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Development of Co-Alloys for Perpendicular Magnetic Recording Media Bin Lu, Dieter Weller, Ganping Ju, Ashwin Sunder, Duane Karns, Meiling Wu, and Xiaowei Wu
Abstract—Exploring new Co-alloy composition to achieve both chemical segregation and suitable magnetic properties is a priority task for the development of the future perpendicular magnetic recording media. With proper interlayer (IL) design, columnar Co-alloy grains with narrow orientation distribution ( 6 ) have been obtained on thin IL (4 nm thick). We 50 demonstrate CoCrPtB-alloys with full remanence squareness and large negative reverse field ( 1.5 kOe) at low IL and magnetic layer thickness of 4 and 11 nm, respectively. It is also shown that carbon overcoat thickness reduction results in a signal-to-noise ratio gain of about 0.5 dB/nm.
1
Index Terms—Co-alloy, microstructure, perpendicular magnetic recording media, recording performance.
I. INTRODUCTION
C
oCr-ALLOYS were first used as perpendicular media (PM) in 1977 [1], [2]. In 2001, 24 years later, such CoCr-based alloys (with addition of Pt) were used to demonstrate an areal density of 52.5 Gb/in in perpendicular recording [3]. One of the key attributes of CoCr-alloys is their hexagonally . close packed (hcp) crystallographic structure with This anisotropic structure is also the origin of relatively large magnetic anisotropy with easy axis along the crystallographic axis. Appropriate seeding techniques have been developed to achieve axis normal texture and consequently perpendicular magnetic orientation. Suitable microstructural features have been obtained at moderate growth temperatures [4], and the path toward thermally stable media at 200 Gb/in has been outlined for these CoCrPt-alloys [3]. The general objective of perpendicular media (PM) development is to achieve thermally stable media with enhanced signal-to-noise ratio (SNR). Requirements are suitable microstructure such as excellent (00.2) orientation, small grain size and narrow grain size distribution, low fcc phase content and low stacking fault (SF) density as well as appropriate magnetic properties, such as sufficient magnetic anisotropy and coercivity , respectively, anisotropy field compatible with write fields, high negative nucleation , full remanence squareness , and field optimized intergranular exchange interaction. Although these requirements are similar to those in longitudinal media, the approaches are different due to differences in crystallographic orientation and layer construction. In PM, Co-alloy crystalline grains are oriented with the axes in the Manuscript received January 29, 2003; revised February 11, 2003. The authors are with Seagate Research, Seagate Technology, Pittsburgh, PA 15222 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/TMAG.2003.813779
film normal direction. The intrinsic perpendicular anisotropy energy needs to be larger than the demagnetization energy, which typically requires a low SF density. It is necessary to explore new compositions and to revisit compositions that have earlier been rejected in longitudinal media (e.g., CoCrPt [5]). In order to concentrate the write flux and increase the field gradient, it is important to minimize the separation between the head and the soft magnetic underlayer (SUL). This requires the interlayer (IL), which is the spacing between the magnetic layer and the SUL, to be as thin as possible. (e.g., IL 0–1 nm for 1 Tb/in [6], [7]). It also requires the magnetic layer (maglayer) to be as thin as possible. These requirements are difficult to meet in Co-alloy media, since only marginal IL thickness is available to control both the (00.2) orientation and the grain size. Moreover, since SF and fcc grains tend to concentrate in the initial growth region of the Co-alloy film [8], [9], it is very challenging to fabricate thin and defect-free maglayers. Similar to longitudinal media, the intergranular exchange coupling of perpendicular media can be reduced either physically or chemically [10]. For example, recent so-called “granular” CoCrPt–O perpendicular media [11] show a similar