Various double perovskites with ordered B-site lattices and a general formula of A₂M’M’’O₆ have recently been synthesized and characterized. The magnetic behavior of these materials is particularly interesting, with half-metallic behavior in Sr₂FeMoO₆, multiferroicity in Bi₂NiMnO₆ and Bi₂CoMnO₆, and semiconducting ferromagnetism in La₂NiMnO₆ and La₂CoMnO₆. The purpose of this joint-use project was to perform structural and dielectric characterization on La₂NiMnO₆ (LNMO) thin films that are semiconducting but become ferromagnetic at an exceptionally high temperature of 280 K. The material is thus a prime candidate for use in room-temperature spintronic device applications. Due to the ordered state of B-site cations with different valence states, such as Ni²⁺ and Mn⁴⁺ in La₂NiMnO₆, double perovskites are also prime candidates for strong magnetodielectric coupling, where the dielectric behavior can be controlled by applied magnetic fields. Due to the semiconducting behavior of La₂NiMnO₆, however, accurate dielectric characterization close to the onset of magnetic order is hampered by the large conductivity. Our purpose, therefore, was to use zero-bias pyroelectric characterization to determine if a polar state exists in La₂NiMnO₆, and to determine if this material might also show a switchable multiferroic response.

Fig. 1. X-ray diffraction analysis of an ordered perovskite La₂NiMnO₆ thin film sample. The reciprocal space mapping in the inset shows that the film is coherent on the SrTiO₃ substrate. Clear odd-index superlattice peaks due to B-site ordering are visible in the asymmetric diffraction pattern (a). The pyroelectric response of La₂NiMnO₆ is shown in (b), together with a rate-dependent thermally-induced current plot in (c).

The ordering of the transition metal ions in double perovskites is often incomplete. It is therefore important to determine the structural quality of a crystal before dielectric measurements. The structural analysis was done by conventional x-ray diffraction and reciprocal space mapping analysis, as illustrated for a well-ordered sample in Fig. 1(a). It is clear that the La₂NiMnO₆ film is coherently grown on the SrTiO₃ substrate and the odd-index superlattice peaks characteristic of an ordered phase are clearly visible in an asymmetric diffraction pattern measured for a (001)-oriented thin film sample. The presence of the expected anisotropic polarization-dependent Raman scattering peaks was also verified for the same thin film samples.

The dielectric characterization was done in a planar capacitor geometry. The temperature dependence of the dielectric permittivity, measured at various frequencies, showed a clear relaxor-like behavior between 100 and 200 K. The pyroelectric measurements were done in the same capacitor geometry, using a chopped laser source for periodically heating and cooling the thin film capacitor. For polar materials, the small temperature modulation results in a change of the surface charge due to a temperature dependence of the spontaneous polarization. Since no bias is applied to a sample during the pyroelectric measurement, it is possible to determine the presence of a polar state even for a semiconducting material, where traditional ferroelectric testing would not be possible due to large leak currents. The measurements showed that a polar state does indeed appear in La₂NiMnO₆ at temperatures below 250 K. The pyroelectric coefficient can be determined from slow temperature sweeps, as illustrated in Fig. 1(b) and looking at the normalized slope of the thermally-induced current as a function of the sweep rate. Analysis of the data in Fig. 1(c) showed that the pyroelectric response in La₂NiMnO₆ is 34 nC/cm^-2K, which is comparable to the well-known lead titanates. This study shows that ordered perovskites, like La₂NiMnO₆, may be useful not only as spintronic materials, but may also be useful for developing lead-free pyroelectric sensors.

Authors

• I. Ohkubo^a, M. Oshima^a, R. Takahashi, and M. Lippmaa
• ^aDepartment of Applied Chemistry, University of Tokyo