MnO3 Single Crystal Research Articles

Influence of the transport current on the magnetoelectric properties of La0.7Pb0.3MnO3 single crystals with giant magnetoresistance in the microwave region

The results of experiment on the influence of a direct current and a low-frequency alternating current, as well as a magnetic field, on the microwave-range conductivity σ MW of Ln0.7Pb0.3MnO3 single crystals with giant magnetoresistance are presented. The greatest sensitivity of the samples toward the effects of a current is observed in the temperature range corresponding to the dc magnetoresistance maximum. The response signal of a sample in the microwave range to the effects of an alternating current of a low frequency f 0 has a nonlinear character. As f 0 is varied in a magnetic field, the amplitude of the response signal varies with the appearance of resonance peaks. The results obtained are interpreted within an approach based on the coexistence of two phases having different conductivities in the doped manganite crystals. This two-phase interpretation is supported by data from magneticresonance investigations, which demonstrate the existence of two magnetic phases over a broad temperature range in Ln0.7Pb0.3MnO3 single crystals.

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρxx and transverse ρxy resistivities and magnetic susceptibility χac of La0.8Sr0.2MnO3 single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρxy∼ρxx, which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χac. As a result, the curve of ρxy versus ρxx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρxx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽TC. The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials.

Hard X-Ray Diffraction Studies of La1−xSrxMnO3

Superstructure reflections at low temperatures in La1−xSr x MnO3 single crystals indicating charge order in samples containing 12.5% and 15% of Sr have been observed. The positions of the reflections in reciprocal space suggest a fourfold unit cell referring to the cubic perovskite structure. The onset temperatures for charge ordering are 150 K and 180 K for x=0.125 and x=0.15, respectively. Especially in the sample with the lower Sr concentration, the onset is correlated to a jump in resistivity measured on the same sample.

Fine structures in the X-ray photoemission spectra of MnO, FeO, CoO, and NiO single crystals

The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced.

Liquidlike Spatial Distribution of Magnetic Droplets Revealed by Neutron Scattering inLa1−xCaxMnO3

Elastic neutron scattering experiments performed in semi-conducting La(1-x)Ca(x)MnO3 single crystals (x=0.05, 0.08), reveal new features in the problem of electronic phase separation and metal insulator transition. Below TN, the observation of a broad magnetic modulation in the q-dependent scattering intensity, centered at nearly identical qm whatever the q direction, can be explained by a liquid-like spatial distribution of similar magnetic droplets. A semi-quantitative description of their magnetic state, diameter, and average distance, can be done using a two-phase model. Such a picture can explain the anomalous characteristics of the spin wave branches and may result from unmixing forces between charge carriers predicted from the s-d model.

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La0.9Sr0.1MnO3 single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and Tc=145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La0.9Sr0.1MnO3 single crystal in the cooling regimes studied shows typical “spin-glass” behavior.

Magnetic anisotropy of doped manganite thin films and crystals

Magnetic anisotropy of La0.7Sr0.3MnO3 thin films and single crystals has been measured using vibrating sample and torque magnetometry. The magnetic anisotropy of thin films is dominated by strain anisotropy due to the lattice mismatch between the film and SrTiO3 substrate. The two and fourfold symmetries in the (110) and (001) films, respectively, can only be explained by stress anisotropy contributions (Kstress∼8.4×104 ergs/cm3). Magnetic anisotropy of La0.7Sr0.3MnO3 single crystals, subject to no external stress, reveals a uniaxial anisotropy in the (100) plane of K1∼1.8×104 ergs/cm3 that can be attributed to magnetocrystalline effects.

Neutron scattering investigation of the structure and spin dynamics in La0.85Sr0.15MnO3

We have performed diffraction and inelastic neutron scattering experiments on a La0.85Sr0.15MnO3 single crystal. In the ferromagnetic phase of this system (Tc=235 K) the long wavelength dispersion relation along the (0,1,0) direction is well represented by E=E0+Dq2, but with the spin wave spectrum almost gapless (E0<0.1 meV), and with DT=0=83.75±0.36 meV Å2. The spin-wave stiffness constant D(T) exhibits a power law behavior as a function of temperature, and appears to collapse as T→Tc. An orthorhombic–rhombohedral structural phase transition is observed at Ts=360 K, and exhibits a hysteretic character suggesting a first-order transformation. Anomalies in magnetization measurements and Bragg peak intensities indicate the existence of a second phase transition at T=200 K.

