Ferroelectric and Magnetic Properties of Hexagonal ErFeO₃ Epitaxial Films

Abstract

Hexagonal rare-earth iron oxides (h-RFeO3) exhibit both magnetic and ferroelectric orders. The ferroic properties depend on the R ionic radius; for example, they are antiferroelectric (AFE) when R = Dy and ferroelectric (FE) when R = Tm, Yb, and Lu. However, insights into the ferroelectric properties of h-ErFeO3, characterized by an intermediate ionic radius, are hampered by its severe electric leakage current. In this study, we successfully prepared h-ErFeO3 films with limited electric leakage current via atomically flat indium tin oxide bottom electrodes. The electric insulation enhancement allowed us to investigate the ferroelectric properties of the h-ErFeO3 films. The h-ErFeO3 films exhibited AFE behavior when the applied maximum electric field (E max) was below 1 MV/cm. Conversely, clear FE polarization reversal appeared for E max > 1 MV/cm. At E max = 1.4 MV/cm, the remnant polarization and coercive electric field (E c) values of the film were 2.1 μC/cm2 and 0.15 MV/cm, respectively. The E c value of the h-ErFeO3 film was smaller than that of the other h-RFeO3 films due to its larger a-axis length. In addition, the antiferromagnetic-to-weak ferromagnetic phase transition was observed by changing the temperature. Thus, uniquely, the h-ErFeO3 film exhibited four types of ferroic properties: antiferroelectric, ferroelectric, antiferromagnetic, and weak ferromagnetic.

Hexagonal RFeO3 (R = Dy, Er, and Lu) Films Grown on Glass Substrates with Both Magnetic and Ferroelectric Orders

“…The calculated lattice parameters ( a , c, and volume) were (6.11 Å, 11.9 Å, 193 Å 3 ), (6.07 Å, 11.9 Å, 190 Å 3 ), and (6.00 Å, 11.9 Å, 186 Å 3 ) for the h -DyFeO 3 , h -ErFeO 3 , and h -LuFeO 3 films, respectively. By contrast, it was reported that the lattice parameters of the epitaxial films of h -DyFeO 3 , h -ErFeO 3 , and h -LuFeO 3 were (6.24 Å, 11.82 Å, 199 Å 3 ), (6.03 Å, 11.86 Å, 187 Å 3 ), and (5.92 Å, 11.77 Å, 179 Å 3 ), respectively. ,, The tendency that the a -axis length and volume decreased with a decreasing R ion size was consistent. However, the decrease in the films on glass was smaller than that in epitaxial films, likely due to the difference in the force applied to the films by the substrates between the epitaxial and polycrystalline growths.…”

“…As shown in Figure (b), the ferromagnetic hysteresis of the film at 20–50 K was more clearly observed than that at 2–10 K. In h-R FeO 3 , there are four types of possible magnetic configurations (Γ 1 –Γ 4 ), and only the Γ 2 -type configuration allows the nonzero net magnetization along the c -axis direction owing to the Dzyaloshinskii–Moriya interaction . Therefore, the magnetic phase transition seems to reflect the reconfiguration of magnetic structures between Γ 2 and Γ x ( x = 1, 3, or 4). ,, …”

“…Finally, we discuss the ferroelectric properties of the h-R FeO 3 films grown on glass. The ferroelectric properties of h-R FeO 3 epitaxial films on single-crystal substrates have been investigated using polarization versus electric field ( P – E ) curve measurements; h -LuFeO 3 and h -ErFeO 3 epitaxial films exhibited room-temperature ferroelectricity, − whereas h -DyFeO 3 exhibited antiferroelectric behavior at room temperature . We first tried to measure P – E curves for the h-R FeO 3 films on the ITO-covered glass.…”

“…12 Therefore, the magnetic phase transition seems to reflect the reconfiguration of magnetic structures between Γ 2 and Γ x (x = 1, 3, or 4). 23,24,41 Finally, we discuss the ferroelectric properties of the h-RFeO 3 films grown on glass. The ferroelectric properties of h-RFeO 3 epitaxial films on single-crystal substrates have been investigated using polarization versus electric field (P−E) curve measurements; h-LuFeO 3 and h-ErFeO 3 epitaxial films exhibited room-temperature ferroelectricity, 23−25 whereas h-DyFeO 3 exhibited antiferroelectric behavior at room temperature.…”

“…h-R FeO 3 nanoparticles can be synthesized by wet-chemical or supercooling methods, although it is difficult to characterize the ferroic properties owing to the existence of impurity phases and their small size. − A more effective method is the epitaxial thin-film growth on single-crystal substrates, such as YSZ(111) and Al 2 O 3 (0001) substrates. A series of h-R FeO 3 ( R = Sc, Y, and Gd–Lu) epitaxial films have been fabricated without impurity phases. − It is suggested that the metastable h-R FeO 3 phase is stabilized owing to the lattice match between the film and single-crystal substrate. Compared to single-crystal substrates, glass substrates are more cost-effective, which is important for future applications.…”