Self-biased cobalt ferrite nanocomposites for microwave applications Abdelkrim Hannour a,n , Didier Vincent a , Faouzi Kahlouche a , Ardaches Tchangoulian a , Sophie Neveu b , Vincent Dupuis b a LT2C Laboratory, Jean-Monnet University, 25 rue Dr. Rémy Annino, F-42000, Saint-Etienne, France b UPMC Univ Paris 06, UMR 7195, PECSA, F-75005, Paris, France article info Article history: Received 3 August 2013 Received in revised form 3 October 2013 Available online 22 October 2013 Keywords: Microwave application Cobalt ferrite nanoparticle Magnetic nanocomposite Ferromagnetic resonance PMMA AAO membrane abstract Oriented CoFe 2 O 4 nanoparticles, dispersed in polymethyl methacrylate (PMMA) matrix, were fabricated by magnetophoretic deposition of functionalized nanocolloidal cobalt ferrite particles into porous alumina membrane. Their magnetic behavior exhibits an out-of-plane easy axis with a large remanent magnetization and coercitivity. This orientation allows high effective internal magnetic anisotropy that contributes to the permanent bias along the wire axis. The microwave studies reveal a ferromagnetic resonance at 46.5 and 49.5 GHz, depending on the filling ratio of the membrane. Ansoft High Frequency Structure Simulator (Ansoft HFSS) simulations are in good agreement with experimental results. Such nanocomposite is presented as one of the promising candidates for microwave devices (circulators, isolators, noise suppressors etc.). & 2013 Elsevier B.V. All rights reserved. 1. Introduction Recently, nanocomposites based on magnetic nanostructures embedded into Anodic Aluminum Oxide (AAO) membrane are becoming increasingly important for developing circulators [1] magnetic photonic band gap materials [2], isolators [3] and noise suppressors [4]. For example, most circulators use the non- reciprocal properties of ferrite materials for obtaining the circula- tion effect [5]. Their technological applications concern essentially wireless communications as mobile phone systems, satellite links, radar duplexers and military applications [6]. However, the main disadvantages of conventional circulators, widely used in micro- wave devices, are that the ferrite needs to be biased by an external dc magnetic field to show a ferromagnetic resonance, as well as the geometry of bulk ferrite, difficult to integrate on future miniature planar circulators. Thus, in order to work in high-frequencies ( 440 GHz) and reduce the circulators size by removing the permanent magnet needed for biasing the ferrite crystal, many research works were done to fabricate unbiased microwave circu- lators using hexaferrite [7,8] or ferromagnetic nanowires [9]. These features present an interesting alternative way to bulky ferrite materials. Indeed, by using a self-biased ferrite nanocomposite based on ferrite inclusions (nanoparticles) in dielectric host matrix, one can control the key physical parameters determining the performances of circulators, namely, anisotropy coefficient K , damping parameter α, saturation magnetization M s and particle shape [10]. Another important fact is that the choice of dielectric matrix with low dielectric losses is necessary for reduction of the insertion losses (PMMA [11] and AAO [12] for example). In this paper, we report an original approach that combines the highest saturation magnetization and magneto-crystalline aniso- tropy of preformed functionalized CoFe 2 O 4 nanoparticles [13,14] and the high-aspect ratio nanostructure offered by AAO template. We focus on the investigation of morphological, magnetic and microwave properties of oriented CoFe 2 O 4 nanoparticles prepared by magnetophoretic deposition of cobalt ferrite nanoparticles into porous alumina template using Scanning Electron Microscope (SEM), Superconducting Quantum Interference Device (SQUID) magnetometer and Vector Network Analyzer (VNA), respectively. 2. Experimental details 2.1. Synthesis of nanocolloidal CoFe2O4 nanoparticles Ferrofluid based on cobalt ferrite nanoparticles was prepared by dispersion of cobalt ferrite nanoparticles in toluene solution. First, nanoparticles with a spherical shape, an average diameter ranging from 18 to 20 nm and a spinel crystal structure (Fig. 1), were synthesized by an alkaline coprecipitation of cobalt hydro- xide and iron hydroxide followed by an hydrothermal decomposi- tion treatment (200 1C, 24 h) [15]. The precipitate obtained was isolated and dispersed in an aqueous solution of nitric acid. BNE surface agent was then added. At the end of this process, the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.10.010 n Corresponding author. Tel.: þ33 61 1927837. E-mail address: abdelkrim.hannour@hotmail.com (A. Hannour). Journal of Magnetism and Magnetic Materials 353 (2014) 29–33