Photo-induced effect in the layered perovskite manganite La_(1.2)Sr_(1.8)Mn_(1.8)Co_(0.2)O_7

1IntroductionWiththediscoveryofcolossalmagnetoresistance(CMR)effectinmanganites,hole-dopingperovskitemanganiteswithunusualelectronictransportandmagneticpropertieshaveattractedconsiderableattention.Thesepropertiesresultfromanintrinsicinteractionbetweencharge,spin,orbitalandlatticedegreesoffreedomthatarestronglycoupledtoeachother[1—6].DoubleexchangemodelcombinedwithJohn-Tellereffectwasusedtoexplainthesepropertiespartly[7—9].InordertogetbetterunderstandingofthemechanismofCMReffect,externals…     

Domain structure and charge ordering in a layered manganite La<Subscript>1.2</Subscript>Sr<Subscript>1.8</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript> crystal

Charge ordering in a layered manganite La_1.2Sr_1.8Mn₂O₇ crystal with structural domains was studied using neutron diffraction in the temperature range 175–700 K. The wave vector of the charge ordering in the crystal is found to be q ? {0.2, 0, 0}2π/a. It is argued that the actual domain structure of the anisotropic crystal affects its charge-ordering state.     

Nanosecond photoelectric effects in all-oxide p-n junction of La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript>/SrNb<Subscript>0.01</Subscript>Ti<Subscript>0.99</Subscript>O<Subscript>3</Subscript>

Nanosecond (ns) photoelectric effects have been observed in all-oxide p-n junctions of La_0.9Sr_0.1MnO₃/SrNb_0.01Ti_0.99O₃ for the first time. The rise time was about 23 ns and the full width at half maximum was about 125 ns for the open-circuit photovoltaic pulse when the La_0.9Sr_0.1MnO₃ thin film in the p-n junction was irradiated by a laser of ≈20 ns pulse duration and 308 nm wavelength. The photovoltaic sensitivity was 80 mV/MJ for a 308 nm laser pulse.     

Two-dimensional antiferromagnetic correlations in an La<Subscript>1.4</Subscript>Sr<Subscript>1.6</Subscript>(Mn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>O<Subscript>7</Subscript> single crystal

The temperature and field dependences of the magnetization, the electrical resistivity, and the magnetostriction of bilayer lanthanum manganite La_1.4Sr_1.6Mn₂O₇ single crystals and cobalt-doped La_1.4Sr_1.6(Mn_0.9Cu_0.1)₂O₇ are measured. The magnetostriction of the cobalt-doped compound increases as compared to the initial La_1.4Sr_1.6Mn₂O₇ compound, and the magnetization and the magnetoresistance of the former compound change substantially. Powder and single-crystal neutron diffraction patterns are used to detect ferromagnetic ordering in La_1.4Sr_1.6(Mn_0.9Co_0.1)₂O₇ at a temperature below T _C ~ 45(2) K, and this ordering coexists with antiferromagnetic correlations, which develop at temperatures below T _C ~ 80(5) K.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

Study on the spin-states of cobalt-based double-layer perovskite Sr<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Co<Subscript>2</Subscript>O<Subscript>7</Subscript>

The spin-states of cobalt based perovskite compounds depend sensitively on the valence state and local crystal environment of Co ions and the rich physical properties arise from strong coupling among charge, spin, and orbital degrees of freedom. While extensive studies have been carried out in the past, most of them concentrated on the isotropic compound LaCoO₃. In this paper, using the unrestricted Hartree-Fock approximation and the real-space recursion method, we have investigated the competition of various magnetically ordered spin-states of anisotropic double-layered perovskite Sr₂Y_0.5Ca_0.5Co₂O₇. The energy comparison among these states shows that the nearest-neighbor high-spin-intermediate-spin ferromagnetically ordered state is the relevant magnetic ground state of the compound. The magnetic structure and sizes of magnetic moments are consistent with the recent experimental observation.     

