BiFeO3 thickness dependence of the exchange bias in polycrystalline BiFeO3/NiFe bilayers

Abstract

A clear exchange bias has been observed at room temperature in a series of BiFeO<inf>3</inf>(t)/NiFe(3.6 nm) bilayers, in which the BiFeO<inf>3</inf> layers are single-phase polycrystalline with t varied from 8 nm to 240 nm. With increasing t, both the exchange bias field (H <inf>E</inf>) and the coercivity (H <inf>C</inf>) increase sharply and approach maxima when t is about 40 nm, which is close to one half the spin cycloidal modulation period (64 nm) of the bulk BiFeO<inf>3</inf> material. The oscillatory variations of H <inf>E</inf> and H <inf>C</inf> with the BiFeO<inf>3</inf> layer thickness suggest that the cycloidal spin structure may exist in polycrystalline BiFeO<inf>3</inf> thin films. The angular dependence of the exchange bias exhibits collinear unidirectional and uniaxial anisotropies in the present BiFeO<inf>3</inf>/NiFe bilayers. © 2013 The Korean Physical Society.