Abstract Body

The Y-Ba-Cu-O family of high-T_c superconductors, including YBa₂Cu₃O_y (Y123), Y₂Ba₄Cu₇O_y (Y247), and YBa₂Cu₄O₈ (Y124), presents significant potential for the advancement of second-generation coated conductors. Among these, Y124 shows a stoichiometric composition and possesses a twin-free micro structure. Although T_c of Y124 is approximately 81 K, slightly lower than that of Y123, it can be substantially increased to around 90 K through Ca doping at the Y site [1]. However, in high-T_c cuprate superconductors, misorientation between grains can significantly reduce intergrain critical current density (J_c), making tri-axial grain alignment along the a, b, and c axes essential for optimizing transport properties. Our previous work has demonstrated a magnetic grain-orientation technique utilizing a modulated rotating magnetic field (MRF) [2], without using the epitaxial growth methods. This technique has proven effective for materials exhibiting tri-axial magnetic anisotropy. To advance this magneto-scientific technique towards practical applications, a comprehensive understanding of factors influencing magnetization axes and magnetic anisotropies is essential. Investigating twin-free Y124 offers valuable insights into the quantitative relationships between magnetic alignment techniques and superconducting properties. This study focuses on synthesizing and aligning (Y_1-xDy_x)Ba₂Cu₄O₈ [(Y_1-xDy_x)124], exploring the impact of Dysprosium (Dy) doping on magnetic anisotropy and grain orientation under various magnetic fields. Such investigations are crucial for enhancing our understanding of superconducting materials and advancing their potential applications in technological innovations.

Single crystals of (Y_1-xDy_x)124 with varying nominal Dy concentration levels, x=0,0.05, 0.1, 0.25, 0.50, 0.75, and 1, were grown using the flux method in ambient pressure, employing KOH as the flux medium [1]. Powder XRD measurements were carried out for synthesized (Y_1-xDy_x)124 samples. Nearly all diffraction peaks observed in the XRD patterns were recognized as reflections of the Y124 structure. This result indicates the use of the KOH flux method facilitates crystal growth in the (Y_1-xDy_x)124 phase, even in an ambient pressure. Pulverized (Y_1-xDy_x)124 microcrystals were mixed with Araldite Standard (η_init ~ 40 Pa s) epoxy resin in a weight ratio of 1:10 and aligned under MRFs of 1-10 T. The magnetization axes and degrees of orientation of the magnetically aligned (MA) powder samples of (Y_1-xDy_x)124 were determined from XRD and (017) pole figure measurements.

The XRD patterns of MA-(Y_1-xDy_x)124 revealed a notable enhancement in the (00l), (h00), and (0k0) peaks corresponding to the α, β, and γ planes under the MRF of 1-10 T, respectively. For (Y_1-xDy_x)124 (x = 0), the magnetization axes followed the order χ_c > χ_a > χ_b, and remained unchanged with change in x. Fig. 1 shows XRD patterns of α and γ planes of MA-(Y1-xDyx)124 samples under the MRF of 1T. As shown in Fig. 1, for (Y_1-xDy_x)124 (x = 0) at MRF of 1 T, alignment was achieved only along the c-axis, suggesting that the static component of the magnetic field was sufficient on its own, whereas the rotating component did not contribute adequately. This insufficiency might be attributed to either a lack of sufficient magnetic orientation energy or the necessity for an extended magnetic alignment period. Pole figure measurements at 10 T showed two-fold symmetric spots. MA-Y124 achieved strong in-plane orientation with a full width at half maximum (FWHM) of less than 4°. Even at 5 T, the sample maintained a high degree of in-plane alignment, with an FWHM under 5°, indicating that 5 T is adequate to generate the required magnetic orientation energy in Y124. Additionally, the degree of in-plane orientation increased progressively with higher x values. The changes in the in-plane orientation degree of MA-(Y_1-xDy_x)124 will be explored further in the presentation.

References

[1] Horii, S. et al., Supercond. Sci. Technol., 28, 105003 (2015).
[2] Horii, S. et al., Physica C supercond., 470, 1056 (2010).

Figure 1. XRD patterns at the (a) α and (b) γ planes for the (Y_1-xDy_x)124 powder samples aligned under MRF of 1 T

Keywords: Magnetic anisotropy, Tri-axial magnetic alignment, YBa₂Cu₄O₈