30 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 1, JANUARY 2000 Magnetic Nanowires in Hexagonally Ordered Pores of Alumina Robert M. Metzger, Valery V. Konovalov, Ming Sun, Tao Xu, Giovanni Zangari, Bin Xu, Mourad Benakli, and W. D. Doyle, Fellow, IEEE Abstract—Acid-anodized aluminum forms amorphous alumina with long and columnar nanopores with approximately hexagonal ordering (“alumite”). Excellent hexagonal ordering of these nanopores has been achieved by 24 hours of anodization, but with restricted domain size (2–4 μm 2 ), which can be increased to 100 μm 2 with longer anodization. We have deposited Fe in disordered pores and Co in ordered pores; we can control the average length and diameter of these nanowires, but there is still a distribution of nanowire lengths. Previously, we described Fe nanowires with diameters down to 11 nm in disordered pores. Here we focus on longer (770 nm) and shorter (64 nm) Co nanowires with diameters of 25 nm in ordered pores with 100 nm pore-to-pore separation. The longer wires have an easy axis out-of-plane, with squareness 0.9, coercivity = 1900 Oe, and a fluctuation field of 5.3 Oe. The shorter wires are more isotropic, with lower coercivities ( 1300 Oe) and larger fluctuation fields 8.4 Oe. Index Terms—Alumite, anodized aluminum, cobalt nanowires, coercivity, extra-high density magnetic recording, fluctua- tion field, iron nanowires, nanopores, perpendicular magnetic recording, squareness. I. INTRODUCTION T HE THERMAL stability limit for magnetic recording density in conventional longitudinal recording may be 50 Gb in [1], but more recent calculations are more optimistic [2]. Linear density limits are currently determined by the medium noise. The power signal-to-noise-ratio (SNR) is determined by the number of grains which constitute the single bit. For a fixed bit size, the number of grains and the SNR could be increased by decreasing their dimensions, but this approach is limited by the thermal stability of the written magnetization transition, which is mainly determined by the demagnetizing fields and by the energy barrier to magnetization reversal for a magnetic grain. This limit could be relaxed to the superparamagnetic limit for the material of interest, if a bit cell consisted of a single and isolated magnetic unit. Such an ideal magnetic medium would consist of ferromagnetic islands Manuscript received August 19, 1999. This work was supported by DOD- URISP-DAA-H04-96-1-0316 and by NSIC-NSF-EHDR-542417-55139. R. M. Metzger, V. V. Konovalov, M. Sun, and T. Xu are with the Department of Chemistry and the Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487-0336 U.S.A. (e-mail: rmet- zger@bama.ua.edu). G. Zangari is with the Department of Metallurgical and Materials En- gineering and also the Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487-0202 U.S.A. (e-mail: gzan- gari@coe.eng.ua.edu). B. Xu, M. Benakli, and W. D. Doyle are with the Department of Physics and the Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487-0209 U.S.A. (e-mail: wdoyle@magnet.mint.ua.edu). Publisher Item Identifier S 0018-9464(00)00707-X. of nm dimensions, placed in an ordered fashion on the sites of a nm-sized 2-dimensional lattice (magnetic array). Current approaches to such magnetic arrays employ e-beam lithography to define the array geometry [3]–[5]. Anodization of aluminum is a much cheaper process for the synthesis of a nm-scale porous structure, consisting of close-packed cells in a local hexagonal arrangement, with pores at their centers. Hexagonally ordered patterns on microm- eter-sized domains can be obtained by cycles of anodization and successive removal of the porous oxide. Such a method appears very promising for the production of hexagonal pat- terns with extended long-range order. Magnetic materials can be electrodeposited into these pores by AC electrodeposition [6], producing isolated needle-like particles which have been of interest for perpendicular magnetic recording. The implication for magnetic recording is that an ordered perpendicular (or lon- gitudinal) medium should exhibit much lower noise, compared to a disordered medium, thus increasing the signal-to-noise ratio [7], although this has been debated [8]. The nanowires that can be introduced into the pores by electrodeposition may potentially produce bit densities in excess of 100 Gbit in . The controlled anodization of aluminum has been an indus- trial process since the 1920’s [9], but the main emphasis was anodization at pH above 7, to yield thick corrosion-resistant passivated oxide “barrier type films.” When Al is anodized in acidic solutions, pores form which have only an approximately hexagonal order [10], which is idealized as truly hexagonal in Fig. 1: this structure has been called “alumite” [6]. Since the pores were relatively uniform (Fig. 2), their use as hosts for mag- netic nanowires of Fe or Co received early attention by many groups worldwide [6], [11]–[15], including our own [16]–[18]. The mechanism for pore formation and its hexagonal ordering (a mesoscopic phenomenon) remains unknown. Early interest by the hard disk industry in magnetic nanowires in alumite faded in the early 1990’s. Since then, a dramatic breakthrough was achieved when Masuda and co-worker showed that prolonged anodization, followed by stripping of the thick oxide, and re-anodization produced ordered nanopores with perfectly hexagonal domains [19]. This result was confirmed by us [20], [21] (Fig. 3), and by others [22], [23]. The Al surface is usually flattened before anodization by an initial electropolishing step, which removes some of the “hills” in the sample, and may leave local thin spots in the air-formed oxide layer [21]. Ordering within this electropolishing step has received experimental [24] and theoretical [25], [26] attention: ordered patterns of small hillocks or stripes (not pores) can 0018–9464/00$10.00 © 2000 IEEE