voided microstructure to early longitudinal CoCrPt media on which a 1-Gb/in areal density demonstration was performed in 1989 [12]. Those longitudinal media were deposited under low-mobility process conditions, corresponding to Zone 1 (or Zone T) in Thornton’s well-known film growth model [13]. Voids generated at grain boundaries were responsible for the decoupling of the magnetic grains. Later, such longitudinal media were replaced by those processed at elevated temperature and low Ar pressure. The latter media developed a smoother surface and, hence, have led to better head–disk interface performance. Magnetic decoupling was achieved by chemical segregation of mainly Cr to the grain boundaries. Although the history of longitudinal media does not necessarily project the course of PM development, it does suggest that alloy perpendicular media may have an indispensable advantage over “granular” CoCrPt–O perpendicular media. Moreover, it has been confirmed that Cr segregation does exist in CoCrTa, CoCrPt, and CoCrPtTa perpendicular media [14], [15]. In this paper, we discuss important issues in the development of Co-alloy perpendicular media, such as magnetic hardness, thermal stability, maglayer thickness, IL thickness, and COC thickness. II. EXPERIMENTAL PROCEDURE Media were sputter deposited using a Unaxis Circulus-M12 T) is system at various heater power settings. FeCoB (
0018-9464/03$17.00 © 2003 IEEE
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Fig. 1. Schematic plot of layer structure of a perpendicular medium.
used as SUL material [16], [17] at a fixed thickness of 200 nm. IL thickness is varied from 4 to 15 nm. Maglayer thickness is varied from 5 to 40 nm and prepared by co-sputtering of CoCr, Pt, and B targets using a Triatron sputter station with three concentric magnetron cathodes [18]. The base pressure of the sputtering system is about 2 10 Pa, the substrate temperature at the time of maglayer deposition is about 250 C. Magnetic properties are measured using a custom-built magnetooptical Kerr effect (MOKE) system and a Quantum Design superconducting quantum interference device (SQUID) magnetometer. Microstructures are characterized using a Philips X’pert X-ray diffraction (XRD) system and Philips CM-200 transmission electron microscope (TEM). Read/write experiments are performed on a Guzik spin stand with a single-turn, single-pole merged head [writer width 150 nm, head to media spacing 15 nm, rotation speed 5400 r/min, 25 mm, minimum bit length 100 nm for track radius pseudorandom bit sequence (PRBS)]. The autocorrelation signal-to-noise (ACSN) ratios are determined from the autocorrelation of successive periods of a PRBS signals with the electronics noise removed. All signals are captured from an oscilloscope at a sampling rate of 1 Gsamples/s. III. RESULTS AND DISCUSSION A. Magnetic Layer To better describe the layer structure and magnetic properties of PM, the schematic diagrams of a medium and definitions of measurables in a hysteresis loop are plotted in Figs. 1 and 2, respectively. Fig. 1 shows a typical PM design. The SUL contributes to the writing process and significantly enhances the readback signal. On top of the SUL are seedlayer and underlayer, which are referred to as interlayer (IL). These layers help to define the orientation and grain size of the maglayer, which is covered by several nanometers of carbon overcoat (COC) and an organic lube for protection and lubing purposes. The measurable field values in a perpendicular hysteresis loop are shown in Fig. 2. They are, as indicated by the arrows, , nucleation field , and reversal field . coercivity is defined [5] as the field of the intercept between the (shown by saturation magnetization level and the tangent at two lines in the figure). Since the significance of a nucleation field is to describe how well the media can resist the erasing cannot field from the return pole during the writing process, as indicated in fulfill this purpose. Here, we rather use
Fig. 2. Hysteresis loop and measurable fields of a perpendicular medium.
Fig. 3. Thickness dependence of
H
and
S for two Cr contents of maglayer.
