A radical approach to promote multiferroic coupling in double perovskites

Abstract

Double perovskites provide a unique opportunity to induce and control multiferroic behaviours in oxide systems. The appealing possibility to design materials with a strong coupling between the magnetization and the polarization fields may be achieved in this family since these magnetic insulators can present structural self-ordering in the appropriate growth conditions. We have studied the functional properties of La₂CoMnO₆ and Bi₂CoMnO₆ epitaxial thin films grown by pulsed laser deposition. Cation-ordered La₂CoMnO₆ films display a magnetic Curie temperature of 250 K while cation-disordered Bi₂CoMnO₆ films present ferromagnetism up to ∼800 K. Such high transition temperature for magnetic ordering can be further tuned by varying the strain in the films indicating an important contribution from the structural characteristics of the materials. Our approach might be generalized for other oxide systems. At this end, our results are compared with other multiferroic systems. The roles of various cations, their arrangements and structural effects are further discussed.

Introduction

Multiferroics form a surprising class of materials displaying simultaneously magnetism, ferroelectricity, and/or ferroelasticity in a single phase [1], [2], [3]. The coexistence of various electronic order parameters that can interact in a given system opens new opportunities to design unconventional spintronic devices and also poses new challenges for physics, chemistry, and technology [1], [2], [3]. However, their applicability depends on the nature of the actual coupling between the electronic, magnetic, and lattice order parameters. In this context, there are various stumbling blocks. The most challenging ones are how we can couple directly their molecular magnetic and electric fields responsible for ferromagnetism and ferroelectricity, and control eventually this coupling? Moreover, if this intimate coupling is achieved, how important is the material's response to an external applied field? Answers to these questions are hence crucial for understanding the microscopic mechanism behind the ferroic coupling and for their applicability in real devices.

In this context, double perovskites A₂B′B″O₆ (where B′ has partially filled e_g-orbitals while B″ has empty e_g-orbitals or vice-versa) provide a unique opportunity to promote strong multiferroic behaviours, because they are ferromagnetic insulators governed by the 180° superexchange process [4] with a potential to generate a polar character. In fact, the polar nature can be obtained if one can promote different chemical valencies for the B′ and B″ cations in a self-ordered phase with B′ and B″ alternating along specific crystallographic directions (i.e. with a B′–O–B″–O–B′ sequence). Of course, such effect is absent in a disordered phase. For example, it has recently been shown that the ordered La₂NiMnO₆ have a Ni²⁺/Mn⁴⁺ cationic configuration leading to large dielectric constant [5] whereas the disordered phase has a Ni³⁺/Mn³⁺ configuration [5], [6], [7]. An alternative path to induce a polar character in double perovskites may be achieved by choosing an appropriate A-site cation (e.g., Bi, Pb) that may induce a long-range molecular electric field in the system without the requirement of B′/B″ self-ordering. For example, the complete substitution of La³⁺ by Bi³⁺ can induce a long-range in-built molecular electric field in A₂CoMnO₆ double perovskites due to the contribution of Bi 6s² lone pair electrons similar to that observed in BiMnO₃ [8]. From this perspective, comparing La₂CoMnO₆ and Bi₂CoMnO₆ properties in thin films becomes an interesting benchmark to understand the important ingredients required to promote useful multiferroic properties.

There are several reports on the structural and magnetic properties of bulk La₂CoMnO₆ double perovskites while very little information can be found on bulk Bi₂CoMnO₆ (BCMO) [4], [5], [6], [7], [9], [10]. Nevertheless, the available data have clearly demonstrated that the cation-disordered materials exhibit multiple magnetic transitions while the ordered ones are characterized by a single ferromagnetic-to-paramagnetic transition (FM-T_c). For example, La₂CoMnO₆ in its cation-ordered phase displays a unique FM-T_c around 245 K, while disordered samples present also a second magnetic transition around 145 K [4], [5], [6], [7], [9], [10]. From a structural point of view, it has been also established that a cation-ordered La-based double perovskite displays monoclinic P2₁/n symmetry while the disordered system presents the orthorhombic Pnma symmetry [5], [6], [7]. The recent demonstration of magnetodielectric effects in the La-based self-ordered double perovskites in the vicinity of their ferromagnetic transition close to room temperature is a clear indication of their potential for novel multifunctional devices [6]. However, they still remain challenging materials and the observation of a coupling between the magnetization and the polarization needs to be correlated to microscopic models. Thus, a deeper understanding and a meticulous correlation between their physical and structural properties become crucial.

Unlike the study of the bulk phases, growth of double perovskite thin films is quite recent [11], [12], [13], [14], [15], [16], [17], [18]. Obviously, these films are of great interest since they represent the first step toward their integration in multifunctional applications for spintronic devices. This has motivated us to compare the functional properties of LCMO and BCMO films. In this paper, we present our results on epitaxial LCMO and BCMO double perovskite thin films grown by pulsed-laser deposition. The impact of the growth parameters on the Co/Mn ordering is presented as well as the functional magnetic properties of the films. The results reveal that the ordered LCMO films display ferromagnetism up to about 250 K while the disordered BCMO films remain ferromagnetic up to anomalously large temperature of 800 K. We present a possible scenario to explain the surprising enhancement of the FM-T_c of disordered BCMO films with respect to LCMO. This mechanism is based on the increase of the superexchange strength arising from the overlap between the Bi 6s² lone pair electrons and the Co–O–Mn, Mn–O–Mn, and Co–O–Co electronic wave functions aided by the structural changes.