Magnetostriction and thermal expansion anomalies near the Curie point of the compound La0.7Sr0.3MnO3 with the perovskite structure

It is found that the bulk part ω of the magnetostriction near the Curie temperature Tc in a La0.7Sr0.3MnO3 single crystal with the perovskite structure is negative and that the temperature dependence of |ω| has a maximum near Tc. The quantity |ω| at the maximum increases rapidly with increasing magnetic field. The thermal expansion coefficient near Tc increases with temperature much faster than linearly. The paramagnetic Curie temperature determined from the Curie-Weiss law, which the paramagnetic susceptibility of this crystal satifies, was found to be lower than Tc. These anomalies and also the near-Tc metal-insulator transition which is characteristic for this material are explained by the existence of a magnetically two-phase state consisting of a conducting ferromagnetic matrix containing antiferromagnetic insulating microregions occupying not more than 5% of the sample volume.

Shock‐induced phase transition of MnO around 90GPa

Shock compression experiments on single crystal MnO grown by the Verneuil method have been carried out for the pressure range of 41–114GPa using both propellant and two‐stage light‐gas guns. The shock states, recorded with the inclined mirror method, were analyzed by the free‐surface approximation and impedance‐match solution. The observed shock compression curve of MnO with the rock salt structure(B1) is found to be slightly more compressible than that measured statically to 60GPa using a diamond‐anvil cell. We observed a phase transition around 90GPa with a volume decrease of approximately 8%. The present phase transition pressure falls on a trend of B1‐B2(CsCl‐type structure) transformation among alkaline earth metal monoxides, rather than transformation to the B8(NiAs‐type structure) that wüstite (FeO) demonstrates.

High resolution electron energy loss spectroscopy of MnO(100) and oxidized MnO(100)

Single crystal MnO(100) substrates can be selectively oxidized to produce Mn2O3- and Mn3O4-like surfaces under mild oxidation/reduction conditions readily accessed under ultrahigh vacuum (UHV). MnO(100) yields a characteristic Mn 2p x-ray photoelectron spectroscopy (XPS) satellite structure and appropriate O/Mn concentrations from O 1s/Mn 2p XPS intensity ratios. Its high resolution electron energy loss (HREEL) spectrum shows a series of Fuchs–Kliewer multiple phonon excitations with a single loss energy of 70.9 meV, characteristic of the cubic manganese monoxide structure. However, the HREEL spectral (HREELS) background is high and the phonons are not as well resolved as those typically observed on comparable metal monoxides. Annealing the MnO(100) substrate at 625 K and 5×10−7 Torr O2 slowly forms Mn2O3, as indicated by O 1s and Mn 2p XPS, and does so without significantly altering the symmetry of the MnO(100) low energy electron diffraction pattern. The MnO(100)-Mn2O3 surface can be selectively reduced to Mn3O4-like composition by heating under UHV to 775 K and to MnO(100) at 1000 K. HREEL spectra for the UHV annealed surfaces are well-resolved, and for the MnO(100)-Mn3O4 substrate a second fundamental phonon loss is observed at 55.6 meV as a result of the lower symmetry of the Mn3O4 spinel structure. The UHV-annealed MnO(100) surface appears to be more highly ordered since its HREELS phonon loss peaks are better resolved. It is also somewhat reduced, however, resulting in a less intense phonon spectrum with a fundamental loss energy of only 65.1 meV.

Satellite structure in the photoemission spectra of MnO(100).

The satellite structure in both core-level and valence-band photoemission spectra from single-crystal MnO(100) has been studied by using both x-ray photoemission spectra and resonant photoemission. Only very weak satellites are present in the Mn 2p core-level spectra, while no satellite structure is evident in the Mn 3p and 3s core-level spectra. The small intensity of the satellite peaks in MnO is consistent with the trend predicted by a recent ligand charge-transfer screening model; such screening is quite weak in MnO in comparison to the heavier transition-metal oxides. The valence-band spectrum, however, exhibits a stronger satellite peak than predicted by the model, and its origin has been uncertain. We have found that, while the satellite in the valence-band spectrum can be resonantly enhanced across the Mn 3p\ensuremath{\rightarrow}3d optical excitation threshold, its intensity depends strongly upon surface treatment. The satellite disappears with only 0.2--1-langmuir exposure of oxygen at 340 \ifmmode^\circ\else\textdegree\fi{}C, confirming that it is essentially a surface-related feature. We suggest that it is associated with point defects on the surface, presumably surface Mn vacancies.

Electronic structure of MnO studied by angle-resolved and resonant photoemission.

