Robert Roth

Ferroelectric 180° Domain Wall Motion Controlled by Biaxial Strain

180° domain wall motion in a tetragonal ferroelectric oxide is accelerated by an order of magnitude using in situ strain in a force microscope. Single-domain PbZr0.2 Ti0.8 O3 films on piezoelectric (001)-oriented 0.72PbMg1/3 Nb2/3 O3 -0.28PbTiO3 substrates allow for direct investigation of strain-dependent domain dynamics. The strain effect depends on the sign of applied field through strain-dependent electrode built-in potentials and a suggested charging of tilted walls.

Strain induced low mechanical switching force in ultrathin PbZr0.2Ti0.8O3 films

Mechanical force has been found to be an alternative way to non-electrically switch the polarization of ultrathin ferroelectric films owing to the flexoelectric effect. Reducing the required force for switching is desirable for a low risk of damage to both sample and tip. Here, the strain dependence of mechanical threshold force has been studied in ultrathin PbZr0.2Ti0.8O3 films. The mechanical threshold force for polarization reversal reduces remarkably by a factor of ∼5 with decreasing the compressive strain, associated with a reduction of coercivity and tetragonality. We attributed such behavior to the reduction of switching barrier and remnant polarization. Our work provides a route to realize ultra-low mechanical writing force for non-volatile memory applications.

Thickness dependence of the magnetoelastic effect of CoFe2O4 films grown on piezoelectric substrates

Epitaxial CoFe2O4 (CFO) films of varying thickness were grown on piezoelectric Pb(Mg1∕3Nb2∕3)0.72Ti0.28O3 substrates. The magnetic anisotropy of the CFO films is controlled by the piezoelectric in-plane strain imposed by the substrate constraint during application of an electric field. We find that the strain-induced change of the remanent magnetization is constant at large thickness, but drops significantly below ∼100 nm. This thickness dependence of the magnetoelastic effect is shown not to be caused by a variation of the as-grown strain state.

Annealing control of magnetic anisotropy and phase separation in CoFe2O4-BaTiO3 nanocomposite films

Multiferroic heteroepitaxial nanocomposite films of BaTiO3 and CoFe2O4 (CFO) have been grown by pulsed laser deposition employing alternating ablation of two ceramic targets. Films grown at temperatures between 650 °C and 710 °C contain columnar CFO grains about 10–20 nm in diameter embedded in a BaTiO3 matrix. The very strong vertical compression of these grains causes large perpendicular magnetic anisotropy. Post-growth annealing treatments above the growth temperature gradually release the compression. This allows one to tune the stress-induced magnetic anisotropy. Additionally, annealing leads to substantial enhancement of the saturation magnetization MS. Since MS of a pure CFO film remains unchanged by a similar annealing procedure, MS is proposed to depend on the volume fraction of the obtained CFO phase. We suggest that MS can be utilized to monitor the degree of phase separation in nanocomposite films.

Strain-induced improvement of retention loss in PbZr0.2Ti0.8O3 films

The retention behavior of nanoscale domains in PbZr0.2Ti0.8O3 thin films is investigated by in-situ controlling the epitaxial strain arising from a piezoelectric substrate. The retention behavior in our sample shows strong polarity-dependence: Upward-poled domains exhibit excellent stability, whereas downward-poled domains reveal a stretched exponential decay. Reversible release of in-plane compressive strain strongly reduced the retention loss, reflected in an enhancement of the relaxation time by up to one order of magnitude. We tentatively attribute the observed behavior to a strain dependence of the built-in field at the interface to the La0.7Sr0.3MnO3 bottom electrode, with a possible further contribution of strain-dependent screening of the depolarizing field. Our work directly reveals the importance of epitaxial strain for reducing ferroelectric domain relaxation which is detrimental for applications such as nonvolatile memory devices.