Yosuke Hamasaki

Epitaxial growth of metastable multiferroic AlFeO3 film on SrTiO3 (111) substrate

GaFeO3-type AlFeO3 is consisted of oxygen octahedra and tetrahedra containing Al and Fe ions and is known to have a non-centrosymmetric polar structure with space group Pna21. We tried to grow epitaxial GaFeO3-type AlFeO3 films on SrTiO3 (111) substrates by pulsed laser deposition technique. Both the atomic arrangement of close-packed and the atomic distance of the substrate surface played important roles in stabilizing GaFeO3-type AlFeO3 on the substrate. Piezoresponse force microscopy measurements clearly showed that GaFeO3-type AlFeO3 films have ferroelectricity at room temperature. In addition, AlFeO3 film also showed pinched-like hysteresis loop with TN ∼ 317 K.

Controllable exchange bias in Fe/metamagnetic FeRh bilayers

We report the studies of tuning the exchange bias at ferromagnetic Fe/metamagnetic FeRh bilayer interfaces. Fe/FeRh(111) bilayers show exchange bias in the antiferromagnetic state of FeRh while no exchange bias occurs at Fe/FeRh(001) interface. The contrasting results are attributed to the spin configurations of FeRh at the interface, i.e., the uncompensated ferromagnetic spin configuration of FeRh appears exclusively for (111) orientation. The exchange bias disappears as the bilayers are warmed above the antiferromagnetic-ferromagnetic transition temperature. The direction of the exchange bias for Fe/FeRh(111) is also found to be perpendicular to the cooling-field direction, in contrast to the commonly observed direction of exchange bias for ferromagnetic/antiferromagnetic interfaces. In view of these results, the exchange bias in Fe/FeRh bilayers with the (111) crystallographic orientation should be useful for the design of rapid writing technology for magnetic information devices.

Control of crystal-domain orientation in multiferroic Ga0.6Fe1.4O3 epitaxial thin films

In multiferroic GaFeO3 (GFO)-type iron oxides, spontaneous polarization and magnetization coexist at room temperature along the [001]GFO and [100]GFO directions, respectively. Due to the large magnetocrystalline anisotropy and polarization direction in GFO, controlling the domain configuration and orientation is crucial when designing the ferroelectric and ferrimagnetic properties. In this study, we fabricate Ga0.6Fe1.4O3 epitaxial thin films on various substrates to investigate the substrate effect on the structural, ferroelectric, and magnetic properties. Multiple domains and their orientations in the films can be controlled in four ways through variations in the substrate. Additionally, decreasing the number of domains reduces the leakage current, allowing ferroelectric measurements for the film at room temperature. Furthermore, tilting the easy magnetic axis from the in-plane direction causes the in-plane magnetic anisotropy of the film to vary from 1.1 × 106 to 1.8 × 105 erg/cm3 at 300 K. Domain cont...

Evidence of ferroelectricity in ferrimagnetic κ-Al2O3-type In0.25Fe1.75O3 films

We report direct evidence of the multiferroic nature of the κ-Al2O3-type InxFe2-xO3 films (x = 0–0.25) grown on the SrTiO3(111) substrates at room temperature. The κ-Al2O3-type InxFe2-xO3 films show a clear saturated and opened P-E hysteresis loop at room temperature with a remnant polarization of 1.8 μC cm−2. Also, the ferrimagnetic-incommensurate magnetic phase transition is observed at 205 K, which is compatible with the recent report on the In0.24Fe1.76O3 nanorods. The results indicate that the substitution of Me3+ ion, which has a larger ionic radius than Fe3+, is effective to grow good insulating ferroelectric κ-Al2O3-type Fe2O3 films, leading to the genuine multiferroic nature.

