Enhanced Coercivity in CoPt Thin Film on Glass ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

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Enhanced Coercivity in CoPt Thin Film on Glass Substrate by Fine-Tuning Pt Underlayer An-Cheng Sun , S. H. Huang , C. F. Huang , Jen-Hwa Hsu , Fu-Te Yuan , H.C. Lu , S.F. Wang , S.N. Hsiao , and H. Y. Lee Department of Chemical Engineering and Materials Science, Yuan-Ze University, Chung-Li, 32003, Taiwan Department of Physics, National Taiwan University, Taipei, 106, Taiwan Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan Perpendicularly magnetized CoPt films with rhombohedral lattice were deposited on glass substrates with a Pt underlayer. The results show that the magnetic properties of CoPt films are substantially affected by post-annealing time and temperature of . Soft magnetic phase is formed as CoPt is directly deposited on the glass Pt underlayer, as well as the thickness of Pt underlayer substrate at 350 C. Besides, a very low coercivity of 103 kA/m is obtained when , and are 5 min, 300 C, and 20 nm, respectively. Further varying , and to 15 min, 350 C, and 25 nm increases to about 207 kA/m. The microstructural studies indicate that the size of CoPt grain is the key factor to determine the magnetic properties, which could be controlled by the formation CoPt phase with high conditions of Pt underlayer. In this study, the optimum deposition conditions for Pt underlayer to obtain are min, C, and – nm. Our study demonstrates that using a Pt underlayer/glass substrate can effectively replace the single-crystal substrate and also enhance of CoPt film, which may increase the application potential of CoPt film in the future. Index Terms—CoPt,

, magnetic property, sputtering condition, thin film, underlayer.

I. INTRODUCTION

T

HE demand for ultrahigh-density magnetic recording media has caused high magnetocrystalline anisotropy (MCA) materials to receive considerable attention in the past ordered structure decade. FePt and CoPt alloy films with are regarded as potential candidates to replace the currently used Co-Cr-Pt in the future magnetic recording applications with the density of over 1 Tbits/in because of their large MCA C) J/m ), high Curie temperature ( ( and chemical stability [1], [2]. However, fabrication of ordered FePt and CoPt phases require high-temperature process normally above 450 C [1], [2] and 520 C [1], [3], respectively, CoPt not suitable for the recording industry. High MCA thin films with perpendicular magnetic anisotropy (PMA) have attracted much attention recently due to their great potential for use in spintronic devices, magnetic sensor, and ultrahigh density CoPt was firstly reported by recording media [4]–[12]. The Iwata et al. [4]. The film exhibits CuPt-type structure with alternative Co and Pt atomic layer stacking along the [111] direction ( J/m ) on single-crystal MgO(111) substrate. High and relatively low formation temperature (300–400 C) are its advantages in various future applications. However, a small coercivity was normally observed in previous studies [4]–[8], even adopting high-cost single-crystal substrates such as a MgO(111) and Al O (11–20), which limits the applicability film. Thus, growth of CoPt(111) textured of CoPt employing glass substrates is film with a large out-of-plane films. In order to crucial for the future application of CoPt Manuscript received November 04, 2012; accepted January 09, 2013. Date of date of current version July 15, 2013. Corresponding author: A.-C. Sun (e-mail: [email protected]). 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.2013.2241032

increase the applicability of CoPt film, glass substrate with Pt(111) textured underlayer was used to deposit stoichiometric of CoPt CoPt films in this study. The results show that phase is significantly increased by fine-tuning the sputtering of Pt underlayer. This work conditions and thickness demonstrates that an optimum Pt(111) underlayer prepared under adequate conditions is a necessary step to obtain CoPt film with high out-of-plane with glass substrate. II. EXPERIMENTAL PROCEDURES CoPt films with a Pt underlayer were deposited on amorphous glass substrates by magnetron sputtering in an ultra-high vacuum sputtering system. The background pressure was better than Pa, and the working pressure was fixed at 1.3 Pa. Prior to sputtering CoPt film, Pt underlayers with thickness in the range of 0 to 30 nm were deposited on glass sub. The strates and underwent an annealing at 200 to 500 C was varied from 5 to 30 min. Afterwards, annealing time the substrate temperature was raised (or reduced) to 350 C to deposit magnetic CoPt layer. The thickness of CoPt layer was varied from 2 to 50 nm. The chemical composition of Co and Pt in CoPt film was controlled by adjusting the sputtering powers of both the targets. The chemical composition of Co and Pt in CoPt films were kept at 49 and 51 at. %, respectively, which was analyzed by calibrated energy dispersion spectroscopy (EDS). The film structure was determined by radiation. The using x-ray diffractometry (XRD) with Cu beam energy is 8 keV with a wavelength of 0.15 nm. The step size of the – scans was 0.01 and the collection time was 3 second per step. Surface morphology and microstructures were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using an accelerating voltage of 200 keV, respectively. Magnetic properties at RT and 5 K were measured using a polar magneto-optic Kerr effect

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Fig. 1. (a) Polar Kerr hysteresis loops and (b) XRD patterns of the CoPt/Pt/ glass substrate with different thicknesses of CoPt layer.

