Exchange bias effect revealed by irreversible structural transformation between the HCP and FCC structures of Cobalt nanoparticles

Abstract

In this study, the exchange bias effect (EBE) revealed by the HCP-FCC structural phase transformation in Cobalt nanoparticles (CoNPs) are modeled and investigated by using the Ising model within the framework of the Effective Field Theory developed by Kaneyoshi. CoNPs are mixed (indistinguishable HCP and FCC) phase at T < 0.36 and T > Tc-FCC. For the non-structural transformation, CoNPs exhibit no exchange bias effect (H-EB = 0.0) and they have normal symmetric coercive field points. But, they have asymmetric coercive field points and they have an exchange bias field for the HCP -> FCC transformation or the inverse (HCP <- FCC). Additionally, an intersection point (Hi) revealed by EBE is obtained between the irreversible M-H curve of the FCC and HCP phase (Hi = Hc1(FCC) = -0.39 and -0.13). Our results show that the exchange bias effect strongly depends on their irreversible structural phase transformations in the materials.