IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 2133

A Simple Route to Fabricate Percolated PerpendicularMagnetic Recording Media

M. Tofizur Rahman, Chih-Huang Lai, D. Vokoun, and Nazmun N. Shams

Department of Material Science and Engineering, National Tsing Hua University, HsinChu 300, Taiwan, R.O.C

A simple approach is proposed for fabrication of percolated perpendicular media (PPM). Nano pore array with pore diameter of12–15 nm and different pore densities has been prepared by anodizing aluminum on silicon wafers. The pore density increases from3.2 1010 cm 2 to 11 0 1010 cm 2 with the decrease in anodizing voltage from 30 to 10 V. Then, Pt (7 nm)/ Co (0.5 nm)/Pt (2) nm 5

multilayers are deposited onto this porous anodized alumina (AAO) by sputtering. The pore size is reduced to 10–12 nm after depositionof magnetic layers. The Co/Pt multilayers on AAO exhibit perpendicular magnetic anisotropy, squarness ratios of unity, and negativenucleation fields. The perpendicular coercivity increases linearly with the increase in pore density due to the pinning effect imposed bythe pores, which is consistent with theoretical calculation for PPM.

Index Terms—Anodized aluminum oxide (AAO), Co/Pt multilayers, percolated perpendicular media (PPM), pinning sites.

I. INTRODUCTION

THE ongoing scale reduction of magnetic recording leadsto a call for alternative advanced recording geometries in

perpendicular recording. Promising candidates include tiltedmagnetic recording [1], exchange coupled composite (ECC)media, [2] and exchange spring media [3]. All of these effortsare mainly concentrated on granular perpendicular media tosuppress the transition noise. Very recently, the percolatedperpendicular media (PPM) has been proposed to further ex-tend the recording density [4]. The media consisting of fullyexchange coupled grains with densely distributed nonmagneticpinning sites, of which thermal stability is determined by theenergy barrier needed to overcome the domain wall pinninginstead of grain volume. Although superiority over otheradvanced recording geometries and a possibility to achievethe recording density beyond 1 Tb/in are already reportedbased on micromangnetic modeling [5], [6], no real fabricationmethods have been realized to date. Pinning behavior of mag-netic thin films deposited on prepatterned substrates has beenreported [7], [8] in which the expensive and time-consuminglithography process is used to pattern substrates and the pinningsite density is poor.

In this paper, we demonstrate a simple way to fabricate PPMmedia by depositing magnetic material on substrates with ultrahigh-density preformatted pinning sites. Unlike other works, weuse anodic aluminum oxide (AAO) templates prepared on sil-icon wafers as substrates. Arrays of various pore diameters anddensities can be prepared by controlling anodizing conditions ofAAO [9], [10]. The effects of pore density on magnetic proper-ties of Co/Pt multilayer PPM media are discussed.

II. EXPERIMENTAL PROCEDURE

The fabrication procedure is schematically shown in Fig. 1.The procedure consists of two main processes: 1) preparation ofordered porous alumina on Si wafer with controlled pore diam-eter and density and 2) subsequent deposition

Digital Object Identifier 10.1109/TMAG.2007.893142

Fig. 1. Schematicdiagrams for preparation of PPM media. (a) Deposition ofAl and Ti films on Si wafer. (b) Preparation of nanopore array by anodization.(c) Deposition of Co/Pt multilayer on porous anodized aluminum oxide.

of magnetic layer on AAO templates. At first, a 50-nm Al filmis deposited on an Si wafer with a 7-nm Ti underlayer by sput-tering. This Al film is anodized in sulfuric acid under the voltagefrom 10 to 30 V to get porous alumina with different pore densi-ties. After anodic oxidation, the size of nanoholes is controlledby wet chemical etching of alumina using diluted phosphoricacid. Then, Co(0.5)/Pt(2) nm multilayers with a 7-nm Ptbuffer layer are deposited onto the surface of nanoporous alu-mina by sputtering at room temperature. We chose Co (0.5 nm)/Pt(2 nm) multilayers for its exchange coupled grains and per-pendicular magnetic anisotropy, which enables us to demon-strate the pinning effects of AAO pores. It is well known thatthe coercivity of the Co/Pt multilayer decreases with in-creasing Pt layer thickness [11]. In this paper, we chose a thickPt layer so that the coercivity of Co/Pt MLs deposited on Sisubstrate was only 150 Oe, and the enhancement of coercivitycan be profound. The deposition parameters of multilayers werechosen to form (111) textured films with a clear layer structurewithout formation of Co–Pt alloys, revealed from transmissionelectron microscopy (TEM) images. The surface morphologyof the anodized alumina with and without a Co/Pt multilayeris observed with a field-emission scanning electron microscope(FE-SEM). Surface roughness is evaluated with an atomic force

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2134 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007

Fig. 2. Plane-view SEM images of aluminum oxide anodized at (a) 30 V and(b) 15 V. (c) Variations of pore density as function of anodizing voltage.

microscope (AFM). Magnetic properties are measured using vi-brating sample magnetometer (VSM) at a maximum field of15 kOe.

