Materials Science and Engineering B 139 (2007) 37–40 High resistive Co–Fe–Hf–O magnetic thin films for high-frequency applications Nguyen Duy Ha a,∗ , Anh-Tuan Le a , Manh-Huong Phan b , Heebok Lee c , Chong-Oh Kim a a Kamerlingh Onnes Laboratory, Faculty of Mathematics and Natural Sciences, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands b Aerospace Composites Group, Department of Aerospace Engineering, University of Bristol, Queen’s Building, Bristol BS8 1TR, United Kingdom c Department of Physics Education, Kongju National University, Kongju 314-701, South Korea Received 1 September 2006; received in revised form 23 November 2006; accepted 6 January 2007 Abstract Here, we present results of a systematic investigation of electrical and magnetic properties of Co–Fe–Hf–O thin films, which were deposited on Si(1 0 0) substrates by the oxygen reactive RF-sputtering method, at varying partial pressure of oxygen from 0 to 13%. Among the compositions investigated, we have achieved the optimal Co 19.35 Fe 53.28 Hf 7.92 O 19.35 film with desired properties of high saturation magnetization, 4πM s ∼ 19.86 kG, low coercivity, H c ∼ 1.5 Oe, high anisotropy field H k ∼ 84 Oe, and high electrical resistivity ρ ∼ 3569  cm. This film also exhibits a stable constant frequency response of the magnetic permeability up to 3 GHz, and reaches a maximum at the ferromagnetic resonant frequency of 4.024 GHz. The excellent properties of this film make it ideal for uses in high-frequency applications of micromagnetic devices. The dependence of the electrical and magnetic properties of Co–Fe–Hf–O film on the oxygen concentration can be understood from the microstructural evolution of this material. © 2007 Elsevier B.V. All rights reserved. Keywords: Co–Fe–Hf–O thin films; Magnetic properties; High resistivity; High-frequency application JEL classification: 75.70.Ak; 85.70Kh 1. Introduction In recent years, the micromagnetic devices, such as magnetic thin-film inductors, transformers for microswitching convert- ers, ultrahigh-density recording heads and thin-film fluxgate sensors [1–4], have been developed to miniaturize the mag- netic components in electronic equipment. However, in order to develop the high-performance magnetic thin films devices, it is necessary to suppress the excess losses due to eddy currents, and consequently, the electrical resistivity (ρ) of ferromagnetic films must be high to minimize energy loss due to eddy cur- rents while the large values of saturation magnetization (4M s ) and anisotropy field (H k ) are required to increase the switch- ing capacity of the films to higher frequencies [1,2]. Among developed ferromagnetic thin-film materials, the Co–Fe–Hf–O ferromagnetic thin films have received the most considerable attention because of their combined excellent properties includ- ∗ Corresponding author. Tel.: +31 71 527 5432; fax: +31 71 527 5404. E-mail address: nguyen@physics.leidenuniv.nl (N.D. Ha). ing high resistivity (ρ ∼ 1000–2500  cm), large saturation magnetization (4M s ∼ 10–16 kG) and hard-axis anisotropy field (H k ∼ 10–60 Oe) [2–7]. It has been shown that a typical Co–Fe–Hf–O film has a granular structure which consists of Fe–Co rich bcc nanograins embedded in a Hf–O rich amor- phous matrix [4–8]. The highly resistive Hf–O rich amorphous matrix contributes to the high resistivity of the film which sig- nificantly suppresses eddy current loss, and consequently, gives rise to good high-frequency performance [2,6]. Addition of rel- atively large Hf and O amounts usually leads to a significant increase in the resistivity of Co–Fe–Hf–O films, but this may considerably deteriorate their magnetic softness [8]. Therefore, to produce such highly resistive Co–Fe–Hf–O films with excel- lent magnetic properties, the main challenge lies in optimizing the volume fraction of the nanocrystalline Fe(Co) ferromagnetic phase and the Hf–O amorphous one. In this context, our efforts have been devoted to improve the high-frequency magnetic performance of Co–Fe–Hf–O films through fine tuning the alloy composition with varying oxygen concentration by using the oxygen reac- tive RF-sputtering method. We have achieved the optimal 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.01.027