Electron- and Hole-Doping Effects on AA-site Ordered NdBaMn2_2O6_6

We have investigated electron- and hole-doping effects on $A$-site ordered perovskite manganite NdBaMn$_2$O$_6$, which has the $A$-type (layered) antiferromagnetic (AFM) ground state. Electrons (holes) are introduced by partial substitution of Ba$^{2+}$ (Nd$^{3+}$) with Nd$^{3+}$ (Ba$^{2+}$). Electron-doping generates ferromagnetic (FM) clusters in the $A$-type AFM matrix. With increasing the electron-doping level, the volume fraction of the FM phase or the number of the FM clusters is abruptly increasing. In contrast, the $A$-type AFM phase is robust against the hole-doping, and no FM correlation is observed in the hole-doped NdBaMn$_2$O$_6$.Comment: 8 pages, 5 figures, to be published in Journal of the Physical Society of Japa

Resonant inelastic x-ray scattering study of hole-doped manganites La1-xSrxMnO3 (x=0.2 and 0.4)

Electronic excitations near the Fermi energy in the hole doped manganese oxides (La1-xSrxMnO3, x=0.2 and 0.4) have been elucidated by using the resonant inelastic x-ray scattering (RIXS) method. A doping effect in the strongly correlated electron systems has been observed for the first time. The scattering spectra show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the energy of the spectral weight shifts toward lower energies. The excitation spectra show little change in the momentum space as is in undoped LaMnO3, but the scattering intensities in the low energy excitations of x=0.2 are anisotropic as well as temperature dependent, which indicates a reminiscence of the orbital nature

Existence of orbital polarons in ferromagnetic insulating La1−x_{1-x}Srx_xMnO3_{3} (0.11<x<<x<0.14) evidenced by giant phonon softening

We present an inelastic light scattering study of single crystalline (La$_{1-y}$Pr$_y$)$_{1-x}$Sr$_{x}$MnO$_3$ ($0\leq x\leq0.14$,$y=0$ and $x=1/8$,$0\leq y\leq0.5$). A giant softening up to 20 - 30 cm$^{-1}$ of the Mn-O breathing mode has been observed only for the ferromagnetic insulating (FMI) samples ($0.11\leq x \leq 0.14$) upon cooling below the Curie temperature. With increasing Pr-doping the giant softening is gradually suppressed. This is attributed to a coupling of the breathing mode to orbital polarons which are present in the FMI phase.Comment: 4 pages, 5 figure

Charge and orbital ordering in underdoped La1-xSrxMnO3

We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0 < x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At x=1/8, several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous ferromagnetic state. The present calculation indicates that a ferromagnetic state with a charge modulation along the c-axis which is consistent with the experiment by Yamada et al. might be responsible for the anomalous behavior around x = 1/8.Comment: 5 pages, 5 figure

Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)

Comprehensive neutron-powder diffraction and Rietveld analyses were carried out to clarify the relation between the crystal and magnetic structures of La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6 octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic easy-axis at low temperature is parallel to the c axis. As the JT distortion becomes suppressed with increasing x, a planar ferromagnetic structure appears at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure at x =< 0.39. The canting angle between neighboring planes continuously increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with decrease of JT distortion can be ascribed to the change in the eg orbital state from d3z^2-r^2 to dx^2-y^2

Orbital polarons and ferromagnetic insulators in manganites

We argue that in lightly hole doped perovskite-type Mn oxides the holes (Mn$^{4+}$ sites) are surrounded by nearest neighbor Mn$^{3+}$ sites in which the occupied $3d$ orbitals have their lobes directed towards the central hole (Mn$^{4+}$) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn$^{4+}$) and Jahn-Teller type (Mn$^{3+}$) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may crystallize into a charge and orbitally ordered ferromagnetic insulating state.Comment: 5 pages, 4 figures, to be published in PR

Aspects of the FM Kondo Model: From Unbiased MC Simulations to Back-of-an-Envelope Explanations

Effective models are derived from the ferromagnetic Kondo lattice model with classical corespins, which greatly reduce the numerical effort. Results for these models are presented. They indicate that double exchange gives the correct order of magnitude and the correct doping dependence of the Curie temperature. Furthermore, we find that the jump in the particle density previously interpreted as phase separation is rather explained by ferromagnetic polarons.Comment: Proceedings of Wandlitz Days of Magnetism 200

Resonant X-ray Study on the Bi-Layered Perovskite Mn Oxide LaSr2Mn2O7

Charge and orbital ordering behaviors in the half doped bi-layered compound LaSr2Mn2O7 have been studied by resonant and non-resonant X-ray scattering. Three different order parameters, which correspond to the A-type antiferromagnetic, a charge and an orbital ordered states, were observed by measuring the magnetostriction and the superlattice peaks characterized by wavevectors (1/2 1/2 0) and (1/4 1/4 0), respectively. The superlattice reflections indicating the charge and orbital ordered states were observed below 210 K. Both the intensities reach a maximum at 160 K on cooling and become very weak below 100 K. The peak width of the charge ordered state agrees with that of the orbital ordered state at all temperatures studied. These results indicate that both the states originate from a single phase and that the charge/orbital ordered islands with definite interfaces disperse in the A-type antiferromagnetic phase. The dimensionality of the charge/orbital ordered phase is discussed using this model.Comment: 9pages, 10 figure

Observation of anomalous single-magnon scattering in half-metallic ferromagnets by chemical pressure control

Temperature variation of resistivity and specific heat have been measured for prototypical half-metallic ferromagnets, R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have found variation of the temperature scalings in the resistivity from T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in terms of the anomalous single-magnon scattering (AMS) process in the half-metallic system.Comment: To appear in Phys. Rev. Lett., 3 pages + 4 EPS figure

Unconventional one-magnon scattering resistivity in half metals

Low-temperature resistivity of half-metals is investigated. To date it has been discussed that the one-magnon scattering process in half-metals is irrelevant for low-temperature resistivity, due to the fully spin-polarized electronic structure at the ground state. If one takes into account the non-rigid-band behavior of the minority band due to spin fluctuations at finite temperatures, however, the unconventional one-magnon scattering process is shown to be most relevant and gives T^3 dependence in resistivity. This behavior may be used as a crucial test in the search for half-metallic materials which are potentially important for applications. Comparison with resistivity data of La_1-x Sr_x MnO_3 as candidates for half-metals shows good agreement.Comment: 4 pages, including 5 eps figures. To be published in J. Phys. Soc. Jpn. vol. 69 No. 7 (2000