III. RESULTS AND DISCUSSION

At first, Al films deposited on Si wafers are anodized at dif-ferent voltages to get a porous structure with different pore den-sities but constant pore diameter . Fig. 2(a) and(b) shows the SEM images of porous alumina anodized at 30and 15 V, respectively. The images show that the average porediameter is about 12–15 nm in both cases. However, theinterpore distance decreases with the decrease in anodizationvoltage [9], [10] that causes higher pore density. The variationof pore density as a function of anodization voltage isplotted in Fig. 2(c). It is found that the increases almostlinearly with the decrease in anodization voltage.

Fig. 3(a) and (b) shows SEM images of porous alumina an-odized at 25 V and Co/Pt deposited on it, respectively. The darkareas in the both figures correspond to the pores. The brightareas correspond to the AlO in Fig. 3(a) and the magnetic layer

Fig. 3. Plane-view SEM images of aluminum oxide anodized at 25 V (a) beforeand (b) after deposition of Co/Pt multilayers.

Fig. 4. M -H loops of Co/Pt multilayer deposited on AAO withD of (11�10 )/cm . Filled circles and open circles represent perpendicular and in-planeloops, respectively.

supported by the pore walls, that is, AlO covered by Co/Pt inFig. 3(b), respectively. The presence of pores after depositionreveals that the pores are not filled by the magnetic material.From the comparison of the plane-view SEM images of an alu-mina template, it is observed that the pore size decreases from12–15 nm to 10–12 nm after deposition of Co/Pt multilayers.The magnetic material is deposited mainly on top of the walland the perimeter of the pores. A very small amount reaches thebottom of the pores, which is reasonable for the pores with as-pect ratios larger than 1:5 [12].

The Co/Pt multilayers on AAO exhibits perpendicular mag-netic anisotropy with - loop having squarness ratios ofunity and negative nucleation fields, shown in Fig. 4. Thein-plane is negligible. Fig. 5 shows the dependence ofperpendicular coercivity of Co/Pt multilayers depositedon AAO as a function of pore density. Variations of averagesurface roughness of AAO with different are also plottedin the same figure. For comparison, of the same Co/Ptmultilayers deposited on a Si wafer without AAO is alsoadded to the figure, which is noted as the zero pore density.All Co/Pt multilayers are grown on the same conditions sothat the variations of mainly originated from the changesof pore density. The value of Co/Pt multilayer increasesdramatically when deposited on porous AAO. increaseslinearly with the increase in and finally reaches about1500 Oe at the of 11 10 /cm . This value can befurther tuned to meet the practical applications by selectingappropriate magnetic materials or layer structures.

Several mechanisms can be conceived for the enhance-ment. One possibility arises from the increase of pinning effectscaused by the amplification of surface roughness with the in-crease in [13], [14]. We have controlled our surface mor-phology of AAO template carefully. The average surface rough-

RAHMAN et al.: A SIMPLE ROUTE TO FABRICATE PERCOLATED PERPENDICULAR MAGNETIC RECORDING MEDIA 2135

Fig. 5. Variations of coercivity and average surface roughness as function ofpore density.

ness value, measured by AFM, is varied between 0.4–0.5 nm forall samples as shown in Fig. 5. Based on this observation, wecan conclude that the enhancement is not due to the surfaceroughness.

Another possibility is that the direction of magneticanisotropy of the Co/Pt multilayer films on the land is dif-ferent from that on the walls near the perimeter of the pores,which results in confinement of domain walls. This may be areasonable assumption in light of the fact that the direction offilm deposition on average is perpendicular to the land regionsbut has a certain obliquity relative to the pore walls [8]. Themagnetization near the rim of the pores, instead of being per-pendicular, inclines towards the surface, acting as a transitionregion and causing the pinning effect [7].

Another consideration is that the film around the perimeter ofpores may be somewhat thinner than that on the land, causingthe reduced domain wall energy in the perimeter region and,thereby, providing potential pinning sites for the domain walls[8]. Based on the above discussion, possible mechanisms for thecoercivity enhancement are due to the induced magnetic pin-ning, which impedes the motion of magnetic domain walls inthe Co/Pt multilayers at the pores. It is well known that the pin-ning strength is proportional to the density of the pinning sitesand the effect is more profound when the size of the pinning sitesis comparable to the domain wall width [15]. The domain wallwidth of Co/Pt multilayers is about 10 nm [16], which is com-parable to the defect size ( 10–12 nm) in this paper. This dataagain confirm that the pores act as the effective pinning sites inthis study.

IV. CONCLUSION

In summary, we have demonstrated an attractive and eco-nomic way to fabricate percolated perpendicular magneticrecording media by depositing magnetic layers onto substrateswith arrays of ultra-high-density nanopores. About one orderincrease of reveals that the pores are effective pinning sites

for the magnetic layer. A Co/Pt multilayer as magnetic layerwas selected in this paper due to its perpendicular magneticanisotropy and exchange coupled grains. However, this fabrica-tion method of percolated perpendicular media can be appliedto various systems, including FePt or CoPt.

ACKNOWLEDGMENT

This work was supported in part by the National ScienceCouncil of R.O.C. under Grant NSC 95-2221-E-007-063-MY2and in part by the Ministry of Economic Affairs of R.O.C. underGrant 93-EC-17-A-08-S1-0006.

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Manuscript received October 30, 2006 (e-mail: chlai@mx.nthu.edu.tw).