Co For Mn Research Articles

The enhanced antiferromagnetic interaction and tunable temperature coefficient of resistivity in antiperovskite compounds Mn3−xCoxAgN (0≤x≤0.2)

The magnetic and electric evolution with the substitution level of Co element has been investigated in Mn[Formula: see text]CoxAgN. The DC magnetic measurements demonstrate that the substitution of Co for Mn enhances the antiferromagnetic (AFM) interaction and weakens the magnetic irreversibility at low temperature. The strengthening of AFM order in Mn[Formula: see text]CoxAgN has been attributed to the enhanced Co–Mn AFM interaction and weakened Co–N–Mn ferromagnetic (FM) interaction. Additionally, with the gradual increase of Co content, the temperature coefficient of resistivity (TCR) in paramagnetic (PM) region changes from a positive one to a negative one gradually. When [Formula: see text], the TCR value is only [Formula: see text]5[Formula: see text]ppm/K, which is one magnitude smaller than that of conventional standard resistor materials. The negative TCR phenomenon is closely associated with the Co-element-induced lattice disorder, which leads to the weak localization of conduction electrons.

Enhancing ferromagnetism in the Sm(Co, Mn)5 system: Impact on phase stability and magnetic properties

This work demonstrates that Mn can be a beneficial candidate for the performance modulation of permanent magnetic materials like SmCo5. Theoretical calculations reveal that Mn has a large positive contribution to the magnetization of SmCo5, owing to the strong ferromagnetic exchange interaction between Co and Mn atoms. It is also found that Mn could occupy both 2c and 3g crystal sites because of the minute energy difference between them can be easily overcome through thermal activation. Co atoms on the same site with Mn have increasing moments with the increase in Mn content, whereas those on the other sites show decreasing moments. The experimental substitution of Mn for Co leads to a contraction of the SmCo5 single-phase region, and thus pure Sm(Co, Mn)5 phase could only be obtained through subtle control of Sm content by annealing. Significantly, a 13 % increase in saturation magnetization of SmCo5 is achieved through Mn substitution. A slight rise in the Curie temperature is also obtained, suggesting that Mn substitution does not strongly disturb the exchange interactions of the Co sublattice. This study offers a new option for enhancing magnetic properties in Sm–Co systems and introduces novel physical phenomena resulting from Mn substitution, which is worthy of further investigation.

Microstructure and hydrogen storage properties of Zr-based AB2-type high entropy alloys

This paper reports the partial substitution of Fe, Co, Ni and Cu for Mn in ZrMnCr0.5V0.5 alloy with to obtain ZrMn0.5Cr0.5V0.5M0.5 (M = Fe, Co, Ni and Cu) high entropy alloys. These alloy samples are prepared by electric arc melting, and their microstructure and hydrogen storage properties are investigated in detail. Structural analysis results show that all the five alloys consist of C14 type Laves primary phase. Metastable cubic phase can be observed in ZrMnCr0.5V0.5 and Fe-, Co-substituted alloys, while ZrNi, Cu10Zr7 and Cu8Zr3 phase secondary phases exist in Ni- and Cu-substituted alloys. Hydrogen storage testing results show that elemental substitution greatly improves the first activation properties of the alloys. All the five alloys can absorb hydrogen rapidly when the temperature is above 100 ℃, while the elemental substitution significantly affects the hydrogen absorption kinetics at lower temperatures. Compared with other four alloys, the Cu-substituted alloy has two kinds of Cu-Zr secondary phases, resulting in a poorer hydrogen absorption performance. Pressure-composition isotherms (P-C-T) test results show that all these alloys have very sloping absorption and desorption plateaus. It is also found that all the alloy particles are pulverized spontaneously during hydrogenation cycles, but there is no clearly change on phase structure of C14 type Laves phase, and the alloys show a good cycling performance. Among these alloys, the ZrMn0.5Cr0.5V0.5Fe0.5 alloy has the best hydrogen storage performance because it has the shortest activation time and fast hydrogen absorption at room temperatures.

