Magneto-Optical Kerr Effect in Multiferroic Nanostructures
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Magneto-Optical Kerr Effect in Multiferroic Nanostructures Matthew T. Corbo1, Florian Straub1, Haimei Zheng1, Maria de la Paz Cruz1,2, and Yuri Suzuki1 1 Materials Science & Engineering, University of California, Berkeley, Berkeley, CA, 94720 2 Centro de Ciencias de la Materia Condensada, Universidad Nacional Autonoma de Mexico, Km. 107 Carretera Tijuana-Ensenada, Ensenada, B.C., 22800, Mexico ABSTRACT We report the study of the magneto-optical properties of composite multiferroic thin films composed of CoFe2O4 nanopillars embedded in a BiFeO3 matrix. The magneto-optical Kerr rotation and Kerr ellipticity in these films have been measured and are in good agreement with magnetization measurements. The Kerr signal has been studied as a function of film composition and nanopillar diameter confirming that the magneto-optical signal is due solely to the CoFe2O4 nanopillars. INTRODUCTION With the continued efforts in the miniaturization of electronic devices, materials that have the ability to incorporate multiple functionalities into a single element have become very attractive. Magnetoelectric (ME) materials [1,2] have the potential of combining electrical and magnetic functionality into a single device. These phenomena have been studied in single-phase oxide materials such as Cr2O3, GdAlO3, TbAlO3, DyAlO3 and more recently in BiMnO3 and BiFeO3 [3-6]. The long-range magnetic order can be antiferromagnetic as in Cr2O3 and BiFeO3 or ferromagnetic as in BiMnO3; however, the magnetoelectric effect has been disappointingly small in these materials. An alternative route to achieving large magnetoelectric effects has been to synthesize composite films of magnetostrictive and piezoelectric materials. Recently, nanostructured, heteroepitaxial composite multiferroic thin films have been fabricated and shown to exhibit significant magnetoelectric coupling [7,8]. In these materials, the magnetoelectric effect is observed as a stress-mediated electric field induced change in the magnetization direction within the film. For example, local magnetoelectric coupling has been demonstrated by measuring the magnetic response after electrically poling 10% of the sample covered with 30 µm diameter test capacitors [8]. Optical characterization, and specifically the use of magneto-optics, provides a noncontact, non-destructive tool for probing the magnetism in thin films. Previous attempts to study the magnetoelectric coupling using optical and magneto-optical (MO) methods in single phase multiferroic materials were limited by the inability to achieve large ME coupling coefficients. Most optical studies have relied heavily on nonlinear optics, including second harmonic generation (SHG) [9,10]. Lottermoser, et al. recently demonstrated electric field induced spontaneous ferromagnetic ordering of Ho3+ atoms in HoMnO3. This was accomplished by observing the change in polarization of the SHG signal transmitted through a bulk HoMnO3 sample at cryogenic temperatures (1.5K). It was concluded that ferromagnetic ordering of Ho3+ ions can b
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