As-prepared, single-crystalline bismuth ferrite nanoparticles show strong size-dependent magnetic properties that correlate with: (a) increased suppression of the known spiral spin structure (period length of approximately 62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Zero-field-cooled and field-cooled magnetization curves exhibit spin-glass freezing behavior due to a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes in our nanoparticle assemblies.

Previously we have presented evidence for stripe order of holes and spins in La1.6−xNd0.4SrxCuO4 with x = 0.12. Here we show, via neutron diffraction measurements of magnetic scattering, that similar order occurs in crystals with x = 0.15 and 0.20. Zero-field-cooled magnetization measurements show that all 3 compositions are also superconducting, with the superconducting transition temperature increasing as the low-temperature staggered magnetization decreases. 74.72.Dn,71.45.Lr,75.50.Ee,75.70.Kw Neutron scattering studies [1,2,3,4,5,6,7] of dynamical magnetic correlations in superconducting La 2−x Sr x CuO 4 have provided important clues to the nature of electronic correlations within the doped CuO 2 planes. The lowenergy magnetic scattering, which is characterized by the two-dimensional antiferromagnetic wavevector Q AF = ( [7]. In one common interpretation [8,9,10,11], the incommensurate peaks are viewed as the dynamical response of a spatially uniform electron liquid with a nearly-nested Fermi surface. From a rather different perspective, the Q-dependent structure is taken as evidence for spatial inhomogeneity associated with charge segregation [12,13,14] or charge-density-wave correlations [15,16,17,18]. Evidence for the latter picture is provided by our recent discovery [19,20] of incommensurate charge and spin order in La 1.6−x Nd 0.4 Sr x CuO 4 with x = 0.12; however, given the claim [21] that bulk superconductivity is absent at this composition, one might choose to argue that these results are not directly relevant to the case of superconducting samples.To test the relationship between charge-stripe order and superconductivity, we have now investigated two other Sr concentrations, x = 0.15 and 0.20. Our neutron diffraction measurements on single-crystal samples reveal elastic incommensurate magnetic peaks for both compositions, thus demonstrating the presence of chargestripe order. Since the x = 0.20 crystal was known to be superconducting from previous work [22], we decided to check the x = 0.12 and 0.15 crystals for superconductivity as well. To our surprise, zero-field-cooled susceptibility measurements exhibit a bulk shielding signal for all three compositions. Since both the incommensurate peak splitting, ǫ, and the superconducting transition temperature vary with x, the results strongly suggest a local coexistence of superconductivity and stripe order. The fact that T c decreases as the staggered magnetization increases indicates that these two types of order compete with one another [23]. Furthermore, since the variation of ǫ with x in the Nd-doped crystals is essentially identical to that obtained from recent inelastic measurements [7] on crystals of La 2−x Sr x CuO 4 , it seems inescapable that dynamical charge-stripe correlations are present in the optimally doped material.The crystals studied in this work were grown at the University of Tokyo using the traveling-solvent floatingzone method. The transport properties of the x = 0.12 and 0.20 compositions were reported several years ago [22...

High-resolution photoemission is used to study the electronic structure of the cuprate superconductor, Bi(2)Sr(2)CaCu(2)O(8+delta), as a function of hole doping and temperature. A kink observed in the band dispersion in the nodal line in the superconducting state is associated with coupling to a resonant mode observed in neutron scattering. From the measured real part of the self-energy it is possible to extract a coupling constant which is largest in the underdoped regime, then decreasing continuously into the overdoped regime.

La2-xBa x Cu04 has been found to undergo the following sequence of transformations upon cooling: tetr.(/4/mmm)-• ovi\\oXBmab) -• telvXP^i/ncm), over a range of composition 0.05 < x < 0.20. The newly discovered low-temperature tetragonal phase can be thought of as a coherent superposition of the twin-related Bmab structures. The system can be modeled as an A'F-spin system with temperaturedependent quartic anisotropy, v(T). Slight differences between orthorhombic and tetragonal structures appear to have large effects upon the superconductivity.

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The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 21 ͞Co 31 charge ordering occur as well as a change in the Co 21 spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering. We report here on structurally related LBaCo 2 O 51x materials, whose structures are derived from perovskites via ordering of the rare earth ͑L͒ and Ba cations into layers along c and removing oxygen exclusively from the L layer [6,7]. This creates an apically connected double layer of corner-sharing CoO 5 pyramids. For x . 0 the extra oxygen ions are incorporated into the L layer of LBaCo 2 O 51x to form disordered octahedra along the c axis. We have synthesized LBaCo 2 O 5.00 and studied the thermal evolution of its structure and properties using synchrotron x-ray [8] (Fig. 1) and the appearance of magnetic superstructure reflections in neutron powder diffraction data. Synchrotron x-ray powder diffraction measurements show that this magnetic transition occurs simultaneously with a tetragonal-to-orthorhombic ͑T -O͒ structural phase transition. Furthermore, as mentioned above, the minority tetragonal phase persists below the T -O transition. The two-phase coexistence was confirmed by ultra-highresolution diffraction measurements using a crystal analyzer. We stress that the observation of two phases in synchrotron x-ray powder diffraction experiments, with an order of magnitude better resolution than available with a standard laboratory x-ray diffractometer, can be traced to minute oxygen-content variations in the order of magnitude around x 0.01 [11]. In the presence of strong lattice coupling these small compositional variations can lead to phase coexistence, which can be observed in high-resolution experiments. This behavior beautifully illustrates how sensitive phase transitions within these systems are to minute variations in the oxygen stoichiometry.At room temperature there is no evidence for any long-range charge ordering. However, upon cooling long-range charge ordering is detected below 200 K, by the appearance of the ͑

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The complex interplay between superconducting and magnetic phases remains poorly understood. Here, we report on the phase separation of doped holes into separate magnetic and superconducting regions in superoxygenated La(2-x)Sr(x)CuO(4+y), with various Sr contents. Irrespective of Sr-doping, excess oxygen raises the superconducting onset to 40 K with a coexisting magnetic spin-density wave that also orders near 40 K in each of our samples. The magnetic region is closely related to the anomalous, 1/8-hole-doped magnetic versions of La(2)CuO(4), whereas the superconducting region is optimally doped. The two phases are probably the only truly stable ground states in this region of the phase diagram. This simple two-component system is a candidate for electronic phase separation in cuprate superconductors, and a key to understanding seemingly conflicting experimental observations.