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.<br/> <br/>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 00<u>20</u> 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/<i>mmm</i> (bulk paraelectric BaFe₁₂O₁₉) to 6<i>mm</i> (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.<br/> <br/>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.<br/> <br/>[1] Wang, P. S., Xiang, H.J. <i>Physical Review X</i> <b>4</b>, 011035 (2014).<br/>[2] Guguschev, C. et al. <i>Crystal Growth & Design </i><b>22 (4)</b>, 2557-2568 (2022).<br/>[3] Shlyk, L. et al. <i>Adv. Mater. </i><b>20</b>, 1315–1320 (2008).