Towards a Potential Room-Temperature Ferrimagnetic Ferroelectric

Multiferroics with coupled magnetic and electric orders, although rare, hold potential for low-energy-consumption materials for logic and memory capable of electric-field control of magnetism. Barium hexaferrite (BaFe₁₂O₁₉), the most common refrigerator magnet, is predicted to gain electric polarization order at room temperature in addition to its robust ferrimagnetism under in-plane, biaxial, compressive strain [1]. The recent realization of single-crystal substrates of Sr_1.03Ga_10.81Mg_0.58Zr_0.58O₁₉ (SGMZ) [2], an insulator that is isostructural to BaFe₁₂O₁₉, enables straining BaFe₁₂O₁₉ as SGMZ has a ~1.1% smaller in-plane lattice constant. In addition to strain, to induce the ferroelectric state, an epitaxial bottom electrode is needed to control the electric state for this hexaferrite multiferroic candidate. SrCo₂Ru₄O₁₁ is a metallic ferromagnetic oxide [3], belongs to the same hexaferrite family as BaFe₁₂O₁₉, and has small (~0.3%) in-plane lattice mismatch to the SGMZ substrate. Consequently, a coherent SrCo₂Ru₄O₁₁ epitaxial thin film on the SGMZ substrate would be ideal for straining BaFe₁₂O₁₉ and serving as the bottom electrode of a metal-insulator-metal structure to test for ferroelectricity in this predicted strain-induced multiferroic.

Here we show that a 1.1% in-plane biaxial compressive strain from SGMZ substrates can be imposed on up to 27.5 nm thick BaFe₁₂O₁₉ films. The full width at half maximum (FWHM) of the X-ray diffraction rocking curve of the 0020 peak in w ranges from 0.006° to 0.009°, the smallest ever reported. Scanning transmission electron microscopy (STEM) multislice ptychography results on a commensurately strained film clearly show local electric polarization arising from the off-center displacement of Fe³⁺ ions in the trigonal bipyramid sites, locally breaking the mirror-plane symmetry perpendicular to the c-axis of BaFe₁₂O₁₉. Second harmonic generation (SHG) proves the symmetry breaking on a larger scale from 6/mmm (bulk paraelectric BaFe₁₂O₁₉) to 6mm (fully strained BaFe₁₂O₁₉). Commensurately strained BaFe₁₂O₁₉ containing 95% enriched Fe⁵⁷ has been synthesized. Mössbauer measurement will be made on this sample to quantify the splitting between the two sites of the iron cations in the trigonal bipyramid sites.

Films of SrCo₂Ru₄O₁₁ were grown by MBE on (0001) SGMZ substrates in an adsorption-controlled regime. With matching substrates and adsorption-controlled growth, we have grown epitaxial, fully strained SrCo₂Ru₄O₁₁ thin films with even lower resistivity than SrCo₂Ru₄O₁₁ single crystals [3]. Our next step is to grow a commensurately strained metal-insulator-metal BaFe₁₂O₁₉/SrCo₂Ru₄O₁₁/SGMZ stack to test for ferroelectricity.

[1] Wang, P. S., Xiang, H.J. Physical Review X 4, 011035 (2014).
[2] Guguschev, C. et al. Crystal Growth & Design 22 (4), 2557-2568 (2022).
[3] Shlyk, L. et al. Adv. Mater. 20, 1315–1320 (2008).