Field-dependent magnetization of $\mathrm{BiFe}{\mathrm{O}}_3$ in an ultrathin $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{BiFe}{\mathrm{O}}_3$ superlattice

We report the observation of field-dependent magnetization of $\mathrm{BiFe}{\mathrm{O}}_3$ (BFO) in an ultrathin $\mathrm{BFO}/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) superlattice using polarized neutron reflectivity (PNR). Our PNR results indicate parallel alignment of magnetization across BFO/LSMO interfaces. The study showed an increase in average magnetization of the BFO layer on increasing applied magnetic field at 10 K, similar to the ferromagnetic nature of LSMO. We obtained a saturation magnetization of $110\pm 15\mathrm{kA}/\mathrm{m}\left(\sim 0.8{\mu }_{\mathrm{B}}/\mathrm{Fe}\right)$ for an ultrathin BFO layer (∼2 unit cells) sandwiched between ultrathin LSMO layers (∼2 unit cells), which is much higher than the canted moment $(0.03{\mu }_{\mathrm{B}}/\mathrm{Fe})$ in the bulk BFO.

Figure 1

(a) Typical θ-2θ (out-of-plane) XRD pattern (in log axis) of the superlattice ${\left[{\left(\mathrm{LSMO}\right)}_2/{\left(\mathrm{BFO}\right)}_2\right]}_{32}$ on a (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrate showing a high-quality, epitaxial multilayer. Inset (i) shows a larger version of data around the (001) STO peak. Inset (ii) shows the rocking curve with FWHM of 0.0923° of BFO (002) diffraction peak. (b) X-ray reflectivity (XRR) from superlattice. Inset shows the depth profile of the electron scattering length density (ESLD) of the superlattice, which gives the best fit to XRR data.

Figure 2

(a) Magnetic hysteresis curve at 10 K of the superlattice ${\left[{\left(\mathrm{LSMO}\right)}_2/{\left(\mathrm{BFO}\right)}_2\right]}_{32}$. (b) Magnetization as a function of temperature for field-cooled (FC) at 1 kOe and zero-field-cooled (ZFC) conditions on warming the superlattice.

Figure 3

(a) Polarized neutron reflectivity (PNR) from superlattice ${\left[{\left(\mathrm{LSMO}\right)}_2/{\left(\mathrm{BFO}\right)}_2\right]}_{32}$ at 300 and 10 K under field-cooled (FC) condition in a field of 1 kOe. Reflectivity data at 300 K and 10 K are shifted by a factor of 5 for the sake of clarity. Normalized spin asymmetry (NSA) data at 300 K (b) and 10 K (c). (d) Nuclear scattering length density (SLD) and (e) magnetization ($M$) depth profile extracted from fitting PNR data at 300 K and 10 K.

Figure 4

(a)–(d) Normalized spin asymetry (NSA), $(R^{+}-R^{-})/R_{\mathrm{F}}$, data (PNR data) at different applied in-plane magnetic fields. (e) NSA data at 4.5 kOe with fits assuming different magnetization depth profiles. (f) Magnetization depth profiles across a bilayer of LSMO/BFO at different magnetic fields, which gave a best fit to NSA data (a)–(d).

Figure 5

Average magnetization of LSMO and BFO layers as a function of applied in-plane magnetic field obtained from PNR. Thickness-weighted magnetization and average magnetization for whole superlattice obtained from PNR (open square, black) and VSM (open star), respectively, as a function of magnetic field.