Journal of Magnetism and Magnetic Materials 239 (2002) 291–293

Capping effect in magnetic properties of Ag ultra-thin filmson Co/Pt(1 1 1)

Y.E. Wu, C.W. Su, F.C. Chen, C.S. Shern*, R.H. Chen

Department of Physics, National Taiwan Normal University, 88 Section 4, Ting-Chou Road, Taipei 116, Taiwan, ROC

Abstract

Magneto-optical Kerr effect was used to study the changes of the magnetic property for Ag ultra-thin films depositedon Co/Pt (1 1 1) surface. The perpendicular magnetic anisotropy has a significant enhancement when the system isannealed and the Co–Pt alloy is formed. The magnetization disappears at lower temperatures, and appears at higher

temperature after 1ML Ag/1ML Co/Pt (1 1 1) ultra-thin film was annealed at 710K. This magnetization is reversible.The thermal energy triggering the motion of the magnetic domain walls is the possible mechanism for the largermagnetization at high temperature. r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Surface magneto-optical Kerr effect; Perpendicular magnetic anisotropy; Ultra-thin film; Coercivity; Thermal annealing

In recent years, thin films with at least one ferromag-netic component have attracted wide attention fromresearchers in both experimental and theoretical fields.

Co–Pt multilayers have some important properties suchas a perpendicular magnetic anisotropy (PMA), highcoercivity, and large magneto-optical effect [1–3]. Ag–

Co thin films exhibit the phenomena of giant magne-toresistance and superparamagnetism [4–6]. Therefore, asystem containing Ag, Co and Pt is especially interest-

ing. In our recent study of Ag/Co/Pt (1 1 1) ultra-thinfilms [7], Ag adatoms become more stable after Co–Ptalloy is formed. The temperature of the formation of theCo–Pt alloy rises as the coverage of Ag increases. These

interesting structural properties motivated us to studythe capping effects in magnetic behavior of Ag ultra-thinfilms on Co/Pt (1 1 1).

In this report, we studied the changes in structural andmagnetic properties after Ag was deposited on Co/Pt(1 1 1). The relationship between the magnetic property

and the structure was investigated.The experiments were carried out in situ in an ultra-

high vacuum (UHV) chamber with the working base

pressure better than 3� 10�10 Torr. The substrate Pt

(1 1 1) surface was cleaned by standard Ar ion bombard-ment and annealing cycles in the UHV chamber. A He–Ne laser with a wavelength of 632.8 nm was used as the

light source for the SMOKE measurement. The deposi-tion rates of Co and Ag were 900 and 320ML/s at roomtemperature, respectively. The detailed cleaning process

and the equipment in UHV chamber have beendescribed elsewhere [8,9].As the Co coverage reaches 1ML, a 6-fold fine

structure of LEED is developed surrounding eachinteger spot of the substrate as shown in Fig. 1(a). ThisLEED pattern containing satellite spots can be inter-preted by a misfit vector of an ¼ nðanCo � anPtÞ [1]. Thesereciprocal lattice vectors anPt ¼ 2p=aPt; anCo ¼ 2p=aCo areequal to 22.7 and 25.0 nm�1, respectively. This incom-mensurate LEED pattern indicates an incoherent

epitaxy of the Co overlayer on the Pt (1 1 1) surface.We then deposited 1ML of Ag on the 1ML Co/Pt (1 1 1)surface at room temperature. As shown in Fig. 1(b), the

LEED pattern does not change except that the threeinner spots of the satellite on the diagonal lines arebrighter compared to 1ML Co/Pt (1 1 1). The symmetry

of these inner spots changes from 6-fold to 3-fold. Afterannealing at 710K, all the satellite spots shown inFig. 1(b) disappear, only the inner spots on the diagonallines persist as shown in Fig. 1(c). Carefully study, the

inner spots are the LEED pattern of Ag and the ones on

*Corresponding author. Tel.: +886-2-29346620-166; fax:

+886-2-29326408.

E-mail address: ching@cc.ntnu.edu.tw (C.S. Shern).

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 6 2 3 - 0

the boundary are that of Pt. At high temperatureannealing, Co atoms diffuse into Pt bulk and Co–Ptalloy is formed, but Ag atoms still reside at the top in an

ordered state. This observation is consistent with ourprevious AES study [7].According to our previous studies [9], the hysteresis

loops occur only in the polar configuration at the initialstage of Co film growth with thickness less than 4ML.The easy axis of the magnetization is out-of-plane. Asthe Co film thickness further increases, the easy axis of

the magnetization gradually changes from the out-of-plane to the in-plane. Comparing the above magneticanisotropy to the growth mode study observed by STM

[10], we found that PMA occurs when the growthpattern of Co on Pt (1 1 1) is that of a layer-by-layer. Incontrast, in-plane magnetic anisotropy occurs as the

growth pattern of Co thin film is that of 3-D islandgrowth.After Ag ultra-thin films were deposited on the 1ML

Co/Pt (1 1 1) surface, SMOKE measurement showedthat the coercivity (Hc) increased as shown in Fig. 2(a)–(c). The thicker the Ag film gets, the larger the coercivityof the magnetic hysteresis loop becomes. The coercivities

are 280, 595 and 745Oe for 0, 0.5 and 1.0ML Agcoverage, respectively. Raising Ag coverage to morethan 1ML did not result in further increase in Hc: Asshown in Fig. 2(d)–(f), the Hc behaviors of Ag ultra-thinfilms deposited on the 2ML Co/Pt (1 1 1) are similar tothat of 1ML Co/Pt (1 1 1). Note thatHc does not change

comparing Fig. 2(e) and (f). Ag capping layer on the topof Co/Pt (1 1 1) can prevent the spin reversal of Co.After all Co sites are completely occupied by Ag atoms,the increasing thickness of the Ag capping layers does

not affect the magnitude of Hc:

