Exchange Bias Effect in FeCo Nanoparticles

Abstract

The exchange bias fields in FeCo nanoparticles are investigated. A series of Fe1−x Co x ( x =0.3, 0.5, and 0.7) NPs were fabricated using the hydrothermal method. XRD studies determined that the structures of the first two NPs were body-centered cubic (BCC) while that of the third was of a disordered structure having a few small disordered BCC crystallites. SEM images of the first two showed that these nanocrystals have well-structured pyramid shapes with planar surfaces on them. FC and ZFC magnetization curves showed the temperature dependences of the magnetization of the first two FeCo NPs were different from that of the third FeCo NPs. The hysteresis loops measured at 300, 150, and 10 K showed horizontal shifts of several of the loops. The exchange bias fields at 10 K of the three FeCo NPs were small but in two of them, the Fe0.7Co0.3 and the Fe0.3Co0.7 NPs, the fields were large at 150 K. The exchange bas field in the first NPs remained large as the temperature was raised to 300 K. HE for the last FeCo NPs became small again at 300 K.