(PMOKE) and a superconducting quantum interference device magnetometer (SQUID) with maximum applied fields of about 1.3 MA/m and 4.0 MA/m, respectively. Magnetic domain structure was studied by magnetic force microscopy (MFM). III. RESULTS AND DISCUSSION Figs. 1(a) and (b) plot the polar-Kerr hysteresis loops and XRD patterns of the CoPt/Pt/glass with varying from 2 to 20 nm. In Fig. 1(a), a low out-of-plane coercivity is obtained at thinnest case. Higher is obtained at nm, which is about 135 kA/m. When nm, it is found that , nucleation field and polar Kerr squareness are reduced with increasing , indicating significantly misalignment of magnetic moments in thicker films. This can be attributed to the spontaneous demagnetization. The magnetization decreases before the applied field is ramped to zero, which has been found in CoPt films especially in thicker films [13]. In Fig. 1(b), the CoPt(222) peak is clearly observed as nm, signaling the presence of the hard magnetic phase with PMA property. The absence of CoPt(222) peak in thinner CoPt film is attributed to the volume effect of the film. Figs. 2(a)–(c) elucidate the dependence of on different sputtering conditions of Pt underlayer. The thickness of CoPt layer is fixed at 3 nm. In Fig. 2(a), and are set at 300 C and 20 nm, respectively. It is noted that initially increases with and peaks at min with of about 135 kA/m. Further increasing decreases . Next, is kept at 15 min. to study the annealing effect on . The results are shown in Fig. 2(b). rises from 95 to 199 kA/m as is increased from 200 to 350 C. Films annealed at temperature higher than 350 C will reduce . Remarkably, can be increased substantially by 50% with an increase of only 50 C of annealing temperature when it is above 300 C. It implies a noticeable change in microstructures with increasing . Saturation magnetization seems independent of annealing condition, the value of which is in the level of 1.1 web/cm . Besides, the value of increases from J/m to J/m as is raised from 300 to 350 C. The thickness dependence of Pt underlayer on is plotted in Fig. 2(c). A very low is found in CoPt/glass without a Pt underlayer, indicative of forming soft magnetic phase in CoPt film. increases with increasing , and a maximum of about 207 kA/m is obtained when nm. At this thickness the value of reaches J/m . Further increasing reduces .

Fig. 2. of CoPt/Pt/glass films under different sputtering conditions of Pt C, nm, and – underlayer. The conditions in (a) min.; (b) nm, min, and – C; and (c) min, and – nm. The is fixed at 3 nm.

Figs. 3(a)–(i) show the surface morphologies of CoPt/Pt/glass substrate with various annealing time , annealing temperature , and thickness of Pt underlayer. Their corresponding hysteresis loops are also plotted in the insets of each image. The is fixed at 3 nm. Therefore, the surface morphology of Pt layer can be observed directly because the epitaxial growth starts from Pt underlayer and extends into magnetic FePt and CoPt layers as demonstrated in previous works [14], [15]. In Fig. 3(a), densely-distributed fine grains with sizes in the range of 10 to 20 nm and some large grains (indicated by white dotted circles) with size greater than 200 nm are also observed. When is increased from 5 to 15 min., the size of the small grains grows to about 100 nm. Additionally, the number of large grain is reduced. The enhancement of at min could be attributed to uniform grain growth of small grains. In Fig. 3(c), many white particles appear on the film plane after long annealing time, which roughens the Pt plane. The variation of surface morphology with is displayed in Figs. 3(d) to (f). The is maintained at 15 min. In Fig. 3(d), at C, the sizes of grains are essentially small than 100 nm. Relatively, many large grains with sizes of 150–200 nm appear as is increased to 350 C, accompanied by higher , and . Further increasing to 500 C induces substantial grain growth in CoPt/Pt film; some grains are larger than 500 nm causing a significant change in the magnetic properties. Regarding the dependence of surface morphology of CoPt film on the thickness of Pt underlayer , CoPt/glass substrate exhibits a large-area flat surface image with no observed contrast at (see Fig. 3(g)) and it has soft magnetic behavior. As nm, small grains and voids are well separated in CoPt/Pt film indicating stress-relaxation in the Pt layer during film annealing. The grains drastically grow about 150–200 nm larger in size, as

