Epitaxial engineering of magnetoelectric multiferroics
Professor Darrell G Schlom, Cornell University, USA
When it comes to high temperature magnetoelectric multiferroics, LuFe2O4 stands out because it is reported to be simultaneously ferrimagnetic and ferroelectric at the highest temperature of any known material, 250 K. The multiferroic status of LuFe2O4 has, however, recently come into question. Nonetheless, we have found an adsorption-controlled regime in which single-phase epitaxial films of LuFe2O4 can be grown by molecular-beam epitaxy on (111) MgAl2O4, (111) MgO, and (0001) 6H-SiC substrates. These LuFe2O4 films exhibit ferrimagnetism below 240 K. A ferroelectric that is closely related to LuFe2O4 is the metastable hexagonal polymorph of LuFeO3. Although metastable, hexagonal LuFeO3 has been grown in thin film form by epitaxial stabilization. It is an improper structural ferroelectric, isostructural to YMnO3, and polar at room temperature. It should, therefore, display similar topologically protected cloverleaf domains. Additionally,below about 120 K hexagonal LuFeO3 orders antiferromagnetically in a pattern in which symmetry allows a slight canting of the spins giving rise to weak ferromagnetism. Our preliminary results on intergrown LuFeO3-LuFe2O4 samples suggest the tantalizing prospect of the existence of a room temperature ferrimagnetic ferroelectric in the LuFe2O4(LuFeO3)n homologous series.
J A Mundy, C M Brooks, H Das, Q Mao, T Heeg, C J Fennie, Cornell University, USA
D A Muller, Cornell University and Kavli Institute at Cornell for Nanoscale Science, USA
R Misra, L A Zhang, V Gopalan, Z-K Liu and P Schiffer, Penn State University, USA
W Zander and J Schubert, Peter Gruenberg Institut (PGI-9), Germany
B S Holinsworth, K R O’Neal, and J L Musfeldt, University of Tennessee, USA
Unraveling the complex phase evolution in highly-strained BiFeO3 thin films: thickness, temperature, and chemical-alloying evolution
Professor Lane Martin, University of Illinois Urbana-Champaign, USA
The parent structure of the multiferroic BiFeO3 is a rhombohedrally distorted perovskite structure, but this material is known to exhibit a strain-induced structural phase transition under large compressive strains to a nearly tetragonally-distorted perovskite phase. At a critical compressive stain level of ~4.5% so-called mixed-phase structures have been observed in which both the tetragonal- and rhombohedral-like phases coexist. It is in these mixed-phase films that reversible electric field induced strains between 4-5% have been reported. Recent studies suggest that such films exhibit an exotic structural evolution accompanied by exciting properties including enhanced electromechanical responses. In this presentation, I will discuss a number of intriguing aspects of these complex materials including the structural evolution of these films as a function of strain, thickness, and temperature. The discussion will include details of new phases of BiFeO3, the implications of these phases for the observed large electromechanical response, a model for the formation of these structures that builds upon the idea of a spinodal-modulated structure, and routes to stabilize these structures. We will examine the temperature- and thickness-dependent nanostructural evolution of the strain-induced phase boundaries in BiFeO3/LaAlO3 (001) heterostructures. It is observed that the fraction of the mixed-phase regions decreases with increasing temperature and that in 40 nm thick films, all evidence of the mixed-phase structure is removed by 300°C. Upon cooling the films, the mixed-phase structures are observed to return. We will discuss the possibility that in some films the mixed-phase structures form via a strain induced spinodal-instability and the resulting mixed-phase structures represent a strain-relaxation mechanism in these films. Details of the thermodynamic landscape and connections with other systems will be discussed. Furthermore, in films > 250 nm, a breakdown of this strain-stabilized metastable mixed-phase structure to non-epitaxial microcrystals of the parent rhombohedral structure of BiFeO3 is observed. By a thickness of 300 nm, the entire film is observed to have experienced epitaxial breakdown. We will discuss a proposed mechanism for this breakdown and present a proposed phase stability map as a function of strain and film thickness at the growth temperature. We will also investigate chemical alloying routes to further stabilize the mixed-phase structures to greater thicknesses and implications of the the mixed-phase structure of magnetoelectric coupling near room temperature.
A R Damodaran, C-W Liang, Q He, C-Y Peng, L Chang, Y-H Chu, L W Martin, Adv Mater 23, 3170 (2011).
A R Damodaran, S Lee, J Karthik, S MacLaren, L W Martin, Phys Rev B85, 024113 (2012).
A R Damodaran, E Breckenfeld, A Choquette, L W Martin, Appl Phys Lett100, 082904 (2012).
Strain-control of local magnetism in manganite films on barium titanate substrates
Dr Xavier Moya, University of Cambridge, UK
Magnetoelectric effects in composites of ferromagnetic alloys and piezoelectric langatate, langasite or quartz
Professor Gopalan Srinivasan, Oakland University, Rochester, Michigan, USA
Mechanical strain mediated magnetoelectric (ME) effects are studied in bilayers and trilayers of magnetostrictive permendur (P) and piezoelectric single-crystal lanthanum gallium tantalate (LGT), lanthanum gallium silicate (LGS) or quartz. It is shown that the ME voltage coefficient which is proportional to the ratio of the piezoelectric coupling coefficient to the permittivity, is higher in LGT, LGS or quart-based composites than for traditional ferroelectrics based ME composites. The piezoelectric LGT and LGS are free of ferroelectric hysteresis, pyroelectric effects and phase transitions up to 1450 0C and is of interest for ultrasensitive, high temperature magnetic sensors.