SI_Thickness dependence of LSMO_PZT

Thickness dependence of La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3
magnetoelectric interfaces
Jinling Zhou1, Vu Thanh Tra2, Shuai Dong3, Robbyn Trappen1, Matthew A. Marcus4, Catherine
Jenkins4, Charles Frye1, Evan Wolfe1, Ryan White5, Srinivas Polisetty1,6, Jiunn-Yuan Lin2,
James LeBeau7, Ying-Hao Chu8,9 and Mikel Barry Holcomb1,*
of Physics, National Chiao Tung University, 30010 HsinChu, Taiwan
of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
of Physics, Southeast University, 211189 Nanjing, China
Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Institute of Standards and Technology, Gaithersburg, MD 20899, USA
of Chemical Engineering and Material Science, University of Minnesota, Minneapolis, Minnesota, MN 55455, USA
of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA
of Materials Science and Engineering, National Chiao Tung University, 30010 HsinChu, Taiwan
of Physics, Academia Sinica, 105 Taipei, Taiwan
Supplementary Material
Film fabrication: The thickness gradients of the top LSMO and PZT layer were controlled by a
programmable shutter1 and the two wedges were grown in orthogonal directions by pulsed laser
deposition. The laser pulse was set at 1 J/cm2 at a repetition rate of 15 Hz for PZT and at 10Hz
for LSMO; the substrate temperature was 620 ο‚°C for PZT and 685 ο‚°C for LSMO. The oxygen
gas pressure was 100 mTorr throughout the growth of all layers. Before the growth of PZT, a 5
nm flat LSMO buffer layer was grown on the SrTiO3 (STO) (001) substrate and monitored by
reflection high energy electron diffraction2 to promote high quality epitaxial growth and to serve
as a bottom electrode. The thickness of each layer was not varied close to the edge of each
sample, leaving flat areas (at least 1 mm2) near the corners of the samples for calibration.
Sample characterization: Structural characterization was performed using x-ray diffraction
(XRD) with Cu Kα X-rays (λ = 0.1541 nm). Figure S1 displays typical XRD curves in blue and
red for two LSMO thicknesses of one monolayer and 10 nm, respectively, along a constant PZT
thickness. The STO peaks correspond to a lattice parameter of 3.905 Å. Because the lattice
parameter of LSMO is close to that of STO, the 10 nm LSMO exhibits as shoulders on the
primary and secondary STO peaks as indicated by blue arrows. The measured out-of-plane PZT
lattice parameter c = 4.270 Å differs from the bulk value c = 4.135Å,3 suggesting a reduced inplane lattice parameter, which is consistent with the epitaxial growth of PZT.4 Atomic force
microscopy (AFM) measurements were taken to map the topographic features of all samples
after growth. Figure S2 shows a typical AFM image of our LSMO/PZT heterostructures where
terraces are formed.5 As shown in figure S2, the width of the terrace is about 0.25 m with a root
mean square roughness of 0.266 nm for 5 nm thick LSMO on top of 200 nm PZT. Microfluorescence element maps were taken at beamline 10.3.2 at the Advanced Light Source of
Lawrence Berkeley National Laboratory to verify the overall wedge quality and to determine the
layers’ thickness at the measured locations. A typical fluorescence map above the Pb L3-edge is
shown in figure S3. For optically thin films, the intensity of the collected fluorescence signal is
proportional to the column density of the specific element. Therefore figure S3 confirms the PZT
wedge shape and the gradual increase in thickness as Pb is only present in the PZT layer. A
typical PFM image for tPZT ~ 20 nm is shown in the figure S4. The area inside the blue box was
poled upwards and the black inner box was poled afterwards in the opposite direction. The
unpoled area outside the blue box shows similar contrast to areas with upward polarization,
indicating a natural upward polarization direction of thin PZT which might be due to the
presence of an n-type interface in our films at thin PZT locations.6 Cross-sectional scanning
transmission electron microscopy (STEM) was performed using a probe corrected FEI Titan 60300 S/TEM equipped with a high brightness field emission gun (X-FEG) and a Super-X energy
dispersive X-ray (EDX) spectrometer. A high-angle annular dark-field image (HAADF) STEM
image across the STO/LSMO/PZT region is shown in figure S5, demonstrating excellent
interface quality. Correspondingly, the atomic resolution EDX maps of the cations are shown in
the lower images. The abrupt elemental distributions indicate chemically sharp interfaces.
