lithography and self-assembly for nanometer scale magnetism

Microelectronic Engineering 61–62 (2002) 569–575www.elsevier.com/ locate /mee

Lithography and self-assembly for nanometer scale magnetisma , b b a a a*S. Anders , S. Sun , C.B. Murray , C.T. Rettner , M.E. Best , T. Thomson ,

a a a aM. Albrecht , J.-U. Thiele , E.E. Fullerton , B.D. TerrisaIBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA

bIBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA

Abstract

The limits to scaling the relevant physical dimensions required to increase the areal density of magneticstorage devices will be reached soon, if the storage density continues to double annually. Two approaches toovercoming the limit of the minimum particle size required for thermal stability are presented. In the firstapproach, a narrow particle size distribution is produced using self-assembled layers of magnetic Fe–Ptnanoparticles. The very narrow particle size distribution offers the potential for increased storage density byutilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. The secondapproach involves patterned magnetic Co–Cr–Pt nanostructures produced using a focused ion beam, whichoffers the possibility of single bit per island storage on thermally stable sub-100-nm islands. 2002 ElsevierScience B.V. All rights reserved.

Keywords: Magnetic recording; Patterned magnetic media; Magnetic nanoparticles; Self-assembly

1. Introduction

Since the introduction of the hard disk drive in 1956 the principle of magnetic data storage has beenbased on writing magnetic bits in plane on magnetic layers. Since then the storage density has

2 2increased 20 million fold from 2 kb/ in to nearly 40 Gb/ in for today’s most advanced products, and2106 Gb/ in have been achieved in laboratory demonstrations [1]. In particular over the past few years,

due to the introduction of giant magneto-resistive (GMR) read heads, the storage density has increasedat a rate of greater than 100% annually. This increase was achieved by scaling the critical physicaldimensions, however, experiments and theory indicate that there is a limit to this scaling that will bereached within the next few years.

One of these limits is the reduction in grain size, where the onset of superparamagnetism at roomtemperature for small particles will define the size of the smallest possible particles. Today’s magnetic

*Corresponding author.E-mail address: [email protected] (S. Anders).

0167-9317/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 02 )00522-1

570 S. Anders et al. / Microelectronic Engineering 61 –62 (2002) 569 –575

storage media consist typically of quaternary alloys, e.g. Co–Cr–Pt–B, with grain sizes in the rangeof 10–12 nm [2]. The magnetic anisotropy energy E per grain is given by the grain volume V and theg

anisotropy energy density K as E 5 K V . The thermal stability of the grain magnetization can beu u g

described by the stability ratio C 5 E /k T where k is Boltzman’s constant and T the temperature.B B

C . 60 [3,4] is required to store information reliably for 10 years. If we assume cylindrical particles5 3with a height of the typical film thickness of 20 nm and K of current media (about 3310 J /m ), thisu

implies a minimum stable particle diameter of about 7 nm for monodispersed particles.To increase the storage density, the grain size needs to be reduced. This together with increasing

magnetic isolation is the conventional path taken to scale media. Since ultimately the transition widthis determined by the grain size, higher densities require smaller grains. Also, for constant signal tonoise, it is desirable to keep the number of grains per bit constant, and thus the grain size is reduced asthe density is increased. The anisotropy energy density can only be increased to some extent tomaintain the thermal stability as the grain size is reduced, as the media switching field increases withK and cannot exceed the write field available from the head.u

Various alternative routes to increase the storage density have been proposed. The stability oflongitudinal media can be improved by recording on two ferromagnetic magnetic layers that areantiferromagnetically coupled through a thin spacer layer. This leads to an effective decrease in themagnetic moment density but an effective increase in the magnetic anisotropy energy and animproved thermal stability [5]. Perpendicular recording, where the easy magnetization axis is orientedperpendicular to the storage medium and a soft underlayer is included, allows higher anisotropymaterials to be used as the media is effectively between the head poles. This permits higher K mediau

and thus improves the thermal stability [6]. In thermally-assisted recording the data storing andreading is performed at room temperature on very high anisotropy materials and writing is performedat elevated temperature and thus reduced anisotropy [7].

