Inverse dependence of exchange bias and coercivity on cooling field caused by interfacial randomization in nanosystems with Co sparsely distributed in CoFe_(2)O_(4) matrix

We conceive a model of nanosystems where Co is uniformly,randomly distributed into a CoFe_(2)O_(4)(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(H_(E))and coercivity(H_(C))on Co/CFO interfacial coupling(J_(Co/CFO)),composition and cooling field(H_(FC)),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(T_(P)),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 J_(CFO) induces an elevated T_(P) and a reduced magnetization,while the opposite results are obtained by a higher Co occupation fraction(P_(Co)).On the other hand,H_(E) is nonmonotonic with J_(Co/CFO )and the peak decreases and shifts to a lower J_(Co/CFO)value for a larger P_(Co).H_(C) increases monotonically with increasing J_(Co/CFO).Moreover,H_(E) is inversely proportional to P_(Co) when PCo varies from 0.11 to 0.30,indicating the exchange bias is still an interfacial effect.Finally,for a nonzero J_(CFO),H_(E) may be suppressed and accompanied by the recovery of H_(C) with increasing H_(FC).Otherwise,H_(C) for zero J_(CFO) becomes high and insensitive to H_(FC).It is ascribed to the uniform contact between Co and CFO,leading to the Co magnetization reversal and the CFO magnetization precisely controlled by H_(FC).This work opens a new avenue to control the exchange bias in nanosystems through interfacial randomization.