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In an antiferromagnetic (core)/ferromagnetic (shell) nanoparticle, the transition behaviors from negative (NEB) to positive exchange bias (PEB) at low temperature after field cooling are studied in detail. The NEB field may exhibit an oscillatory behavior due to the competition between interfacial coupling and cooling field. The critical cooling fields, at which the transitions occur, exhibit a logarithmic decrement with the decrease of interfacial coupling, but indicate a linear decrease with the decrease of antiferromagnetic coupling or with the further dilution in the antiferromagnetic core. With the further increase of cooling field, the PEB field increases linearly and finally levels off. Moreover, the weaker antiferromagnetic coupling may enhance the NEB field value, whereas the suppression of PEB may be observed by diluting the antiferromagnetic core. The magnetization reversal by coherent rotation strongly depends on the variation of the magnetic parameters, because its occurrence just needs to consume a lower additional energy, which is the main reason for these unique phenomena. We have shed new light on the microscopic origin of the peculiar magnetic properties in the nanoparticles with such an inverted magnetic structure.

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