Facile preparation of single-crystalline nanowires of γMnOOH and βMnO2

A facile method has been developed to synthesize single-crystalline manganese (oxyhydr)oxide nanowires. A pure phase of single-crystalline MnOOH nanowires with the lengths of several hundred nanometres to several micrometers and the diameters of 5–40 nm was synthesized by hydrothermal transformation of commercial granular - or MnO₂ in water, and single-crystalline MnO₂ and polycrystalline Mn₂O₃ nanowires formed after subsequent calcination at 300 and 600 °C of the as-prepared MnOOH nanostructures, respectively. These low-dimensional nanostructured materials may find novel properties and provide more possible applications in lithium batteries and catalysis. PACS 81.07.-b; 81.10.Dn; 81.16.Be     

Magnetotransport properties of La0.67Ca0.33MnO3//La0.67Sr0.33MnO3 bilayers

The perovskite bilayers La0.67Ca0.33MnO3 （LCMO） （100 nm） / La0.67Sr0.33MnO3（LSMO） （100 nm） and LSMO （100 nm） / LCMO （100 nm） are fabricated by a facing-target sputtering technique. Their transport and magnetic properties are investigated. It is found that the transport properties between them are different obviously due to distinguishable structures, and the different lattice strains in both films result in the difference of metal-to-insulator transition. Only single-step magnetization loop appears in our bilayers from 5K to 320K, and the coercive force of LSMO/LCMO varies irregularly with a minimum ～ 2387A/m which is lower than that of LCMO and LSMO single layer films. The behaviour is explained by some magnetic coupling.     

Paramagnetic shift of μ+ in MnO and its time dependence

Conclusion The paramagnetic shift K of ⁺ in MnO is measured from 140K to 420K. K is negative at 14OK, increases rapidly with increasing temperature, but does not show a linear relation to the susceptibility . At T = 170K and 230K, the value of K depends greatly on the fitting time-range of the spectrum. These features are explained by a site change of ⁺ caused by its diffusion and trapping in the crystal.     

Infrared Absorption of Phase Separated La0.67-xPrxCa0.33MnO3

Infrared absorption spectra of La0.67-xPrxCa0.33MnO3 （x= 0, 0.18 and 0.36） are experimentally studied in the temperature range 20 -300K. Absorption peak splitting corresponding to the stretching oscillation of the Mn-O bond, together with a shift of peak position, is observed below the Curie temperature. These features weaken and even disappear as the samples are warmed up to the Curie temperature, which indicates that this anomaly may be a result of phase separation in the compounds.     

Effects of Ag Doping on Magnetoresistance of La0.833 K0.167MnO3 Polycrystalline Perovskite Manganites

The microstructure,magnetic and transport properties of the heterogeneous system with a nominal composi-tion (1/m)Ag2O-La0.833K0.167MnO3 (1/m is molar ratio with m=32,16,8,4,and 2) have been studied.The x-ray diffraction patterns show that all the samples are two-phase composite and consist of a magnetic La0.833K0.167MnO3 (LKMO) perovskite phase and a nonmagnetic metal Ag phase.The room-temperature magnetoresistance (MR) effect is enhanced significantly due to the addition of Ag.For the m=4 sample,we obtain MR values up to 32％ under a lower field of 0.5 T and 64％ under a higher field of 5.5 T at the temperature of 300K,which are much larger than the corresponding MR values (10％and 35％) of the LKMO sample withoutAg.In the low-temperature range from 4.2K to 250K,the MR values of the Ag-added samples however decrease with the increasing Ag content in the samples.This effect is discussed qualitatively by using the relative contribution from the intrinsic MR effect and the extrinsic MR effect including the spin-polarized-tunnelling and spin-dependent scattering effects in different temperature regions.     

Influence of grain boundaries on resistivity of manganites La1 − x K x MnO3

Results of investigation of resistivity and magnetoresistance of manganites La_{1 − x} K _x MnO₃ (x = 0.050–0.175) are presented. Behavior of resistivity ρ(T) in the paramagnetic and ferromagnetic phases has been described. To describe ρ(T) near the phase-transition temperature, notions of the percolation theory have been used. Two maxima have been found in the dependence ρ(T); their appearance has been attributed to the ceramic nature of the studied samples. The observed increase in magnetoresistance with a decrease in temperature is caused by intergranular spin-polarized tunneling of charge carriers.     