Fig. 2 to describe the onset of reversal [19]. We define as the reverse field strength that causes the magnetization to drop . to 98% of Choosing a right composition for the maglayer has traditionally been a key task for any type of media development. CoCr has been studied as a maglayer for many years [20]–[32]. However, due to inferior intrinsic magnetic properties of CoCr, it is very difficult for CoCr media to achieve full remanence square200–400 emu/cm , ness ( ). Typical magnetic data are 4–6 kOe, and 1–2 kOe for maglayer thickness 50–300 nm [33]. In addition to CoCr, many CoCrXY alloys have been studied, for example, CoCrTa [34]–[38], CoCrPt [39]–[48], CoCrPtTa [49]–[55], CoCrPtB [56], CoCrNb [57], [58], CoCrNbPt [59], [60], CoCrTaMo [61], CoV [62], CoCrC [63], CoCrWC [64], and CoCrPr [65]. The main conclusions of these studies are as follows. 1) Pt plays a significant role in determining the magnetic anisotropy. CoCr-alloy media without Pt usually have low squareness, high dc noise, positive nucleation field, and poor thermal stability. 2) Elements like Cr, Ta, Nb, B help to exchange decouple the magnetic grains, resulting in significant performance improvement. For example, an addition of 2% Ta can reduce noise by 25% [56]. However, to maintain the magnetic anisotropy of the films, the amount of these additions may not be as high as in longitudinal media.
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H
Fig. 4. Thickness dependence of
H
for two Cr contents of maglayer.
Fig. 6. Media
Fig. 5. Thickness dependence of
H
for two Cr contents of maglayer.
Fig. 7. FWHM of XRD rocking as the CoCrPt thickness increases. The rightmost data point (wrapped by a square) belongs to a maglayer deposited directly onto a glass substrate.
Fig. 3 shows thickness rolloff curves for CoCrPt-media with two different Cr compositions—15 at% and 23 at%. The media are prepared without SUL on Ta (5 nm)/Ru (10 nm) IL. It can be seen that the maglayers have full squareness at the low Cr content. At 23 at% Cr, the media cannot maintain full squaredrops by about 4 kOe compared to 15 at% Cr. The reness. sults demonstrate a dilemma in CoCr-alloy media development: is high [65], but is high When the Cr content is low, and decoupling of the grains is poor; and vice versa. Therefore, to find other elements to enhance chemical segregation without of the maglayer is very important in alloy media reducing development. of the same Fig. 4 shows the thickness dependence of mirrors the rolloff. Fig. 5 samples presented in Fig. 3. for the same samples. It shows the thickness dependence of fluctuates through the thickness range. can be seen that is independent of and rolloff curves. Boron is widely used nowadays in longitudinal media to enhance the grain separation while at the same time maintaining or even increasing the magnetic anisotropy. However, in perpendicular media, boron causes growth of SFs [8], [9]. Hence, rapidly, though it has been shown that it will decrease the moderate boron addition may improve media performance [66].
and ACSN against boron content.
Fig. 6 plots the media coercivities and recording performance in terms of ACSN of four media with different boron additions. drops quickly as boron is added to the maglayer. ACSN increases up to 8 at% of boron addition before dropping off sharply due to low loop squareness and low coercivity of the media. B. Interlayer For a thin film grown at thickness 10 nm, the maglayer may contain a significant amount of defects and misoriented grains. This puts strong microstructural requirements on both the seedlayer and the underlayer. Fig. 7 shows the full-width at half-maximum (FWHM) of XRD rocking curves of the CoCrPt (00.2) peak as a function of the CoCrPt thickness. The maglayer is deposited onto a Ta (5 nm)/Ru (10 nm) IL except for the 38-nm thick film (open circle), which was deposited onto glass. It is seen that the value of FWHM remains constant as the maglayer thickness increases. This indicates that the grains grow in a “nucleation texture,” in which case the grains of the CoCrPt orient in (00.2) direction in the initial stage of the film growth and maintain that orientation subsequently during film growth. There is no growth competition as in a “growth
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Fig. 9. IL thickness dependence of coercivity. Fig. 8. Cross-sectional TEM image of a CoCrPt media showing the columnar growth of the Ru underlayer and the maglayer.
