90° Rotation of orbital stripes in bilayer manganite PrCa2Mn2O7 studied by in situ transmission electron microscopy
Graphical abstract
The intermediate state of the thermally induced 90° rotation of orbital stripes between two charge/orbital ordering CO1 and CO2 phase in the bilayer manganite PrCa2Mn2O7 is revealed by in situ TEM. A new state with two sets of satellite spots in the [001] electron diffraction patterns is found to bridge this transition. The reasons to this transition are explained.
Highlights
► Intermediate states to bridge the 90° rotation of the orbital stripes in bilayer manganite PrCa2Mn2O7 were identified. ► Some restricted conditions for the orbital rotation to occur were found and reasons were discussed. ► The reported CO2 phase at high temperatures in PrCa2Mn2O7 can also exist at room temperature.
Introduction
Multiferroics has drawn extensive attention because of the elusive but intriguing underlying physics as well as the potential applications [1], [2], [3], [4], [5], [6], [7], [8]. Different multiferroic materials have been discovered in recent years, showing various multiferroic mechanisms, e.g. charge ordering induced multiferroicity in the strongly correlated electron system Pr1−xCaxMnO3 [9]. Recently, the ferroelectricity in highly Ca-doped bilayer manganite series Pr(Sr1−xCax)2Mn2O7 for 0.5≤x≤1 was found to be coupled with a rotation of the orbital ordering (OO) in this system, which makes this system a unique multiferroic material [10], [11]. More importantly, the OO rotation could find some potentially industrial applications such as memory devices by using orbital orientation [12]. However, neither the origin nor the mechanism of the OO rotation has been well understood yet.
The X-ray diffraction results show the presence of two charge ordering-orbital ordering (COOO) phases in Pr(Sr0.1Ca0.9)2Mn2O7: the high temperature phase (CO1) at TCO1>T>TCO2 (TCO1=330 K, TCO2=295 K) with lattice parameters a=0.5412, b=1.0921, c=1.9234 nm and Pbnm space group; the lower-temperature phase (CO2) at T<TCO2 with a=1.0812, b=0.5475, c=1.9203 nm and Am2m space group [10]. We will adopt the same lattice setting for CO1 and CO2 phase in this study. The orbital zigzag chains in the CO1 phase run along the b direction (accordingly, along the a direction for the orbital stripes), and are rotated to the a direction in CO2 by a 90° rotation (along the b direction for the orbital stripes) [10], [11]. Upon this rotation, the centrosymmetry of the crystal is broken, generating ferroelectricity in the CO2 phase [10].
Even though some efforts have been made to explain the rotation of the OO stripes, there is no settled explanation to this phenomenon due to the complication of the interplay of lattice, orbital, charge, and spin as well as the very limited experimental evidences of OO rotation in this system. In order to understand how and why the rotation of the OO stripes occurs, there are at least two fundamental questions to be clarified experimentally: (1) what is the mechanism of the OO rotation? (2) are there any restricted conditions for the OO rotation to occur? X-ray diffraction was used to determine the structure [13], but only average information is obtained. Moreover, abundant twin formation and even impurities could bring some confusion to the details of the OO rotation. Therefore, we carried out a detailed in situ transmission electron microscopy (TEM) study on PrCa2Mn2O7 single crystals, focusing on local single domains, to answer these questions. In this way the evolution of the OO stripes and the limited conditions for the OO rotation to occur were found. These findings may shed some light on the understanding of the mechanism(s) of the OO rotation.
Section snippets
Experimental
Bulk PrCa2Mn2O7 single crystals have been successfully synthesized using a traveling solvent floating zone method with high oxygen background pressure [14], [15]. Both powdered samples, deposited on a Cu grid coated with a holey carbon film, and Ar-ion milled TEM samples were used for the TEM investigation using a CM20 and a Tecnai G2 (field emission gun) electron microscope operated at 200 kV. In situ heating and cooling TEM observations were performed in a Tecnai G2 electron microscope
Twins and CO1 phase at room temperature (RT)
PrCa2Mn2O7 has a Ruddlesden–Popper (RP) structure with (AO) (ABO3)n (n=2), where ABO3 is a perovskite and AO a rock salt layer. The MnO6 bilayers are stacked along the long axis and inserted by a rock salt layer in-between, as shown schematically in Fig. 1. Similar to previous TEM observations in PrCa2Mn2O7 [11], abundant lamellar twins with sharp (1 –1 0)A or (1 1 0)B twin interfaces were found when viewing along the [001] direction (Fig. 2a). These twins were believed to originate from the
Discussion
Fig. 9 gives a brief summary of our in situ TEM observations. When the CO1 phase is present at RT, it cannot be changed to the CO2 phase neither by heating nor by cooling (Fig. 9a). No OO rotation occurs because the 90° rotation of OO stripes is invariably accompanied with the phase transition between the CO1 and CO2 phase [10], [11]. When the CO2 phase is present at RT, it can be changed to the CO1 phase, and the accompanying OO rotation, by heating (Fig. 9b). However, the inverse phase
Conclusion
The CO1 and CO2 phases in PrCa2Mn2O7 can coexist at room temperature. The thermally induced rotation of the OO stripes is closely related with the stable phases at RT: for the CO1 phase at RT, no OO rotation occurs neither on heating nor cooling the samples; for the CO2 phase at RT, the OO rotation could be realized through the CO2–CO1 phase transition by heating. However, the OO rotation can be blocked by heating the samples into the CDO phase.
The detailed process of the rotation of the OO
Acknowledgments
We thank financial support from the European Research Council under the Seven Framework Programme (FP7), ERC Grant no. 246791 – COUNTATOMS.
References (29)
- et al.
J. Cryst. Growth
(2012) - et al.
Trans. Nonferrous Met. Soc. China
(2011) - et al.
Nature
(2003) - et al.
Nat. Mater.
(2004) - et al.
Science
(2005) J. Phys. D: Appl. Phys.
(2005)- et al.
Nature
(2006) - et al.
Nat. Mater.
(2007) - et al.
Nat. Mater.
(2007) J. Phys. Soc. Jpn.
(2010)
J. Phys.: Condens. Matter
Nat. Mater.
Phys. Rev. B
Phys. Today
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