Enhanced flux pinning in HgBa₂CuO_4+x by neutron irradiation and its relationship to magnetic anisotropy

Our previous results indicate that neutron irradiation is an effective method for improving the pinning strength and critical current density of HgBa₂CuO_4+x, demonstrating that the mercury-based materials are not intrinsically limited to low current density. Here we expand our investigations of random HgBa₂CuO_4+x powders with a study of the effects of further neutron irradiation and annealing on the magnetization hysteresis and flux creep of powder samples. We also report the effects of irradiation on aligned powder samples and thus understand the relationship between flux pinning and magnetic anisotropy. We find that the critical current density continues to increase with increasing neutron fluence but at a decreasing rate. The irradiation enhancements in ground-then-aligned powders are not as large as in the as-sintered material, which we attribute to small defects introduced during the grinding process that inhibit the formation of defect clusters during irradiation. Lastly, we find that the neutron irradiation decreases and inverts the anisotropy as measured by magnetization hysteresis.