W. T. Hsieh

Two‐dimensional magnetic order in Pb2Sr2TbCu3O8

Neutron diffraction techniques have been used to investigate the magnetic ordering of the Tb ions in polycrystalline Pb2Sr2TbCu3O8. Significant magnetic correlations are found to develop below ∼10 K, and these correlations are two dimensional in nature, representative of the strongly anisotropic magnetic interactions in these layered materials. The correlation length increases with decreasing temperature, and long‐range order is observed to develop at TN≊5.5 K. However, the sawtooth Bragg profile for the scattering demonstrates that the long‐range order is two dimensional, rather than three dimensional, with a spin configuration where nearest‐neighbor spins are antiparallel. The scattering profile can be explained quantitatively by assuming long‐range order within the ab plane, with no significant correlations along the c axis to the lowest temperatures measured (1.36 K). Our results suggest that the system is well described by a two‐dimensional Ising model. The only other (pure) system where a crossover ...

Magnetic ordering of Pr in Pb2Sr2PrCu3O8

The magnetic ordering of the Pr ions in Pb2Sr2PrCu3O8 has been studied using neutron‐diffraction and ac‐suspectibility measurements. An imperfect three‐dimensional magnetic ordering of the Pr spins was observed at a temperature well below its Neel temperature of TN≊7 K. The magnetic intensities observed at T=1.4 K can be explained by assuming long‐range order in the ab plane with short‐range correlations, of correlation length ξc=20 A, along the c axis and a moment directed along the c‐axis direction. The magnetic ordering is 2D in nature, and the basic magnetic structure consists of nearest‐neighbor spins that are aligned antiparallel along all three crystallographic directions. The magnetic transition is also evident in the ac‐susceptibility versus temperature measurements, where a cusp that is typical of antiferromagnetic ordering is clearly observed, which matches the TN obtained by neutron diffraction.

Dependence of powder neutron scattering on the dimensionality of magnetic order

The peak profiles of coherent scattering obtained in powder diffraction experiments that reflect the dimensionality of the ordered system are discussed. The well known expression that generates the powder diffraction pattern of a two-dimensionally ordered system is generalized to include couplings along the third dimension. Attention is concentrated on the magnetic systems, and the interactions between the adjacent layers are allowed to be either ferromagnetic or antiferromagnetic. A two-dimensionally ordered system gives Bragg peaks with characteristic sawtooth profiles at the (hk) positions. As the correlations between the ordered layers develop, new Bragg peaks at the (khl) positions appear and their widths are closely related to the correlation length along the third axis. A finite correlation length gives a width which is broader than the instrumental resolution, and its intrinsic width is inversely proportional to the correlation length. In the limit for long-range correlation along the third axis, symmetric peaks with their widths consistent with the instrumental resolution are then obtained. Neutron magnetic diffraction patterns taken at low temperatures on various high-Tc oxides are used as examples to illustrate the expression obtained.

Cu spin reorientation in Tl(BaSr)PrCu2O7

The Cu spins in TlBa2PrCu2O7 order at TN≊370 K with a spin structure that is collinear and is characterized by the {1/21/21} wave vector, where the nearest neighbor spins are aligned antiparallel along all three crystallographic directions. If 50% of the Ba atoms are randomly replaced by the smaller Sr atoms to form Tl(BaSr)PrCu2O7, the TN of the Cu spins reduces to 350 K but the magnetic structure that forms below TN is the same. However, at T≊20 K the Cu spins undergo a change in structure, and the spin arrangement is then characterized by the {1/21/21/2} wave vector below T≊12 K. The ground state spin structure of the Cu ions in Tl(BaSr)PrCu2O7 is hence noncollinear, where the spin directions of the nearest neighbor Cu ions in the ab plane remain collinear and antiparallel while along the c axis they are orthogonal. These results demonstrate that the atoms in the BaO layers are also actively participating in the coupling between the Cu ions.

Superconductivity and magnetic ordering in Pb2Sr2PrCu3O8.01

Abstract Superconductivity as well as magnetic ordering of the Pr spin were observed in polycrystalline Pb2Sr2PrCu3O8.01. High resolution neutron profile refinement analysis shows that the sample is single phase with a nominal oxygen concentration of 8.01(2). The AC susceptibility measurements reveal diamagnetic responses, that indicate the onset of superconductivity, at temperatures below 60 K. and a DC applied magnetic field of 10 kG destroys the diamagnetic screening of the superconducting state. As the temperature is further reduced, the AC susceptibility shows an upturn at T = 20 K below which χAC increases with decreasing temperature, and then a cusp that is typical of antiferromagnetic ordering is evident at T = 5 K. The upturn in AC susceptibility corresponding to the magnetic ordering of Pr spins is clearly demonstrated by neutron diffraction measurements. The Pr spins are aligned antiparallel along all three crystallographic directions with TN ≈ 7 K, and the ordering is two-dimensional in nature.