Exchange coupling with the multiferroic compound BiFeO3 in antiferromagnetic multidomain films and single-domain crystals

The exchange coupling observed between a soft ferromagnetic layer and the antiferromagnetic multiferroic compound BiFeO3 (BFO) is investigated. Results obtained on BFO ferroelectric and antiferromagnetic multidomain films and monodomain single crystals are compared. A significant interface coupling occurs in the two systems whose anisotropy however differs significantly. In thin film based heterostructures, the measured twofold anisotropy of the FM layer imposed by the magnetic field during deposition is well accounted for using a double macrospin model describing the role of uncompensated spins, pinned or reversible, in the vicinity of the interface. In contrast, no macroscopic bias is observed in thin films deposited on BFO single crystals where the anisotropy direction is imposed by the underlying antiferromagnetic structure. This highlights the fundamental difference between exchange coupling with a single domain antiferromagnet and with a much more magnetically disordered multidomain state.