The promotional role of Mn in CO2 hydrogenation over Rh-based catalysts from a surface organometallic chemistry approach.

Rh-based catalysts modified by transition metals have been intensively studied for CO2 hydrogenation due to their high activity. However, understanding the role of promoters at the molecular level remains challenging due to the ill-defined structure of heterogeneous catalysts. Here, we constructed well-defined RhMn@SiO2 and Rh@SiO2 model catalysts via surface organometallic chemistry combined with thermolytic molecular precursor (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO2 hydrogenation. We show that the addition of Mn shifts the products from almost pure CH4 to a mixture of methane and oxygenates (CO, CH3OH, and CH3CH2OH) upon going from Rh@SiO2 to RhMn@SiO2. In situ X-ray absorption spectroscopy (XAS) confirms that the MnII is atomically dispersed in the vicinity of metallic Rh nanoparticles and enables to induce the oxidation of Rh to form the Mn-O-Rh interface under reaction conditions. The formed interface is proposed to be key to maintaining Rh+ sites, which is related to suppressing the methanation reaction and stabilizing the formate species as evidenced by in situ DRIFTS to promote the formation of CO and alcohols.

The Ni Catalyst Supported on the FSP-made Transition Metal (Co, Mn, Cu or Zn) Doped La2O3 Material for the Dry Reforming of Methane

The transition metal (Co, Mn, Cu or Zn) doped La2O3 material was prepared by flame spray pyrolysis (FSP) technique. The 2 wt.% Ni catalyst supported on this material was characterized by XRD, N2 physisorption, TPR, H2 chemisorption and TGA, and evaluated by the dry reforming of methane (DRM). The perovskite structure was certainly formed when either Co or Mn was introduced. The Cu can generate the La2CuO4 spinel phase while the Zn showed a mixed phase of La2O3, ZnO and La(OH)3. The Ni/Co-La2O3 catalyst was more active for the DRM because of high amount of active dual sites of Ni and Co metals dispersed on the catalyst surface. The formation of La2O2CO3 during the reaction can inhibit the coke formation. The cooperation of La2O2CO3 and MnO phases in the Ni/Mn-La2O3 catalyst was promotional effect to decrease carbon deposits on the catalyst surface. The partial substitution of Co for Mn with a small content of Mn can enhance the catalytic activity and the product yield. The Ni/Mn0.05Co0.95-La2O3 catalyst showed the highest CH4 conversion, H2 yield and H2/CO ratio. The Mn inserted into the perovskite structure of LaCoO3 was an important player to change oxygen mobility within the crystal lattice to maintain a high performance of the catalyst. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Investigation on the thermal expansion behavior of FeCoNi and Fe30Co30Ni30Cr10-xMnx high entropy alloys

This work investigates the thermal expansion behaviors of Fe30Co30Ni30Cr10-xMnx High Entropy Alloys (HEAs) (x = 0, 5, 10 at.%) from 400 K to 1200 K. Interestingly, comparing to that of Cantor alloy, a decrease in Cr and Mn in Co–Cr–Fe–Mn–Ni system could significantly decrease the thermal expansion coefficient by 45.2%; Fe30Co30Ni30Cr10-xMnx (x = 0, 5, 10) also showed an abrupt change in thermal expansion behaviors similar to that of Invar alloys. Experimental and theoretical analysis suggest the abrupt change in thermal expansion behaviors of these HEAs were associated with the transition from ferromagnetism to paramagnetism, and these alloys appear to exhibit the dimensional stability of the Invar effect. Since the Invar effect is related to ferromagnetic properties, the amount of Cr and Mn in the HEAs would influence the suppression of thermal expansion due to different intensity of anti-ferromagnetic coupling effect. This research contributes to the understanding of the thermal expansion behaviors of Co–Cr–Fe–Mn–Ni high entropy alloys and the effects of antiferromagnetic elements.

