Enhanced Curie temperatures in Fe and Co magnetic nanoparticle assembly on single-crystalline Al

(1)

Enhanced Curie temperatures in Fe and Co magnetic nanoparticle assembly on single-crystalline Al

₂

O

₃

/ NiAl „100… with normal metal capping layer

Wen-Chin Lin

Department of Physics, National Taiwan University, Taipei 106, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

Po-Chun Huang

Department of Physics, National Taiwan University, Taipei 106, Taiwan Ker-Jar Song

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan Minn-Tsong Lin^a兲

Department of Physics, National Taiwan University, Taipei 106, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

共Received 15 November 2005; accepted 16 March 2006; published online 12 April 2006兲

The ferromagnetism of Fe nanoparticle assembly on Al₂O₃/ NiAl共100兲 is observed above 150 K with the coverage larger than 5 ML共monolayer兲. Cu capping layer induces an enhancement of the Curie temperature 共TC兲 in both Fe and Co magnetic nanoparticle assembly. The TC of Fe nanoparticle assembly with 2 and 6 ML Cu capping layer is enhanced by⬃20 K and even higher, indicating the critical effects of metallic capping layer in such magnetic nanostructures as nanoparticle assembly. The capping layer effect would be crucial for the ex situ measurements and the nanostorage-related applications. © 2006 American Institute of Physics.

关DOI:10.1063/1.2195111兴

In low dimensional systems, such as the ultrathin films and nanoparticles, the magnetic properties are strongly af- fected by variation of the surface condition, for example, the capping of normal metal共NM兲 such as Cu, due to the large ratio of surface to bulk atom number. In the reported studies, usually both the magnetic moment and Curie temperature 共TC兲 of magnetic ultrathin films are reduced by the NM capping layer.^1–6The proposed reasons include the modification of surface anisotropy,⁷ the surface interdiffusion,⁸ etc. Al- though in some cases, there exists a nonzero magnetic moment in the NM layer right beside the FM film, the reduction in the magnetization of the FM film is always even larger.^3,5,6 As the nanoparticle systems are concerned, the NM capping effect is considered to be more complicated. The larger ratio of surface to bulk atom number in nanoparticles than the ultrathin films might result in more significant reduce of magnetization and TC. In another aspect, for the Fe nanoparticle assembly prepared by buffer layer assistant growth 共BLAG兲,⁹ the metallic substrate, such as Cu共111兲, is shown to provide an additional coupling between the magnetic nanoparticles and significantly enhances the TC, as compared with the substrate of Ge共111兲.⁹The questions thus raise that what kind of effect might be indeed caused by capping of NM layer on magnetic nanoparticle assembly and which one will dominate the final results? The magnetization and TC

might be either reduced just as the cases of ultrathin films^1,2 or enhanced since the NM metallic layer may provide additional coupling through the metallic connecting as the case of Fe dots/Cu共111兲 prepared by BLAG.⁹It is a very crucial and practical issue for the magnetic nanoparticle assembly in both the scientific and technical aspects, since the protective

capping layer is unavoidable for the ex situ measurements and applications.

Recently many methods are developed for the fabrica- tion of nanoparticle assembly.^9–19However, most of the mag- netic samples are ex situ measured with a protective capping layer,^15,18which, as indicated in this letter, actually induces complicated effects on the magnetic behavior of nanoparticle assembly. In this experiment, the Fe and Co nanoparticle assembly are prepared on Al₂O₃/ NiAl共100兲. The morphol- ogy and ferromagnetic behavior are in situ characterized.

The insulating Al₂O₃layer not only provides a superior template for the growth of nanoparticles but also terminates the possible magnetic interaction through the metallic substrate between the nanoparticles. Thus as compared with the nanoparticle assembly grown on bare metallic substrate,^9,11 the more direct comparison between the magnetic behaviors before and after NM capping共without and with NM connection between nanoparticles兲 can be made in this experiment.

