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Antiferromagnetic order in tetragonal bismuth ferrite–lead titanate

https://doi.org/10.1016/j.jmmm.2011.06.009Get rights and content

Abstract

Neutron powder diffraction of particulates of 0.7BiFeO3–0.3PbTiO3 in the tetragonal P4mm phase has been used to determine the type of antiferromagnetic order that occurs below 220 K. It is shown that G-type antiferromagnetic ordering occurs, with magnetic propagation along the 121212 direction. Unlike the rhombohedral R3c phase the direction of antiferromagnetic propagation and the ferroelectric order parameter are not parallel in the tetragonal phase, but at an angle of 49.9°. The ground state (at 4 K) magnetic moment is 4.1 μB.

Highlights

► G-type antiferromagnetic ordering below 220 K in 0.7BiFeO3–0.3PbTiO3. ► Ground state magnetic moment=4.1 μB. ► Magnetic propagation vector k=(121212). ► Ferroelectric ordering (0 0 1) at 49.9° to magnetic propagation vector.

Introduction

Bismuth ferrite, BiFeO3, is one of the few known room-temperature multiferroics, displaying both G-type antiferromagnetic and ferroelectric order [1]. It readily forms a solid solution with ferroelectric PbTiO3, resulting in the system xBiFeO3−(1−x)PbTiO3, which is also multiferroic at room temperature [2], [3], [4], [5], [6], [7], [8] for a wide range of x. Single phase perovskite is more easily achievable in the solid solution than in BiFeO3 [9], whilst DC resistivity values and electric-field induced strains are of the same order as one would expect for undoped lead zirconate titanate (PZT) [10].

We have shown previously that for 0.7BiFeO3–0.3PbTiO3, which is close to the R3c–P4mm phase boundary, the crystallographic phase content is form dependent [11]. Particulate material has a predominantly tetragonal perovskite structure (P4mm) and is paramagnetic at room temperature, as determined using neutron diffraction, whereas dense ceramics are predominantly rhombohedral and display antiferromagnetic order under ambient conditions. Zhu et al. [6] have confirmed the existence of antiferromagnetic order for tetragonal compositions in the range 0.45<x<0.69 using zero-field cooling with SQUID (superconducting quantum interference device) measurements at sub-ambient temperatures. For the composition x=0.69, a Néel temperature of 220 K was recorded.

The identity and magnetic structure of this low temperature tetragonal perovskite antiferromagnetic phase are unknown. There are a number of reported primitive tetragonal perovskites in which magnetic ordering occurs: PbVO3 (P4mm) shows two-dimensional antiferromagnetic order below 43 K [12], plus the observation of either spin-glass or G-type antiferromagnetic order in thin film form [13]; BiCoO3 (P4mm) is a C-type antiferromagnetic with a Néel temperature of 470 K [14]; 0.5PbFeO2F–0.5PbTiO3 (P4mm) shows G-type antiferromagnetic order with a Néel temperature of ∼450 K, and a propagation vector k=(121212) [15].

Although 0.5PbFeO2F–0.5PbTiO3 displays the same space group as tetragonal 0.7BiFeO3–0.3PbTiO3, the spontaneous strain or tetragonality (ca)/a is very different. At room temperature and at 4 K, the spontaneous strain of 0.5PbFeO2F–0.5PbTiO3 is extremely low, ca. 0.2%—splitting of the 001/100 peaks in the diffraction data is not evident; 0.7BiFeO3–0.3PbTiO3, however, shows a colossal spontaneous strain of ca. 19% [4], two orders of magnitude larger.

The objective of the work reported here was to solve the structure of the antiferromagnetic tetragonal phase of 0.7BiFeO3–0.3PbTiO3 as a function of temperature, and determine the magnetic ground state. Differences in magnetic structure will be sought with the rhombohedral BiFeO3 end member.

Section snippets

Experimental

The powders were formed by rapidly cooling dense ceramic bodies, leading to disintegration as reported previously, generating a predominantly tetragonal material [11]. Neutron diffraction data were gathered using G4.1, a reactor neutron facility at Laboratoire Léon Brillouin (LLB), France. The powder samples were loaded into vanadium cans, and measurements were taken at room temperature, at 4 K, then upon continuous heating back to room temperature; each temperature point presented is the

Results and discussion

Fig. 1 and Table 1 show the data collected at 4 K, along with refinement and residual. In order to generate a satisfactory fit, it was necessary to generate a large magnetic super-cell containing eight Fe3+ ions with an occupancy of 0.7 per site, akin to the nuclear phase, with lattice constants a and c double that of the nuclear tetragonal model.

Fig. 2 shows an enlarged region of Fig. 1 displaying magnetic Bragg peaks at 30.4° and 54.9°, which are absent in data collected at room temperature.

Conclusions

The tetragonal phase that exists in 0.7BiFeO3–0.3PbTiO3 displays G-type antiferromagnetic ordering below ca. 220 K. At 4 K, the magnetic moment is 4.1 μB. Contrary to observations in the rhombohedral R3c phase in BiFeO3 and 0.9BiFeO3–0.1PbTiO3, the ferroelectric order parameter and direction of antiferromagnetic propagation in the tetragonal phase are misaligned by 49.9°. The structure is similar to that reported for the composition 0.5PbFeO2F–0.5PbTiO3.

Acknowledgments

The authors gratefully acknowledge Aziz Daoud-Aladine and the Science and Technology Facilities Council, for the data collected on HRPD.

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