Synthesis, Battery Characteristics and Crystal and Electronic Structure of Cathode Material Spinel Mg(Co, Ni, Mn)2O4 for Mg Secondary Battery

1.IntroductionA magnesium secondary battery based on two-electron reaction is expected as a next-generation battery. In this study, we focused on Mg(Co, Ni, Mn)2O4 with spinel structure as a new cathode material. In the previous work, since the energy density of the Li-ion battery tended to increase when Ni is substituted for Co in their cathode materials, a Ni-substituted MgCo2O4 system[1] have been studied. It was also clarified that the discharge capacity increased by substituting Mn for Co in the MgCo2O4[2]. In this study, we adopted the advantages of Ni and Mn to the synthesized new cathode materials of MgCo2-x-yNixMnyO4 in which Co were replaced with Ni and Mn. The purpose of this study was to clarify the relationship between the battery characteristics and the crystal structure by using the three-electrode cell and quantum beam, synchrotron X-ray and neutron, analyses.2.ExperimentalAll samples were synthesized via the reverse coprecipitation method. The respective metal nitrates were mixed in the secondary distilled water at a predetermined ratio, further made up to 200 mL and added dropwise to the aqueous sodium carbonate solution at 70 °C, the precipitate was dried, pulverized with a ball mill and then calcined at 400°C. The phase of the obtained sample was identified by powder X-ray diffraction and ICP-AES. The charge/discharge test for synthesized sample was performed using a three-electrode cell (Toyo System Co., Ltd.) (90 ℃, Reference electrode: Ag, Negative electrode: AZ31, Electrolyte: 0.3 M [Mg(G4)][TFSA]2/P13TFSA, Separator: glass fiber filter). Crystal structures of the samples were analyzed by Rietveld method for the results obtained by synchrotron X-ray diffraction (BL19B2, SPring-8) and neutron diffraction (BL20, J-PARC). Furthermore, the valence of the transition metals was estimated by XAFS measurement (BL14B2, SPring-8).3.Results and DiscussionSamples were synthesized in the different metal ratios of Co, Ni and Mn in MgCo2-x-yNixMnyO4. As a result of powder X-ray diffraction, all samples were assigned to a spinel structure (Fd-3m). From the ICP-AES, although most of the composition ratios could be controlled, the Mg content in all samples tended to be slightly less than 1.0. The lattice constant increased with increasing Ni and Mn substitution. This is considered to be due to the divalent Ni on the hexacoordinated site with a larger ionic radius than those of Co and Mn. From the results of charge and discharge tests, it was found that the Ni-substituted sample showed an increase in the discharge potential, and the both Ni- and Mn-substituted samples showed an increase in the discharge capacity. As a result of Rietveld analysis of the Ni substituted samples, it was found that the substituted Ni also occupied at 8a site of the Mg diffusion path. Therefore, it is considered that the Mg diffusion was hindered and the discharge capacity became lower than that of the non-substituted product [1]. The electrochemical tests using [Mg(G4)][TFSA]2/P13TFSA electrolyte with high oxidation resistance showed the increased charge / discharge capacities after 2 cycles. The Rietveld analysis using neutron diffraction patterns clarified cation mixing and refined the mixing amounts of Co, Ni, and Mn into the 8a site. The overall tendency was that Ni was substituted at the 16d site and Mn was substituted at the 8a site. The valences of each transition metals estimated in the charge / discharge process by XAFS, all the transition metals contributed to the charge / discharge because the valences of Co, Ni, and Mn increased or decreased after the initial charge or discharge. The main feature was that Mn showed a large valence change from 4+ to 3+ [2], and it was found that it greatly contributes to the Mg insertion. Although viewpoint of deterioration needed further improvement, it was concluded that MgCo0.8Ni0.4Mn0.8O4 and MgCo0.5Ni0.5MnO4 showed the discharge capacity of 200mAh/g and were the optimum composition in this samples.This work was supported by JST ALCA-SPRING Grant Number JPMJAC1301, Japan.References1) Okamoto, T. Ichitsubo, T. Kawaguchi, Y. Kumagai, F. Oba and S. Yagi, K. Shimokawa, N. Goto, T. Doi, E. Matsubara, Adv. Sci., 2, 1500072 (2015).2) Y. Idemoto, Y. Mizutani, C. Ishibashi, N. Ishida, N. Kitamura, Electrochemistry, 87, 220 (2019). Figure 1

