Enhanced ferromagnetism in ferroelectric {\&} ultrathin ferromagnetic digital superlattices*

Electric control of magnetism has generated much activity due to both intellectual and technical interests. Epitaxial growth of transition-metal oxides with different ordering parameters offers unique platform to study magnetoelectric effect. In particular, quantum confinement of correlated electrons lead to interesting phenomena such as metal-insulator transition. In this work, we explore such effect in quantum confined regime by growing digital superlattices. We selected ferroelectric BaTiO$_{\mathrm{3\thinspace }}$(BTO) and La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3\thinspace }}$(LSMO) which exhibits ferromagnetic to paramagnetic transition at critical thickness. We explore how confined LSMO behaves when sandwiched by BTO. Superlattices are fabricated in formula [(BTO)$_{\mathrm{24}}$/(LSMO)$_{x}$]$_{y}$. BTO thickness is constant {\&} LSMO thickness are varied from 40u.c. to 2u.c. For all samples, total thickness of LSMO remains 40 u.c. Stable RHEED oscillations and sharp patterns ensure layer-by-layer growth and decent structural ordering. We find that LSMO has the ferromagnetic ground state down to 2 u.c. In addition, enhancement of ferromagnetism on ultrathin LSMO samples is observed compared to thicker films. Possible explanation involving interfacial magnetization will be discussed.