Magnetic Properties of Polycrystalline Bismuth Ferrite Thin Films Grown by Atomic Layer Deposition Pasi Jalkanen,* ,† Vladimir Tuboltsev, † Benoît Marchand, † Alexander Savin, ‡ Manjunath Puttaswamy, § Marko Vehkama ̈ ki, § Kenichiro Mizohata, † Marianna Kemell, § Timo Hatanpa ̈ a ̈ , § Valentin Rogozin, ∥ Jyrki Ra ̈ isa ̈ nen, † Mikko Ritala, § and Markku Leskela ̈ § † Department of Physics, Division of Materials Physics, University of Helsinki, P.O. Box 43, FI-00014, Helsinki, Finland ‡ School of Science, O.V. Lounasmaa Laboratory, Aalto University, P.O. Box 15100, FI-00076, Espoo, Finland § Department of Chemistry, Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, FI-00014, Helsinki, Finland ∥ Structural Materials Technology Faculty, Volgograd State Technical University, RU-400005, Volgograd, Russia ABSTRACT: The atomic layer deposition (ALD) method was applied to grow thin polycrystalline BiFeO 3 (BFO) films on Pt/SiO 2 /Si substrates. The 50 nm thick films were found to exhibit high resistivity, good morphological integrity, and homogeneity achieved by the applied ALD technique. Magnetic characterization revealed saturated magnetization of 25 emu/cm 3 with temperature-dependent coercivity varying from 5 to 530 Oe within the temperature range from 300 to 2 K. Magnetism observed in the films was found to change gradually from ferromagnetic spin ordering to pinned magnetic domain interactions mixed with weak spin-glass-like behavior of magnetically frustrated antiferromagnetic/ ferromagnetic (AFM-FM) spin ordering depending on the temperature and magnitude of the applied magnetic field. Antiferromagnetic order of spin cycloids was broken in polycrystalline films by crystal sizes smaller than the cycloid length (∼60 nm). Uncompensated spincycloids and magnetic domain walls were found to be the cause of the high magnetization of the BFO films. SECTION: Physical Processes in Nanomaterials and Nanostructures T echnologically viable multiferroic materials exhibiting simultaneous coupled electric, magnetic, and elastic order parameters at room temperature (RT) are of high interest for the development of next-generation micro- electromechanical devices. At present, one of the most studied multiferroics is perovskite bismuth ferrite BiFeO 3 (BFO) and its derivatives since this is the only single-phase material simultaneously presenting ferroelectricity (T Curie = 1103 K) and magnetic order (G-type antiferromagnetism, T Neel = 643 K) above RT. 1 Wide use of a material in microelectronics is determined to a large extent by the compatibility of the production method with the large scale device fabrication paradigm. Various synthesis methods have been applied to fabrication of good quality BFO with the purpose of achieving enhanced magnetization and ferroelectric polarization. 2−7 Magnetic properties of BFO have been proven to be affected by the material dimensionality and strain. Thus, the cycloid-like modulation canceling out a net magnetic moment in bulk BFO was shown to be suppressed in films and grains having a dimension smaller than the cycloid wavelength of about 60 nm. 8,9 This reveals a critical role of the internal morphology at the nanoscale for achieving the desired magnetic and ferroelectric properties that dictate in turn the choice of the appropriate synthesis method. 10,11 Therefore, a lot of attention has been focused on the study of interfaces and domain boundaries exhibiting versatile multiferroic proper- ties. 12−16 Controllable ferroelectric and magnetic domain walls stabilized by optimized boundary conditions were suggested to be utilized in domain wall-based logical circuits and racetrack memories. 14,17 The atomic layer deposition (ALD) method provides additional degrees of freedom in design and fabrication of devices depending on domain wall optimization, adding the option of conformity in deposition of various geometries with unprecedented thickness control. 18 Recently, first attempts to synthesize thin BFO films by ALD have been reported. 5,6,19 However, magnetic properties of the ALD-grown films have not been addressed so far. Here, we present our study of morphology and magnetism in just few tens of nanometers thick BFO films grown by ALD. Good dielectric properties exhibited by the grown films prove the applicability of ALD as a versatile technique for synthesis of complex BFO oxides. The capability of ALD to provide ultrathin films may pave the route to achieving BFO electromagnetic properties desired for practical applications via manipulation with the material dimensionality affecting the magnetic and ferroelectric domain structure. 17 Figure 1 shows a cross-section of one of the ALD-grown BFO films. As it follows from the figure, the film is comprised of densely packed polycrystalline grains having irregular hexagonal shape and average size of ∼30 nm. Grains A−G were found to have lattice fringe spacings in the (012) and (110)/(104), observable at different specimen tilt angles. There Received: October 29, 2014 Accepted: December 1, 2014 Published: December 1, 2014 Letter pubs.acs.org/JPCL © 2014 American Chemical Society 4319 dx.doi.org/10.1021/jz502285f | J. Phys. Chem. Lett. 2014, 5, 4319−4323