Study On The Magnetoelectric Properties Of SrMnO₃ Thin Films

Designing novel multiferroic materials with strong ferroelectric and magnetoelectric coupling has been one of the focal points over the last decade due to the promising applications and rich physics involved.Despite the fact that conventional ferroelectric and magnetic orders have very different symmetric and chemical requirements,a rich amount of novel multiferroic materials and exotic multiferroic mechanisms have been discovered and developed in the last decade.Among variable approaches?e.g.Bi 6s lone pairs,spin ordering,superlattice,octahedral rotation,etc.?,strain engineering stands out as one of most effective and simple pathways to achieve the multiferroicity.Recently it was theoretically proposed that epitaxial strain could stabilize the polar state in antiferromagnetic-paraelectric SrMnO₃ and other manganite systems and lead to superior multiferroicity with higher transition temperature.In this study,using epitaxial strain?up to 3.8%?as a tuning knob,we successfully introduce novel multiferroicity with prominent high temperature ferroelectricity into the paraelectric SrMnO₃.More interestingly,the experimental temperature dependent ferroelectric and magnetic results suggest that the emergent antiferromagnetic order below 80 K greatly enhances the ferroelectric polarization due to the spin-order induced ions displacements.We envision that the strain mediated spin-phonon coupling can be utilized as a new pathway to discover novel phases and functionalities in a wide range of antiferromagnetic insulators with delicate epitaxial manipulations.Moreover,using ionic liquid gating and forming gas annealing methods,we demonstrate a distinct structural phase transformations within the SrMnO₃ thin films with the formation of oxygen vacancies.It is interesting to note that the newly formed phase shows a notable magnetism despite the fact that the SrMnO₃ is antiferromagnetic,which suggest a novel pathway to manipulate the magtic ground state?possibily to otain ferromagnetism?in the interesting SrMnO₃ system.Additionaly,we observe that the strained SrMnO₃ can relax the strain through the formation of oxygen vacancies.As a consequence,a maze-like domain structure can be identified from the topography of the film.More interestingly,the domain wall is highly conducting due to the accumlated oxygen vancancies depite the fact that the domain region?or the SrMnO₃ bulk?is insulting.Furthermore,the conductivity of domain can also be modified dramantically by controlling the oxygen vacancies distribution.This discovery provides an ideal platform for the study of correlation of electronic and magnetic properties with quantum confinement effect into the multiferroic model system.