4008 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009 Magnetic Anisotropy of FeCo Films Induced by Obliquely Sputtered Ru Underlayers Z. Lu , Y. Fukuma , W. H. Butler , H. Fujiwara , G. J. Mankey ,and S. Matsunuma MINT Center, University of Alabama, Tuscaloosa, AL 35487 USA Hitachi Maxell, Ltd., R&D Division, Ibaraki, Osaka 567-8567, Japan The magnetic properties of Si/Ru(t nm)/Fe Co (3.0 nm)/Ru (4.0 nm) films with obliquely sputtered Ru underlayers were studied as a function of Ru underlayer thickness (t). It was found that an obliquely sputtered Ru underlayer produces high uniaxial anisotropy in the Fe Co layer with extremely small hysteresis in the hard axis direction. The anisotropy field can be increased by increasing Ru underlayer thickness. X-ray reflectometry showed that the interfaces of the samples were quite smooth with rms roughness as small as 0.35 nm. The high uniaxial anisotropy of the samples shows high thermal stability and endures e-beam lithography for microscopic device fabrication. We discuss the origin of this high anisotropy in terms of a magnetostatic effect arising from an anisotropic surface morphology. Index Terms—Multilayered media, thin films. I. INTRODUCTION H IGH anisotropy is an important factor in determining the upper limit of high frequency operation for magnetic de- vices. The natural resonance frequency in a thin film is roughly proportional to , where is the in-plane uniaxial anisotropy field and is the saturation magnetization of the film. Thus, a magnetic thin-film device that is required to op- erate at high frequency requires the use of magnetic films with high and controllable anisotropy and magnetization. High and controllable in-plane anisotropy is also needed for the storage layer of a magnetic random access memory cell. It has been known for many years that oblique deposition can induce a high in-plane uniaxial magnetic anisotropy in magnetic films [1]–[5]. The observed uniaxial anisotropy easy axis direc- tion is either parallel or perpendicular to the incidence plane during the deposition depending on the incidence angle. The crystallites formed by the oblique deposition are elongated ei- ther parallel or perpendicular to the incidence plane and the easy axis direction coincides with that of the elongated crystallites. The proposed mechanisms include magnetostatic and magne- tostrictive effects originating from anisotropic morphology and crystalline anisotropy due to inclined texturing of the crystal- lites formed during deposition. Recently, it was reported that a high uniaxial anisotropy can also be induced by normal-in- cidence sputtering of Co on top of an underlayer of Ta, Pt, or Co sputter deposited by oblique incidence [6]–[9]. The pro- posed mechanism involves the magnetostatic interactions re- sulting from anisotropic undulation of the surface caused by the obliquely deposited underlayer [6]. More recently, we found that a Co Fe (fcc) film deposited by normal-incidence sputtering on top of a very thin Ru under layer (2.5 nm) grown by oblique sputtering exhibits a very high in-plane uniaxial anisotropy with highly square hysteresis curve Manuscript received March 06, 2009; revised May 02, 2009. Current version published September 18, 2009. Corresponding author: Z. Lu (e-mail: zlu@mint.ua.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2024161 in the easy axis direction and extremely small hysteresis loss and linearity approaching the saturation field in the hard axis direction [10]. The anisotropy field could be controlled up to at least 200 Oe. This paper reports that much higher anisotropy with similar hysteresis loop characteristics can be attained by using (bcc) Fe Co instead. Some comments on the possible origins of the anisotropy will also be given. Because (bcc) Fe Co has a higher saturation magnetization than Co, or (fcc) Co Fe , the (bcc) Fe Co films presented here may be advantageous for high frequency devices or other applications that require high in-plane magnetic anisotropy. II. EXPERIMENTAL PROCEDURE The samples used in this study were Fe Co films de- posited by normal-incidence sputtering on obliquely sputtered Ru underlayers on Si substrates with native oxides. The films were deposited at room temperature by dc magnetron sput- tering with an Ar pressure of 2 mTorr. The base pressure of the sputtering system was approximately Torr. The Ru underlayers were deposited on the substrate with an incident angle (angle between incident beam and surface normal of substrate) of 60 . This angle was chosen based on previous experience with Co Fe films and involved a compromise between anisotropy, which we expected to increase with angle, and deposition rate, which decreases. The Fe Co layers and Ru capping layers were deposited by normal-incidence sputtering. The thickness of the Ru underlayer varied from 1 to 8 nm. The thicknesses of Fe Co and Ru capping layers were fixed at 3.0 nm and 4.0 nm, respectively. The deposition rate was approximately 0.18 nm/min. The magnetic properties of the samples were characterized by using vibrating sample magnetometry (VSM). The interface of the samples was studied by using X-ray reflectometry (XRR) and transmission electron microscopy (TEM). Endurance for lithographic processing was checked by making the films into 200 nm circular elements by means of electron beam (e-beam) lithography and Ar ion milling. The magnetization switching in the individual patterned element was observed using the magneto-optical Kerr effect (MOKE). 0018-9464/$26.00 © 2009 IEEE