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Journal of Magnetism and Magnetic Materials 310 (2007) 2333–2335

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Magnetic properties of FePd films grown on Si antidots

C.C. Yua,�, Y.D. Yaob, S.C. Choub

aDepartment of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROCbInstitute of Physics, Academia Sinica, Taipei 115, Taiwan, ROC

Available online 27 November 2006

Abstract

In this study, 15 nm thick Si antidots were first fabricated by nanosphere lithography and reactive-ion etching on MgO (1 0 0)

substrates. Then, 30 nm thick FePd alloy films were deposited on the Si antidots at 500 1C for 1 h. Through this method, we controlled the

formation of FePd sub-micro dot arrays and FePd:Si matrix that formed by the interdiffusion between FePd alloy and Si antidots.

Meanwhile, the investigation of magnetic force microscopy showed that the magnetic domains of FePd sub-micro dot arrays were

separated by FePd:Si matrix.

r 2006 Elsevier B.V. All rights reserved.

PACS: 75.30.Gw; 75.60.-d; 75.50.Ss

Keywords: FePd; Si antidots; Perpendicular magnetic anisotropy

1. Introduction

The fabrication of magnetic nanostructures with magne-tically decoupled dot arrays has been paid attentionrecently due to the application of high-density magneticrecording. Especially, L10 (0 0 1) films display a largeperpendicular magnetic anisotropy (PMA) constant, forexample, L10-type FePt (0 0 1) films have a Ku�6.6–10�107 erg/cm3 [1,2]. Many researchers fabricate the dots arrayon patterned substrates that are prepared by the laserinterference lithography or e-beam lithography [3,4]. Onthe other hand, directly ion milling on the magnetic film isalso a modern technique to fabricate magnetic dot arraysand nanostructures [5]. There are also some studies thatfocus on the chemically synthesized FePt and CoPtnanoparticles [6,7].In this study, we try to fabricate and study the PMA

behavior of FePd sub-micro dot arrays without inter-dotmagnetic coupling. Therefore, a magnetically decoupledmatrix should be established during the growth of FePdfilms. The nanosphere lithography was used to fabricate Siantidots that serve as a seeding layer to produce the

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/j.jmmm.2006.11.070

onding author. Tel.: +886 7 5919473; fax: +886 7 5919357.

ddress: yucc@nuk.edu.tw (C.C. Yu).

magnetically dead FePd:Si matrix. The correlation betweenthe structure and magnetism of FePd sub-micro dot arrayswere then investigated by X-ray diffraction (XRD),perpendicular magneto-optical Kerr effect (PMOKE),and scanning probe microscopy.

2. Experiments and results

Firstly, 500 nm polystyrene nanospheres were spun onthe MgO (1 0 0) substrates at 3000 rpm to create mono-layered arrays. The detailed experimental parameters canbe found in Refs. [8,9]. Then, the nanospheres were thinnedby reactive-ion etching (RIE) with oxygen at a power of80mW/cm2 for 10min as sketched in Fig. 1(a). Throughthis etching process, the separation between Si antidots waswell controlled. A 15 nm thick Si film was then depositedby the e-beam evaporation under a deposition rate ofaround 0.1 A/s as sketched in Fig. 1(b). The Si-antidotarrays were obtained by removing the nanospheres in anultrasonic bath of dichloromethane as sketched in Fig. 1(c).Then, Si-antidot templates were outgassed at 500 1C forhalf an hour in a thermal deposition system with a basepressure of around 8� 10�8 Torr. A 30 nm thick FePd filmwas then deposited on the MgO (1 0 0)/ Si-antidot templateat 500 1C under a deposition rate of around 0.1 A/s in the

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Fig. 1. The experimental procedure in this study: (a) deposition and

thinning of monolayered nanospheres, (b) deposition of Si films, (c) Si-

antidots templates after lift-off process, and (d) deposition of FePd films.

20 25 30 35 40 45 50 55

2θ (degree)

Lo

g I

nte

nsity (

cp

s.)