The grain size distribution for current granular media is very wide with a typical standard deviationfor the grain diameter of about s 5 30–35%. The smallest grains are superparamagnetic and notuseful for magnetic storage while the large grains contribute disproportionately to the noise. Anincrease in thermal stability and storage density and a reduction in noise can be achieved by reducingthe median grain size with a simultaneous narrowing of the size distribution. Magnetic nanoparticles[8] can have a very narrow particle size distribution, and in the first part of the paper we describe thefabrication of magnetic nanoparticle films, and the requirements for their application to magneticstorage.

Ultimately, one would like to record one bit per nanoparticle, and this requires a long-range regularassembly of the nanoparticles that has not been achieved as yet. However, various lithographicmethods can be used to produce regular arrays of magnetic nanostructures [9]. In the second part ofthe paper we report on the fabrication and properties of magnetic nanostructures produced using afocused ion beam. It is shown that the patterning leads to improved thermal stability and thus opensup a further route to increase the magnetic storage density.

2. Self-assembled layers of magnetic nanoparticles

2.1. Film preparation

A possible method to create new magnetic storage media is the deposition of self-assembled layers

S. Anders et al. / Microelectronic Engineering 61 –62 (2002) 569 –575 571

of magnetic nanoparticles [8] with a very narrow size distribution. In particular we have investigatedFe–Pt nanoparticles because the L1 phase of Fe–Pt in thin film form is known to have very high0

magnetic anisotropy [10]. Fe–Pt nanoparticles were prepared by the combination of reduction ofPt(acac) (acac5acetylacetonate) and thermal decomposition of Fe(CO) in the presence of oleic acid2 5

and oleyl amine. The nanoparticles can be easily dispersed in a variety of solvents. A drop of thesolution is placed on the substrate and during the evaporation of the solvent a regular assembly ofnanoparticles is formed. The elemental composition of the Fe–Pt nanoparticle materials is tuned byvarying the molar ratio of Fe(CO) and Pt(acac) . The Fe–Pt particle size can be varied from 3 to 105 2.

nm by first growing 3 nm monodisperse seed particles in situ and then adding more reagents toenlarge the existing seeds to the desired size. The size distribution of the particles is very narrow(s # 5%) as can be seen in Fig. 1 for a self-assembled layer of 6-nm particles.

As deposited, the Fe–Pt nanoparticles are in the disordered fcc phase and are superparamagnetic atroom temperature. Annealing at various temperatures and durations in nitrogen or forming gastransforms the particles to the ferromagnetic, ordered L1 phase.0

2.2. Magnetic and chemical properties

The coercivity of the annealed films typically increases with anneal time and temperature, and aminimum temperature of about 500 8C is required to begin forming the ordered L1 phase. An0

increased number of layers also leads to an increase in the film coercivity. For magnetic recordingthin, high anisotropy layers of small nanoparticles are most desirable. The thinnest layers that showedcoercivities .500 Oe (as measured by polar Kerr effect) were one-layer films of 8-nm particles andtwo-layer films of 4-nm particles. Superconducting quantum interference device (SQUID) measure-ments show that coercivities as high as 17 kOe can be achieved for three-layer /4-nm particle thinfilms (annealed at 800 8C for 5 min in N ). The easy axis of the particles is 3D randomly oriented,2

leading to identical hysteresis loops for in-plane and perpendicular measurements and a remanentmagnetization M 5 0.5M where M is the saturation magnetization (Fig. 2).r s s

The chemical nature of the films was studied using near edge X-ray absorption fine structurespectroscopy (NEXAFS) measured in total electron yield [11]. The data indicate that all films containa fraction of oxidized Fe. Fig. 3 shows an example of the NEXAFS spectra for one- and seven-layer

Fig. 1. Transmission electron microscope image of self-assembled layer of 6 nm diameter Fe–Pt nanoparticles.


