A. Prodi

Site Specific X-ray Anomalous Dispersion of the Geometrically Frustrated Kagomé Magnet, Herbertsmithite, ZnCu3(OH)6Cl2

Structural characterization, exploiting X-ray scattering differences at elemental absorption edges, is developed to quantitatively determine crystallographic site-specific metal disorder. We apply this technique to the problem of Zn-Cu chemical disorder in ZnCu(3)(OH)(6)Cl(2). This geometrically frustrated kagomé antiferromagnet is one of the best candidates for a spin-liquid ground state, but chemical disorder has been suggested as a mundane explanation for its magnetic properties. Using anomalous scattering at the Zn and Cu edges, we determine that there is no Zn occupation of the intralayer Cu sites within the kagomé layer; however there is Cu present on the Zn intersite, leading to a structural formula of (Zn(0.85)Cu(0.15))Cu(3)(OH)(6)Cl(2). The lack of Zn mixing onto the kagomé lattice sites lends support to the idea that the electronic ground state in ZnCu(3)(OH)(6)Cl(2) and its relatives is nontrivial.

Searching for stable Na-ordered phases in single-crystal samples of γ-NaxCOO2

We report on the preparation and characterization of single-crystal {gamma} phase Na{sub x}CoO{sub 2} with 0.25{<=}x{<=}0.84 using a nonaqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x<0.84), we find six distinct stable phases with Na levels corresponding to x{approx} 0.75, 0.71, 0.50, 0.43, 0.33, and 0.25. The composition with x{approx_equal}0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below x{sub c}{approx}0.53, where the diffusion process above x{sub c} has a diffusion coefficient about five times larger than that below x{sub c}. The series of crystals were studied with x-ray diffraction, susceptibility, and transport measurements. The crystal with x=0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T=51 and 88 K. The resistivity of the Curie-Weiss metallic Na{sub 0.71}CoO{sub 2} composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram.

Magnetoelectric coupling driven by inverse magnetostriction in multiferroic BiMn3Mn4O12

Inserting both polar A and magnetic B ions in a same crystalline phase, such as A = Bi3+, B = Fe3+ or Mn3+ in simple perovskites ABO(3), has been successful in achieving multiferroic properties with large ferroelectric and magnetic orders. However, modest magnetoelectric couplings have been hitherto reported, thus preventing any application for future electronics. By means of neutron diffraction, we found a large uniform C-type modulation of an E-type antiferromagnetic structure of the Mn3+ ions in the quadruple perovskite BiMn3Mn4O12. A symmetry analysis indicates that this modulation is induced by the internal strain created by the polar Bi3+ ion, which gives evidence of a large magnetoelectric coupling driven by inverse magnetostriction. This modulation is indeed absent in the isomorphic and isovalent compound LaMn3Mn4O12 containing the nonpolar La3+ ion. Our analysis indicates that this coupling mechanism is effective owing to the symmetry-limited structural distortions and inhomogeneities characteristic of the quadruple perovskite structure, thus preventing the release of the strain. We conclude that internal strain is a key control parameter to achieve large magnetoelectric couplings in proper ferroelectrics.

Synthesis and characterization of single crystals of the spin- 1 2 kagome-lattice antiferromagnets Zn x Cu 4 − x (OH) 6 Cl 2

The Zn-paratacamite family, Zn

Magnetic response of the CE structure in the doping-free system Na Mn 7 O 12

_{x}

Synthesis and Characterization of Single Crystal Samples of Spin-

Cu

1/2

_{4-x}

Kagome Lattice Antiferromagnets in the Zn-Paratacamite Family Zn

(OH)

_{x}

_{6}

Cu

Cl