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Double Perovskite Ferroelectrics

Lippmaa Group

La2NiMnO6 is a double perovskite with a pseudo-cubic structure where the Ni and Mn atoms at the perovskite B-site form a natural superlattice along the [111] direction (Fig. 1). Such B-site-ordered double perovskites open interesting materials design possibilities since transition metals with dissimilar d-electron configurations can be integrated in a crystal, allowing for tuning of transport, dielectric, or magnetic properties. In our recent work, we explore the influence of epitaxial strain on the dielectric behavior of the La2NiMnO6 perovskite. This material is ferromagnetic in bulk form and, as is common for oxides with non-zero d-electron configurations, does not support a polar ferroelectric state. However, first-principles calculations show that when the rhombohedral crystal is strained, a polar state can emerge due to a shift of the A-site La atoms from the unstrained equilibrium position, giving rise to macroscopic ferroelectricity. The structural differences between the bulk nonpolar and strained polar structures are illustrated in Fig. 1.

Fig. 1. Crystal structure of an ordered bulk La2NiMnO6 double perovskite crystal with Ni (gray) and Mn (blue) atoms arranged in layers along the [111] direction. The rhombohedral unit cell outline is shown in black. The La atoms are at symmetric positions and the lattice is nonpolar (top). Under epitaxial strain of about 6%, the A-site La atoms are displaced, inducing a polar state.

Fig. 2. Ferroelectric and ferromagnetic hysteresis loops of strained La2NiMnO6 thin films measured at 10 K, showing that the material is multiferroic.

The strained crystal structure can be obtained by growing La2NiMnO6 thin films on a lattice-mismatched SrTiO3 substrate. The lattice expansion in an epitaxial thin film does not have a significant effect on the magnetism and a clear ferromagnetic hysteresis loop can be observed at low temperatures. The magnetic state survives up to room temperature, with a Curie temperature close to 300 K. The ferroelectric state is observable below about 50 K, as illustrated in Fig. 2. The ferroelectricity in La2NiMnO6 films is caused by an offset of the A-site cations, while the magnetism originates from the transition metals at the B-site of the crystal lattice. The material is thus multiferroic, although there is no direct coupling between the magnetism and the polarity. Double perovskites are thus an interesting class of materials, where multiple ferroic orders can be integrated in the same lattice on a microscopic scale by associating the different types of order with different lattice sites.

A consequence of this differentiation of lattice sites opens the possibility to tune the polarity and the magnetism independently of each other. The ferroelectric polarization is strongly dependent on the magnitude of the applied strain, which has little effect on the magnetic order. The strain sensitivity can be quite dramatically illustrated by a noticeable shift in the pyroelectric polarization measurement response when an in plane strain change by just 0.015% is applied. This small strain change can be reliably generated by a structural phase transition that occurs in the SrTiO3 substrate at 105 K. The magnetism, on the other hand, can be controlled by the level of disorder among the B-site Ni and Mn cations. For typical thin films, the level of Ni/Mn ordering can be adjusted between about 20% and 80% by adjusting the film growth parameters. This variation leads to a nearly linear change of the saturation magnetization, reaching 2.5 µB/B-site for a perfectly ordered lattice and vanishing in a completely disordered material.

A potential limitation in the use of double perovskites such as La2NiMnO6 is the coexistence of rhombohedral and monoclinic crystal phases. The multiferroic behavior is only observed in a strained rhombohedral lattice, while the monoclinic phase remains paraelectric at all temperatures. The phase distribution can be imaged on a macroscopic scale by scanning nonlinear dielectric microscopy, which visualizes the distribution and polarity of ferroelectric domains in the films. Further work is required to find ways of controlling the phase formation in thin films by process parameter variations and substrate mismatch selection.


References
  • [1] R. Takahashi, I. Ohkubo, K. Yamauchi, M. Kitamura, Y. Sakurai, M. Oshima, T. Oguchi, Y. Cho, and M. Lippmaa, Phys. Rev. B 91, 134107 (2015).
Authors
  • R. Takahashi, I. Ohkuboa, K. Yamauchib, M. Kitamura, Y. Sakurai, M. Oshima, T. Oguchib, Y. Choc, and M. Lippmaa
  • aNational Institute for Materials Science
  • bOsaka University
  • cTohoku University