The CMO crystal structure consists of double CaMnO 3 perovskite blocks stacked along the direction, with an extra CaO rocksalt sheet inserted after each double perovskite block. Ca 3Mn 2O 7 is an n = 2 member of the Ruddlesden Popper layered perovskite family with the general formula (ABO 3) n(AO) or A n+1B nO 3n+1. Hybrid improper ferroelectricity at room temperature was first experimentally confirmed in the n = 2 Ruddlesden-Popper material Ca 3Ti 2O 7 10 and was subsequently observed in Sr 3Sn 2O 7 and other layered perovskites 11, 12, 13.Ĭa 3Mn 2O 7 (CMO) is another hybrid improper ferroelectric candidate, which was the first theoretically proposed system for achieving room temperature multiferroicity 2. In particular, oxygen octahedron rotations and tilts, which are ubiquitous in perovskite materials and couple strongly to magnetism, combine with a layered crystal structure to induce polarization 8, 9. The principle of the hybrid improper ferroelectric mechanism is to induce ferroelectricity, ferromagnetism, and magnetoelectricity simultaneously with the same set of lattice instabilities 2, 7. Recently, a new class of materials, called hybrid improper ferroelectrics, was theoretically proposed as a potential room-temperature multiferroic system 2. However, identifying stable and single-phase multiferroic materials to achieve this objective has been challenging because electrical polarization and magnetization must be strongly coupled together in such systems 1, 2, 3, 4, 5, 6. The development of multiferroic materials aims to realize direct electric-field controlled switching of magnetization at room temperature 1, 2. These findings open the door to manipulating phase coexistence and achieving exotic properties in hybrid improper ferroelectric. Electron energy loss spectroscopy (EELS) studies coupled with first-principles calculations demonstrate that the stabilization mechanism of the db-PNRs is directly related to an Mn oxidation state change (from 4+ to 2+), which is linked to the presence of Mn antisite defects. In-situ TEM heating experiments show that the db-PNRs can exist up to 650 ☌.
In this work, we report the direct observation of double bilayer polar nanoregions (db-PNRs) in Ca 2.9Sr 0.1Mn 2O 7 using aberration-corrected scanning transmission electron microscopy (S/TEM). However, little is known about the atomic-scale structure of the polar/nonpolar phase coexistence or the underlying physics of its formation and transition. The layered perovskite Ca 3Mn 2O 7 (CMO) is a hybrid improper ferroelectric candidate proposed for room temperature multiferroicity, which also displays negative thermal expansion behavior due to a competition between coexisting polar and nonpolar phases.
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