Ferroelectricity in the 1 µC cm −2 range induced by canted antiferromagnetism in (LaMn3)Mn4O12

International audiencePyroelectric current and field-dependent specific heat measurements on polycrystalline samples of the quadruple per-ovskite (LaMn 3)Mn 4 O 12 give evidence of ferroelectricity driven by the canted antiferromagnetic ordering of the B-site Mn 3+ ions at T N,B =78 K with record large remnant electric polarization up to 0.56 µC cm −2. X-ray diffraction measurements indicate an anomalous behavior of the monoclinic β angle at T N,B , which suggests that the polarization lies in the ac-plane, where the moments are collinear, and that symmetric exchange striction is the mechanism of spin-driven ferroelectricity. Polarization values up to ∼3-6 µC cm −2 are expected in single crystals or epitaxial films, which would enable the development of practical multiferroic applications. In the search for multiferroic materials for applications, magnetic ferroelectrics, where ferroelectricity is induced by magnetic order, are promising for their inherently strong mag-netoelectric couplings 1. The challenge is to enhance the remnant polarization, which is typically small in these materials, P ∼ 0.1µC cm −2. This has been attributed to the weakness of the spin-orbit interaction in non-collinear spin structures 2 , where P arises from the antisymmetric exchange striction (in-verse Dzyaloshinskii-Moriya interaction) between neighbouring spins. More promising are collinear spin structures, where values up to ∼10 µC cm −2 are predicted due to the comparatively strong symmetric exchange striction 3. This prediction is supported by the large value P ∼ 1µC cm −2 reached under pressure in the perovskitelike compound TbMnO 3 , characterized by a collinear E-structure 4. The symmetric exchange mechanism has been invoked to account for a remarkable P-enhancement up to 0.27 µC cm −2 in another manganese oxide , (CaMn 3)Mn 4 O 12 (CMO) 5,6 , with quadruple perovskite (QP) AA 3 B 4 O 12 structure, characterized by two distinct A and B Mn sites 7. Though, the role of this mechanism in CMO remains controversial 8,9 possibly because of a complex interplay of incommensurate helicoidal spin structure and charge and orbital ordering of the Mn 3+ and Mn 4+ ions 10. Here we show that (LaMn 3)Mn 4 O 12 11,12 (LMO) is a model system to investigate the role of symmetric exchange stric-tion on magnetic ferroelectricity. LMO shares with CMO a similar QP structure, but exhibits simpler structural and electronic properties. LMO is a single-valent Mn 3+ system with neither charge orderings nor incommensurate structural modulations 11. The Im3 cubic structure stable at high temperatures undergoes a monoclinic I2/m distortion at 653 K 12. The B-site Mn 3+ ions antiferromagnetically (AFM) order forming a canted C-type structure at T N,B =78 K; the A-sites Mn 3+ ions also order AFM at lower temperatures T N,A =21 K 11. Here we report on record high electric polarizations ∼ 0.5µC cm −2 in polycrystalline LMO samples induced by the magnetic order at T N,B , so even higher values promising for applications are expected in single-crystalline samples. The LMO samples studied in the present work are 4 mm diameter cylinders synthesized under high-pressure as described elsewhere 11. Depending on synthesis conditions, either powders or small, 0.5 mm-sized, single crystals are obtained. Powders are typically 95 % pure and contain minor LaMnO 3 and Mn 3 O 4 impurities. One powder sample was studied by x-ray diffraction as a function of temperature in a D8 Bruker diffractometer equipped with a focusing primary Ge(111) monochromator for Cu K α1 radiation and a close-cycle He cryostat. Pyrocurrent, I p , measurements were carried out on three 0.2 mm thin disks of sintered powders of the same batch. The size of the available single crystals was too small for these measurements. Electrodes were realized by depositing a 0.1µm thick Au layer on both sides of the disks by magnetron sputtering. The disks were cooled in a close-cycle cryogenic system down to T p =100 K, at which a poling field E p = 3-36 kV cm −1 was applied for 30 min. At this temperature , the dc resistance of the samples exceeds 1 GΩ, which ensures their full polarization. We subsequently cooled the samples down to 15 K, removed the poling field and applied a short-circuit for 10 min to remove space charges