Structural and magnetic phase transitions in Bi_1−x Pr _x FeO₃ perovskites

Excerpt

Crystal structure and physical properties of materials demonstrating simultaneous spin and electric dipole ordering have recently attracted much attention [1, 2]. The interest is explained by promising possibilities of exploitation of the intrinsic cross-coupling effects (induction of magnetization by an electric field or of electric polarization by a magnetic field) in practical applications. Most of the principal mechanisms underlying the combined ferroelectromagnetism in a single-phase compound [1] can be realized in complex oxides with the perovskite-like (ABO₃) structure to make this class of materials one of the most popular from the viewpoint of multiferroic research. Among multiferroic perovskites, BiFeO₃ is distinguished by its extremely high magnetic and ferroelectric transition temperatures (T _N ≈ 640 K, T _C ≈ 1100 K). In ferroelectric phase, the compound has a rhombohedrally distorted structure (space group R3c) [3] and possesses a large polarization P _S ~ 100 μC/cm² [4] directed along the [001]_h axis. Superexchange interactions between magnetically active Fe³⁺ ions give rise to G-type antiferromagnetism. Due to the flexomagnetoelectric interaction [5], the antiferromagnetic structure is modulated with a long-range (~620 Å) cycloid [6]. The magnetic moments of iron ions retain their local antiferromagnetic G-type orientation and rotate along the propagation direction of the modulated wave in the plane perpendicular to the hexagonal basal plane. Such a modulation prevents the observation of weak ferromagnetism (allowed by symmetry of the space group R3c [7]) and of linear magnetoelectric effect [8]. The modulated structure can be suppressed by applying a strong magnetic field H _C ~ 180–200 kOe to release a weak ferromagnetic moment of ~0.25 emu/g [8]. Alternative effective way to suppress the spatial spin modulation is a lanthanide (Ln) A-site substitution [9]. Recognition of the possibility to tune and control multiferroic properties of bismuth ferrite via the “chemical pressure” motivated numerous investigations of Bi_1−x Ln _x FeO₃ solid solutions [10‐13]. However, many early conclusions related to the structural phase evolution in these systems need to be carefully checked. A tendency to the structural phase separation characteristic of the materials [14] can hamper structural identification and can strongly influence the properties of the compounds under study. For instance, presence of a minor amount of the parent R3c phase can be a possible reason for the observation of ferroelectric-like behavior in the intrinsically nonferroelectric strongly-doped Bi_1−x Ln _x FeO₃ compounds [10]. These circumstances seem to underlie the scattering of the structural and ferroelectric data reported for BiFeO₃-based compounds, so substituted BiFeO₃ is still an area of fruitful research. Among Bi_1−x Ln _x FeO₃ series, praseodymium-containing system remains less well studied. Existing articles describe properties of Bi_1−x Pr _x FeO₃ multiferroics in a rather limited compositional range [15], reported data not always being consistent with some general trends observed in Bi_1−x Ln _x FeO₃ series [16]. To contribute to deeper understanding of the effect of Pr substitution on crystal structure and magnetic properties of bismuth ferrite, we performed solid-state synthesis, X-ray diffraction, and magnetization measurements of the Bi_1−x Pr _x FeO₃ (x ≤ 0.3) compounds. …