Effect of cobalt on the microstructure and hydrogen sorption performances of TiFe0.8Mn0.2 alloy

The microstructures and the hydrogen sorption performances of TiFe0.8Mn0.2Cox (x = 0, 0.05, 0.10, 0.15) and TiFe0.8Mn0.2-yCoy (y = 0.05, 0.10) alloys have been investigated. For TiFe0.8Mn0.2Cox alloys, the lattice parameters of TiFe phase decreased and the Laves phase contents increased with the addition of Co. With the increase of Co content in TiFe0.8Mn0.2Cox alloys, the maximum hydrogen storage capacities of TiFe0.8Mn0.2Co0.05 and TiFe0.8Mn0.2Co0.10 alloys decreased, but the effective hydrogen capacities increased, which is ascribed to the improved flatness of the α-β desorption plateau. Substitution of Co for Mn in TiFe0.8Mn0.2-yCoy alloys can effectively lead to single phase of TiFe alloys. Therefore, TiFe0.8Mn0.2-yCoy alloy showed a deteriorated activation property, but its effective hydrogen capacity increased remarkably due to the obviously improved flatness of the α-β desorption plateau. The addition of Co might adjust the change of the octahedral intersitial environment caused by Mn doping in TiFe phase, which contributes to the improved flatness of the α-β desorption plateau and hence the increased effective hydrogen capacity.

La–Sr–Mn–Co–O Films for High Pulsed Magnetic Field Measurements at Cryogenic Temperatures

The magnetoresistance of nanostructured La1– x Sr x (Mn1– y Co y ) z $O_{3 \pm \delta}$ (La–Sr–Mn–Co–O) films with substitution of Co for Mn with amount of Co/(La + Sr) = 0.12 and different Mn excess Mn/(La + Sr) = 1.05, 1.07, and 1.11 was investigated at temperatures of 4–230 K in pulsed magnetic fields up to 60 T. It was found that the manganite–cobaltite films exhibit larger magnetoresistance in comparison with manganite films without Co doping. The largest magnetoresistance values and sensitivity to the magnetic field are obtained for La–Sr–Mn–Co–O films having Mn content close to the stoichiometric ratio for manganites: Mn/(La + Sr) = 1.05. It was found that magnetoresistance at high fields (20–60 T) has a minimum at (50–80 K) and increases with the decrease of temperature. The possibility to use these films for magnetic field measurements at cryogenic temperatures is demonstrated.

Microstructure, magnetic and magnetocaloric properties in magnetic-field-induced martensitic transformation Mn1−xCo1+xGe alloy ribbons

The sharp field-induced metamagnetic martensitic transformation plays a pivotal role in magnetoresponsive effects for ferromagnetic shape memory alloys (FSMAs). Homogeneous polycrystalline Mn1−xCo1+xGe alloy ribbons are prepared by using the arc-melting and melt-spinning technique in this work. The microstructure, crystalline structure, magnetic and magnetocaloric properties are studied in these ribbons. The substitution of Co for Mn reduces the structural transformation temperature, which leads to the realization of coincident magnetostructural transformation. Strikingly, a magnetic-field-induced martensitic transformation is observed in these ribbons. Therefore, large magnetocaloric effect, including maximum magnetic entropy change (ΔSM,max) value of ∼28 and 25 Jkg−1K−1 and effective refrigeration capacity (RCeff) of ∼168 and 181 Jkg−1 are obtained for x=0.035 and 0.09 samples, respectively, under the field change of μ0ΔH=0–7T. Furthermore, the average hysteresis loss of these ribbons reduces largely in contrast to their bulk counterparts. These results would provide the new experimental data for magnetic refrigerant materials and pave the way for the further investigation of other FSMAs.

