Magnetostatic Interactions in Self-Assembled Co_<i>x</i>Ni_1–<i>x</i>Fe₂O₄/BiFeO₃ Multiferroic Nanocomposites

Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition Co_<i>x</i>Ni_1–<i>x</i>Fe₂O₄ was varied over the range 0 ≤ <i>x</i> ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a “checkerboard” ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results

Multiferroic Behavior of Templated BiFeO₃–CoFe₂O₄ Self-Assembled Nanocomposites

Self-assembled BiFeO₃–CoFe₂O₄ nanocomposites were templated into ordered structures in which the ferrimagnetic CoFe₂O₄ pillars form square arrays of periods 60–100 nm in a ferroelectric BiFeO₃ matrix. The ferroelectricity, magnetism, conductivity, and magnetoelectric coupling of the ordered nanocomposites were characterized by scanning probe microscopy. The insulating BiFeO₃ matrix exhibited ferroelectric domains, whereas the resistive CoFe₂O₄ pillars exhibited single-domain magnetic contrast with high anisotropy due to the magnetoelasticity of the spinel phase. Magnetoelectric coupling was observed in which an applied voltage led to reversal of the magnetic pillars

Combinatorial Pulsed Laser Deposition of Fe, Cr, Mn, and Ni-Substituted SrTiO₃ Films on Si Substrates

Combinatorial pulsed laser deposition (CPLD) using two targets was used to produce a range of transition metal-substituted perovskite-structured Sr­(Ti_1–<i>x</i>M_<i>x</i>)­O_3−δ films on buffered silicon substrates, where M = Fe, Cr, Ni and Mn and <i>x</i> = 0.05–0.5. CPLD produced samples whose composition vs distance fitted a linear combination of the compositions of the two targets. Sr­(Ti_1–<i>x</i>Fe_<i>x</i>)­O_3−δ films produced from a pair of perovskite targets (SrTiO₃ and SrFeO₃ or SrTiO₃ and SrTi0_0.575Fe_0.425O₃) had properties similar to those of films produced from single targets, showing a single phase microstructure, a saturation magnetization of 0.5 μ_B/Fe, and a strong out-of-plane magnetoelastic anisotropy at room temperature. Films produced from an SrTiO₃ and a metal oxide target consisted of majority perovskite phases with additional metal oxide (or metal in the case of Ni) phases. Films made from SrTiO₃ and Fe₂O₃ targets retained the high magnetic anisotropy of Sr­(Ti_1–<i>x</i>Fe_<i>x</i>)­O_3−δ, but had a much higher saturation magnetization than single-target films, reaching for example an out-of-plane coercivity of >2 kOe and a saturation magnetization of 125 emu/cm³ at 24%Fe. This was attributed to the presence of maghemite or magnetite exchange-coupled to the Sr­(Ti_1–<i>x</i>Fe_<i>x</i>)­O_3−δ. Films of Sr­(Ti_1–<i>x</i>Cr_<i>x</i>)­O_3−δ and Sr­(Ti_1–<i>x</i>Mn_<i>x</i>)­O_3−δ showed no room temperature ferromagnetism, but Sr­(Ti_1–<i>x</i>Ni_<i>x</i>)­O_3−δ did show a high anisotropy and magnetization attributed mainly to the perovskite phase. Combinatorial synthesis is shown to be an efficient process for enabling evaluation of the properties of epitaxial substituted perovskite films as well as multiphase films which have potential for a wide range of electronic, magnetic, optical, and catalytic applications

Magnetic and Photoluminescent Coupling in SrTi_0.87Fe_0.13O_3−δ/ZnO Vertical Nanocomposite Films

Self-assembled growth of SrTi_0.87Fe_0.13O_3−δ (STF)/ZnO vertical nanocomposite films by combinatorial pulsed laser deposition is described. The nanocomposite films form vertically aligned columnar epitaxial nanostructures on SrTiO₃ substrates, in which the STF shows room-temperature magnetism. The magnetic properties are discussed in terms of strain states, oxygen vacancies, and microstructures. The nanocomposites exhibit magneto-photoluminescent coupling behavior that the near-band-edge emission of ZnO is shifted as a function of magnetic field

Hierarchical Templating of a BiFeO₃–CoFe₂O₄ Multiferroic Nanocomposite by a Triblock Terpolymer Film

A process route to fabricate templated BiFeO₃/CoFe₂O₄ (BFO/CFO) vertical nanocomposites is presented in which the self-assembly of the BFO/CFO is guided using a self-assembled triblock terpolymer. A linear triblock terpolymer was selected instead of a diblock copolymer in order to produce a square-symmetry template, which had a period of 44 nm. The triblock terpolymer pattern was transferred to a (001) Nb:SrTiO₃ substrate to produce pits that formed preferential sites for the nucleation of CFO crystals, in contrast to the BFO, which wetted the flat regions of the substrate. The crystallographic orientation and magnetic properties of the templated BFO/CFO were characterized