Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO3 films

Eun-Mi Choi,Shukai Yu,Rui Wu,Ahmed Kursumovic,Bonan Zhu,Venkatraman Gopalan,Tuhin Maity,Yoonsang Park,Judith L Macmanus-Driscoll,Zenxhing Bi,Haiyan Wang,Ping Lu

doi:10.1038/s41467-020-16101-2

Abstract

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO3. Here, using 3D straining in nanocomposite films of (SmMnO3)0.5((Bi,Sm)2O3)0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with TC,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (−3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)2O3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO3 films. Also, while bulk SmMnO3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.

Highlights

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order
In type II orthorhombic rare-earth manganites (o-RMnO3) ferroelectricity is induced by inversionsymmetry breaking of the magnetic order through the Dzyaloshinskii-Moriya interaction3. o-RMnO3 has a very rich functional phase diagram due to a changing magnetic spin state, from an A-type antiferromagnet (A-AFM) to an E-type antiferromagnet (E-AFM) and electrically from paraelectric (PE) to ferroelectric (FE)[3]
Three different thin films were deposited by pulsed laser deposition (PLD) onto (001) SrTiO3 (STO) and Nb-doped SrTiO3 (Nb) doped (001) STO substrates: (a) 100 nm reference film of SMO, (b) 20 nm SMO0.5:BSO0.5 vertically aligned nanocomposite (VAN), and (c) 100 nm SMO0.5:BSO0.5 VAN

Summary

Introduction

The electric polarisation (P) in these materials is much smaller (P < 0.1 μC cm−2) compared to Bi-based FE originating from the ordering of lone pairs, even at very low temperature[1,4,5,6,7,8,9] For device applications such as energy efficient non-volatile random access memory (RAM) (whether ferroelectric RAM or multistate multiferroics RAM), a high P (>1 μC cm−2) is strongly desired at room temperature (RT) and above[10]. Achieving multiferroicity requires a delicate balance of bond angle and bond length tuning Such tuning has not been demonstrated to date, and there are no reports of RT ferroelectricity in o-RMnO313,14. Very uniform and high strain states can be engineered into the self-assembled VAN films[22]

Methods

Results

Conclusion