A strong ferroelectric ferromagnet created by means of spin–lattice coupling

Abstract

Nature 466, 954-958 (2010) This Letter determined that EuTiO 3 , when appropriately strained, becomes a strong ferroelectric ferromagnet, in agreement with prediction. Strong ferroelectrics are proper ferroelectrics, having polarization as their order parameter, with high paraelectric-to-ferroelectric transition temperatures (T c ). Such ferroelectrics are manifested by a high T c and a high peak at T c in the dielectric constant versus temperature behaviour, signifying that ferroelectricity is driven by the soft mode, which is indicative of proper ferroelectricity. Our measurements of strained EuTiO 3 demonstrate both of these characteristics (shown in Fig. 3 of our Letter), and led us to conclude that strained EuTiO 3 is a strong ferroelectric. In contrast, all well-established prior single-phase ferroelectric ferromagnets are improper or pseudoproper ferroelectrics (that is, with weak ferroelectricity resulting in minuscule P s ). We did not present P s values in our Letter. Second harmonic generation measurements do not provide quantitative values of P s and attempts to determine P s via pyroelectric measurements (Yan, L., Li, J. F. & Viehland, D., personal communication)1 resulted in unphysically high values, presumably owing to electrical leakage. Nonetheless, the magnitude of the P s of our strained EuTiO 3 films can be estimated as follows. In their classic work, Abrahams, Kurtz, and Jamieson 1 established a correlation between P s and T c for displacive ferroelectrics. By studying numerous displacive ferroelectrics they found 5) where T c is the paraelectric-to-ferroelectric transition temperature in K, Dz is the atomic displacement of the 'homopolar' metal atom in Å , and P s is the spontaneous polarization of the ferroelectric in mC cm 22 . Combining these equations to eliminate Dz allows P s to be estimated from T c in displacive ferroelectrics. The huge anomaly of the soft optical phonon near T c that we observe ( Supplementary Fig. 1 of our Letter) shows that strained EuTiO 3 is a displacive ferroelectric, making the aforementioned correlation applicable. Plugging in our measured value of T c (Fig. 3 in our Letter) yields P s 5 29 6 2 mC cm 22 for our strained EuTiO 3 films from this established correlation. This rough estimate is consistent with our first-principles theoretical predictions-P s 5 21 mC cm 22 for EuTiO 3 under 11.1% biaxial tension, corresponding to the strain of our commensurate EuTiO 3 films grown on (110) DyScO 3 . Thus, the data in our Letter shows that appropriately strained EuTiO 3 is a strong ferroelectric ferromagnet.

Electrically induced enormous magnetic anisotropy in Terfenol-D/lead zinc niobate-lead titanate multiferroic heterostructures

“…12 Strong ME coupling, large magnetostriction and high piezoelectric coefficients are critical for enhanced performance of these materials, and more efforts are needed to develop new types of ultralow power spintronics, memory devices and tunable microwave devices. 6,[12][13][14][15][16][17][18][19][20][21][22][23] In this study, we report a significant E-field-induced effective magnetic anisotropy up to 3500 Oe in Terfenol-D/PZN-PT which is four times more than that we demonstrated in Fe 3 O 4 /PMN-PT heterostructures.…”

Goldstone-like phonon modes in a (111)-strained perovskite

“…Coherent epitaxial strain is a powerful way to enhance, control and even induce new functionality, especially in perovskite oxides [21]. Well-known examples include enhanced polarization in BaTiO 3 [22], ferroelectricity in SrTiO 3 [23] and coexistence of metallic and insulating phases in perovskite manganites [24], as well as multiferroicity in EuTiO 3 [25] and SrMnO 3 [26]. While most perovskite thin films have been grown along the [001] direction, recent progress has enabled the exploration of a variety of symmetry constraints at oxide interfaces [27].…”

Multiferroics beyond electric-field control of magnetism

“…The converse approach -a d 0 B-site transition metal and a magnetic A site -is exemplified by modifications of EuTiO 3 . While not ferroelectric under ambient conditions due to the small size of the Eu 2+ ion providing insufficient space for the Ti 4+ to off center, both tensile strain [15] and negative chemical pressure [16] have been shown to induce ferroelectricity. The f 7 Eu 2+ ions have large spin magnetic moments, which do not interact strongly because of the highly localized nature of the f electrons.…”