Mechanical and high-temperature (920 °C) magnetic field grain alignment of polycrystalline (Ho,Y)Ba2Cu3O7−δ

Abstract

The effects of uniaxial compression, high‐temperature (920 °C) heat treatment, and heat treatment plus magnetic field on grain alignment in bulk ceramic pellets of HoBa₂Cu₃O_7−δ and YBa₂Cu₃O_7−δ were studied and separated. Uniaxially cold pressing prereacted powder into pellets at room temperature is found to yield significant grain alignment on the flat end surfaces of the pellets perpendicular to the pressing axis, where the c axes of the grains are parallel to this axis, in agreement with previous reports. A simple method to quantitatively determine the degree of surface grain alignment from the powder x‐ray diffraction data from these surfaces is used. These data were augmented by x‐ray rocking curve measurements. The degree of alignment of the cold pressed samples was found to increase with increasing pressure up to our pressure limit of 310 MPa. However, bulk magnetization anisotropy measurements on these pellets indicate that the degree of bulk alignment is only about 1/3 to 1/2 that inferred from the above surface measurements, which in turn suggests that the degree of alignment decreases with distance from the flat end surfaces. We find that a pronounced additional surface grain alignment is produced by heat treating the cold‐pressed samples at high temperature (920 °C). We also observed a significant influence during the high‐temperature annealing of a small (0.7 T) applied magnetic field on the alignment of the surface grains. A model to explain these observations is presented and discussed.