Switching phenomena in a new 90-K superconductor

Recently, Wu et al.¹ and Hor et al.² have shown that Y_1.2Ba_0.8CuO_4-δ is a superconductor with a superconducting onset temperature at ∼92 K as determined by their resistivity and a.c. susceptibility measurements. Because the magnetic properties are important in describing the nature of superconductivity, we have measured the d.c. magnetic moment of this material. Here we show that this material cooled in zero field or in a high field (H_cool > 90 G) is diamagnetic below T_cm ≈90 K, consistent with the previous measurements^1,2. However, when the sample is cooled in a small field (≤85 G), the magnetization, M, first becomes negative (diamagnetic) below T_cm, but further cooling results in a jump of M to a positive value at low temperature. We have also observed this switching by the application of an additional small field when the sample was cooled in a small field.