Weak dipolar interaction between CoPd multilayer nanodots for bit-patterned media application

Materials Letters 182 (2016) 185–189

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Weak dipolar interaction between CoPd multilayer nanodots for bit-patterned media application X.T. Zhao a, W. Liu a,n, Z.M. Dai a, D. Li a, X.G. Zhao a, Z.H. Wang a, D. Kim b, C.J. Choi b, Z.D. Zhang a a b

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Powder & Ceramics Division, Korea Institute of Materials Science, 797 Changwon-daero, Changwon 642-831, South Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 27 March 2016 Received in revised form 23 June 2016 Accepted 28 June 2016 Available online 28 June 2016

The morphology and magnetic performance of CoPd multilayer nanodots have been investigated by a combination of DC sputtering, anodic aluminum oxide(AAO) template method and Ar ion etching. The nanodots exhibit a normal switching field distribution(SFD) of 17%, which is comparative to Bit-patterned media (BPM) made by e-Beam. It is found that the first-order reverse curve (FORC) data of CoPd nanodots have a good match with the mean-field model, which shows dipolar interaction contributes as small as 8.4% to the total SFD. Magnetic force microscope(MFM) imaging at a certain location confirms the FORC results by observing the effect of dipolar interaction directly. Our CoPd multilayer nanodots can be a good candidate for BPM application. & 2016 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials Multilayer structure Nanodot Anodic aluminum oxide(AAO) Magnetic force microscopy (MFM) First order reverse curves(FORC)

1. Introduction Bit-patterned media(BPM) for high density data storage has been extensively investigated [1–5], and one key issue of a BPM candidate is the variation for reversing different dots, namely switching field distribution(SFD). The SFD should be narrow enough to avoid undesired switching of dots adjacent to the dot being written. One part of SFD is contributed by fluctuation in structure, like film composition or crystallization [2]. Other contributions mainly come from exchange-coupling and dipolar interaction [1,6]. Since these interactions affect BPM differently relying on the structural properties of specific sample, an alternative way that can eliminate interaction between BPM units should be found. As an economic and efficient tool to create nanostructures, selforganized porous anodic aluminum oxide(AAO) [7,8] has been widely used as template in fabricating BPM [4,5,9]. However, unavoidable large aspect-ratio makes it hard to control thickness of film deposited into the pores of AAO. That is why nanodots made by AAO are mainly restricted to elementary substance, alloy and bi-layer systems [4,10,11]. In this work, we designed a new n

Corresponding author. E-mail address: [email protected] (W. Liu).

http://dx.doi.org/10.1016/j.matlet.2016.06.121 0167-577X/& 2016 Elsevier B.V. All rights reserved.

method combining AAO, sputtering and Ar ion etching to fabricate BPM with well preserved multilayer structure. Interaction between nanodots was investigated to evaluate its potential for BPM application.

2. Experiment Fig. 1 shows the diagrams of fabrication of CoPd nanodots. At first, the desired multilayer structure was deposited by DC sputtering with a Ar pressure of 2 mTorr after reaching a 2
10  7 Torr base pressure. We prepared sample in structure of Si substrates/Pd (3 nm)/[Co(0.5 nm)/Pd(1 nm)]19Co(0.5 nm)/Pd(3 nm) to obtain perpendicular magnetic anisotropy(PMA). Then a piece of ultrathin AAO template was transferred onto the film. The AAO template was prepared by a conventional two-step anodization, and details can be found elsewhere [9,12]. The thickness of AAO was about 160 nm. The root mean square roughness was around 3.5 nm and below 1 nm for AAO and films either. After transferring the AAO onto the films, 25 nm Ti was deposited to form nanodots through pores in AAO. After deposition the AAO templates was peeled off by tape. Finally, the films were etched by Ar ion under 0.02 Pa pressure for 200 s. The acceleration voltage and grid voltage were 300 V and 200 V.

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Fig. 1. The brief fabricating process of CoPd nanodots.(a) Growing the CoPd Film; (b) transferring the AAO and growing the Ti nanodots; (c) peeling off the AAO and Ar ion etching; (d) CoPd nanodots.

Scanning electron microscope(SEM), transmission electron microscope(TEM) were employed to characterize morphology of the nanodots. Hysteresis loops and first-order reverse curves (FORC)were measured by a polar MOKE system. Micro magnetization arrangements of the nanodots were observed by atomic force microscope(AFM) and magnetic force microscope(MFM). Marks were made by lithography to get MFM images of the same location at remnant state after applying field. In MFM measurement the tip had a constant distance of 30 nm away from the surface.

3. Results and discussion Fig. 2(a) gives a SEM image of the Ti dots deposited on film before etching. The dots were arranged in hexagonal close-packed lattice of AAO template, while the diameter of nanodots was obviously larger than the diameter of AAO pores shown in lower half of the image. Ar gas and pore wall of AAO make Ti atoms scattered and rough AAO surface [13] give scattered Ti atoms space to form larger dots. In Fig. 2(b, c), after etching, a much clear contrast indicates that CoPd nanodots have been formed via the protection of Ti layer. Fig. 2(d) shows a cross-sectional TEM image of the CoPd multilayer nanodots. The cone shape comes from diverse thickness

