2686 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009 Effects of the Crystalline Structure on the Switching Field and Its Distribution of Bit Patterned Co/Pt Multilayer Media Vickie W. Guo , Hwan-Soo Lee , Yi Luo , Matthew T. Moneck ,and Jian-Gang Zhu Data Storage Systems Center, CarnegieMellon University, Pittsburgh, PA 15213 USA Samsung Electro-Mechanics, Suwon 443-743, South Korea We studied the effects of medium microstructure on the switching field (SF) and its distribution (SFD). Specifically, quasi-single crys- talline and polycrystalline Co/Pt based multilayers, chosen for its high perpendicular anisotropy and high exchange coupling, have been prepared via sputtering technique. The quasi-single crystalline was achieved by utilizing an Ag underlayer. Using electron-beam lithog- raphy, films were patterned into arrays of small islands, ranging from 1 m to 0.050 m( nm) in size. The hysteresis loop for the both types of film, as unpatterned, showed similar anisotropy field and coercivity whereas, as patterned, the quasi-single crystalline samples consistently exhibited a higher SF than that of the polycrystalline samples. However, the interpretation on the SFD was rather complex. Additionally, a critical size where remanent magnetization changes a multi domain to a single domain state appeared to be notably smaller for the polycrystalline samples than that for the quasi-single crystal samples, suggesting a different nucleation volume in the films. Index Terms—Co/Pt multilayers, medium microstructure, switching field (SF), switching field distribution (SFD). I. INTRODUCTION B IT PATTERNED media (BPM) that has a reasonably high switching field (SF) and the sufficiently narrow distribu- tion will have a major impact on the future of high density mag- netic data storage and is currently the subject of active research [1]–[4]. Understanding the origin of the SF and the switching field distribution (SFD) for patterned elements, which are influ- enced by fundamental physical and intrinsic magnetic proper- ties such as anisotropy, magnetization, exchange coupling, and magnetostatic interaction, can be a key to designing desirable BPM media. There is, so far, no agreement on the optimal properties of a practical patterned medium, however, either Co/Pt or Co/Pd based multilayers have been extensively studied owing to their high perpendicular anisotropy and strong exchange coupling [3]–[6]. For BPM, highly exchange coupled systems have been favorably adopted since strong exchange coupling is most likely to lead to a single-domain state. In this work, two types of sample possessing distinctively different microstructure, namely, quasi-single crystalline (QSC) and polycrystalline (PC) films were prepared. The QSC and PC Co/Pt multilayer films were fabricated via sputtering technique, and the films were subsequently patterned into periodic arrays of dot or square shaped features, using electron-beam lithog- raphy. The patterns were ranged from 1000 nm ( m) to 50 nm in size. The QSC microstructure was achieved by employing an Ag underlayer on Si (111). Our primary focus is to study the behavior of magnetization switching for patterned elements and the effects of medium microstructure on the SF and the SFD. Moreover, a pattern size where remanent magnetization changes a multi domain to a single domain state was identified for the both films. Manuscript received October 16, 2008. Current version published May 20, 2009. Corresponding authors: V. W. Guo and H.-S. Lee (e-mail: wguo@an- drew.cmu.edu; jisoo725@naver.com). 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.2009.2018640 Fig. 1. XRD spectrum of the QSC sample. Use of the Ag underlayer resulted in (111) textured Co/Pt multilayers. The inset indicates a schematic of the lattice planes of Si(111), Ag(111), and Pt(111). II. EXPERIMENTAL SETUP All films were prepared using dc magnetron sputtering in a 5mtorr Ar. A Si wafer was rinsed using buffered HF for re- moving the native oxide and achieving a hydrophobic surface. For the fabrication of QSC films, prior to a 6-nm-thick Pt layer, a 15-nm-thick Ag seed layer was deposited onto a single crystal Si (111) substrate for the purpose of alleviating the lattice mis- match between the Pt (111) and the Si (111). Subsequently, 10 times of repeats of a Co(6 Å)/Pt(18 Å) stack were carried out. The deposition rates for the Co and the Pt were 0.23 Å/s and 0.27 Å/s, respectively. The Co/Pt multilayer stack was finally capped with a 3-nm-thick Pt layer. In Fig. 1, an X-ray diffraction spectrum for the QSC sample [Si/Ag/Pt/(Co/Pt) ] is shown. Peaks corresponding to the Si (111), the Ag (111), the Pt (111) reflections were observed, in- dicating the prominent [111] texture of the QSC film. The Co (111) peak was not clearly discernible since the Co layer was probably too thin to provide any detectable signal. The Ag was reported to grow on the Si (111) with a cube-on- cube orientational relationship [7]. A misfit between the Ag (fcc, Å) and Si (diamond cubic, Å) is %. However, a 4 4 Ag unit cell fits remarkably well onto a 3 3 Si unit cell, revealing a mismatch of only 0.4%. Lattice parame- ters for Pt and Co are 3.92 Å and 3.55 Å, respectively. The good lattice match between the Pt and the Ag permits the Si (111) as 0018-9464/$25.00 © 2009 IEEE