Results are reported from angle-integrated, angle-resolved, and resonant photoemission measurements on cleaved MnO single crystals using synchrotron radiation. Normal-emission spectra from a cleaved (100) surface indicate that the relative dispersion of Mn 3d states in the valence band is less than \ifmmode\pm\else\textpm\fi{}0.1 eV along \ensuremath{\Gamma}--\ensuremath{\Delta}--X in the Brillouin zone, which supports the view of highly localized 3d electrons on the ${\mathrm{Mn}}^{2+}$ cations. The results of polarization-dependent and off-normal-emission measurements are also consistent with a localized Mn 3d orbital picture. To explain the presence of satellite photoemission peaks, a hybridization between Mn 3d and O 2p valence states must be invoked. The degree of hybridization is evident from resonant-photoemission measurements at the Mn 3p\ensuremath{\rightarrow}3d absorption threshold. The antiresonant behavior of the 3d-derived states near the top of the valence band in constant-initial-state spectra suggests that MnO is a charge transfer rather than a Mott insulator. The experimental results are discussed in the context of recent cluster and band theories which have been proposed to explain the insulating nature and electronic structure of MnO.

Photoconductivity of MnO

The photoconductivity spectra and the carrier drift mobilities of MnO single crystals are measured at various temperatures from 83 to 299 K in the spectral region from 300 to 650 nm by the synchronous detection and transit time methods. The photoconductivity spectra show the photoconductivities associated with the intrinsic inter-band excitations and with the crystal-field adsorptions of Mn2+ ions. The photocarriers are both electrons and holes with high drift mobilities. The results are discussed in terms of recent theories describing the band gaps and the electronic structures of the 3d transition-metal compounds.

ChemInform Abstract: Investigations for Determination of Phase‐Extension of Transported MnO‐Crystals.

AbstractThe O/Mn ratio of transported MnO single crystals has been determined by means of solid electrolyte coulometry.

Electrical resistivity and Seebeck coefficient of La1?x M x MnO3 (M = Ca, Sr) single crystals

The temperature dependence of the electrical resistivity and Seebeck coefficient was measured on single crystals of La1−xCa x MnO3(0 <x ≤ 0.3) and La1−xSr x MnO 3 (0 <x ≤ 0.4) grown by the arc-image floating zone method. The electrical conduction for La1−x crystals withx ⩾ 0.2 was of the activation type aboveTc and of the degenerate type belowTc, while that for the crystal withx = 0.1 was of the activation type over the whole measured temperature range between −170 and 400°C. The conduction behaviour of La1− x Sr x MnO3 was essentially the same as that of La1−xCa x MnO3 except that the conduction of the crystals withx = 0.3 and 0.4 was of the degenerate type aboveTc. A distinct difference in Seebeck data was observed between the calcium and the strontium compounds.

High-temperature deformation of MnO

Steady-state flow stresses have been measured for {100}-oriented MnO single crystals for temperatures from 900 to 1400°C and strain rates from 2 × 10 −6 to 2 × 10 −3 s −1. The crystals were equilibrated in oxygen partial pressures ranging from 10 11 to 10 2Pa, which induced deviations from stoichiometry of 4 × 10 −4 to 9 × 10 −2. Deformation rates are controlled by oxygen diffusion. Pipe diffusion along dislocation cores is predominant for T ≤ 1000° C; volume diffusion is predominant for T ≥ 1200 dgC. In general, the primary diffusing oxygen species are singly charged vacancies for low Po 2 and neutral interstitials for high Po 2. At 1400°C, ionization states decrease for the oxygen point defects.

Dislocation structures in deformed CoO and MnO

Transmission electron microscopy (TEM) and etch-pitting techniques have been used to examine dislocation structures of CoO and MnO single crystals deformed into steady state from 1000 to 1400° C. In this temperature range, studies have shown that there are low- and high-temperature activation energies (Q) and stress exponents (n). No change was observed in the structures of samples deformed within this temperature range. It is concluded, therefore, that any change inQ andn values is due to a change in oxygen diffusion path from the bulk at higher temperatures to along dislocation cores at lower temperatures, rather than being due to a change in the rate-controlling mechanism of deformation

Creep of Mn1‐δO

Constant‐load creep tests have been performence nonstoichiometnc (100) oriented MnO single crystals between 900° and 1400°C. Tests were conducted under a controlled oxygen partial pressure to ensure that dislocation‐climb recovery was controlled by diffusion of either oxygen vacancies (low pO2) or oxygen interstitials (high po2). Within experimental error, the activation energies and the stress exponents are independent of oxygen partial pressure. The activation energy measured for T≤1000°C is about half of that measured for higher temperatures.

Effect of [formula omitted] atmospheres on high-temperature deformation of MnO single crystals

The steady-state deformation of MnO single crystals has been measured at 1400°C in both creep and constant-strain-rate tests. For oxygen partial pressures near 10 Pa, the flow stress is larger, or the steady-state strain rate is lower, in a CO 2 CO atmosphere than in an O 2 Ar atmosphere. This effect may be the first experimental evidence that carbon dissolution can affect behavior of dislocations in non-stoichiometric oxide single crystals.