Glassy nature of stripe ordering in La(1.6-x)Nd(0.4)Sr(x)CuO(4)

We present the results of neutron-scattering studies on various aspects of crystalline and magnetic structure in single crystals of La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12 and 0.15. In particular, we have reexamined the degree of stripe order in an x=0.12 sample. Measurements of the width for an elastic magnetic peak show that it saturates at a finite value below 30 K, corresponding to a spin-spin correlation length of 200 A. A model calculation indicates that the differing widths of magnetic and (previously reported) charge-order peaks, together with the lack of commensurability, can be consistently explained by disorder in the stripe spacing. Above 30 K, the width of the nominally elastic signal begins to increase. Interpreting the signal as critical scattering from slowly fluctuating spins, the temperature dependence of the width is consistent with renormalized classical behavior of a 2-dimensional anisotropic Heisenberg antiferromagnet. Inelastic scattering measurements show that incommensurate spin excitations survive at and above 50 K, where the elastic signal is neglible. We also report several results related to the LTO-to-LTT transition.Comment: 13 pp, 2-col. REVTeX, 11 figures embedded with psfig; expanded discussion of T-dep. of magnetic peak width; version to appear in Phys. Rev. B (01Jun99

The structural and magnetic properties of some transition metal compounds

SIGLEAvailable from British Library Document Supply Centre- DSC:D81505 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Robust antiferromagnetism and structural disorder in Bi_xCa_1-xFeO₃ perovskites

An investigation of Ca substitution in the multiferroic material BiFeO3 shows that a wide range of perovskites BixCa1-xFeO3 (0.4 &lt;= x &lt;= 1.0) can be prepared by sintering in air at 810-960 degrees C. 0.4 &lt;= x &lt; 0.8 samples are cubic Pm&lt;(3)over bar&gt;m, whereas x = 0.8 and 0.9 show a coexistence of the cubic and the rhombohedral R3c BiFeO3-type structure. Considerable disorder arising from Bi3+ lone-pair distortions is evidenced by synchrotron X-ray and neutron studies of the average structures of the cubic phases, and electron microscopy reveals commensurate and incommensurate local superstructures. The BixCa1-xFeO3 (0.4 &lt;= x &lt;= 1.0) materials show a remarkably robust antiferromagnetic order with T-N = 623-643 K and ordered moments of 3.6-4.1 mu(B). They are "leaky" dielectrics with relative permittivities of similar to 30-100 and bulk resistivities similar to 50-500 k Omega cm at room temperature. The activation energy for bulk conduction increases from 0.27 eV for x = 0.4 to 0.5 eV for x = 1, but with a discontinuity at the cubic-rhombohedral boundary. Further processing of the x = 0.8 and 0.9 compositions to reduce conductivity through control of oxygen content could lead to improved BiFeO3-based multiferroics.</p