A Strain-Enabled Novel Multiferroic State in Barium Hexaferrite through Suppression of Quantum Paraelectricity

Barium hexaferrite (BaFe₁₂O₁₉) is predicted to harbor a novel multiferroic phase within which frustrated antiferroelectricity and ferrimagnetism coexist. However, experimental studies have shown that in bulk barium hexaferrite, quantum fluctuations instead lead to a quantum paraelectric ground state at low temperature [1-2]. Strain is a popular method for tuning functional properties in epitaxial thin films and has previously been used to coax ferroic electric dipole order, for example in SrTiO₃ [3]. Similarly, recent theoretical exploration has suggested that strain may enhance antiferroelectricity in barium hexaferrite [4]. In this theoretical work, we combine first-principles calculations with quantum Monte Carlo simulations to explore the temperature- and strain-dependent properties of barium hexaferrite. We find good agreement between our model and available bulk experimental data. In the investigation of strained barium hexaferrite, we find that a modest compressive strain of approximately 1% enables the transition to a frustrated antiferroelectric phase with a critical temperature of greater than 10 K. Our results suggest that further investment in epitaxy and development of compressive substrates for hexaferrites may provide a promising route towards room temperature multiferroics.<br/><br/>[1] Shen et al. <i>Phys. Rev. B</i> <b>90</b>, 180404(R) (2014)<br/>[2] Zhang et al. <i>Phys. Rev. B</i> <b>101</b>, 104102 (2020)<br/>[3] Haeni et al. <i>Nature</i> <b>430</b> 758–761 (2004)<br/>[4] Wang et al. <i>Phys. Rev. X</i> <b>4</b>, 011035 (2014)