Hysteresis anomalies and exchange bias in 6.6 nm CuO nanoparticles

In the 6.6 nm particles of CuO with Néel temperature TN≃40 K, hysteresis loops are observed up to 330 K. With decrease in temperature, the coercivity Hc increases slowly up to TN below which rapid increases in Hc and exchange bias He are observed. At 5 K, the ratio He/Hc≃2, decreasing to zero as T→TN. At 10 K, the variation of He, Hc, and the ratio He/Hc with the cooling field show behavior similar to that of the weak ferromagnetic moment resulting from the uncompensated surface spins.