Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution to realize magnetoelectric coupling between ferromagnetic and ferroelectric order parameters. Despite having antiferromagnetic order, BiFeO3 (BFO) has nevertheless been a key material due to excellent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La0.7Sr0.3MnO3 (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO, an insulating superlattice, is demonstrated using polarized neutron reflectometry. Remarkably, the magnetization enables magnetic field to change the dielectric properties of the superlattice, which we cite as an example of synthetic magnetoelectric coupling. Importantly, controlled creation of magnetic moment in BFO is a much needed path toward design and implementation of integrated oxide devices for next generation magnetoelectric data storage platforms.

Fig. S1 Atomic resolution characterization in real space in real space was carried out by aberration-corrected scanning transmission electron microscopy (STEM). High angle annular dark-field (HAADF) Z-contrast imaging was combined with electron energy-loss spectroscopy (EELS). Observations were carried out in a Nion UltraSTEM200, equipped with a cold fieldemission electron source and a fifth-order corrector, operated at 200 kV. EEL spectra were collected using a Gatan Enfinium spectrometer, the convergence semi-angle for the incident probe was 30 mrad, with an EELS collection semi-angle of 48 mrad. The figures shows a highresolution HAADF image from the whole BFO/LSMO superstructure grown on a STO substrate. The green rectangle marks the area where an EEL spectrum image was acquired, with a dispersion of 0.25 eV per channel and a dwell time of 0.05 s per pixel. Together with the image there are EELS maps for the different elements. From left to right, La M 4,5 (green), Mn L 2,3 (red), Fe L 2,3 (blue) & Ti L 2,3 (magenta) maps together with the overlay.

Polarized neutron reflectometry
Polarized neutron reflectometry (PNR) measurements of the superlattice and thick BFO film samples were performed at the Spallation Neutron Source and the Lujan Neutron Scattering Center, respectively. Briefly, PNR involves measurement of the specular reflectivity, R, of the sample as a function of wave vector transfer Q = 4πsinθ/λ, where θ is angle of incidence between incident wavevector and its projection onto the sample's surface, and λ is neutron wavelength. The reflectivity is measured for polarized neutron beams with spin parallel (R + ) or opposite to (R -) the applied field. The reflectivity is related to square of the Fourier transform of the spin-dependent scattering length density (SLD) depth profile , where is the nuclear scattering length density depth profile, C = 2.911×10 -9 Å -2 m/kA, and M(z) is the magnetization (in kA/m) depth profile.
PNR measurements were made after first cooling the sample in a 5 kOe field to 10 K. The measurements were made as the sample was warmed to above ~170 K in the 5 kOe field. The measurements were fitted to a model in which the magnetizations of the BFO and LSMO layers were optimized to minimize the 2 metric for goodness of fit [P.R. Bevington D.K. Robinson, Data Reduction and Error Analysis in the Physical Sciences (3 rd ed. McGraw-Hill, New York) (2003) p. 65]. R ± (Q) were calculated using the dynamical formulism of Parratt [Parratt, L. G. Surface studies of solids by total reflection of x-rays. Phys. Rev. 95, 359-369 (1954)]. Error bars on the extracted magnetizations degrade the value of 2 by the number four (i.e., a 2-sigma error).

Capacitance measurement
Capacitance measurements were performed in a Quantum Design physical property measurement system using a home built probe at the national high magnetic field laboratory. AH 2500A, 1 kHz automatic capacitance bridge was used for accurate measurement of capacitance and loss. Magnetic field up to 12T was applied using the VSM.

Thin film growth
The composite targets used in this work were prepared by a conventional ceramic sintering process. PLD (KrF excimer laser, λ = 248 nm) was employed to grow nanocomposite films on STO (0 0 1) substrates. Substrate temperatures of 500-800 °C and suitable oxygen pressures of 10-350 mTorr were maintained during the film deposition. Following that, the samples were cooled down to room temperature in an oxygen-rich atmosphere (60-200 Torr) to ensure proper film stoichiometry.