Louis-Philippe Carignan

Growth, structure, and properties of epitaxial thin films of first-principles predicted multiferroic Bi2FeCrO6

The authors report the structural and physical properties of epitaxial Bi2FeCrO6 thin films on epitaxial SrRuO3 grown on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The 300nm thick films exhibit both ferroelectricity and magnetism at room temperature with a maximum dielectric polarization of 2.8μC∕cm2 at Emax=82kV∕cm and a saturated magnetization of 20emu∕cm3 (corresponding to ∼0.26μB per rhombohedral unit cell), with coercive fields below 100Oe. The results confirm the predictions made using ab initio calculations about the existence of multiferroic properties in Bi2FeCrO6.

Ferromagnetic Nanowire Metamaterials: Theory and Applications

An overview of ferromagnetic nanowire (FMNW) metamaterials is presented. First, FMNW metamaterials are placed in the historical context of antique composites and 20th Century artificial dielectrics, and presented as an example of second-generation metamaterials following the microstructured metamaterials developed in the first part of the decade. Next, the fabrication processes of FMNW metamaterials and subsequent planar devices are detailed. It is then shown how the geometrical properties of the FMNW structure, such as the wire diameter and the wire nanodisk thicknesses, determine the dc and RF responses of the material. Upon this basis, the modeling of the metamaterial is presented, using a two-level approach where the microscopic (with respect to the wires) susceptibility is derived by solving the Landau-Lifshitz equation and the macroscopic (metamaterial) permittivity and permeability tensors are obtained by effective medium theory. Next, a review of FMNW microwave devices, such as circulators, isolators, and phase shifters, is provided, and the example of an FMNW dual-band edge-mode isolator is studied. Finally, spintronic effects and applications of FMNW metamaterials, such as dc to RF generators and detectors based on the spin-torque transfer phenomenon, are reviewed.

Magnetic anisotropy in arrays of Ni, CoFeB, and Ni/Cu nanowires

An effective field model based on intrawire and interwire dipolar interactions has been developed in order to describe the magnetic anisotropy in arrays of homogeneous and multilayer nanowires. Variable angle ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) characterization techniques were used to determine the effective interaction field acting on Ni, CoFeB, and Ni/Cu nanowires. FMR spectra are well described by a rigid magnetization model and VSM data are in rough agreement with a mean longitudinal field model. FMR and VSM values of the effective fields are mutually consistent and in fair agreement with the values calculated with the model. The results show that the anisotropy of our arrays is strongly dominated by the dipolar interactions.

Double ferromagnetic resonance in nanowire arrays

Microstrip line measurements are used to determine the frequency dependent microwave response of 40 nm diameter CoFeB ferromagnetic nanowire arrays, with external static applied field parallel to the nanowire axis. The ferromagnetic resonance (FMR) response of the wires is obtained for applied fields below and above magnetization saturation. For applied magnetic fields above saturation, a single FMR peak is observed, while below saturation, two sets of peaks are obtained. The two FMR peaks below saturation are associated with two magnetization populations, one for nanowires with upward magnetization and one with downward magnetization. A model based on a Maxwell–Garnett homogenization procedure has been established and used to predict the frequency response of the FMR peaks. There is good agreement between the model and experimental results.

Epitaxial thin films of the multiferroic double perovskite Bi2FeCrO6 grown on (100)-oriented SrTiO3 substrates: Growth, characterization, and optimization

The influence of the deposition pressure (PO2) and substrate temperature (TS) during the growth of Bi2FeCrO6 thin films grown by pulsed laser deposition has been investigated. It is found that the high volatility of Bi makes the deposition very difficult and that the growth of pure Bi2FeCrO6 thin films on SrTiO3 substrates is possible only in a narrow deposition parameter window. We find that the pure Bi2FeCrO6 phase is formed within a narrow window around an oxygen pressure PO2=1.2×10−2 mbar and around a substrate temperature TS=680 °C. At lower temperature or higher pressure, Bi7.38Cr0.62O12+x (also called b∗Bi2O3) and Bi2Fe4O9/Bi2(Fe,Cr)4O9+x phases are detected, while at lower pressure or higher temperature a (Fe,Cr)3O4 phase forms. Some of these secondary phases are not well known and have not been previously studied. We previously reported Fe/Cr cation ordering as the probable origin of the tenfold improvement in magnetization at saturation of our Bi2FeCrO6 film, compared to BiFeO3. Here, we address...

