Magnetic phase transition and clustered state in Ca-doped lanthanum cobaltite and manganite with insulator ground states.

The transport and magnetic properties (ac linear and nonlinear (second and third orders) susceptibilities) are presented for La0.8Ca0.2MnO3 and La0.8Ca0.2CoO3 single crystals with insulator ground states. The ferromagnetic (FM) clusters with similar magnetic characteristics originate in the paramagnetic phases of both compounds below some temperature T(∗). At high temperatures the FM clusters arise at the preferable sites that can be attributed to the chemical inhomogeneities, their density being weakly T-dependent. On cooling a homogeneous nucleation of the FM clusters develops below a definite temperature T(#) that is characterized by a fast growth of their density. These two stages are observed in both compounds. At the third stage a coalescence of the FM clusters starts in the doped cobaltite, whereas in the manganite the development of matrix FM ordering occurs which changes a cluster's behavior. The indicated features support the common nature of the cluster state in the doped cobaltite and manganite. The difference in their evolution is a consequence of the different magnetic properties of the matrices in the manganite and cobaltite.

Electric field driven destabilization of the insulating state in nominally pure LaMnO3.

We report an electric field driven destabilization of the insulating state in nominally pure LaMnO3 single crystal with a moderate field which leads to a resistive state transition below 300 K. The transition is between the insulating state in LaMnO3 and a high resistance bad metallic state that has a temperature independent resistivity. The transition occurs at a threshold field (Eth) that shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves a large change in resistance, it softens on heating and is eventually absent above 280 K. When the Mn(4+) content is increased by Sr substitution up to x = 0.1, the observed transition, although observable in a certain temperature range, softens considerably. This observation has been explained as a bias driven percolation type transition between two co-existing phases, where the majority phase is a charge and orbitally ordered polaronic insulating phase and the minority phase is a bad metallic phase. The mobile fraction f of the bad metallic phase deduced from the experimental data follows an activated kinetics as f = fo(E)exp(-Δ/kBT) with the activation energy Δ ≈ 200 meV, and the pre-factor fo(E) is a strong function of the field that leads to a rapid enhancement of f on application of field, leading to the resistive state transition. We suggest likely scenarios for such co-existing phases in nominally pure LaMnO3 that can lead to the bias driven percolation type transition.

The x-ray magnetic circular dichroism spin sum rule for 3d4 systems: Mn3+ ions in colossal magnetoresistance manganites.

The colossal magnetoresistance manganites La(0.87±0.02)Sr(0.12±0.02)MnO(3+δ), La(0.78±0.02)Sr(0.17±0.02)MnO(3+δ), and La(0.66±0.02)Sr(0.36±0.02)MnO(3+δ) (δ close to 0) were investigated by using soft x-ray magnetic circular dichroism (XMCD) and magnetometry. Very good agreement between the values for the average Mn magnetic moments determined with these two methods was achieved by correcting the XMCD spin sum rule results by means of charge transfer multiplet calculations, which also suggest a charge transfer of ~50% for Mn(4+) and approximately equal to 30% for Mn(3+). The magnetic moment was found to be localized at the Mn ions for x = 0.17 and 0.36 at 80 K and for x = 0.12 in the temperature range from 80 to 300 K. We discuss our findings in the light of previously published data, confirming the validity of our approach.

Optical and magneto-optical properties of ferromagnetic La(1-x)Ba(x)MnO3 single crystals.

The optical and magneto-optical properties of ferromagnetic La(1-x)Ba(x)MnO(3) single crystals with x=0.15, 0.20 and 0.25 are studied. The components of the permittivity tensor are obtained by spectral ellipsometry techniques and transverse Kerr effect measurements. The Kerr effect spectra depend substantially on the Ba content. The plasma frequency is estimated. In the paramagnetic semiconductor state, the small polarons contribute to conductivity in La(0.85)Ba(0.15)MnO(3), in La(0.75)Ba(0.25)MnO(3) no evidence for polarons is found even in the semiconductor state. For La(0.85)Ba(0.15)MnO(3) (T(C)=214 K), the metallic phase is estimated to occupy less than 1% of the total volume at T=190 K. It is shown that in La(0.75)Ba(0.25)MnO(3), the energy gap vanishes and the metal-semiconductor transition occurs somewhat below T(C) rather than at T=T(C).

Electronic structure of A- and B-site doped lanthanum manganites: a combined X-ray spectroscopic study.

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.

Direct investigation of orbital ordering in a colossal magnetoresistance manganite by means of X-ray linear dichroism at the Mn L edge.

We have investigated for the first time the orbital ordering in a three-dimensional colossal magnetoresistance manganite, namely La(7/8)Sr(1/8)MnO3, by applying soft X-ray linear dichroism (XLD) to the Mn L edge. We found that the cooperative Jahn-Teller distorted orthorhombic phase, which is present at a temperature of 240 K, is probably accompanied by a predominantly cross type (x2 - z2)/(y2 - z2) orbital ordering. This result is discussed in the light of different exchange interaction models.

Giant volume magnetostriction and colossal magnetoresistance at room temperature in La(0.7)Ba(0.3)MnO(3).

Giant volume magnetostriction at room temperature is found for the first time in a La(0.7)Ba(0.3)MnO(3) single crystal, achieving 2.54 × 10(-4) in a magnetic field of 8.2 kOe. An even greater value of the volume magnetostriction, equal to 4 × 10(-4) in the same magnetic field, is observed at the Curie point T(C) = 310 K.Volume magnetostriction and magnetoresistance exhibit similar dependences on temperature and magnetic field in the T(C)-region, that is explained by the presence in this compound of a magnetic two-phase ferromagnetic-antiferromagnetic state due to strong s-d exchange.