Emergent phenomena in perovskite-type manganites

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Abstract

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn4+ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO3 and Sr1−xBaxMnO3 are discussed. In the YMnO3, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr0.5Ba0.5MnO3, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.

Introduction

Close interplay among spin, charge, orbital, and lattice degrees of freedom in correlated electron systems has been known to produce intriguing phenomena. Perovskite-type manganites are representative materials that show such interesting phenomena as colossal magnetoresistance (CMR) [1] and multiferroicity [2], [3]. CMR can be viewed as the magnetic-field induced melting of the long-range charge-/orbital-ordering or its short-range fluctuation, and subsequent phase transformation to the ferromagnetic metallic phase that competes with the charge-/orbital-ordered state. Therefore, phase competition is one of the fundamental features in the CMR physics and has been widely found in the phase diagram of hole-doped manganites. However, the situation is less clear in the electron-doped regime in the vicinity of Mn4+ state, mainly due to the difficulty in synthesizing single crystals without oxygen deficiency in such a high valent regime.

Another interesting phenomena is the multiferroicity exhibited by R MnO3 (R=rare earth element) with cycloidal spin structure [4], [5], [6]. Recently, perovskite-type R MnO3 with smaller R-ion that show the E-type spin structure have been predicted to possess larger polarization [7], [8], [9]. Indeed, several experimental studies have revealed the phase diagrams as well as the polarization values [10], [11], [12]. Nevertheless, the crystal structure of the ferroelectric phase has never been clarified due to the lack of single crystal sample as well as small displacement associated with the ferroelectric lattice distortion.

In addition to the spin-driven (improper) ferroelectricity with Mn3+ mentioned above, displacement-type (proper) ferroelectricity with off-center Mn4+ ion has also been theoretically predicted for A MnO3 with large A2+-ion [13], [14], [15]. This type of proper ferroelectrics with off-center magnetic ions would be very important since they are expected to have both large polarization value and the strong coupling between magnetism and ferroelectricity. Thus far, Sr1−xBaxMnO3 with x less than 20% have been synthesized, but proved to be paraelectric [16]. Materials with larger Ba content tend to crystallize in a hexagonal polymorph form, and perovskite-type structure has not yet been obtained.

Section snippets

Phase competition in Sr1−xCexMnO3

We have successfully grown single-crystalline samples of Sr1−xCexMnO3 by using two-step procedure [17]; first, oxygen-deficient single crystals of Sr1−xCexMnO3 were grown by the floating-zone method, and then the single crystal was treated under high oxygen-pressure. For these crystals with controlled electron-doping level (2x), we have investigated the temperature dependence of various properties, such as resistivity, magnetization, and lattice constants. Based on these data, the phase diagram

Determination of atomic displacements in perovskite-type YMnO3 with E-type spin structure

It is well known that YMnO3 and other rare-earth manganites with small rare-earth ion tend to crystallize in a hexagonal form under the ambient pressure, and the perovskite-type structure is stabilized only under the high pressure. Single crystals of perovskite-type YMnO3 were successfully obtained by high-pressure hydrothermal condition, as reported elsewhere [20]. Temperature dependence of magnetization clearly indicated two anomalies; one is at 42 K, which corresponds to the transition to the

Discovery of ferroelectricity with off-center magnetic ion in perovskite Sr1−xBaxMnO3

SrMnO3 has been predicted theoretically to be ferroelectric, but it is known to be paraelectric in reality. We tried to partially substitute Ba for Sr, and succeeded in synthesizing Sr1−xBaxMnO3 single crystals up to x=0.5 [24]. Also in this case, we used the two-step procedure to obtain the fully oxidized single-crystalline sample. As the Ba concentration is increased, the G-type antiferromagnetic transition temperature is gradually reduced. For the compounds with x larger than 0.4,

Summary

We have discussed three topics of the various emergent phenomena which are exhibited by perovskite-type manganites. In the electron-doped regime close to Mn4+, keen competition between G-type antiferromagnetic metal phase and C-type orbital-ordered insulator phase has been observed for the single crystals of Sr1−xCexMnO3. In the spin-driven ferroelectric YMnO3, atomic displacements as small as 10−3 Å order have been observed by means of synchrotron x-ray diffraction for the first time. New

Acknowledgment

This work was in part supported by supported by KAKENHI 22740244 and “Funding Program for World-Leading Innovative R & D on Science and Technology (FIRST Program)” on “Quantum Science on Strong Correlation” from JSPS. Single crystal x-ray diffraction experiment was performed at BL02B1, SPring-8 with approval of JASRI (Proposal nos. 2009B1304 and 2010A1795).

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    Present address: ISIR-SANKEN, Osaka University, Japan.

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