Manipulation of Antiferromagnetism in Freestanding BiFeO₃ Membranes

BiFeO₃ (BFO) is arguably the only material that exhibits two order of parameters above room temperature with a Curie temperature of 1100 K and Nèel temperature of 653 K. Moreover, these two order parameters, ferroelectricity, and antiferromagnetism, are strongly coupled, known as magnetoelectric coupling. Owing to this magnetoelectric coupling, extremely low switching energy to alter the states of orderings can be realized by reducing the ferroelectric switching voltage as well as the ferroelectric polarization of BFO. Recent studies show that by optimizing the chemical composition of BFO, size scaling, and the removal of substrate clamping effect, the BFO ferroelectric switching voltage can be well improved[1]. Freestanding BFO membranes exhibit ~30% coercive voltage reduction and ~50% switching speed enhancement[2]. However, the methods to obtain large area of freestanding BFO films are still challenging as well as the understanding of its antiferromagnetic ordering. Here, we synthesized high-quality BFO epitaxial thin films on SrTiO₃ single crystal substrates by pulsed laser deposition, and optimized the freestanding process of BFO membranes by introducing various oxide buffer layers, such as SrRuO₃, and SrTiO₃, in between the sacrificing layer, La_0.7Sr_0.3MnO₃, and the BFO layer. Moreover, we used XMLD (X-ray Magnetic Linear Dichroism) to characterize the antiferromagnetic orientations of BFO membranes, we found that the antiferromagnetic axis of BFO is lifted from in-plane toward out-of-plane direction ~&lt;112&gt;_C, where C is noted as a cubic system, after the substrate was removed. We also notice that the tilting angle of the antiferromagnetic axis of BFO is strongly influenced by the thickness of the BFO layer and the buffer layer and that implies the antiferromagnetic axis degree of freedom is tunable by altering the elastic boundary conditions. Finally, we further conduct magnetotransport measurements to confirm whether the efficiency of magnetoelectric coupling can be improved after the freestanding process.<br/><br/>References:<br/>1. Manipatruni, S., Nikonov, D.E., Lin, CC. <i>et al.</i> Scalable energy-efficient magnetoelectric spin–orbit logic. <i>Nature</i> 565, 35–42 (2019).<br/>2. Shi, Q., Parsonnet, E., Cheng, X. <i>et al.</i> The role of lattice dynamics in ferroelectric switching. <i>Nat Commun</i> <b>13, </b>1110 (2022).