Fe

Pd

(00

1)

Fe

Pd

(20

0)

Fe

Pd

(00

2)

a

b

Mg

O(0

02

)

Fig. 2. The y–2y X-ray spectra of (a) FePd films and (b) FePd sub-micro

dot arrays surrounded by FePd:Si.

-10k -5k 0 5k 10k

Ke

rr I

nte

nsity (

a.u

.)

Applied Field (Oe)

-10k -5k 0 5k 10k

Applied Field (Oe)

a b

Fig. 3. The curves of perpendicular magneto-optical Kerr effect of (a)

FePd films and (b) FePd sub-micro dot arrays surrounded by FePd:Si.

Fig. 4. The morphology (left) and magnetic domain (right) of (a)–(b)

FePd films and (c)–(d) FePd sub-micro dot arrays surrounded by FePd:Si.

The dashed rings in (d) indicated the boundaries of Si antidots.

C.C. Yu et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2333–23352334

thermal deposition system using Fe50Pd50 alloy sources. Atlast, FePd sub-micro dot arrays surrounded by FePd:Siwere obtained under high temperature growth as sketchedin Fig. 1(d). As a reference, a 30 nm thick FePd film wasalso grown onto the MgO (1 0 0) substrates directly.The structure of FePd films was characterized by the

y–2y XRD. As shown in Fig. 2(a), the L10 (0 0 1) and (0 0 2)diffractions indicated that FePd films were ordered FCTstructures. Fig. 3(a) showed the PMOKE curve of FePd

films grown on MgO (1 0 0) substrates directly. It showed ahysteresis loop of FePd films with perpendicular magneticanisotropy. Figs. 4(a) and (b) are the morphology andmagnetic domain of as-grown FePd films, respectively.They were taken under a tapping-mode scanning with liftoperation in a NT-MDT P47 system. The shape ofmagnetic domains was stripe-like with a periodicity ofaround 72 nm. These features were almost the same asprevious reports [10]. That ensures that our growthtechnique on fabricating FePd alloy with PMA is suitable.In addition to FCT (0 0 2) structures, the sample of FePd

sub-micro dot arrays surrounded by FePd:Si possessedFCC phases, as evidenced by the FCC (2 0 0) diffractionthat located at 2y ¼ 48.21 in Fig. 2(b). However, the XRDspectrum were contributed from both FePd and FePd:Si.Therefore, it is hard to know the individual structures ofFePd sub-micro dots and FePd:Si only from y–2y XRD.The FePd grown on Si-antidot templates showed a poorPMA characteristic that was measured by PMOKE asshown in Fig. 3(b). It indicated that the magnetic proper-ties of FePd films were strongly influenced by the under-lying Si-antidot templates. Fig. 4(c) showed themorphology of FePd sub-micro dot arrays. It displayedregular antidots that resulted from the underlying Sitemplates. The depth of these antidots was of around14 nm that coincident with the thickness of Si antidots. Themagnetic domains of FePd sub-micro dot arrays, afterapplying a strong perpendicular magnetic field, were shownin Fig. 4(d). By comparing to the morphology, themagnetic phase signal only came from the FePd films thatgrown on MgO substrates directly. For the FePd filmgrown on Si, it showed a poor-magnetic phase signal. Itmeans that FePd sub-micro dot arrays were obtained undera deposition temperature ¼ 500 1C. The low magneticphase signal of FePd grown on Si also tell us that FePd:Siis magnetically dead or in-plane magnetized. Because of thepoor PMA characteristic of FePd sub-micro dots, Si as ananti-dot material seems not a good choice. Maybe thePMA behaviors of FePd can be preserved if FePd sub-micro dots were completely separated. Therefore, the

ARTICLE IN PRESSC.C. Yu et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2333–2335 2335

traditional photoresist can be served as the materials ofanti-dot templates. After a photoresist lift-off process,completely separate FePd sub-micro dot arrays can beachieved.

Acknowledgments

The authors are grateful for the financial support by theAcademia Sinica and the National Science Council (underGrant Nos. 94-2119-M-390-010), Taiwan, ROC.

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