MnO3 Crystals Research Articles

Investigation of Sodium Doped Lanthanum Manganite in Optical, Dielectric and Capacitive Perspective Prepared by Flux Method

A comprehensive analysis of optical and dielectric properties of a material is very much essential for the research of device fabrication. From this perception, the sodium (Na) doped lanthanum manganite with 15% substitution of La by Na (La0.85Na0.15MnO3) has been synthesized using the flux method and investigated to observe the optical absorbance, refractive index, capacitance and dielectric constant. In this study, we have reported better optical absorbance for the monovalent Na doping with a slightly lower value of the energy band gap (~ 1.57 eV) compared to the divalent Sr doping on parent LaMnO3. The structural analysis has been accomplished using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) to confirm the ability of the flux method to grow higher order La0.85Na0.15MnO3 crystal satisfactorily. Optical properties of La0.85Na0.15MnO3 have been estimated using UV-Vis spectroscopy and the frequency dependent electronic properties (ac conductivity, dielectric constants and capacitance) have been investigated using a precision impedance analyzer. The capacitance has been reported in Pico farad range for La0.85Na0.15MnO3 crystal with the low value of dielectric constant throughout the high-frequency region.

Reduction of critical field for magnetic and orbital-ordering phase transition in impurity-substituted Nd0.45Sr0.55MnO3 crystal

We have investigated the Mn-site substitution effect in Nd0.45Sr0.55MnO3 single crystal, which has an A-type layered antiferromagnetic (A-AFM) phase with the 3\documentclass[12pt]{minimal}\begin{document}$d_{x^2-y^2}$\end{document}dx2−y2-type orbital-order. Substitution of Fe or Ga for Mn-site suppresses both the A-AFM order and competing ferromagnetic (FM) correlation, whereas Cr substitution suppresses only the A-AFM order but reactivates the underlying FM correlation via double-exchange mechanism along the AFM coupled c-direction. In Nd0.45Sr0.55Mn0.95Cr0.05O3, the A-AFM state with the orbital-order is changed into the orbital-disordered three-dimensional FM metallic state by applying magnetic field of μ0H = 12 T, which is much smaller than that of the parent compound Nd0.45Sr0.55MnO3.

Spin glass behavior in hole- and electron-doped bismuth manganite single crystals

The hole- and electron-doped bismuth manganites Bi0.55Ca0.45MnO3, Bi0.55Sr0.45MnO3 and Bi0.95Ce0.05MnO3 single crystals were grown using the flux-growth method. Their structural, magnetic and electrical transport properties have been compared studied. All samples show spin-glass magnetic behavior at low temperatures. In the immediate temperature region, an antiferromagnetic transition at TA and a charge-ordering state at TCO are observed for Bi0.55Ca0.45MnO3 crystal, whereas only an antiferromagnetic transition exists in Bi0.95Ce0.05MnO3 and Bi0.55Sr0.45MnO3 crystals. Bi0.55Ca0.45MnO3 and Bi0.55Sr0.45MnO3 samples show semiconducting transport behavior in the whole studied temperature range, whereas Bi0.95Ce0.05MnO3 is an insulator at room temperature. In addition, near TCO a positive magnetoresistance as large as 70.7% is observed for Bi0.55Ca0.45MnO3 sample under 5T applied magnetic field. The obtained results may originate from the rotation of the polarized Bi-6s2 lone pair electrons in the magnetic field.

‘Griffiths phase’ versus chemical disorder in low-doped manganites: La0.9Sr0.1MnO3 crystal revisited

Magnetic properties, electron paramagnetic and ferromagnetic resonance were studied in the vicinity and above ferromagnetic-paramagnetic (FM-PM) phase transition of the La0.9Sr0.1MnO3 crystal. It appears that complex chemical/structural disorder inherent to manganites influences strongly on both magnetic and resonance properties of this crystal. In particular, it changes the nature of a FM–PM transition to the first- or mixed-order one and induces FM clustering in their PM state. The latter effect results in coexistence of resonance signals, and non-linearity of the PM inverse magnetic susceptibility versus temperature. This model seems to be much more realistic for description of the PM state in La0.9Sr0.1MnO3 than an idealized Griffiths phase approach.