Martensitic transformation and magnetoresistance in Ni40Mn44-xCoxAl16 Heusler alloys

In this work, we focus on the magnetic properties and martensitic transformation behaviors in Ni40Mn44-xCoxAl16 (x = 5, 7) Heusler alloys. By partial substitution of Co for Mn, the martensitic transformation (MT) can be tuned in a wide temperature range and shifts to the higher temperature with the increase of Co content in these alloys. The reverse MT can be induced by magnetic field from the weak-magnetic martensite phase to ferromagnetic austenite phase in these alloys. The large magnetoresistance effect of −46.39% and −41.75% are acquired under 90 kOe for Ni40Co5Mn39Al16 and Ni40Co7Mn39Al16 alloys, respectively. The Ni40Mn44-xCoxAl16 alloys with large low-field magnetoresistance and low cost show promising applications for multifunctional materials.

Three to tango requires a site-specific substitution: heterotrimetallic molecular precursors for high-voltage rechargeable batteries.

Design of heterotrimetallic molecules, especially those containing at least two different metals with close atomic numbers, radii, and the same coordination number/environment is a challenging task. This quest is greatly facilitated by having a heterobimetallic parent molecule that features multiple metal sites with only some of those displaying substitutional flexibility. Recently, a unique heterobimetallic complex LiMn2(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) has been introduced as a single-source precursor for the preparation of a popular spinel cathode material, LiMn2O4. Theoretical calculations convincingly predict that in the above trinuclear molecule only one of the Mn sites is sufficiently flexible to be substituted with another 3d transition metal. Following those predictions, two heterotrimetallic complexes, LiMn2-x Co x (thd)5 (x = 1 (1a) and 0.5 (1b)), that represent full and partial substitution, respectively, of Co for Mn in the parent molecule, have been synthesized. X-ray structural elucidation clearly showed that only one transition metal position in the trinuclear molecule contains Co, while the other site remains fully occupied by Mn. A number of techniques have been employed for deciphering the structure and composition of heterotrimetallic compounds. Synchrotron resonant diffraction experiments unambiguously assigned 3d transition metal positions as well as provided a precise "site-specific Mn/Co elemental analysis" in a single crystal, even in an extremely difficult case of severely disordered structure formed by the superposition of two enantiomers. DART mass spectrometry and magnetic measurements clearly confirmed the presence of heterotrimetallic species LiMnCo(thd)5 rather than a statistical mixture of two heterobimetallic LiMn2(thd)5 and LiCo2(thd)5 molecules. Heterometallic precursors 1a and 1b were found to exhibit a clean decomposition yielding phase-pure LiMnCoO4 and LiMn1.5Co0.5O4 spinels, respectively, at the relatively low temperature of 400 °C. The latter oxide represents an important "5V spinel" cathode material for the lithium ion batteries. Transmission electron microscopy confirmed a homogeneous distribution of transition metals in quaternary oxides obtained by pyrolysis of single-source precursors.

