electrochemical properties of NASICON cathodes for aqueous secondary batteries.