COMMUNICATION © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 7) 1603131 Ferromagnetism at Room Temperature Induced by Spin Structure Change in BiFe 1−x Co x O 3 Thin Films Hajime Hojo,* Ryo Kawabe, Keisuke Shimizu, Hajime Yamamoto, Ko Mibu, Kartik Samanta, Tanusri Saha-Dasgupta, and Masaki Azuma* Dr. H. Hojo, [+] R. Kawabe, K. Shimizu, H. Yamamoto, Prof. M. Azuma Materials and Structures Laboratory Tokyo Institute of Technology Yokohama 226-8503, Japan E-mail: hojo.hajime.100@m.kyushu-u.ac.jp; mazuma@msl.titech.ac.jp Prof. K. Mibu Graduate School of Engineering Nagoya Institute of Technology Nagoya 466-8555, Japan K. Samanta, Prof. T. Saha-Dasgupta Department of Condensed Matter Physics and Materials Science S. N. Bose National Centre for Basic Sciences JD Block, Sector III, Salt Lake, Kolkata 700106, India DOI: 10.1002/adma.201603131 caused by spin canting and a linear magnetoelectric effect. Modifying the spin structure is therefore the key to realizing BFO-based ferromagnetic ferroelectrics. It is generally believed that epitaxial strain [8–10] and/or chem- ical substitution [11–14] can change the spin structure of BFO thin films, and most researchers have speculated that these films are (weakly) ferromagnetic. However, the origin of the observed magnetism is often unclear because even a small amount of magnetic impurities, e.g., γ-Fe 2 O 3 , can contribute to macro- scopic magnetic signals. [13,15] It was not until quite recently that the sensitivity of the spin structure of epitaxial BFO thin films to structural distortion due to epitaxial strain was confirmed experimentally; a collinear spin structure was stabilized in films grown on perovskite (100) (in pseudocubic notation, which we use throughout the article) substrates with a compressive strain of more than 1.7%. [16] Recently, it was reported that in the mag- netization of the BFO film the “canted spins” could be reversed by performing a two-step switching of the ferroelectric polariza- tion. [17] However, a macroscopic magnetization measurement was absent. We have recently succeeded in observing a spin structure transition from a low-temperature cycloidal one to a high-temperature collinear one at ≈120 K in a rhombohedral BiFe 0.8 Co 0.2 O 3 bulk sample by using neutron powder diffrac- tion. [18] Interestingly, magnetization measurements revealed that the collinear phase shows a weakly ferromagnetic behavior with the remanent moments of ≈0.02 µ B f.u. −1 (=µ B per Fe/Co ion) at room temperature, indicating that the spins are canted. Therefore, BiFe 0.8 Co 0.2 O 3 is expected to be ferromagnetic and ferroelectric at room temperature. If epitaxial BiFe 1−x Co x O 3 (BFCO) films with the spin structure change could be produced, it would serve as a promising playground for studying the cou- pling between ferroelectric and magnetic orderings. Although there have been several reports that claimed the enhanced mag- netization in cobalt-substituted BFO thin films, [11–14] the origin of the magnetization is unclear because of lack of detailed spin structure analysis and/or even temperature dependence of the magnetization. In order to realize a spin structure change in the thin film form, the choice of substrate is crucial since the spin structure of BFO is sensitive to structural distortion as mentioned above. We have chosen the SrTiO 3 (STO) (111) as the substrates to sta- bilize BFCO films with the rhombohedral structure, [19–21] which is the crystal structure of bulk BFCO (x < 0.3). Epitaxial BFCO films were fabricated by using pulsed laser deposition (PLD). We set the thickness to 200 nm to obtain fully relaxed films. [20,21] Besides, 200 nm is expected to be sufficiently thick to sustain the cycloidal modulation. An epitaxial SrRuO 3 (SRO) layer of 10 nm thickness was grown as a bottom electrode. The θ–2θ Multiferroic materials, where ferromagnetic and ferroelec- tric orders coexist, promise various improvements over singly ordered ferroic materials for the next generation of memory, sensing, and actuation applications. Here, the coexistence of such orders is confirmed at room temperature in epitaxial thin films of BiFe 1−x Co x O 3 (x ≤ 0.15), which manifests a spin struc- ture change from a low-temperature cycloidal one to a high- temperature collinear one with canted ferromagnetism. This is the first demonstration that BiFeO 3 -based films show ferromag- netism due to spin canting. BiFeO 3 (BFO) is the most widely studied multiferroic mate- rial, with robust ferroelectricity well above room temperature. [1] A cycloidal space-modulation with a periodicity of 62 nm is superimposed on the G-type antiferromagnetic structure as schematically illustrated in Figure 3c. [2] The origin of the cycloidal modulation has been ascribed to an inhomogeneous magnetoelectric coupling which arises from the fact BFO is noncentrosymmetric. [3,4] Although the origin is different, this spin structure is essentially the same as that of TbMnO 3 , [5] the material in which electric polarization induced by magnetic ordering was first found. Indeed, a sharp change in electric polarization in accordance with the appearance of a collinear spin structure with an estimated remanent magnetization of 0.03 µ B f.u. −1 has been observed in a single-domain crystal of BFO in a magnetic field of 18 T. [6] Recently, novel transverse electric polarization that is coupled with the domains of the cycloidal spin order have been discovered. [7] On the other hand, the presence of cycloidal modulation in a zero-magnetic field prohibits the appearance of a net ferromagnetic magnetization [+] Present address: Department of Energy and Material Science, Kyushu University, Kasuga 816-8580, Japan Adv. Mater. 2017, 1603131 www.advancedsciencenews.com www.advmat.de