SUN et al.: ENHANCED COERCIVITY IN

COPT THIN FILM ON GLASS SUBSTRATE BY FINE-TUNING PT UNDERLAYER

Fig. 3. Surface morphologies of CoPt/Pt/glass films under different sputtering conditions of Pt underlayer. The conditions in (a)–(c) are nm, and , and 30 min.; (d)–(f) are nm, min, and , and 500 C; and (g)–(i) are , and 25 nm. The is fixed at 3 nm. The insets of each image are their corresponding hysteresis loops.

Fig. 4. Plane-view TEM images and magnetic domain images of CoPt/Pt/glass films under different sputtering conditions of Pt underlayer. The preparation C, nm, and min.; (c) condition in (a) and (b) are C, nm, and min. The thickness of and (d) are CoPt is kept at 3 nm.

is further increased to 25 nm (see Fig. 3(i)). The hysteresis loop is similar to the one in the inset of Fig. 3(e), which could be ascribed to similar surface structures in both films. Plane-view TEM images and magnetic domain structures of CoPt/Pt/glass films under various sputtering conditions of Pt underlayer are displayed in Figs. 4(a)–(d). The MFM images were obtained from the as-grown samples with dimensions of 5 m 5 m. In Fig. 4(a), clear grain boundaries without observable defects are observed at nm, min, and C. The grain size is about 50–150 nm, which can be considered as the sizes of CoPt and Pt grains due to epitaxial effect for both of them [14], [15]. The magnetic do-

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C, C,

min, and

main structure under above sputtering conditions is presented in Fig. 4(b). Coarse maze-like domains with average size ranging from 200–300 nm are observed, which is a typical structure for epitaxial grown ordered FePt and CoPt films with PMA [3], [16]–[18]. The sharpened contrast boundary between anti-parallel polarized domains reveals the strong stray field at the surface of the film. This is normally observed in the magnetic thin films with uniaxial magnetic anisotropy. The domains are almost separated by 180 walls [17]–[20]. The grain size is smaller than the domain size; the resistance of wall motion is insufficient. Upon varying and to 25 nm and 350 C, respectively, the grain structure is different from that in Fig. 4(a). Visible defects such as contours and stripes (marked by arrows) are found in Fig. 4(c), indicating large internal stress in CoPt/Pt bilayer film as and are further increased. This local straininduced-stress could be the source of impedance for domain wall motion. The magnetic domain mapping is presented in Fig. 4(d) revealing that the average domain size of CoPt films is about 150–200 nm, smaller than the one in Fig. 4(b), indicating creation of impedance of domain wall propagation. Therefore, a higher is obtained under this condition. Comparing the surface morphologies, microstructures, and magnetic domains, two sources may account for the enhanced in this study. One is the local strain-induced-stress, generating from the contours and stripes (see Fig. 4(c)), which also impedes the wall motion [21], [22]. Another is the grain boundary pinning effect for domain wall motion in CoPt/Pt/glass samples [16], [23]–[26]. In this study, polycrystalline CoPt grains are epitaxially grown on Pt(111) films. The CoPt grain size is between 50 and 150 nm, close to the CoPt domain size. Therefore, the domain wall motion is hindered easily at the grain boundary, which results in enhanced due to the pinning effect.

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IV. CONCLUSION In summary, improvement in magnetic properties of CoPt film is observed on fine-tuned sputtering conditions of Pt underlayer on glass substrate. The is lower than 20 kA/m without inserting a Pt underlayer, suggesting formation of soft CoPt phase. When the Pt layer is deposited between CoPt film and glass substrate, the perpendicular coercivity is significantly increased. The sources of coercivity enhancement may be owing to strong magnetic domain-wall impedance at grain boundary and local strain-induced-stress between grains. The improvement in magnetic properties of CoPt film with well-tuned Pt underlayer suggests the usefulness of CoPt/Pt/glass films in future magnetic recording media.

ACKNOWLEDGMENT This work was supported by the National Science Council of Taiwan (NSC Grants 101-2221-E-155-020-MY3 and NSC-992112-M-002-020-MY3).

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