Effect of Cobalt on the Crystal Structure and Magnetism of Electron-Doped Sr0.8Ce0.2MnO3 Oxide

The Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.2, 0.3, 0.4, 0.5, and 0.6) complex oxides with the perovskite structure obtained from simple oxides by the solid–state reaction technique have been examined using structural analysis and magnetic measurements. Substitution of Co for Mn in the dilute Sr0.8Ce0.2MnO3 antiferromagnet with a Neel temperature of TN = 210 K leads to a decrease in the degree of tetragonal distortion of the crystal structure and transition to the cubic cell. The degeneracy of the antiferromagnetic interaction (TN = 138 K at y = 0.2) observed at the first stage of the substitution of Co for Mn changes for its gradual enhancement with an increase in the magnetic transformation temperature up to 239 K at y = 0.6. An increase in the Co content weakens the competition between the ferromagnetic and antiferromagnetic couplings and reduces the temperature of the transition to the spin-glass-like state. The magnetic inhomogeneity and formation of Co2+–Mn4+ ferromagnetic clusters in Sr0.8Ce0.2Mn0.4Co0.6O2.69 at 140 K have been observed.

Влияние кобальта на кристаллическую структуру и магнетизм электрон-допированного оксида Sr-=SUB=-0.8-=/SUB=-Ce-=SUB=-0.2-=/SUB=-MnO-=SUB=-3-=/SUB=-

AbstractThe Sr_0.8Ce_0.2Mn_1 – _ y Co_ y O_3 – δ ( y = 0.2, 0.3, 0.4, 0.5, and 0.6) complex oxides with the perovskite structure obtained from simple oxides by the solid–state reaction technique have been examined using structural analysis and magnetic measurements. Substitution of Co for Mn in the dilute Sr_0.8Ce_0.2MnO_3 antiferromagnet with a Néel temperature of T _N = 210 K leads to a decrease in the degree of tetragonal distortion of the crystal structure and transition to the cubic cell. The degeneracy of the antiferromagnetic interaction ( T _N = 138 K at y = 0.2) observed at the first stage of the substitution of Co for Mn changes for its gradual enhancement with an increase in the magnetic transformation temperature up to 239 K at y = 0.6. An increase in the Co content weakens the competition between the ferromagnetic and antiferromagnetic couplings and reduces the temperature of the transition to the spin-glass-like state. The magnetic inhomogeneity and formation of Co^2+–Mn^4+ ferromagnetic clusters in Sr_0.8Ce_0.2Mn_0.4Co_0.6O_2.69 at 140 K have been observed.

Effects of Mn substitution on crystal structure, magnetic and magnetocaloric properties of (Co1−xMnx)2B compounds

The crystal structure, magnetic and magnetocaloric properties of (Co1−xMnx)2B (x = 0.0, 0.1, 0.2, 0.25, 0.3, 0.35, 0.40) compounds were investigated by x-ray powder diffraction (XRD) and magnetization measurements. All compounds crystallize in a body-centred tetragonal single phase with the CuAl2-type structure. Due to the changes of lattice stability and ferromagnetic–antiferromagnetic exchange coupling interaction, the room temperature magnetization and the Curie temperature (TC) of (Co1−xMnx)2B compounds first increases at about x = 0.1 and then decreases with the substitution of Mn for Co. Second-order phase transition was confirmed based on the isothermal magnetization curves and the isothermal entropy change (ΔSM) was determined for the samples with x = 0.3, 0.35 and 0.4. The maximal magnetic entropy (|ΔSM|) under a field change from 0 to 5 T are found to be 2.49, 2.37 and 2.02 J kg−1·K for x = 0.3, 0.35 and 0.4, respectively. The corresponding relative cooling powers (RCP) are evaluated to be 59.74, 78.94 and 113.50 J kg−1. Given the low cost of Co, Mn and B, (Co1−xMnx)2B compounds may come as an base compound for a more efficient magnetocaloric material.

Enhanced battery performance in manganese hexacyanoferrate by partial substitution

Manganese hexacyanoferrate (Mn-HCF) is a promising cathode material for sodium-ion secondary batteries (SIBs). We investigated the effects of partial substitution of Fe, Co, and Ni for Mn in Mn-HCF on the SIB rate and cycle performance. The discharge capacity is discernible at 50C in the substituted samples, while it disappears at 20C in the non-substituted samples. We ascribed the enhancement of the rate performance to the suppression of the Jahn-Teller distortion of Mn3+ and the resultant activation of the Mn reduction reaction.