Influence of oxygen vacancies on magnetoresistance properties of bulk La0.67Ca0.33MnO3−δ

Bulk polycrystalline samples of La_0.67Ca_0.33MnO_3−δ were annealed in controlled oxygen atmospheres. Two types of conditions were chosen, firstly with the aim of taking oxygen from the grain boundaries and secondly with the aim of removing oxygen uniformly from the sample. Reducing conditions increase the overall resistivity and suppress the Curie–Weiss temperature (T_c). A second peak at temperatures below T_c appears in the resistivity and magnetoresistivity when the samples are deoxygenated. However, the presence of this peak does not strongly influence the low-temperature magnetoresistance.     

Effect of High Resistance Phases on Metal—Insulator Transition of La0.67Ca0.33MnO3

The temperature dependence of resistivity ρ of YSZ doping composite of (1-x) La0.67Ca0.33MnO3 xYSZ and Y2O3 doping composite of (1-y) La0.67MnO3 yY2O3 is investigated,respectively,in a temperature range 77-300K,where the YSZ represents yttria-stabilized zirconia(8mol%Y2O3 92mol% ZrO2).Experimental results show that the YSZ doping level has important effects on both the metal-insulator(M-I) transition temperatures and zero field resistivity of the composites of (1-x) La0.67Ca0.33MnO3 xYSZ.However,the Y2O3 doping level has little effect on the M-I transition temperatures and the zero field resistivity of (1-y)La0.67Ca0.33MnO3 yY2O3 only increases slightly.The difference between the two types of composites may mainly results from the different distribution of high resistance phases at the grain boundaries and /or surfaces of La0.67Ca0.33MnO3 grains rather than the substitution of La^3 ions with Y^3 ions.     

Phase diagram of perovskite manganites Sm1 − x Sr x MnO3

Physics of the Solid State - The magnetic and structural phase diagram of perovskite manganites Sm1 ? x Sr x MnO3 (0.16 ≤ x ≤ 0.67) are constructed based on systematic studies by...     

Features of magnetic properties of La x MnO3 + δ (0.815 ≤ x ≤ 1.0)

Magnetic and resonance studies of the system of polycrystalline samples of self-doped manganites La _x MnO_{3 + δ} (x = 0.815, 0.90, 0.94, 0.97, and 1.0) have been performed in a temperature range of 77–300 K. According to ⁵⁵Mn NMR data, all samples contain a ferromagnetic phase at 77 K. As the defect density increases (x changes from 1.0 to 0.815), samples become more magnetically ordered. In this case, the ferromagnetic state of the system gradually changes from a mixed state in which both ferromagnetic insulating (basic) and ferromagnetic metal (for x = 0.97 and 1.0) phases coexist to only the ferromagnetic metal state (for x = 0.815 and 0.90). It has been shown that both ferromagnetic metal and ferromagnetic insulating phases are inhomogeneous, and either phase consists of two phases with different dynamics of nuclear spins and different Curie temperatures. The diagram of the magnetic phase state of the La _x MnO_{3 + δ} system (x = 0.815, 0.90, 0.94, 0.97, 1.0) has been constructed for a temperature range of 120–240 K and Mn⁴⁺ contents of 12–30%.     

Effect of oxygen content on the magnetic state of La0.5Ca0.5MnO3−γ perovskites

The magnetization and magnetoresistance are studied in La_0.5Ca_0.5MnO_3−γ as a function of oxygen content. It is found that as the oxygen content is decreased, this compound undergoes a sequence of transitions from an antiferromagnetic to a ferromagnetic state (γ⩾0.04), from the ferromagnetic to a spin-glass state (γ ⩾0.14), and from the spin glass to an inhomogeneous ferromagnetic state (γ⩾0.25). Strongly reduced samples (γ⩾0.25) show a large magnetoresistance despite the absence of Mn³⁺–Mn⁴⁺ pairs. It is suggested that the oxygen vacancies in the strongly reduced samples (γ⩾0.25) are long-range ordered. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 583–587 (10 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Ultrafast Photoelectric Effects in Heterojunctions of La0.7Sr0.3MnO3 and Si

Ultrafast photoelectric effects have been observed in p-n heterojunctions of La0.7Sr0.3MnO3(LSMO)/Si and LSMO/SrTiO3_/Si for the first time. The rise time was about 1 ns and the full width at half maximum was about 2ns for the photovoltaic pulse when the heterojunction was irradiated by a laser of -25 ps pulse duration and 1064 nm wavelength. The photovoltaic sensitivity was as large as 435 mV//mJ for a 1064 nm laser pulse. No such pulse was observed with irradiation from a pulsed 10.6 μm CO2 laser.     

