Evidence for Large Electric Polarization From Collinear Magnetism in TmMnO3

There has been tremendous research activity in the field of magnetoelectric (ME) multiferroics after Kimura et al (2003 Nature 426 55) showed that antiferromagnetic and ferroelectric orders coexist in orthorhombically distorted perovskite TbMnO3 and are strongly coupled. It is now generally accepted that ferroelectricity in TbMnO3 is induced by magnetic long-range order that breaks the symmetry of the crystal and creates a polar axis (Kenzelmann et al 2005 Phys. Rev. Lett. 95 087206). One remaining key question is whether magnetic order can induce ferroelectric polarization that is as large as that of technologically useful materials. We show that ferroelectricity in orthorhombic (o) TmMnO3 is induced by collinear magnetic order, and that the lower limit for its electric polarization is larger than in previously investigated orthorhombic heavy rare-earth manganites. The temperature dependence of the lattice constants provides further evidence of large spin–lattice coupling effects. Our experiments suggest that the ferroelectric polarization in the orthorhombic perovskites with New Journal of Physics 11 (2009) 043019 1367-2630/09/043019+09$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft 2 commensurate magnetic ground states could pass the 1μC cm−2 threshold, as predicted by theory (Sergienko et al 2006 Phys. Rev. Lett. 97 227204; Picozzi et al 2007 Phys. Rev. Lett. 99 227201). Multiferroic materials are defined as materials with more than one switchable spontaneous order parameter such as ferromagnetism and ferroelectricity. It has become custom to include materials with coexisting spontaneous antiferromagnetic and ferroelectric order in the class of magneto-electric (ME) multiferroics. One can distinguish two major classes of ME multiferroics: those where the onset of ferroelectricity is unrelated to magnetic order, and those where ferroelectricity is induced by magnetic order. Hexagonal YMnO3 is an example of a multiferroic material where the onset of ferroelectricity is completely unrelated to the onset of magnetism, and probably arises from geometrical effects [5]. Orthorhombic TbMnO3 is an example of a multiferroic material where ferroelectricity arises from magnetic spiral order [1, 2]. Ferroelectricity from magnetic order is related to competing magnetic interactions, whose competition at low temperatures is reduced through small lattice distortions that result in switchable electric polarization. Magnetically induced ferroelectricity has been observed for structurally very different materials, most notably in rare-earth (R) manganites RMn2O5 [6], the kagome staircase magnet Ni3V2O8 [7] and the triangular lattice antiferromagnet RbFe(MoO4)2 [8]. This suggests that the mechanism to obtain ferroelectricity from magnetic order is quite general and should be present in many materials. In all these materials, ferroelectric polarization arises, at least partly, from incommensurate spiral magnetic structures that lead to polar structures. The ME interaction in these materials is believed to be mediated by spin–orbit interactions, and so the ferroelectric polarization is relatively small. Much larger ferroelectric polarizations were predicted for materials where ferroelectricity arises from collinear magnetic order [3, 4]. In such materials, ME coupling may be mediated by the symmetric exchange, which is larger than spin–orbit related interactions. An example is orthorhombic (o) HoMnO3 where ferroelectricity arises from commensurate, collinear magnetic order [9, 10]. However, the ferroelectric polarization in o-HoMnO3 was observed to be much smaller than predicted [4], and arises partly from rare-earth magnetic order [9]. Here, we present the case of o-TmMnO3 for which we observed a ferroelectric polarization that arises from collinear Mn magnetic order, and that is at least 15 times larger than observed for o-HoMnO3. We provide evidence for spin–lattice coupling effects that are larger than in other magnetically induced ferroelectrics. Polycrystalline samples of perovskite TmMnO3 were prepared under high pressure as described in [11]. Neutron powder diffraction measurements were performed on a large amount (5.4 g) of TmMnO3 sample using the HRPT [12] and DMC diffractometers at the Paul Scherrer Institute, and incident neutrons with a wavelength of 1.89 and 4.5Å, respectively. The magnetic structures were determined using the Fullprof Suite [13]. No texture effects were observed during the analysis. The ferroelectric polarization was determined using a 0.4mm thin hardened pellet of polycrystalline TmMnO3 covered with an area 3.12× 10−6m2 of silver epoxy. The sample was cooled from 50 to 2K in poling electric fields of up to E = 3750 kVm−1, after which the electric field was reduced to zero and the sample was allowed to discharge for 5min. After the discharge at 2K the residual current was reduced to 10−14A, which suggests that trapped charges did not affect the pyroelectric measurement. Then the sample was heated at different constant rates New Journal of Physics 11 (2009) 043019 (http://www.njp.org/)