texture” (open circle) [10]; it is concluded that the grains of the CoCrPt film grow in columns. Fig. 8 shows a cross-sectional TEM image of such media with layer structure glass/SUL (200 nm)/Ta (2 nm)/Ru (6 nm)/ CoCrPt (16 nm). The image on top is a bright field diffraction contrast image, while the one at the bottom is its corresponding dark field image. The images clearly show that the Ru and CoCrPt layers develop into columns on top of the Ta seedlayer. A series of IL thickness samples have been fabricated varying both seedlayer (2, 4, 6 nm) and underlayer (1, 2, 3 nm) thickness. To further optimize maglayer performance, a new seedlayer/underlayer combination different from Ta/Ru is selected. The media structure is glass/SUL (200 nm)/seedlayer ( nm)/underlayer ( nm)/CoCrPtB (11 nm)/COC (5 nm). dependence of the IL thickness. All media Fig. 9 plots the have full loop squareness ( ) and negative , which fluctuates between 1.1 and 1.9 kOe. This demonstrates the magnetic hardness and structural quality of the CoCrPtB layer. It can be depends monotonically on both seedlayer and unseen that derlayer thickness. We attribute this observation as due to the grain diameter increasing while at the same time defects (mostly SFs) are continuously reduced. The FWHM of the CoCrPtB (00.2) peak decreases from 6.0 to 5.2 as the IL thickness increases from 3 to 9 nm. Fig. 10 shows the variation of media performance in terms of ACSN versus the IL thickness. Since the recording performance is a complicated function of many parameters, including spacing loss, magnetic properties, and microstructure, it is not surprising that the trend of ACSN differs significantly from that of in Fig. 9. At seedlayer/underlayer thickness of 4 nm/1 nm and 2 nm/2 nm, the media exhibits the highest ACSN. C. Overcoat Reducing carbon overcoat (COC) thickness is a continuing effort. A series of media have been fabricated with structure: glass/SUL (200 nm)/seedlayer (4 nm)/underlayer (1 or 2 nm)/ CoCrPtB (11 nm)/COC (3.5 nm). These media are compared with a corresponding series with COC 5 nm instead of 3.5 nm.
Fig. 10. Media recording performance in ACSN against seedlayer and underlayer thickness. TABLE I ACSN OF THE MEDIA WITH DIFFERENT COC THICKNESS
Table I lists spin-stand testing results. It shows that reduction of 1.5-nm COC increases ACSN by 0.7 dB. Figs. 11 and 12 show the PRBS power spectrum density of media B and D listed in Table I. The dc noise is obtained by dc-erasing the media before readout. The ac noise is obtained by recording a sufficiently high-frequency tone before readout ( 1000 kFCI). The value of PRBS ACSN, excess ac noise, and excess dc noise are listed under each spectrum. It can be seen that reducing COC thickness results in an increased PRBS ACSN of 0.7 dB. This increased PRBS ACSN is attributable to a combination of many factors. First, the reduced COC yields
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REFERENCES
Fig. 11. PRBS power spectrum density of the CoCrPtB media with IL and COC 3.5 nm.
=
Fig. 12. PRBS power spectrum density of the CoCrPtB media with IL and COC 5 nm.
=
= 6 nm
= 6 nm
a reduced head-to-media spacing (HMS). The reduced HMS increases the resolution of the system. Second, the reduced head-to-SUL spacing also allows the write field to deliver a sharper gradient during recording. A sharper recording gradient produces transitions that are located closer to their intended location and, thus, the transition jitter is reduced. The increase in PRBS ACSN from a reduced COC is observed to be about 0.5 dB/nm. ACKNOWLEDGMENT B. Lu would like to thank Dr. H. Zhou, Dr. R. Chantrell, Dr. K. Howard, Dr. K. Wierman and Dr. T. Klemmer at Seagate Research for helpful discussions.
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