arXiv:1008.0315v3 [cond-mat.mtrl-sci] 24 Aug 2010 Domain wall magnetism in thin films of orthorhombic manganites C.J.M. Daumont * and S. Venkatesan, B.J. Kooi, J.Th.M. De Hosson and B. Noheda † Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands. (Dated: August 25, 2010) Thin films of orthorhombic TbMnO3, as well as other orthorhombic manganites, epitaxially grown on cubic SrTiO3 substrates display an induced magnetic moment that is absent in the bulk (an- tiferromagnetic) counterpart. Here we show that there is a clear correlation between the domain microstructure and the induced magnetic moment in TbMnO3 films on SrTiO3. In addition, the distinct dependence of the magnetization with the film thickness is not consistent with domain magnetism and indicates that the domain walls, rather than the domains, are the origin of the net magnetic moment. Since the orientation of the domain walls can be designed by the film-substrate relationship and its density can be tuned with the film thickness, these results represent a significant step forward towards the design of devices based on domain wall functionality. PACS numbers: 75.85.+t,75.70.-i,75.60.Ch Magnetoelectric and multiferroic materials re-emerged about ten years ago [1, 2] holding promise for novel devices and new physics [3–8]. Among these, the rare earth manganites (RMnO 3 ) are being intensively studied [9–12]. In these materials, the magnetic spin order can break inversion symmetry and directly induce ferroelectricity [13, 14]. Therefore, the cross-talking between the magnetic and electric degrees of freedom can be made very efficient [9]. Due to the crucial role that the structure plays determining the magnetic and ferroelectric order, it was naturally expected that the properties of multiferroics, and in particular manganites, could be greatly modified (an possibly improved) in thin films grown under epitaxial strain. Most multiferroics are ferroelectric antiferromagnets and present the limitation of lacking a net magnetization to be directly addressed by the electric or magnetic field. Remarkably, although the bulk materials are indeed antiferromagnetic, most existing reports on epitaxial manganite thin films coincide in a puzzling feature: the existence of an induced magnetic moment [15–21]. Recently, it has been proposed that the magnetic moment originates in the strain modification of the balance between the different magnetic exchange interactions [22], and that a canting of the Mn spins is induced [21, 23, 24]. Interestingly, the magnetic moment seems to be induced independently of the ground state magnetic structure of the bulk material: TbMnO 3 displays a cycloidal spin structure whereas YbMnO 3 and YMnO 3 have a collinear spin structure of E-type. In both types, the induced magnetization has been reported to follow a similar trend with the unit cell volume [24]. This opens the question if the magnetic moment arises from the modification of the magnetic structure via the strain modification of the bond lengths and bond angles, as expected, or if it originates from a more general feature of epitaxial orthorhombic manganites, such as the domain microstructure. Indeed, it has been shown that orthorhombic manganites grown on cubic SrTiO 3 substrates show crystallographic twins [23]. Twinning is mainly determined by the symmetry relationships between the film and substrate materials but it is affected by the growth kinetics and it can differ depending on the growth conditions. TbMnO 3 thin films grown on (001)-SrTiO 3 by Daumont et al. with low deposition rates (approaching thermodynamic conditions) are reported to form four types of twin domains [25]. Despite the very large mismatch strain (+ 4.1% along [100] o and -5.7% along [010] o ), this microstructure allows the film to keep partial coherence with the substrate, either along the [100] c or the [010] c (substrate) directions and, importantly, it determines the evolution of the lattice parameters with increasing thickness: The partial coherence with the substrate and the crystal twinning are able to maintain the unit cell in-plane area constant, and thus the out-of-plane lattice parameter and the unit cell volume, basically unchanged for a large range of thicknesses from 5 to 70nm [25]. This domain/twin configuration relaxes the film by allowing a o and b o to change in opposite directions and by the same amounts. It can be rationalized that this is very efficient minimizing the elastic energy of the system [26]. In fact, similar twinning patterns have been observed in BiFeO 3 films grown on SrTiO 3 [27]. Here we investigate the possible relationship between the microstructure and the magnetic properties of epitaxial TbMnO 3 in order to learn about the origin of the ferromagnetic component observed in thin film manganites. It is worth to mention that the twin boundaries become antiferromagnetic domain boundaries below the magnetic ordering temperature and that it is reasonable to expect that the symmetry breaking that takes place at the domain walls could allow them to host net magnetic moment [28–31]. Since the domain walls can be as small as a few nanometers, * Also at Oxides Fonctionnels, Unit´ e mixte de Physique CNRS/Thales UMR137, 91767 Palaiseau, France. † Electronic address: b.noheda@rug.nl