Fig. 2. In-plane and perpendicular hysteresis loop measured by vibrating sample magnetometer of three layers of 4-nmFe–Pt particles prepared from Fe(CO) , after annealing at 580 8C in N for 30 min.5 2

films of 8-nm particles together with a metallic Fe reference spectrum. One can see that theseven-layer film shows a much more metallic character which is correlated to a much highercoercivity compared to the one-layer film. The best fit to the experimental data is obtained for aweighted superposition of Fe and Fe O reference spectra but due to the similarity of the spectra of Fe3 4

and FeO and of Fe O and Fe O we cannot exclude the presence of other oxidation states.3 4 2 3

Optimization of annealing conditions and chemistry led to a reduction of the oxidized Fe content andan increase in metallic Fe present in the ordered L1 phase. Both correlate strongly with increase in0

the magnetic coercive field.It has been demonstrated previously [8] that it is possible to perform magnetic recording on these

films in a way similar to granular media with many nanoparticles per bit. To optimize the magnetic

Fig. 3. NEXAFS spectra for one- and seven-layer films of 8-nm particles annealed at 580 8C in N for 30 min, together with2

a metallic Fe reference spectrum. The particles were prepared from FeCl .2


recording properties it will be necessary to find a way to orient the particle easy axes all perpendicularto the film. Ultimately, high-density recording on self-assembled nanoparticles would consist ofstoring one bit in one individual nanoparticle which would correspond to storage densities of about

210–20 Tbit / in .

3. Patterned nanostructures

3.1. Nanostructure fabrication

Patterned magnetic media with predefined single-domain islands can be produced by variousmethods including electron beam lithography [12], interferometric lithography [13], or nanoimprintlithography [14]. We have fabricated magnetic nanostructures using focused ion beams [15,16] topattern perpendicular media. The perpendicular granular media consisted of a 20-nm Co Cr Pt70 20 10

magnetic film protected by 5 nm of CN sputtered onto a Cr–Ta (1 nm)/Ni–Al (6 nm) underlayerx

structure grown on glass substrates. Arrays of square islands with sizes ranging from 50 to 730 nmand periods from 70 to 750 nm were fabricated and studied using atomic force and magnetic forcemicroscopy (AFM and MFM). As previously observed [15,16] small islands with periods ,130 nmare single domain while larger islands show multidomain states similar to the unpatterned film.

3.2. Thermal stability and switching behavior of patterned islands

Fig. 4 shows as an example AFM and MFM images for a pattern that consisted of square islands of

Fig. 4. (a) AFM and (b) MFM images of patterns formed using a focused ion beam in Co–Cr–Pt granular media after5saturation and aging for 4310 s in zero field at room temperature.


various sizes ranging from periods of 100–750 nm. The sample was magnetized in a strong field (205kOe) and aged at room temperature in zero field for 4310 s. The sample and MFM tip magnetization

were opposite so that areas of the sample that are in the original state of magnetization appear brightin the MFM image. It can be seen that all the 100-nm period islands (top left-hand side of Fig. 4) arestill in their original state of magnetization while some of the larger islands are locally reversed (darkin the image). The largest islands show a decay and domain structure similar to that of the unpatternedfilm. We observe generally a very stable thermal behavior for the small, single domain islands with nota single reversal observed (out of a total of 1000 islands monitored) during the duration of the

6experiment of 1.5310 s (about 17 days). This corresponds to a decay rate smaller than 0.03% per3decade of time (the first image was acquired about 1310 s after removal of the field after saturation).

The decay rate increases with island size and large, multidomain islands decay at rates similar to theunpatterned film of 2.4% per decade (as measured by vibrating sample magnetometer). The highthermal stability of the islands can in part be explained by a substantial reduction in the demagnetizingfields caused by the patterning. The observation that small islands are always single domain suggeststhat it is energetically unfavorable to place a domain wall within the island, and this furthercontributes to the thermal stability. However, the reversal mechanism of these islands, which consistof many individual grains, is still unclear. It is very likely that the thermal stability could be improvedfurther if the islands were fabricated consisting of fewer, larger grains, and ideally of one large grain.

Using a static read /write tester [17] magnetic recording on similar islands fabricated from 20-nmgranular Co Cr Pt media was demonstrated [15,18]. The highest island density at a period of 6770 18 12

2nm corresponds to a storage density of .140 Gb/ in but due to the comparatively large width of theread and write element of the GMR head (800 and 2000 nm, respectively) only rows of about 20islands could be addressed. Fig. 5 shows rows of islands written with a square wave bit patterncompared to a square wave bit pattern written on unpatterned media at the same density of 10 fluxchanges /mm. The MFM images suggest that in addition to improving thermal stability the patterningalso reduces the transition noise and improves the signal-to-noise ratio.