Specific features of magnetic states of impurity iron ions in the perovskite La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript> <Superscript>57</Superscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript>

Single-phase polycrystalline La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ samples have been prepared by solidstate ceramic technology. The samples have the rhombohedral structure (space group \(R\bar 3c\)). The studies of perovskite La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ by Mössbauer spectroscopy on impurity ⁵⁷Fe nuclei in the temperature range of 5–293 K have revealed the existence of a superparamagnetic relaxation in the temperature range of 100–210 K. The parameters of hyperfine interactions (hyperfine magnetic fields, line shifts, and quadrupole shifts) and the anisotropy energy have been measured, and the frequencies of magnetic moment relaxation of iron ions have been estimated.     

Metal-insulator transition in a radiation-disordered La<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> manganite

The temperature dependences of the electrical resistivity ρ(T) and the ac magnetic susceptibility χ(T, H = 0) are thoroughly investigated for a perovskite-like lanthanum manganite, namely, La_0.85Sr_0.15MnO₃, which is preliminarily exposed to neutron irradiation with a fluence _F = 2 × 10¹⁹ cm^?2 and then annealed at different temperatures ranging from 200 to 1000°C. The results of the electrical resistance measurements demonstrate that neutron irradiation of the samples leads to the disappearance of the low-temperature insulating phase. As the annealing temperature increases, the insulating phase is not restored and the manganite undergoes a transformation into a metallic phase. Analysis of the magnetic properties shows that, under irradiation, the ferromagnet-paramagnet phase transition temperature T_C decreases and the magnetic susceptibility is reduced significantly. With an increase in the annealing temperature, the phase transition temperature T_C and magnetic susceptibility χ(_{T, H} = 0) increase and gradually approach values close to those for an unirradiated sample. This striking difference in the behavior of the electrical and magnetic properties of the radiation-disordered La_0.85Sr_0.15MnO₃ manganite is explained qualitatively.     

Facile synthesis of hierarchical porous Ni-rich LiNi<Subscript>0.6</Subscript>Co<Subscript>0.2</Subscript>Mn<Subscript>0.2</Subscript>O<Subscript>2</Subscript> cathode material with superior high-rate capability

A hierarchical porous nickel-rich LiNi_0.6Co_0.2Mn_0.2O₂ cathode is successfully prepared for the first time using a facile ammonia-induced method, wherein ammonia molecules play a key role in fabricating the complex architecture, and neither templates nor precipitants are employed. Hierarchical flower-like precursor with ultra-thin nanosheets is formed during the ammonia-induced reaction, and then, the porous product is obtained during the sintering process. The X-ray diffraction pattern demonstrates that the sample has a well-defined α-NaFeO₂ structure with very low-cation disorder. The peculiar hierarchical porous morphology and ideal structure endow this material-enhanced electrochemical performance. It delivers discharge capacities of 173, 138, 111, 97, and 82 mAh g^?1 at 0.1 C, 1 C, 5 C, 10 C, and 20 C, respectively, and maintains 91 % of its initial discharge capacity after 100 cycles at 1 C. The results reveal that this method is facile and feasible to synthesize high-rate Nickel-rich material.     

Particle size effect on magnetotransport properties of nanocrystalline Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

Nanocrystalline samples with an average particle size of 40 and 52 nm have been synthesized by citrate-complex auto-ignition method. Magnetic properties of the samples show para- to ferromagnetic transition at around 135 K. The electron magnetic resonance (EMR) study on these samples indicates the presence of coexistence of two magnetic phases below 290 K. Electrical resistivity follows variable range hopping (VRH) mechanism in the paramagnetic regime. The magnetoresistance (MR) data has been analysed by spin dependent hopping between the localized spin clusters together with the phase-separation phenomenon. These clusters are assumed to be formed by distribution of canted spins and defects all over the nanoparticle. In addition, the hopping barrier depends on the magnetic moment orientation of the clusters. The magnetic moments of the clusters are narrowly oriented in ferro- and are randomly oriented in paramagnetic phase. The ferromagnetic phase contributes to the total MR at low applied magnetic fields whereas the paramagnetic phase contributes at relatively high fields in both the samples. The average cluster size in ferromagnetic phase is bigger than that in paramagnetic phase. It is also observed that the cluster size, in ferromagnetic phase, in 52 nm sample is bigger than that in the 40 nm sample. However, the average cluster size in paramagnetic phase is almost same in both the samples.     

Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.     

Peculiarities of magnetic phase separation in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The temperature and field dependences of the specific magnetic moment of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite have been measured. It is established, that the magnetic state of the sample studied is a cluster spin glass and it is the result of frustration of exchange Mn³⁺-O-Mn³⁺ interactions due to the redistribution of oxygen vacancies. The increase of the magnetic field leads to an increase in the degree of polarization of local spins of manganese. It is established using the magnetic criterion that a phase transition into the paramagnetic state for the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite is a thermodynamic second order phase transition. The causes and mechanism of the magnetic phase separation are discussed.     

Magnetocaloric effect in (La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>)<Subscript>0.9</Subscript>Mn<Subscript>1.1</Subscript>O<Subscript>3</Subscript>

The results of investigations of the magnetization, susceptibility, and magnetic-field-induced changes in the entropy of polycrystalline manganite (La_0.6Ca_0.4)_0.9Mn_1.1O₃ near the magnetic phase transition have been presented. Magnetic measurements have been carried out at temperatures in the range from 210 to 310 K in magnetic fields of up to 9 T. The magnetocaloric effect has been revealed by measuring the magnetic-field dependences of magnetization. The magnitude of the magnetocaloric effect is compared with similar results obtained for other manganites.     

Investigation of the structural and electrochemical performance of Li<Subscript>1.2</Subscript>Ni<Subscript>0.2</Subscript>Mn<Subscript>0.6</Subscript>O<Subscript>2</Subscript> with Cr doping

Cr-doped layered oxides Li[Li_0.2Ni_0.2???x Mn_0.6???x Cr_2x ]O₂ (x?=?0, 0.02, 0.04, 0.06) were synthesized by co-precipitation and high-temperature solid-state reaction. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TRTEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). XRD patterns and HRTEM results indicate that the pristine and Cr-doped Li_1.2Ni_0.2Mn_0.6O₂ show the layered phase. The Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ shows the best electrochemical properties. The first discharge specific capacity of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 249.6 mA h g^?1 at 0.1 C, while that of Li_1.2Ni_0.2Mn_0.6O₂ is 230.4 mA h g^?1. The capacity retaining ratio of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 97.9% compared with 93.9% for Li_1.2Ni_0.2Mn_0.6O₂ after 80 cycles at 0.2 C. The discharge capacity of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂ is 126.2 mA h g^?1 at 5.0 C, while that of the pristine Li_1.2Ni_0.2Mn_0.6O₂ is about 94.5 mA h g^?1. XPS results show that the content of Mn³⁺ in the Li_1.2Ni_0.2Mn_0.6O₂ can be restrained after Cr doping during the cycling, which results in restraining formation of spinel-like structure and better midpoint voltages. The lithium-ion diffusion coefficient and electronic conductivity of Li_1.2Ni_0.2Mn_0.6O₂ are enhanced after Cr doping, which is responsible for the improved rate performance of Li_1.2Ni_0.16Mn_0.56Cr_0.08O₂.     