Anisotropic magnetoresistance in low-doped La0.78Ca0.22MnO3 crystals

Unusual behavior of anisotropic magnetoresistance (AMR) has been encountered in low-doped La0.78Ca0.22MnO3 single crystals. In contrast with previous studies of AMR in manganites, as the maximal effect was observed around TC, the AMR of La0.78Ca0.22MnO3 single crystals (TC ≈ 189 K) increases monotonously with decreasing temperature, reaches a maximum around 140 K, and then decreases with further temperature decrease. Moreover, around the maximum, AMR increases almost linearly with magnetic fields and only at fields exceeding H ∼ 7 kOe does it start to saturate. The observed unusual enhancement of AMR may be ascribed to the changes in magnetic domain structure at temperatures below 150 K as observed previously by us using a magneto-optical imaging technique.

Transformation of the acoustic mode near the structural and magnetic phase transitions in La1–x Sr x MnO3 (x = 0.175) crystal

The peculiarities of the phase transitions in La0.825Sr0.175MnO3 single crystal have been investigated by acoustic methods at the frequency f = 770 MHz. Generation of a magnetoelastic wave near the phase transition at 305 K is revealed, which is assigned to quasi-transverse modes. The most likely cause of the phase transition near T = 305 K and the formation of the quasi-transverse mode is the suppression of local Jahn-Teller distortions.

Response of the electrical resistivity of La0.67Ba0.33MnO3(20 nm)/LaAlO3(001) films to magnetic field and variation in temperature

The structure, electrical resistivity, and magnetotransport parameters of 20-nm-thick epitaxial La0.67Ba0.33MnO3 films grown by laser ablation on LaAlO3(001) substrates are studied. The unit cell volume V eff = 58.80 A3 of the as-grown manganite films is found to be less than that for bulk La0.67Ba0.33MnO3 crystals. Maximum values of the negative magnetoresistance MR(μ0 H = 1 T) = −0.27 for La0.67Ba0.33MnO3 films are observed at a temperature of about 225 K. For 5 < T < 100 K, the film magnetoresistance depends only weakly on temperature and is on the order of −0.1. At temperatures below 100 K and for 3 < μ0 H < 5 T, the electrical resistivity of the as-grown films decreases linearly with increasing magnetic field.

Phase separation in a manganite La0.95Ba0.05MnO3 crystal

The structure and magnetic states of a crystal of lightly doped manganite La0.95Ba0.05MnO3 were studied using thermal-neutron diffraction, magnetic measurements, and electrical resistance data in a wide temperature range. It is shown that, in terms of its magnetic properties, the orthorhombic crystal is characterized by two order parameters, namely, antiferromagnetic (TN = 123.6 K) and ferromagnetic (TC = 136.7 K). The results obtained differ in detail from known information on the manganites La0.95Ca0.05MnO3 and La0.94Sr0.06MnO3. Two models of the magnetic state of the La0.95Ba0.05MnO3 crystal are discussed, one of which is a model of a canted antiferromagnetic spin system and another is associated with the phase separation of the manganite. Arguments are advanced in favor of the coexistence in this crystal of the antiferromagnetic phase (about 87%) with a Mn4+ ion concentration of 0.048 and the 1/16-type charge-ordered ferromagnetic phase (about 13%) with a Mn4+ ion concentration of 0.0625. The specific features of the manganite studied are due to self-organization of the La0.95Ba0.05MnO3 crystal lattice caused by the relatively large barium ion size.

Nonequilibrium 1∕f noise in low-doped manganite single crystals

1 ∕ f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature, suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multistep indirect tunneling.

