3112 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 Synthesis and Magnetic Properties of Co Ir Alloy Nanoparticles for High-Frequency Applications C. N. Chinnasamy , T. Ogawa , D.Hasegawa , H. T. Yang , S. D.Yoon , V. G. Harris , and Migaku Takahashi Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115 USA Department of Electrical and Electronic Engineering, Tohoku University, Sendai 980-8579, Japan National Industry Creation Hatchery Center (NiCHe), Tohoku University, Sendai 960-8579, Japan Co Ir ( and ) alloy nanoparticles were prepared using a high-temperature chemical reduction technique in an inert atmosphere for the first time. High-resolution electron microscopy studies show the formation of nearly monodispersed Co Ir alloy nanoparticles having the hexagonal close packed structure. The sizes of the particles were dictated by controlling the reaction param- eters and the type of surfactants. Superconducting quantum interference device (SQUID) magnetometry studies showed soft magnetic behavior at 300 K and also at 5 K. The as-prepared, nearly dispersed nanoparticles were assembled and fixed on a polymer substrate. The microwave properties measured by ferromagnetic resonance at 9.61 GHz indicate a minimum line width of about 300 Oe. Index Terms—Chemical synthesis and high-frequency applications, Co-Ir alloy nanoparticles, nanoparticles. I. INTRODUCTION H EXAGONAL close packed (hcp)-Co-based metallic al- loys are important magnetic materials for the high-den- sity magnetic recording media [1]. The recording characteristics have been shown to be improved by the addition of 5d transition elements to the Co based thin film media. On the other hand, for high-frequency device applications, much research has been done on the magnetization dynamics in ferromagnetic mate- rials [2]–[4]. However, to improve the high-frequency isotropic magnetic properties using ferromagnetic materials, the ferro- magnetic resonance frequency, , of the material should be in- creased by increasing the magnetic anisotropy field. Superpara- magnetism has isotropic magnetic properties and no hysteresis loss in a dc field and its thermal equilibrium magnetic suscepti- bility, can be described as (1) Here, is the volume of the superparamagnetic nanoparticles, is the saturation magnetization, is the Boltzmann con- stant, and is the temperature. In an ac field, the magnetic sus- ceptibility can be described as (2) (3) where is the frequency, is the blocking frequency defined as the magnetic potential barrier height per unit volume, and is the maximum relaxation frequency which is roughly the same as the ferromagnetic resonance frequency [4]. According to (2) and (3), the superparamagnetic properties are maintained up to . Therefore, in order to make and higher, high Digital Object Identifier 10.1109/TMAG.2007.893868 and low E are needed. However, in the case of magnetic materials with positive uniaxial magnetocrystalline anisotropy, and E are described as (4) where is magnetic anisotropy energy and is gyromagnetic constant. Thus, both high and low cannot be satisfied simultaneously. Since 5d transition elements possess large spin-orbit coupling, it is expected that these elements signif- icantly influence the magneto crystalline anisotropy energy of the ferromagnetic 3-d alloys. For example, face centered tetragonal (fct) 3-d–5d alloys with L1 structure, viz., FePt and CoPt, the 5d elements greatly enhance the magnetocrystalline anisotropy emergy through its large spin-orbit coupling and strong 3-d–5d hybridization [5]. In contrast, Kikuchi et al. showed that as the Ir content increases, the sign of changes from positive to negative indicating that the easy axis of mag- netization rotates from the c axis to the c-plane in the case of thin films [6]. We have also proposed the hexagonal close packed structured C-plane oriented soft magnetic Co Ir alloy thin films for high-frequency applications [7]. In order to realize the same phenomena in nanoparticles, we synthesized Co Ir alloy nanoparticles and studied its structural and magnetic properties. II. EXPERIMENTAL PROCEDURE The experiments were carried out using standard airless pro- cedures. In a typical synthesis: 1) 25 ml octyle ether, 2 mmol of 1,2 hexadecanediol, anhydrous CoCl and Ir(acac) in the respective ratios were added together and temperature was in- creased to 100 C and kept for 10 min. 2) 0.5 mmol of oleic acid (OA) and 0.5 mmol of oleylamine (OY) or triphenylphosphine (TOP) were added and the temperature was increased to 200 C and kept for 15 min. 3) A solution of super hydride (LiBEt H of 1 M THF solution, 2 ml) was added dropwise, and the reaction temperature was increased to 260 C. The reaction was held at 0018-9464/$25.00 © 2007 IEEE