of Ti. Clear and flat multilayer structures indicate that the impact of fabricating craft exerted on the designed layer structure is negligible. However, there are only 15 [Co/Pd] cycles left in this nanodot and much more inferior nanodots can be found in the inset of Fig. 2(d). High aspect ratio and contamination in AAO pores cause the Ti covers not thick enough to protect the CoPd multilayers. Fig. 3(a) represents hysteresis loops at room temperature of CoPd continuous multilayer films and nanodots. The hysteresis loop of CoPd films shows that the reversal mechanism is domain wall propagation, which is typical for PMA multilayer films. The loop of CoPd nanodots keeps good perpendicular anisotropy, but the reversal mechanism is changed. Since domain wall cannot propagate between isolated nanodots anymore, the nanodots have to reverse separately, leading to an increased coercivity of 2.6 kOe. It should be noted that the slope on the descending branch before 2.0 kOe is a false appearance caused by the optical system. SFD calculated by differentiating the magnetization curve along ascending branch and its Gaussian fit are shown in Fig. 3(b). The absolute SFD, i.e., standard deviation is 431 Oe, then the normal SFD defined as sSFD/HC is 17%, which is comparative to that of BPM made from pre-patterned pillars(around 10%) [1]. To analyze the composition of SFD, we employ FORC for its power in telling details of hysteresis systems [11,14–17]. Raw data of FORC is a series

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Fig. 2. SEM and TEM images of the CoPd nanodots. (a)Ti nanodots before etching and a piece of AAO left on the sample; (b) top view of CoPd nanodots; (c) a tilted view of CoPd nanodots; (d) cross-sectional TEM image of CoPd nanodots, the inset of (d): overall profile of nanodots.

of minor loops measured from reverse field(HR) back to saturation. The distribution of FORC is defined as mixed second order derivative [18].

ρ(HR ,H) ≡ −

2 1 ∂ M( HR ,H) ) 2 ∂HR ∂H

(1)

This distribution represents the mapping of all irreversible components of magnetization. A redefinition

HB =

H+HR H − HR , HC = 2 2

(2)

rotates the coordinate axes by 45°. HB represents for effect of local interaction and HC represents for local coercive field. FORC data of CoPd multilayer nanodots shown in Fig. 3(c) are derived from a polynomial fitting [14] with smoothing factor of 4. According to a mean-field model [15,17], nanodots without interaction only have a ridge distributing symmetrically along the HC axis. But the ridge in Fig. 3(c) rotates clockwise, indicating a dipole-dominating interaction. The vague area below the ridge is a result of mismatching of minor loops shifted by dipolar interaction. We trace

the apexes of the ridge and project them onto the HC-HB plane in Fig. 3(d). The linear part of the projection in the range of 2280 Oe to 3160 Oe is fitted with a straight line with a slope k of  0.081. According to the model, the dipolar contribution to sSFD is k/(1k) ¼8.4%(36 Oe). This value is quite small in comparison with the previous work [1,6]. It is reasonable to say that the isolation between nanodots totally rule out exchange coupling and distance between nanodots is relatively large, so the dipolar interaction is small. To directly observe the switching of CoPd nanodots, a series of AFM and MFM images obtained at remnant state of the same location are shown in Fig. 4. Before each scanning, the sample is saturated first then exerted to a demagnetizing field. Remnant state is of same magnetization arrangement with in-situ condition since FORC does not change while decreasing field to zero in either direction. The AFM image of the selected area is shown in Fig. 4 (a) to locate dots and defects. MFM image in Fig. 4(b) is measured after demagnetizing by  2.0 kOe, nanodots magnetized in up and down directions have very distinguishable contrast and good resolution. Compared with MFM image in Fig. 4(c) which has a larger demagnetizing field, it is obvious that reversed nanodots have

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Fig. 3. (a) Hysteresis loops of CoPd film and nanodots at room temperature, M/Ms ¼1 represents for up-saturation; (b) the normalized SFD of CoPd nanodots, solid line is a Gaussian fit; (c) FORC data in HC-HB coordinate axes; (d) projection of the apexes of the ridge in FORC data onto the HC-HB plane, straight dash-dot line presents a linear fitting.

light contrast. Most reversed dots do not depend on each other, indicating that the exchange coupling between nanodots is eliminated. Since sample is demagnetized from a saturated state every time, it is interesting to find no obvious difference between Fig. 4(c) and (d), which implies that the reversing sequence is stable. Coinciding with the FORC analysis, this result indicates that SFD should be dominantly contributed by structural properties.

between nanodots only contributes to 8.4% of the total SFD. The independence of magnetic nanodots results from the structural isolation and well defined multilayers. A direct FORC analysis is supported by MFM images. Since the weak dipolar interaction and good MFM resolution are all important merits for storage application, CoPd multilayer nanodots made by the present procedure can be a good alternative way to make BPM.

4. Conclusion

Acknowledgments

As a conclusion, we report a method to make BPM by combining DC sputtering, AAO template and Ar ion etching. FORC data analyzed by a mean-field model show that dipolar interaction

This work was supported by the National Natural Science Foundation of China under projects 51590883, 51471167 and the project of Chinese Academy of Sciences with Grant number KJZD-

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Fig. 4. AFM and MFM images of the CoPd nanodots. (a) AFM image of the selected location; (b-d) MFM image of the selected location captured after applying a demagnetizing field of (b)  2.0, (c)  2.4 and (d)  2.8 kOe after saturation.

EW-M05-3. This work was also supported by a Joint Research Project from Ministry of Science, ICT and Future Planning/Korea Research Council for Industrial Science and Technology.

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