||Lei Fang, JuneHyuk Lee, E. Vlahos, X. Ke, Y. W. Jung, L. Fitting Kourkoutis, P. Ryan, J. W. Freeland, T. Heeg, M. Roeckerath, V. Goian, M. Bernhagen, R. Uecker, C. Hammel, K. M. Rabe, S. Kamba, J. Schubert, D. A. Muller, C. J. Fennie, V. Gopalan, P. Schiffer, D. G. Schlom, Ezekiel Johnston-Halperin
Recently biaxial strain has been predicted to induce a multiferroic ground state in EuTiO<sub>3</sub> and change its normally paraelectric and antiferromagnetic ground state into a state that is simultaneously ferromagnetic and ferroelectric. This multiferroic state is predicted to be a strong ferromagnet and also a strong ferroelectric. To assess these predictions that would establish EuTiO<sub>3</sub> as the world’s strongest ferromagnetic ferroelectric, epitaxial EuTiO<sub>3</sub> thin films are grown on (001) SrTiO<sub>3</sub>,(001) (LaAlO<sub>3</sub>)0.29—(SrAl1/2Ta1/2O<sub>3</sub>)0.71 (LSAT) and (110) DyScO<sub>3</sub> substrates by reactive molecular-beam epitaxy (MBE). Testing for ferromagnetism in the strained EuTiO<sub>3</sub> films is complicated by the large paramagnetic response of the DyScO<sub>3</sub> substrate. If a superconducting quantum interference device (SQUID) magnetometer is used under typical measurement magnetic fields, the paramagnetic response of the thick substrate swamps the signal from the strained EuTiO<sub>3</sub> film. For this reason magneto-optic Kerr effect (MOKE) is used to measure the magnetization of the strained films. Longitudinal continuous-wave (CW) MOKE geometry is used to measure the in-plane magnetization in a spectral window (690 nm to 750 nm) that is sensitive to the EuTiO<sub>3</sub> epilayer but not the DyScO<sub>3</sub> substrate. The MOKE response from the strained EuTiO<sub>3</sub> film exhibits a clear ferromagnetic hysterisis loop, with sharp switching to full saturation. The temperature dependence reveals a Curie temperature (TC) of 4.3 K. In comparison, unstrained EuTiO<sub>3</sub> films grown on SrTiO<sub>3</sub> and LSAT substrates were also measured and no ferromagnetic feature was observed for the control samples at all temperatures. Optical second harmonic generation (SHG) reveals that the unstrained EuTiO<sub>3</sub> films are not polar, as expected, but that the strained EuTiO<sub>3</sub>/DyScO<sub>3</sub> passes through a phase transition at about 250 K to polar point group mm2, in agreement with theory, and in this state domain switching by electric fields is observed. Our results thus establish that strain induces a strong multiferroic state in EuTiO<sub>3</sub> in agreement with predictions.