Mechanisms of Exchange Bias with Multiferroic ${\mathrm{BiFeO}}_3$ Epitaxial Thin Films

We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in $\mathrm{CoFeB}/{\mathrm{BiFeO}}_3$ heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the ${\mathrm{BiFeO}}_3$ films, as expected from Malozemoff’s model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the ${\mathrm{BiFeO}}_3$ film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using ${\mathrm{BiFeO}}_3$.

Figure 1

(a), (b) Neutron diffraction scans close to the $[\frac{1}{2}\frac{1}{2}\frac{1}{2}]$ reflection for a 70 nm (001)- and a 300 nm (111)-oriented BFO films on STO, at 300 K. Magnetic field dependence of the magnetization of $\mathrm{CoFeB}/\mathrm{BFO}(70\mathrm{nm})$ (c), (d) and $\mathrm{CoFeB}/\mathrm{BFO}(70\mathrm{nm})/\mathrm{LSMO}$ samples (e), (f) grown on (001)-oriented STO (c), (e) and (111)-oriented STO (d), (f), at 300 K.

Figure 2

(a) Dependence of the exchange field on the inverse of the domain size for several $\sim 65\mathrm{nm}$ BFO films. (b) Thickness dependence of the exchange field for $\mathrm{CoFeB}/\mathrm{BFO}$ stacks grown on STO(001). The dotted lines are guides to the eye.

Figure 3

Polarized neutron reflectivity data measured in a saturating field of 1.2 T ($R_{++}$: squares; $R_{--}$: circles) fitted with a three-layer model (a) and a four-layer model (b). Introducing an interface layer of $\sim 2\mathrm{nm}$ carrying a moment of $1\pm 0.5{\mu }_B/\mathrm{f}.\mathrm{u}.$ significantly improves the quality of the fit for up-up reflectivity (see arrows).