Magnetic, magnetocaloric and structural properties of manganese based monoborides doped with iron and cobalt – A candidate for thermomagnetic generators

Refractory borides are good candidates for thermomagnetic energy conversion due to their high chemical stability and good magnetic properties. A systematic study focusing on the crystallographic, magnetic and magnetocaloric properties of cobalt and iron substituted MnB was conducted. The pure MnB sample shows an orthorhombic structure with a spontaneous magnetization (Ms) of 156 Am2kg−1 and a sharp magnetic transition (TC) of 567 K that yields a large magnetic entropy change of 7.5 J kg−1K−1 in a 2T field change at this temperature. Both the substitution of Co and Fe elements for Mn were found to effectively modify TC accompanied by a lower magnetization (Ms) leading to a reduced magnetocaloric effect (MCE). The differences in magnetic properties with substitution are described by calculation of the density of states and interatomic distances. It is found that in contrast to the Co atoms the Fe atoms develop large and stable moments similar to the Mn atoms which is consistent with the experimental findings. The sharp and significant change of Ms(T), the very stable nature of these refractory borides and abundant availability makes some of these compositions suitable for thermomagnetic power generation applications.

Power factor enhancement induced by Bi and Mn co-substitution in NaxCoO2 thermoelectric materials

Polycrystalline Na0.81−xBixCo1−yMnyO2 (0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.05) thermoelectric materials have been synthesized by solid-state reaction method using Bi and Mn as the property-manipulated substitution elements. The thermoelectric ceramics were characterized by X-ray diffraction (XRD) for phase composition, field emission scanning electron microscopy (FE-SEM) for grain morphology and cross section of sintered samples, and inductive-coupled plasma atomic emission spectroscopy (ICP-AES) for stoichiometric composition. The bulk density and average grain size of the system were also examined. The effects of partial substitution of Bi for Na and/or Mn for Co on the thermoelectric properties of Na0.81CoO2 have been systematically investigated. The power factor was substantially improved by Bi and Mn co-substitution in Na0.81CoO2, and the power factor value of Na0.66Bi0.15Co0.99Mn0.01O2 was about three times as large as that of Na0.81CoO2 in the whole measured temperature range. It is evident that the Bi and Mn co-substitution is effective for optimizing thermoelectric properties of Na0.81CoO2.

A scheme for spin-selective electron localization in Mn3Ga Heusler material

We demonstrate that tetragonal Mn3Ga Heusler material allows for a new possibility of adjusting an electric current by means of the so-called spin-selective localization of conduction electrons. On the basis of a first-principles analysis, we propose possible chemical substitutes for Mn, which, when used in small quantities, can lead to a disorder-induced localization of the conduction electrons in a single spin channel. Replacement of the Mn in Mn3 − xYxGa with other 3d transition metals Y is known not to change the tetragonal structure for a certain range of x. For Y = Co the range is x ⩽ 0.5. Therefore, substitution of Co for Mn is used in the present work as a prototype procedure for a detailed demonstration of the underlying physical mechanisms.

Structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds

The structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction, differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations. Results indicate that all samples undergo a martensitic transition from the Ni2In-type parent phase to TiNiSi-type orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in Mn1−xCoxNiSi (x = 0.2, 0.3, and 0.4) and Mn1−yFeyNiSi (y = 0.26, 0.30, 0.36, 0.46, and 0.55) samples decreases the structural transition temperature and Curie temperature of martensite. The martensite phases show a typical ferromagnetic behavior with saturation field being basically unchanged with increasing Co(Fe) content, while the saturation magnetization shows a decreasing tendency. The theoretically calculated moments are in good agreement with the experimentally measured results. The orbital hybridizations between different 3d elements are analyzed from the distribution of density of states.