Fig. 5. (a) MFM image of square wave bit pattern written on patterned media and (b) unpatterned media at 10 fluxchanges /mm.


4. Conclusions

Two approaches to novel materials for ultrahigh density magnetic storage have been described.Both are driven by the fact that conventional magnetic recording will soon reach a limit in thepossible storage density. This limit is set principally by thermal decay effects of the magnetization. Inthe first approach thin films of magnetic nanoparticles were investigated because they exhibit a verynarrow size distribution and ultimately the potential of storing one bit per nanoparticle. By optimizingthe chemistry and anneal conditions we were able to produce very thin (one layer /8-nm particles andtwo layers /4-nm particles) ferromagnetic films with coercivities of .500 Oe. Thicker films (threelayers /4-nm particles) can have coercivities of up to 17 kOe. Further optimization is required to reach

2the goal of magnetic storage in the density range of 100 Gb/ in and above. In the second approachgranular perpendicular Co–Cr–Pt media was patterned into small square islands using a focused ionbeam. The smallest islands exhibit a thermal stability far superior to the continuous media (about threeorders of magnitude higher). Focused ion beam is an efficient technology for fast prototyping ofpatterned media, and the challenge is to find cost-efficient large area patterning technologies to replacefocused ion beam for disk production.

References

[1] B.R. Acharya, E.N. Abarra, A. Inomata, I. Okamoto, Joint European Magnetism Symposium, Grenoble (France), 2001,paper B046.

[2] Y. Kubota, L. Folks, E. Marinero, J. Appl. Phys. 84 (1998) 6202–6207.[3] S.H. Charap, P.-L. Lu, Y. He, IEEE Trans. Magn. 33 (1997) 978–983.[4] D. Weller, A. Moser, IEEE Trans. Magn. 35 (1999) 4423–4439.[5] E. Fullerton, D.T. Margulies, M.E. Schabes, M. Carey, B. Gurney, A. Moser, M. Best, G. Zeltzer, K. Rubin, H. Rosen,

M. Doerner, Appl. Phys. Lett. 77 (2000) 3806–3808.[6] D.A. Thompson, J. Magn. Soc. Jpn. 21 (Suppl. S2) (1997) 9–15.[7] J.J.M. Ruigrok, R. Coehoorn, S.R. Cumpson, H.W. Kesteren, J. Appl. Phys. 87 (2000) 5398–5403.[8] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989–1992.[9] R.L. White, R.M.H. New, R.F.W. Pease, IEEE Trans. Magn. 33 (1997) 990–995.

[10] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M. Toney, M. Schwieckert, J.-U. Thiele, M.F. Doerner, IEEE Trans.Magn. 36 (2000) 10–15.

¨[11] J. Stohr, NEXAFS Spectroscopy, Springer Verlag, New York, 1992.[12] R.M.H. New, R.F.W. Pease, R.L. White, J. Vac. Sci. Technol. B 13 (1995) 1089–1094.[13] T.A. Savas, M. Fahoud, H.I. Smith, M. Hwang, C.A. Ross, J. Appl. Phys. 85 (1999) 6160–6162.[14] W. Wu, B. Cui, X.-Y. Sun, W. Zhang, L. Zhuang, L. Kong, S. Chou, J. Vac. Sci. Technol. B 16 (1998) 38825–38829.[15] J. Lohau, A. Moser, C.T. Rettner, M.E. Best, B.D. Terris, Appl. Phys. Lett. 78 (2001) 990.[16] C.T. Rettner, M.E. Best, B.D. Terris, IEEE Trans. Magn. 37 (2001) 1649–1651.[17] A. Moser, D. Weller, IEEE Trans. Magn. 35 (1999) 2808.[18] M. Albrecht, S. Anders, T. Thomson, C.T. Rettner, M.E. Best, A. Moser, B.D. Terris, J. Appl. Phys., in print.

lithography and self-assembly for nanometer scale magnetism

Documents