IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 2895 Electric and Magnetic Properties of Multiferroic BiFeO and YMnO Thin Films J. Zapata , J. Narváez , W. Lopera , M. E. Gómez , G. A. Mendoza , and P. Prieto Department of Physics, Universidad del Valle, A. A. 25360 Cali, Colombia Department of Physics, Universidad Nacional de Colombia, Bogotá D.C., A.A. 14490, Colombia Excellence Center for Novel Materials, Calle 13 No. 100-00, Edif. 320 Colombia We grew rhombohedrally distorted BiFeO and hexagonal YMnO thin films on Pt/TiO /SiO /Si substrates via RF magnetron sput- tering technique in a pure oxygen atmosphere. BiFeO and YMnO targets were self-made by the usual solid-state reaction method. We investigated the effects of deposition temperature upon crystalline structure, surface morphology, magnetization, and electrical polar- ization of BiFeO and YMnO thin films. The crystalline structure was studied by X-ray diffraction, and the topography of film surface was analyzed by atomic force microscopy. We also conducted measurements of ferroelectric and ferromagnetic hysteresis loops to study the electrical and magnetic behavior of the samples. Polarization, as a function of electric field in capacitor structures based on our BiFeO films, shows hysteretic behavior with a coercive field of 54 kV/cm and a remanent polarization of 21 C/cm ; whereas, YMnO films show hysteretic behavior with coercive field of 2.4 kV/cm, remanent polarization of 1.2 C/cm , and saturation polarization of 3.5 C/cm . Magnetization measurements of the BiFeO films evidence weak ferromagnetism, that can be related to the presence of a small quantity of ferromagnetic impurities. Index Terms—Ferroelectric and magnetic films, magnetoelectric materials, materials oxide thin films, sputtering. I. INTRODUCTION T HERE are very few materials exhibiting multiple func- tional properties; one such class of materials is called mul- tiferroics [1]. Multiferroics are interesting because they exhibit, simultaneously, two or more “ferroic” properties (i.e., ferroelec- tric, ferromagnetic, ferroelastic, or ferrotoroidicity [2]) in the same phase. Additionally, some multiferroics exhibit the mag- neto-electric effect (ME) [3], [4], which describes the induction of magnetization by means of an electric field and the induction of polarization by means of a magnetic field. In consequence, multiferroics offer rich physics and novel device concepts of re- cent interest to researchers. Perovskite BiFeO (BFO) has attracted much attention given the coexistence of ferroelectric and magnetic orders at room temperature. It possesses a rhombohedrally distorted perovskite structure with space group [5] and an antiferromagnetic/ ferroelectric phase with high Curie ( K) [6], and Néel ( K) temperatures [5]. Transport measurements in the bulk are very difficult due to leakage problems resulting from defects and nonstoichiometry, limiting the applications of this material. However, recently, highly resistive bulk samples with excellent single phase BFO have allowed measuring large intrinsic electric polarization on polycrystalline [7] and high-pu- rity single-crystal samples [8]. On the other hand, yttrium manganese oxide—YMnO (YMO) exhibits two stable crystallographic modifications: orthorhombic and hexagonal [9]. While magnetic ordering occurs in both phases, ferroelectric ordering occurs only in the hexagonal, which belongs to systems without central symmetries with the P63cm space group, besides displaying a ferroelectric transition with Curie temperature, Digital Object Identifier 10.1109/TMAG.2008.2002473 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. K [10] and a rather low antiferromagnetic Néel temperature, K [10]. The magnetic structure of YMnO consists of an -type antiferromagnetic order with an helical arrangement of the Mn [11]. Multiferroic thin films have attracted much attention for their possible applications in design of several practical devices [12]–[14]. Most BFO and YMO films have been fabricated via different techniques, such as: pulsed-laser deposition (PLD) or chemical solution deposition (CSD) methods. The sputtering process has advantages in terms of having a reduced process temperature and uniformity over a large area. Additionally, few investigations have addressed the highly oriented growth of BFO [15] and YMO [16] films by RF magnetron sputtering. In this paper, we report the effects of deposition temperature on the crystalline structure, surface morphology, electrical po- larization, and magnetic behavior of rhombohedrally distorted BFO and hexagonal YMO thin films grown via RF magnetron sputtering technique in a pure oxygen atmosphere. II. EXPERIMENTAL BFO and YMO thin films were grown via RF magnetron sput- tering method. The targets were prepared by the usual solid-state reaction method. Bi O and Fe O oxides were used as precur- sors of the BFO, while for the YMO we employed Y O and MnO . The purity of raw materials used was 99.99%. Mixtures of oxide precursors were prepared under air by weighing in sto- ichiometric proportions (1:1 mole ratio) and thoroughly mixing and grinding in an agate mortar. The BFO sample was precal- cined at 650 C for 1 h in an alumina crucible. The calcined ma- terial was once again macerated prior to heating to 810 C for 3 h [17]. The presence of bismuth oxide (Bi O ) impurity phase was detected in the X-ray diffraction (XRD) pattern in addition to the major BiFeO phase. Also, we detected phases at trace levels of (Bi Fe O ) and (Bi FeO ) [18]. For YMO, the first heating treatment was up to 1400 C for 3-h. The second heating treatment was up to 1400 C for 24-h, ensuring reaction among the components. 0018-9464/$25.00 © 2008 IEEE