The temperature dependence of the normalizedsaturated Kerr intensities for 1ML Ag/1ML Co/Pt

(1 1 1) thin film is shown in Fig. 3(a) (curve I). The Kerrsignals were measured at each selected temperature afterthe system was in thermal equilibrium. We did not

observe any longitudinal Kerr intensities up to 710K.The polar Kerr intensities decrease as the sampletemperature increases. It is interesting that the signalenhances between 500 and 650K. The signal reaches a

maximum near 630K and then drops rapidly after the

Fig. 1. The photograph and schematic drawing of LEED

patterns observed in incident electron energy of 70.5 eV: (a)

1ML Co/Pt (1 1 1); (b) 1ML Ag/1ML Co/Pt (1 1 1) and (c)

1ML Ag/1ML Co/Pt (1 1 1) after annealing at 710K. (Note

that: the area of the circle is proportional to the spot size of

LEED, and the open circle is brighter than the dark one in the

schematic drawings.)

Fig. 2. The hysteresis loops taken at 325K in the polar

configuration: (a) 1ML Co/Pt (1 1 1); (b) 0.5ML Ag/1ML

Co/Pt (1 1 1); (c) 1ML Ag/1ML Co/Pt (1 1 1); (d) 2ML Co/Pt

(1 1 1); (e) 1ML Ag/2ML Co/Pt (1 1 1) and (f) 2ML Ag/2ML

Co/Pt (1 1 1).

Fig. 3. (a) The polar saturated Kerr intensity as a function of

the sample temperature: I for 1ML Ag/1ML Co/Pt (1 1 1), II

for 1ML Co/Pt (1 1 1). (b) The Kerr intensity for both the polar

and longitudinal configuration measured at 325K after

annealing at different temperature for 1ML Ag/1ML Co/Pt

(1 1 1).

Y.E. Wu et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 291–293292

temperature exceeds 650K. It then disappears at about710K. The Curie temperature of 1ML Ag/1ML Co/Pt

(1 1 1) is about 710K. The temperature dependence ofthe normalized saturated Kerr intensities for 1ML Co/Pt (1 1 1) is shown in Fig. 3(a) (curve II). Comparing the

two curves in Fig. 3(a), the Ag capping layer causes anenhancement in the magnetization, and a Curie tem-perature increase from 625 to 710K.The out-of-plane magnetization can enhance due to

the formation of Co–Pt alloy for Co ultra-thin filmsdeposited on Pt (1 1 1) [9,11]. The hybridization of theelectron states in the formation of the Co–Pt alloy is the

possible mechanism for this enhancement [12]. It isinteresting that the enhancement is augmented when Agatoms are deposited on top. For further investigation on

the phenomenon of the enhancement, we studied theannealing effect of the 1ML Ag/1ML Co/Pt (1 1 1)system. The Kerr intensities were measured at 325K

after the sample was hold for 5min at each temperature.The remanence of magnetization (Mr) at differentannealing temperatures is shown in Fig. 3(b). Mr isenhanced when the annealing temperatures are higher

than 500K. The maximum enhancement of Mr occursnear 630K during which Mr is about twice the originalvalue. No in-plane magnetization was observed during

the annealing process. It is interesting that Mr decreaseswhen the annealing temperature is higher than 650K,and subsequently disappears at 710K. After careful

analysis, we found that Mr disappears at temperatureslower than 325K, but it can recover reversibly andenhance to a maximum at 425K after the ultra-thin filmswere annealed at 710K. This anomalous behavior of

zero Mr at low temperatures and enhanced Mr at hightemperatures allows the ultra-thin film to be used as amagnetic valve by tuning of the temperature.

For understanding of this unusual property, wefurther studied the behavior of coercivity of the system.We found that Hc increases as the temperature is

lowered after the sample has annealed at 710K. Forexample, Hc increases to 650Oe at 500K and 850Oe at375K. By extrapolation based on the plot of Hc versus

temperature, Hc can surpass 1 kOe at room temperature.At this stage, we can understand the unusual phenom-enon for zeroMr near room temperature and highMr athigh temperatures after the thin film is annealed at

710K. At room temperature, the magnetic domain wallscannot be removed by the applied magnetic field used inthe experiment under the limitation of 950Oe in our

UHV chamber. Therefore, we cannot find hysteresis

loop near room temperature after the system is annealedat 710K. On the other hand, thermal energy can trigger

the motion of the magnetic domain walls at highertemperatures. Therefore, the applied magnetic field canreverse the spin orientation of Co.

In summary, a significant enhancement of PMA wasobserved when Co–Pt alloy is developed for 1ML Ag/1ML Co/Pt (1 1 1). The coercivity increased after Agdeposition, but it did not increase if Ag coverage is more

than 1ML. The Ag capping layer on Co/Pt (1 1 1) canprevent the spin reversal of Co. The Curie temperatureincreases from 625 to 710K after 1ML Ag ultra-thin

film is deposited on 1ML Co/Pt (1 1 1). The out-of-planemagnetization disappears at temperatures lower than325K and it recovers at higher temperatures after the

ultra-thin films are annealed at 710K. This anomalousbehavior in magnetization is reversible. This property ofthe system can be used as a magnetic valve by tuning

temperature.

This research was supported by National ScienceCouncil of ROC under Grant No. NSC 89-2112-M-003-039.

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