Influence of Bi3+ doping on electronic transport properties of La0.5−xBixCa0.5MnO3 manganites

Electronic transport properties of La_0.5?xBi_xCa_0.5MnO₃ (x=0, 1/16, 1/8, 1/4, 3/8 and 1/2) compounds have been studied systematically to investigate their charge ordering (CO) behaviors. The results show that the CO temperature increases with the substitution of Bi³⁺ ion for La³⁺ ion, suggesting that the charge ordering is enhanced. This is attributed to the special role of the 6s² lone pair of Bi³⁺. It is found that for all the samples the adiabatic small polaronic conduction mechanism is responsible for the transport behavior above CO transition, whereas Mott's variable range hopping mechanism dominates below the CO transition. In addition, the electronic transport behavior of La_0.5?xBi_xCa_0.5MnO₃ compounds is high sensitive to an external magnetic field, which could raise fresh opportunities for application in magnetic sensors.     

Fe3O4 Segregation and Transport Properties of La0.67Ca0.33MnO3

Effect of Fe3O4 segregation at grain boundaries on the electrical transport and magnetic properties of La0.67Ca0.33MnO3 is investigated. The experimental results show that the Fe3O4 segregation not only shifts the paramagnetic-ferromagnetic transition temperature of La0.6TCa0.33MnO3 to a lower temperature region but also induces a new transition in a lower temperature region. Meanwhile, the transition processes observed in both the resistivity and magnetization curves are obviously widened. Compared to pure La0.67Ca0.33MnO3, we assume that the Fe3O4 segregation level at the grain boundaries can modify the electrical transport and magnetic properties of La0.67Ca0.33MnO3.     

Synthesis and Gas-Sensing Properties of MnO2 – x and MnO2 – x/CuO-Coated Multiwalled Carbon Nanotube Nanocomposites

Physics of the Solid State - Nanocomposites based on multiwall carbon nanotubes (MWCNTs) impregnated with manganese oxide MnO2 – x and copper oxide CuO are synthesized and...     

Electroresistance and field effects in epitaxial thin films of Pr0.7Sr0.3MnO3

Highly epitaxial thin films of Pr_0.7Sr_0.3MnO₃ were grown on (100) SrTiO₃ single crystal substrates by laser ablation. Similar to other manganite compounds, these Pr_0.7Sr_0.3MnO₃ films exhibited remarkable magnetoresistance. Application of electric currents could induce a remarkable reduction in resistivity, demonstrating a strong electroresistance effect. The ratio of the resistance variation, ER=[R(0)−R(I)]/R(I), is about 33% at metal-insulator transition temperature. Using a planar field effect configuration, significant field modulation of the metal-insulator transition was achieved. The observed field effects were discussed based on the strong interactions between carrier spins and localized spins in Mn ions, as well as the percolative mechanism of phase separation.     

Enhancing low-field magnetoresistance of La0.67Ca0.33MnO3 films deposited on anodized aluminium-oxide membranes

In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.     

Spontaneous reorientation of the electric polarization in Eu1 − x Ho x MnO3 multiferroics

The magnetic, dielectric, and ferroelectric properties of Eu_{1 ? x} Ho _x MnO₃ single crystals (0 < x ≤ 0.5), where magnetic ordering can be varied from the canted antiferromagnetic phase to modulated spin structures, have been studied. It has been found that a ferroelectric state appears at x ≥ 0.2 and low temperatures. As the temperature decreases and the holmium content increases, the electric polarization in this state is reoriented from the a axis to the c axis. It has been shown that the polarization is reoriented owing to a change in the spin rotation plane in the cycloidal phase from the ab to cb plane because of the stabilization of the latter upon an increase in the rare-earth contribution to the anisotropy energy. The T-x phase diagram of magnetic and ferroelectric states has been constructed.     

Structure determination of MnO2 films grown on single crystal α-Al2O3 substrates

Manganese oxide films have been grown by atomic layer deposition and investigated using electron diffraction and high-resolution electron microscopy (HREM). The films were deposited on the (001) surface of monocrystalline α-Al₂O₃. The films were found to consist of an ordered version of the hexagonal ε-MnO₂ (Akhtenskite) type. Using X-ray diffraction, the cell parameters were determined to be a?=?2.75(2)?Å and c?=?4.302(5)?Å. The films are epitaxial with a specific orientation relative to the Al₂O₃ substrate. The [210] and [001] axes of ε-MnO₂ are parallel to the [110] and [001] axes of α-Al₂O₃, respectively. Evidence of cation ordering was found by parallel beam electron diffraction. The ordered domains are needle shaped with widths of 2–10?nm. The unit cell of the ordered structure was found to be orthorhombic with cell dimensions a?=?2.75, b?=?4.76, c?=?4.302?Å and space group Pmnn (No. 58).