Strain transfer in porous multiferroic composites of CoFe2O4 and PbZrxTi1−xO3

Abstract

This manuscript examines the mechanism of strain-coupling in a multiferroic composite of mesoporous cobalt ferrite (CFO), conformally filled with lead zirconate titanate (PZT). We find that when the composites are electrically poled, remanent strain from the piezoelectric PZT layer can be transferred to the magnetostrictive CFO layer. X-ray diffraction shows that this strain transfer is greatest in the most porous samples, in agreement with magnetometry measurements, which show the greatest change in sample saturation magnetization in the most porous samples. Strain analysis shows that porosity both accommodates greater lattice strain and mitigates the effects of substrate clamping in thin film strain-coupled composites.

Increased Magnetoelectric Coupling in Porous Nanocomposites of CoFe2O4 and BiFeO3 with Residual Porosity for Switchable Magnetic Devices

“…In the plane of the applied magnetic field, we saw no change in magnetization (Figure S9). This observation is in agreement with previous results on porous CFO-PZT nanocomposites and is likely due to substrate clamping. − …”

“…The magnetoelectric coefficients were calculated from the lowest electric field at which samples exhibited a saturated magnetization change (0.23 MV/m in all samples except the fully filled sample, which required a higher field of 0.71 MV/m to saturate). The magnetoelectric coefficient of the fully filled sample seemed to be on par with other reported BFO-CFO nanocomposites, which tend to be on the order of 10 –7 s m –1 . − However, for the sample with the most residual porosity, we calculate a large, out-of-plane magnetoelectric coefficient of 1.14 × 10 –6 s m –1 , an order of magnitude larger than dense multiferroic nanocomposites (both previously reported in the literature − ,− and seen in the fully filled sample described in this work). The magnetoelectric coefficients described here are likely an underestimate since samples were poled ex situ, and so, the saturation magnetization changes were observed at the remnant (not saturation) polarization state of the piezoelectric.…”

“…Unfilled cobalt ferrite thin films exhibit a saturation magnetization of about 340 emu/cc and a coercivity of approximately 800 Oe, which is close to the literature values in thin films. 38,39,41,53,54 The piezoelectric properties of the ALD BFO were also investigated using polarization−electric field (PE) curves (Figure S7b). Since the BFO layers in this work are very thin, analogous thickness, planar films of the same thickness would easily short.…”

“…Synthesis of porous CFO ,, and ALD deposition of BFO , have been discussed previously. A schematic of the synthesis can be found in Figure a.…”

“…In epitaxial composites, there is maximum contact between the ferroelectric and the magnetostrictive materials. This results in very high strain transfer between the two materials and has thus resulted in a wide range of systems that exhibit large magnetoelectric coupling. − However, one of the primary limitations in epitaxial films is that they suffer from substrate clampingthe thin composite film is still bound by the lattice constant of the substrate that the films are grown on. Porosity helps alleviate substrate clamping because porous structures are more mechanically flexible than their dense counterparts, which will make them easier to strain in magnetoelectric composites. − This theory was reinforced by previously performed finite element modeling on similar multiferroic composites with residual porosity .…”