Papers by Author: Wolfgang Schäfer

Abstract: Evidence of the coexistence of an ordered antiferromagnetic and a spin glass state is given by studying a polycrystalline Tb₂CuIn₃ sample with neutron diffraction in a non-magnetic and a magnetic environment. Tb₂CuIn₃ is an antiferromagnet with competing exchange interactions (J₁> 0 for nearest neighbours and J₂ < 0 for next nearest neighbours) and an ordering temperature T_N =33 ± 1 K. Powder diffraction measurements reveal a broad diffuse peak - due to short-range magnetic order - underneath the (100) long-range order magnetic Bragg reflection. The exposure of the sample in magnetic fields up to 7.5 T, under both zero field and field-cooled conditions, show that neither the long-range magnetic order nor the short-range order are destroyed by the outer magnetic fields. The almost field independent behaviour of the magnetic intensities is explained by the existence of a spin glass state. Support on the simultaneous presence of antiferromagnetic and spin glass states is given by recent ac-dc magnetic measurements. The infinite range model for spin glasses with strong uniaxial anisotropy has predicted this kind of magnetic behaviour.

Abstract: Neutron powder diffraction on the binary intermetallics Er₅Mg₂₄ and Tm₅Mg₂₄ confirms their isostructural and pure crystallization in the Ti₅Re₂₄-type structure (space group I 4 3m, Z = 2)with rare earths located in 2a(0,0,0) and 8c(x,x,x) and Mg in two different 24g(x,y,x) sites. Room temperature lattice constants are 11.263(2) Å and 11.215(1) Å for the Er and Tm compound, respectively. Atomic positions have been refined. Both compounds order ferromagnetically below Curie temperatures of 17.5(5) K and 7.5(5) K for Er₅Mg₂₄ and Tm₅Mg₂₄, respectively. The magnitudes of the Er moments at 4.2 K are 7.5(2) µ_B and 4.4(2)µ_B on the 2a and the 8c sites, respectively. The Tm moments which have been refined from 2 K measurements amount to 3.0(3)µ_B and 2.8(2) µ_B, respectively. The temperature dependencies of the magnetic Bragg intensities reveal distinct deviations from Brillouin curves for J = 15/2 (Er) and J = 6 (Tm) systems and indicate a complex magnetic exchange.

Abstract: The longtime stability of copper textures has been periodically monitored over 12 years by neutron diffraction pole figure measurements on a cold rolled and a partially recrystallized copper specimen, always using the same diffractometer and the same procedure of data analysis. The texture component fractions of 'rolled' copper suffered a continuous decrease over about 8 years and seem to reverse since then. The decrease is accompanied by recrystallization from initial 3 to 34 vol. % at present. Texture changes in the partially recrystallized specimen are less pronounced. In this sample, the fraction of random orientations changed from initially 11 % to 31 % over the about first six years and followed by a decrease to 24 % at present.