Chemical tuning of room-temperature ferrimagnetism and ferroelectricity in ε-Fe2O3-type multiferroic oxide thin films

e-Fe2O3-type iron oxide is a promising room-temperature multiferroic material. However, it is difficult to achieve the coexistence of large magnetization and reversible polarization due to large leakage current. In this study, we fabricated highly crystalline codoped e-Fe2O3 films of A0.2Ga0.4Fe1.4O3 (A = Al, Ga, Sc, and In) and systematically investigated the doping effects on the magnetic and ferroelectric properties. All films are pure e-phase and simultaneously exhibit in-plane ferrimagnetism and out-of-plane ferroelectricity at room temperature. Unlike the e-Fe2O3 film, the films do not contain secondary phases such as α- or γ-phases, because the e-phase is stabilized by Ga at the tetrahedral site. The Curie temperature, saturated magnetization, coercive field, and magnetic anisotropy of the films are enhanced upon decreasing the ionic radius of A, despite the same content of magnetic element (Fe3+ 3d5). These enhancements are derived from the increase in the amount of Fe ions at octahedral sites. Furthermore, we found that the leakage current significantly decreases with Sc doping, resulting in clear ferroelectric hysteresis loops in a wide frequency range at room temperature. Among the dopants in this study, codoping of Sc and Ga is the most promising method for obtaining a highly crystalline film with room-temperature large magnetization and reversible polarization with low leakage current.

Crystal structure and magnetism in κ-Al2O3-type AlxFe2-xO3 films on SrTiO3(111)

We prepared κ-Al2O3-type structured AlxFe2-xO3 films in the range of x = 0 – 1.70 deposited on SrTiO3(111) substrates and investigated their crystal structures and magnetic properties. All films could be stabilized in the κ-Al2O3-type orthorhombic phase, and the lattice parameters were found to be monotonically decreased with an increase in the Al content. Neel temperature of AlxFe2-xO3 films was found to decrease with an increase in Al content, until the Al1.70Fe0.30O3 film showed paramagnetic behavior. On the other hand, saturation magnetization showed a maximum 0.79 μB/Fe at 10 K in the Al0.91Fe1.09O3 film, manifesting the preferential occupation of Al in the tetrahedral site. Cross sectional TEM observation has revealed the columnar growth of AlxFe2-xO3 films with an average width of ∼10 nm on the bottom layer that may have a similar cation arrangement with a bixbyite-type structure.

Effect of Cr substitution on ferrimagnetic and ferroelectric properties of GaFeO3 epitaxial thin films

To control the properties of multiferroic materials, substitution of magnetic elements is a useful technique. In this study, we fabricated GaCrxFe1−xO3 and Ga1−yCryFeO3 films by substituting Cr3+ (3d3) ions in GaFeO3 for magnetic Fe3+ (3d5) and nonmagnetic Ga3+ (d10) sites, respectively. The effect of the Cr substitution on the magnetic and dielectric properties was systematically investigated. The obtained GaCrxFe1−xO3 and Ga1−yCryFeO3 films (x ≤ 1/4 and y ≤ 1/2) simultaneously exhibit ferrimagnetism and ferroelectricity. For the GaCrxFe1−xO3 films, the magnetic transition temperature (TC) and magnetic anisotropy decrease with increasing x. On the other hand, for the Ga1−yCryFeO3 films, TC increases with increasing y. Unlike the GaFeO3 film, the Cr-substituted GFO films show a peak in the magnetization versus temperature curves and exhibit a sudden decrease in the coercive field near the peak, probably due to the formation of Fe3+–O–Cr3+ coupling which prefers to form ferromagnetic interactions in the antiferromagnetic Fe3+–O–Fe3+ networks. Furthermore, we found that the Cr-substituted GaFeO3 films show out-of-plane ferroelectricity at room temperature.To control the properties of multiferroic materials, substitution of magnetic elements is a useful technique. In this study, we fabricated GaCrxFe1−xO3 and Ga1−yCryFeO3 films by substituting Cr3+ (3d3) ions in GaFeO3 for magnetic Fe3+ (3d5) and nonmagnetic Ga3+ (d10) sites, respectively. The effect of the Cr substitution on the magnetic and dielectric properties was systematically investigated. The obtained GaCrxFe1−xO3 and Ga1−yCryFeO3 films (x ≤ 1/4 and y ≤ 1/2) simultaneously exhibit ferrimagnetism and ferroelectricity. For the GaCrxFe1−xO3 films, the magnetic transition temperature (TC) and magnetic anisotropy decrease with increasing x. On the other hand, for the Ga1−yCryFeO3 films, TC increases with increasing y. Unlike the GaFeO3 film, the Cr-substituted GFO films show a peak in the magnetization versus temperature curves and exhibit a sudden decrease in the coercive field near the peak, probably due to the formation of Fe3+–O–Cr3+ coupling which prefers to form ferromagnetic interactions in the an...