Double exchange model simulation: The simulated lattice is 4x4xN (N = 10 for 4 nm LSMO),
under twisted boundary conditions within the 4x4 a-b plane (k-mesh: 15x15) and open boundary
conditions along the c-axis. The usage of twisted boundary conditions in-plane is important to
reduce the artificial finite-size effects.7 A proper dielectric constant (ο₯r ~ 45) and an ideal
ferromagnetic background8 were adopted for LSMO.
TEY data treatment: Mn valences for TEY spectra are determined using L3/L2 ratio method.
The L3 and L2 peaks are first separated from the raw spectra by subtracting a linear pre-edge
π‘“π‘π‘Ÿπ‘’−𝑒𝑑𝑔𝑒 (𝐸) = π‘ŽπΈ + 𝑏
and an arctangent term9:
π‘“π‘Žπ‘Ÿπ‘π‘‘π‘Žπ‘› (𝐸) = β„Ž1 βˆ™ [
arctan(πœ‹ βˆ™ (𝐸 − 𝐸1 )) 1
arctan(πœ‹ βˆ™ (𝐸 − 𝐸2 )) 1
+ ] + β„Ž2 βˆ™ [
+ ]
where E is the incident x-ray energy, a and b are the slope and interception of pre-edge, h1 and h2
are the height differences between the minima of the L2 and L3 peaks, E1 and E2 are energies
where the first maximum of 𝑑𝐼 ⁄𝑑𝐸 take place. The peak intensities are obtained by integrating
the resulting L3 and L2 peaks. Other methods such as linear combinations were also completed
and showed similar results as the L3/L2 methods. The peak features of our spectra are compared
to the reference spectra and confirm the validity of our valence estimation.
Fluorescence data treatment: For LSMO thickness dependence, the LSMO buffer
contributions were measured at the edge of the double wedge samples where no top LSMO is
present and they were subtracted from all XAS fluorescence spectra for a direct comparison to
the results from surface sensitive TEY results. The linear combination method was used to
estimate the valence. Totally eighteen spectra were used as references. Two of them were from
dichotic LSMO bulk sample. Figure S6 shows a representative fitting curve of a buffer
subtracted spectrum, which was taken at tLSMO = 10 nm. For the demonstrated figure of PZT
thickness dependent Mn valence measurements in the main article, the contributions from the
buffer LSMO layer were not subtracted.
Proof of interfacial magnetoelectric coupling: A remnant shift in the Mn valence was observed
after electrically poling the sample, which indicates the magnetoelectric coupling. Arrays of
electrodes with a diameter of 50 m were grown on a PZT/LSMO/Si double wedge sample
where a PZT wedge is on top of a LSMO wedge. External electric fields were applied to selected
electrodes in two opposite directions and the sample was measured after each poling. Mn valence
at tPZT = 80 nm with a poling voltage of 2.4 V are shown in figure S7. The Mn valence shift to a
larger value when the field was applied upwards on these electrodes. This shift gets smaller with
thicker LSMO, likely due to the presence of more dominate bulk regions which are not affected
by the PZT polarization and support the bulk behavior (in other words, having an increased bulk
region reduces the effect of the interface, similar to smaller magnetic dead layers seen in thick
films). The results are consistent with observations reported on known similar magnetoelectric
bilayers10 and indicate an interfacial magnetoelectric coupling in our systems.
Supplementary Figures
Figure S1, XRD spectra at two LSMO thicknesses consistent with the epitaxial growth of
LSMO and PZT on STO substrate.
Figure S2, AFM image for 5 nm LSMO on 200 nm PZT with a root mean square roughness of
0.266 nm.
Figure S3, Micro-fluorescence map of element Pb demonstrates smooth increase of PZT wedge.
Similar maps were observed for the LSMO wedges.
Figure S4, PFM image for 20nm PZT. The area inside the black box was poled downwards, then
the area between the black and blue box was poled upwards. The unpoled area beyond the blue
box shows similar contrast to areas poled upward which indicates a natural upward polarization
direction for thin PZT films.
Figure S5, Element-specific electron microscopy images of our wedged sample demonstrate
good sample quality and a sharp interface.
Figure S6, Best fit for a buffer subtracted FL spectrum at tLSMO= 10 nm, which demonstrates
excellent matching to the K-edge structure with small residuals.
Figure S7, Mn valence after PZT is poled in two opposite directions indicates a larger change in
valence for thinner LSMO films.
*Corresponding author:
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