Chemical bonding and electronic structure of LaMnO3 and La0.75MnO3 orthorhombic crystals

The electronic structure of the LaMnO3 orthorhombic crystal of a stoichiometric composition and of La0.75MnO3 crystals with a La vacancy in the unit cell is calculated in the LSDA+U approximation of density functional theory. The calculations showed that LaMnO3 is an insulator with a forbidden gap of 0.5 eV and with antiferromagnetic ordering of magnetic moments. The magnetic moment on the manganese ions is 3.78 BM. The La atom has ionic bonds in the lattice, while the bond between oxygen and manganese is covalent. After lanthanum has been removed, geometry optimization of the unit cell leads to La0.75MnO3 stable structures. In one of the structures, which is lower in energy, the states of manganese may be attributed to Mn4+ ions. In both structures with removed lanthanum, the oxygen ions have reduced effective charge, so that one can speak about O− ions appearing along with O2− in the structure. The oxygen, as well as lanthanum and manganese, ions are nonequivalent in these structures; their nonequivalence is primarily reflected by the local densities of states. This leads to charge and magnetic nonequivalence of ions. In La0.75MnO3 crystals, the degree of bond covalence between manganese and oxygen decreases.

Room temperature low-field colossal magnetoresistance in La0.7Sr0.3MnO3

It is found that La0.7Sr0.3MnO3 crystal shows hysteretic nonlinear V-I characteristics at high currents. The authors have reproduced such a behavior by using a simple thermal equation. Utilizing magnetic field dependence of the V-I curves, magnetoresistance ratio of R(0Oe)∕R(2.8kOe)=230% is realized at room temperature in the presence of bias currents.

Defects and the structure of a manganite La0.85Sr0.15MnO3 crystal

Defects in a ferromagnetic crystal of manganite La0.85Sr0.15MnO3 were created by irradiation with fast neutrons (E > 0.1 MeV). Fast neutrons produce defect clusters in a crystal lattice. The volume fraction of the clusters in the crystal after irradiation to a dose F = 2 × 1019 cm−2 (Tirr = 340 K) was ≅40%. The structural and magnetic states of the modified manganite were studied using thermal-neutron diffraction and magnetic measurements. It was revealed that neutron irradiation of a crystal suppresses the cooperative Jahn-Teller effect and the initial charge modes and decreases the temperature of ferromagnetic ordering. Under irradiation with fast neutrons, the crystalline structure of the manganite changes from the orthorhombic O′ to the pseudocubic O* phase. Arguments are advanced in favor of the specific features of the irradiated-manganite structural state being determined by long-wavelength strains induced in the crystal by antisite defects.

Imaging of percolative conduction paths and their breakdown in phase-separated (La1-yPry)0.7Ca0.3MnO3 with y=0.7.

Local magnetization and current distribution in (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) crystals are studied by a magneto-optical (MO) imaging technique. MO images below 120 K visualize inhomogeneous magnetization and conduction paths that manifest the percolative conduction originated from the mesoscopic phase separation into ferromagnetic metals and antiferromagnetic insulators. Application of large amounts of current switches the current distribution from inhomogeneous to homogeneous concomitantly with a steep increase in resistivity. These phenomena are discussed in view of current induced collapse of the phase separation through a local heating.

High-pressure synthesis and magnetic properties of Bi0.5D0.5MnO3 (D = Pb, Ba) solid solutions

Perovskites Bi0.5D0.5MnO3(D = Pb, Ba) were prepared under high pressure (4 GPa) at 1200–1300°C. According to the X-ray diffraction data, crystalline Bi0.5Pb0.5MnO3 has a tetragonal unit cell with the parameters a = 3.940 A and c = 3.800 A, whereas Bi0.5Ba0.5MnO3 crystals are cubic with a = 3.940 A. It is concluded from magnetic studies that lead-containing manganite is an antiferromagnet with TN = 120 K, whereas Bi0.5Ba0.5MnO3 is a spin glass with spin-freezing temperature Tf = 38 K. Both compounds are decomposed upon heating in air at temperatures above 500°C. With the use of synthesis in air, Bi0.5Ca0.5 − xDxMnO3 solid solutions with x as high as 0.25 were obtained.

