Enhanced flux pinning in Zr-doped MgB ₂ bulk superconductors prepared at ambient pressure

We prepared a series of Mg _1-xZr _xB ₂ bulk samples with different x values by the solid-state reaction at ambient pressure and investigated the effects of the Zr doping on phase compositions, microstructure, and flux pinning behavior. The results indicate that the a axis increases with the increase of the Zr doping and the c axis remains unchanged. ZrB ₂ phases form and a small amount of Zr atoms may substitute Mg in Zr-doped samples. Critical temperature drops with increasing the Zr doping. It is found that critical current density J _c can be significantly enhanced by the Zr doping and the best data are achieved in Mg _0.9Zr _0.1B ₂. For this sample, J _c values are remarkably improved to 1.83×10 ⁶A/cm ² in the self field and 5.51×10 ⁵A/cm ² in 1 T at 20 K. Also, the irreversibility field is increased by the Zr doping. The high density, fine grain size of MgB ₂, and small ZrB ₂ particles in Zr-doped samples may be responsible for the enhancement of J _c. Also, the field dependence of J _c at different temperatures was briefly discussed.