Spin Dynamical Properties of the Layered Perovskite La1.2Sr1.8Mn2O7

Inelastic neutron-scattering measurements were performed on a single crystal of the layered colossal magnetoresistance (CMR) material La1.2Sr1.8Mn2O7 (Tc ~ 120K). We found that the spin wave dispersion is almost perfectly two-dimensional with the in-plane spin stiffness constant D ~ 151meVA. The value is similar to that of similarly doped La1-xSrxMnO3 though its Tc is three times higher, indicating a large renormalization due to low dimensionality. There exist two branches due to a coupling between layers within a double-layer. The out-of-plane coupling is about 30% of the in-plane coupling though the Mn-O bond lengths are similar.Comment: 3 pages, 3 figures J. Phys. Chem. Solids in pres

Interplay of the CE-type charge ordering and the A-type spin ordering in a half-doped bilayer manganite La{1}Sr{2}Mn{2}O{7}

We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7} exhibits CE-type charge-ordered and spin-ordered states below $T_{N, CO}^A = 210$ K and below $T_{N}^{CE} \sim 145$ K, respectively. However, the volume fraction of the CE-type ordering is relatively small, and the system is dominated by the A-type spin ordering. The coexistence of the two types of ordering is essential to understand its transport properties, and we argue that it can be viewed as an effective phase separation between the metallic $d(x^{2}-y^{2})$ orbital ordering and the charge-localized $d(3x^{2}-r^{2})/d(3y^{2}-r^{2})$ orbital ordering.Comment: 5 pages, 4 figures, submitted to Phys. Rev.

Temperature and Field Dependence of Magnetic Domains in La1.2_{1.2}Sr1.8_{1.8}Mn2_2O7_7

Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic Magnetic Force Microscopy, domain structures for the layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature T$_C$, and appear either via the application of a c-axis magnetic field, or by decreasing the temperature further. At temperatures well below T$_C$, new domain walls, stable at zero field, can be formed by the application of a c-axis field. Magnetic structures are seen also at temperatures above T$_C$: these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.Comment: 6 figure

Transport and magnetic properties of GdBaCo_{2}O_{5+x} single crystals: A cobalt oxide with square-lattice CoO_2 planes over a wide range of electron and hole doping

Single crystals of the layered perovskite GdBaCo_{2}O_{5+x} (GBCO) have been grown by the floating-zone method, and their transport, magnetic, and structural properties have been studied in detail over a wide range of oxygen contents. The obtained data are used to establish a rich phase diagram centered at the "parent'' compound GdBaCo_{2}O_{5.5} -- an insulator with Co ions in the 3+ state. An attractive feature of GBCO is that it allows a precise and continuous doping of CoO_{2} planes with either electrons or holes, spanning a wide range from the charge-ordered insulator at 50% electron doping (x=0) to the undoped band insulator (x=0.5), and further towards the heavily hole-doped metallic state. This continuous doping is clearly manifested in the behavior of thermoelectric power which exhibits a spectacular divergence with approaching x=0.5, where it reaches large absolute values and abruptly changes its sign. At low temperatures, the homogeneous distribution of doped carriers in GBCO becomes unstable, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation. We also find that throughout the composition range the magnetic behavior in GBCO is governed by a delicate balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions, which can be easily affected by temperature, doping, or magnetic field, bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon. An exceptionally strong uniaxial anisotropy of the Co spins, which dramatically simplifies the possible spin arrangements, together with the possibility of continuous ambipolar doping turn GBCO into a model system for studying the competing magnetic interactions, nanoscopic phase separation and accompanying magnetoresistance phenomena.Comment: 31 pages, 32 figures, submitted to Phys. Rev.

Novel stripe-type charge ordering in the metallic A-type antiferromagnet Pr{0.5}Sr{0.5}MnO{3}

We demonstrate that an A-type antiferromagnetic (AFM) state of Pr{0.5}Sr{0.5}MnO{3} exhibits a novel charge ordering which governs the transport property. This charge ordering is stripe-like, being characterized by a wave vector q ~ (0,0,0.3) with very anisotropic correlation parallel and perpendicular to the stripe direction. This charge ordering is specific to the manganites with relatively wide one-electron band width (W) which often exhibit a metallic A-type AFM state, and should be strictly distinguished from the CE-type checkerboard-like charge ordering which is commonly observed in manganites with narrower W such as La{1-x}Ca{x}MnO{3} and Pr{1-x}Ca{x}MnO{3}.Comment: REVTeX4, 5 pages, 4 figure

Spin and orbital ordering in double-layered manganites

We study theoretically the phase diagram of the double-layered perovskite manganites taking into account the orbital degeneracy, the strong Coulombic repulsion, and the coupling with the lattice deformation. Observed spin structural changes as the increased doping are explained in terms of the orbital ordering and the bond-length dependence of the hopping integral along $c$-axis. Temperature dependence of the neutron diffraction peak corresponding to the canting structure is also explained. Comparison with the 3D cubic system is made.Comment: 7 figure