Magnetic domain structure of a La0.7Sr0.3MnO3 (001) surface observed by a spin-polarized scanning electron microscope

The magnetization vector distribution at a cleaved surface of La0.7Sr0.3MnO3 (001) crystal has been quantitatively analyzed by using a newly developed low-temperature spin-polarized scanning electron microscope. The magnetic structure essentially consists of two kinds of domains, where magnetizations are parallel or antiparallel to the [110] direction with no surface-normal component. The rhombus-shaped domains range from a few to several tens of micrometers across. The domain structure can be considered to be made by laying down the magnetization from the out-of-surface-plane easy axis to the surface plane to reduce the magnetostatic energy without forming closure domains.

Current-induced phase control in charged-ordered Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 crystals

Single crystals of Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 show current-induced insulator–metal (IM) transitions at low temperatures. In addition, the charge-ordering transition temperature, Tco, decreases with increasing current. The electroresistive ratio, defined as ρ0.5/ρI where ρ0.5 is the resistivity at a current of 0.5 mA and ρI the resistivity at a given applied current, I, varies markedly with temperature and the value of I. Thermal hysteresis observed in Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 at the IM transition indicates that the transition is first order. The current-induced changes are comparable to those induced by magnetic fields, and accordingly the IM transition in Pr0.6Ca0.4MnO3 is associated with a larger drop in resistivity.

Magnetic and transport properties of Pr0.8Ca0.2MnO3 crystal

Abstract Magnetic and transport properties of Pr 0.8 Ca 0.2 MnO 3 crystal have been studied. It was found that the magnetic anisotropy in (1 0 0) plane increases with decreasing temperature and increasing pressure. The resistivity for T >140 K obeys Arrhenius law with an activation energy E a =130 meV, while at T E a =62 meV. The magnetoresistance maximizes near the Curie temperature of Mn moments T C1 =130 K. The dynamic resistivity shows a marked dependence on a bias current I at T T C1 . The results are discussed in the frame of magnetic phase separation and spin-density wave scenario.

Erratum: “Phase separation of the spin system in the La0.93Sr0.07MnO3 crystal” [Phys. solid state 45 (12), 2297 (2003)

Erratum: “Phase separation of the spin system in the La0.93Sr0.07MnO3 crystal” [Phys. solid state 45 (12), 2297 (2003)

Phase separation of the spin system in the La0.93Sr0.07MnO3 crystal

The magnetic state of the manganite La0.93Sr0.07MnO3 in the range 4.2–290 K was studied using elastic neutron scattering. The magnetic state of this compound was found to occupy a particular place in the La1−x SrxMnO3 solid-solution system, in which the antiferromagnetic type of order (LaMnO3, T N=139.5 K) switches to ferromagnetic ordering (La0.9Sr0.1MnO3, T C=152 K) with increasing x. In the transition state, this compound contains large-scale spin configurations of two types. A fractional crystal volume of about 10% is occupied by regions of the ferromagnetic phase with an average linear size of 200 A, while the remainder of the crystal is a phase with a nonuniform canted magnetic structure. Arguments are presented for the phase separation of the La0.93Sr0.07MnO3 spin system being accounted for by Mn4+ ion ordering.

Tunneling magnetoresistance in a Eu0.7Pb0.3MnO3 (single crystal)-Fe (film) structure

We have studied the magnetoresistive properties of a structure comprising single crystal manganite Eu0.7Pb0.3MnO3 covered with an epitaxial iron film. At temperatures below T C of the manganite crystal, the structure exhibits positive magnetoresistance. The behavior of the resistance as a function of the magnetic field is characteristic of a tunneling junction with ferromagnetic electrodes separated by a thin insulating film. The observed effect is related to the formation of a transition layer at the manganite-Fe interface, which is depleted of oxygen and possesses dielectric properties. The sensitivity of the resistance with respect to the magnetic field is determined both by the negative magnetoresistance of the manganite crystal and by the tunneling contribution to the conductivity, whereby the tunneling current depends on the mutual orientation of magnetic moments of the electrodes (Eu0.7Pb0.3MnO3 crystal and Fe film).