Magnetic Behavior of Ni/Cu Multilayer Nanowire Arrays Studied by First-Order Reversal Curve Diagrams

Arrays of Ni/Cu multilayer nanowires were fabricated by electrodeposition into alumina templates (pore diameter = 175 nm, interpore distance = 300 nm), with an Ni disc thickness between 20 and 50 nm, and a Cu thickness between 10 and 35 nm. Vector major hysteresis loops were measured with an out-of-plane (along the nanowire axis) and inplane (perpendicular to the nanowire axis) applied field. During the magnetization reversal, we did not observe any net component of transverse magnetization (My) , perpendicular to the applied field Hx. Extrapolated first-order reversal curve (FORC) diagrams were measured, out-of-plane and inplane. While the major hysteresis loops of all arrays have similar shapes, their FORC diagrams reveal distinct patterns, suggesting different magnetic behavior. The magnetic anisotropy can be controlled by the ratio of Cu to Ni thickness, the nanowire axis becoming harder as the ratio increases. In the out-of-plane direction, the maximum interaction field is proportional to the Ni thickness. The FORCs show evidence of a large coercive field distribution, which we attribute to the smaller interaction fields in the nanodiscs along the edges of the samples. In the inplane direction, depending upon the easy axis direction, there can be a mixture of nanodiscs reversed by coherent and incoherent rotation, or by incoherent rotation only.

Epitaxial thin films of multiferroic Bi2FeCrO6 with B-site cationic order

Epitaxial thin films of Bi2FeCrO6 have been synthesized by pulsed laser deposition on SrRuO3 on (100)- and (111)-oriented SrTiO3 substrates. Detailed X-ray diffraction and cross-section transmission electron microscopy analysis revealed a double perovskite crystal structure of the Bi2FeCrO6 epitaxial films very similar to that of BiFeO3 along with a particularly noteworthy Fe3+/Cr3+ cation ordering along the [111] direction. The films contain no detectable magnetic iron oxide impurities and have the correct cationic average stoichiometry throughout their thickness. They however exhibit a slight modulation in the Fe and Cr compositions forming complementary stripe patterns, suggesting minor local excess or depletion of Fe and Cr. The epitaxial BFCO films exhibit good ferroelectric and piezoelectric properties, in addition to magnetic properties at room temperature, as well as an unexpected crystallographic orientation dependence of their room temperature magnetic properties. Our results qualitatively confirm the predictions made using the ab-initio calculations: the double-perovskite structure of Bi2FeCrO6 films exhibit a Fe3+/Cr3+ cation ordering and good multiferroic properties, along with the unpredicted existence of magnetic ordering at room temperature.

Magnetization Reversal in Arrays of Ni Nanowires With Different Diameters

Arrays of Ni ferromagnetic nanowires of three different diameters (20, 40, and 170 nm) are obtained by electrodeposition into nanoporous alumina templates. Hysteresis curves parallel and perpendicular to the applied field are studied by angle dependent vector vibrating sample magnetometry. Hysteresis curves, from high remanence and coercivity for the smallest diameter, to nearly anhysteretic curves for the largest diameter, can be tuned by varying the diameter of the nanowires. The results show that the magnetic response of these arrays is a combination of coherent and incoherent rotation of magnetization of the nanowires.

Anomalously large ferromagnetic Curie temperature of epitaxial Bi2CoMnO6 thin films

We present the structural and physical properties of epitaxial Bi2CoMnO6 films grown on SrTiO3 and LaAlO3. They display magnetic hysteresis loop with 170 Oe coercivity and a 3.1μB∕f.u. saturation magnetization which indicates a disordered Co∕Mn state. Nevertheless, these films display an unusually large ferromagnetic Curie temperature of about 800 K. We propose that the large Curie temperature is the result of a strong coupling between the magnetization and the polarization order parameters of Bi2CoMnO6 promoted by substantial structural changes in the films with respect to the bulk. These structural modifications amplify the overlap of the electronic wave functions of the Mn–O–Mn, Mn–O–Co and Co–O–Co bonds and the Bi 6s2 lone pair electrons providing the long-range spin order through the minimization of the magnetoelastic energy.

Epitaxial Bi2FeCrO6 multiferroic thin films

We present experimental results on Bi2FeCrO6 (BFCO) epitaxial films deposited by laser ablation directly on SrTiO3 substrates. It has been theoretically predicted by Baettig and Spaldin [Appl. Phys. Lett. 86 012505 (2005)], using first-principles density functional theory, that BFCO is ferrimagnetic (with a magnetic moment of 2 µB per formula unit) and ferroelectric (with a polarization of ∼80 µC/cm2 at 0 K). The crystal structure has been investigated using X-ray diffraction, which shows that the films are epitaxial with a high crystallinity and have a degree of orientation depending on deposition conditions determined by the substrate crystal structure. Chemical analysis, carried out by X-ray microanalysis and X-ray photoelectron spectroscopy (XPS), indicates the correct cationic stoichiometry in the BFCO layer, namely (Bi:Fe:Cr = 2:1:1). XPS depth-profiling revealed that the oxidation state of Fe and Cr ions in the film remains 3+ throughout the film thickness and that both Fe and Cr ions are homogeneously distributed throughout the depth. Cross-section high-resolution transmission electron microscopy images plus selected area electron diffraction confirm the crystalline quality of the epitaxial BFCO films with no identifiable foreign phase or inclusion. The multiferroic character of BFCO is demonstrated by ferroelectric and magnetic measurements showing that the films exhibit ferroelectric and magnetic hysteresis at room temperature. In addition, local piezoelectric measurements carried out using piezoresponse force microscopy (PFM) show the presence of ferroelectric domains and their switching at the sub-micron scale.