Acacia gum-assisted co-precipitating synthesis of LiNi<Subscript>0.5</Subscript>Co<Subscript>0.2</Subscript>Mn<Subscript>0.3</Subscript>O<Subscript>2</Subscript> cathode material for lithium ion batteries

LiNi_0.5Co_0.2Mn_0.3O₂ particles of uniform size were prepared through carbonate co-precipitation method with acacia gum. The precursor of carbonate mixture was calcined at 800 °C, and a well-crystallized Ni-rich layered oxide was got. The phase structure and morphology were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The micro-sized particles delivered high initial discharge capacity of 164.3 mA h g^?1 at 0.5 C (1 C?=?200 mA g^?1) between 2.5 and 4.3 V with capacity retention of 87.5 % after 100 cycles. High reversible discharge capacities of 172.4 and 131.4 mA h g^?1 were obtained at current density of 0.1 and 5 C, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to further study the LiNi_0.5Co_0.2Mn_0.3O₂ particles. Anyway, the excellent electrochemical performances of LiNi_0.5Co_0.2Mn_0.3O₂ sample should be attributed to the use of acacia gum.     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

Improving the rate performance and stability of LiNi<Subscript>0.6</Subscript>Co<Subscript>0.2</Subscript>Mn<Subscript>0.2</Subscript>O<Subscript>2</Subscript> in high voltage lithium-ion battery by using fluoroethylene carbonate as electrolyte additive

Fluoroethylene carbonate (FEC) is investigated as the electrolyte additive to improve the electrochemical performance of high voltage LiNi_0.6Co_0.2Mn_0.2O₂ cathode material. Compared to LiNi_0.6Co_0.2Mn_0.2O₂/Li cells in blank electrolyte, the capacity retention of the cells with 5 wt% FEC in electrolytes after 80 times charge-discharge cycle between 3.0 and 4.5 V significantly improve from 82.0 to 89.7%. Besides, the capacity of LiNi_0.6Co_0.2Mn_0.2O₂/Li only obtains 12.6 mAh g^?1 at 5 C in base electrolyte, while the 5 wt% FEC in electrolyte can reach a high capacity of 71.3 mAh g^?1 at the same rate. The oxidative stability of the electrolyte with 5 wt% FEC is evaluated by linear sweep voltammetry and potentiostatic data. The LSV results show that the oxidation potential of the electrolytes with FEC is higher than 4.5 V vs. Li/Li⁺, while the oxidation peaks begin to appear near 4.3 V in the electrolyte without FEC. In addition, the effect of FEC on surface of LiNi_0.6Co_0.2Mn_0.2O₂ is elucidated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The analysis result indicates that FEC facilitates the formation of a more stable surface film on the LiNi_0.6Co_0.2Mn_0.2O₂ cathode. The electrochemical impedance spectroscopy (EIS) result evidences that the stable surface film could improve cathode electrolyte interfacial resistance. These results demonstrate that the FEC can apply as an additive for 4.5 V high voltage electrolyte system in LiNi_0.6Co_0.2Mn_0.2O₂/Li cells.     

Critical behavior of La<Subscript>0.825</Subscript>Sr<Subscript>0.175</Subscript>MnO<Subscript>2.912</Subscript> anion-deficient manganite in the magnetic phase transition region

For La _0.825 ³⁺ Sr _0.175 ² +Mn³⁺O _2.912 ^2? anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point T _C ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (T _C ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, V _fr ≈ 22%, of the indirect Mn³⁺-O-Mn³⁺ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La _0.825 ³⁺ Sr _0.175 ²⁺ Mn³⁺O _2.912 ^2? anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

Improving the electrochemical performance of Li-rich Li<Subscript>1.2</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>Mn<Subscript>0.54</Subscript>O<Subscript>2</Subscript> cathode material by LiF coating

To suppress the capacity fade of Li-rich Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ material as cathode materials for lithium-ion battery, we introduce a LiF coating layer on the surface to improve the cycling performance of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ material. The modified sample shows a capacity of 163.2 mAh g^?1 with a capacity retention of 95% after 100 cycles at a current density of 250 mA g^?1, while the pristine sample only delivers a capacity of 129.9 mAh g^?1 with a capacity retention of 82%. Compared with the pristine material, the LiF-modified sample exhibits an obvious enhancement in the electrochemical performance, which will be very beneficial for this material to be commercialized on the new energy vehicles and other related areas.