Characterization of the metastable magnetic phase of Li_xNi_1-xO using non-linear susceptibility

Metastable magnetism is observed in two important compounds (those with x = 0.35 and x = 0.5) of the Li_xNi_1-xO series, with similar experimental features found in bulk susceptibility measurements. For each of these two compositions, a frequency dependence in the first-order alternating-current (ac) susceptibility (χ₁), a negative peak in the third-order ac susceptibility (χ₃), and a field-cooled/zero-field-cooled bifurcation in the direct-current magnetization were seen. We have used χ₃ to discern the nature of the metastability for these samples. We find that for x = 0.35, χ₃ becomes critical as a function of field (h), frequency (f), and temperature (T), confirming the existence of a spin-glass-like phase. However, χ₃ for x = 0.5 does not become critical as a function of h and f, and shows a -T^-3-dependence as predicted by Wohlfarth's model of superparamagnetism. Thus Li_xNi_1-xO shows magnetic phases of different types in two important composition ranges which are identified using χ₃. The results also indicate the importance of non-linear susceptibility for distinguishing the two metastable phases by means of bulk susceptibility measurements.