Multiferroic Thin Films and Heterostructures
In the past decade intensive work was invested in multiferroic materials and in particular in BiFeO3, due to its high ferroelectric and antiferromagnetic ordering temperatures and large ferroelectric polarization. Moreover, it was demonstrated that in hybrid Co0.90Fe0.10/BiFeO3 heterostructures, an electric field-induced magnetization reversal could be obtained, opening up avenues for next-generation, low-energy consumption spintronics. The concept relies, however, on a particular configuration of stripes of two variants of 71° ferroelastic/ferroelectric domains in the BiFeO3 underlayer (see Fig. 1), whose electric field-induced switching leads to rotation of the antiferromagnetic domains. The latter are coupled to the ferromagnetic domains in the ferromagnetic CoFe top layer, resulting ultimately in reversal of the direction of CoFe magnetization. Hence, the following questions arise: how can we obtain this particular domain pattern in BiFeO3 in a controlled manner? How stable are these stripes of 71° ferroelectric domains upon repeated switching under electric field pulses?
We subjected epitaxial BiFeO3 films that exhibit stripe domains (Figure 1) to fatigue experiments. Metal contacts were evaporated through a shadow mask on top of epitaxial BiFeO3/SrRuO3 heterostructures grown on SrTiO3(100) crystals. The polarization was switched by applying electric field pulses perpendicular to the BiFeO3 film surface. After repeated switching cycles we removed the top metal electrodes by wet etching the metal and investigated the ferroelectric domains patterns in those areas, monitoring the changes in the domain patterns as a function of the number of switching pulses. The domain patterns were investigated by piezoresponse force microscopy (PFM), both on the pristine BiFeO3 film and on the switched areas where the capacitor had formerly been located.
Figure 2 summarizes the evolution of the in-plane stripe-like 71° domains upon repeated switching. The out-of-plane piezoelectric response (VPFM) and the in-plane (LPFM) signal of the pristine state of a BiFeO3 film are shown in Figs. 2(a) and (b), respectively. The evolution of the stripe domains after 5000 switching cycles at different frequencies is displayed in Fig. 2(c). At higher frequencies (10 kHz), the 71° domains become completely disordered after the 5000 switching events. At lower frequencies (0.1 kHz), the stripe domains preserve their morphology, but they grow almost twice as large laterally. Figure 2 (d) compares the evolution of the 71° ferroelectric stripe domains after switching the capacitor at 0.1 kHz from 100 to 10,000 times. Already after 100 switching processes, the domains increased in size and the size seems to stabilize after about 1000 repetitions of the switching. Another difference is that after more than 1000 switching events, the net polarization swaps direction with respect to the as-grown state.
Our results indicate that the out-of-plane electrical switching configuration of BiFeO3 capacitors leads to severe modifications of the domain patterns of the active film and therefore devices that rely on particular domain configurations may suffer degradation.
I. Vrejoiu, F. Johann*, A. Morelli*
*Max Planck Institute of Microstructure Physics, Halle, Germany