Inverse dependence of exchange bias and coercivity on cooling field caused by interfacial randomization in nanosystems with Co sparsely distributed in CoFe2O4 matrix

Abstract We conceive a model of nanosystems where Co is uniformly, randomly distributed into a CoFe2O4 (CFO) matrix, and study the influences of composition and CFO antiferromagnetic exchange coupling (JCFO) on zero-field-cooled/field-cooled magnetization behavior and the dependencies of exchange bias field (HE) and coercivity (HC) on Co/CFO interfacial coupling (JCo/CFO), composition and cooling field (HFC), based on a modified Monte Carlo method. With decreasing temperature, the zero-field-cooled and field-cooled magnetization curves are overlapped and increasing initially, and then splitting at a critical temperature (TP), below which the field-cooled magnetization continues increasing while the zero-field-cooled magnetization exhibits a peak and drops, indicating a magnetically frozen state. A larger JCFO induces an elevated TP and a reduced magnetization, while the opposite results are obtained by a higher Co occupation fraction (PCo). On the other hand, HE is nonmonotonic with JCo/CFO and the peak decreases and shifts to a lower JCo/CFO value for a larger PCo. HC increases monotonically with increasing JCo/CFO. Moreover, HE is inversely proportional to PCo when PCo varies from 0.11 to 0.30, indicating the exchange bias is still an interfacial effect. Finally, for a nonzero JCFO, HE may be suppressed and accompanied by the recovery of HC with increasing HFC. Otherwise, HC for zero JCFO becomes high and insensitive to HFC. It is ascribed to the uniform contact between Co and CFO, leading to the Co magnetization reversal and the CFO magnetization precisely controlled by HFC. This work opens a new avenue to control the exchange bias in nanosystems through interfacial randomization.