The experimental apparatus and the preparation of Al₂O₃/ NiAl共100兲 template are described in our previous report.¹⁹ The Fe, Co nanoparticles, and Cu capping layers were deposited at room temperature共RT兲. Since the deposition rates were calibrated from the epitaxial growth on Cu共100兲,^2,201 ML共monolayer兲 was defined as the atom den- sity on Cu共100兲 surface: 1.54⫻10¹⁵at./ cm². The morphology of the sample was investigated by scanning tunneling microscopy 共STM兲 with V=1.6–2.0 V and I=0.8–1.0 nA.

The magnetic properties were characterized by magneto- optical Kerr effect 共MOKE兲. Figure 1 shows the STM images of Fe nanoparticle assembly on Al₂O₃/ NiAl共100兲共Ref.

19兲 with the deposition coverage ranges between 3 and 13 ML. With the higher coverage, the particle size increases obviously. However, the morphology still retains the shape

a兲Electronic mail: [email protected]

APPLIED PHYSICS LETTERS 88, 153117共2006兲

Downloaded 13 Apr 2006 to 140.112.101.80. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

(2)

of nanoparticle even with the deposition coverage up to 13 ML. Although the average size increases, the full width half maximum 共FWHM兲 of size distribution dose not increase significantly with coverage. As shown in Fig. 1, no particle of very large size can be observed with 3–13 ML Fe coverage, implying that the growth mechanism might be different from the classical behavior. In the sample of 3 ML coverage, the alignment of nanoparticles is still observable. But for the higher coverage the alignment becomes more and more in- visible. Although there exists the enlargement effect due to the finite size of STM tip, the gaps between nanoparticles are still observable, and the shape of nanoparticles also sustains.

The significant reduction in the magnetic long-range exchange coupling of nanoparicale assembly as compared with the thin film system 共Figs. 2 and 3兲 will also evidence the fact that the nanoparticles are still separated, at least in the sense of long-range exchange coupling.

Figure 2 exhibits the in-plane MOKE hysteresis loops of Fe nanoparticle assembly recorded at 150 K with different deposition coverage of 3–13 ML. Since the absence of hysteresis loop in polar MOKE is also confirmed, the 3 ML Fe nanoparticle assembly is nonferromagnetic above 150 K, and its T_Cshould be much lower. Not until 5–7 ML the onset of hysteresis loops is observed at 150 K, which is much lower than that in the Fe and Co/ Cu共100兲 thin film systems.^1,2 There is no preferred easy axis in the plane for the longitu- dinal magnetization. It is reasonable since the shape of nanoparticles is isotropic in the plane and the alignment is also not significant. Since the nanoparticles are deposited at RT, the assembly is supposed to be stable at and lower than RT.

In order to avoid the possible diffusion or percolation caused by annealing to temperature much higher than RT, the experiment is focused on the investigation of Fe nanoparticle assembly with 5–7 ML coverage, in which the TCis near RT.

Figure 3共a兲 shows the Kerr remanence 共MR兲 of 6.7 ML Fe nanoparticle assembly as a function of temperature. As indi- cated, T_Cis determined to be⬃281 K. For comparison, the MR of the 6.7 ML Fe/ NiAl共100兲 thin film grown and mea- sured at RT is also indicated. Obviously both the MRand TC

of 6.7 ML Fe thin film are much larger than that of 6.7 ML Fe nanoparticle assembly, indicating a weak long-range coupling in the nanoparticle assembly, which is probably due to the separation of individual particles particularly in the sense of electron overlapping. In Fig. 3共a兲, after capping 2 ML Cu on the 6.7 ML Fe nanoparticle assembly, the significant in- crease in TCfrom 281 to 308 K is observed. The sequential capping of another 4 ML Cu layer共6 ML in total兲 also helps increasing the magnetization signal.共Due to possible anneal- ing effect, further measurement for TC is not performed.兲 Figure 3共b兲 exhibits the corresponding STM images and MOKE hysteresis loops 共at ⬃260 K兲 for the 6.7 ML Fe nanoparticle assembly with 0, 2, and 6 ML Cu capping layer.

After capping 2 ML Cu, the MR increases but keeps nearly the same MSwith the uncapped sample. With the 6 ML Cu capping layer, both the MR and MS increase. The STM images also reveal that the capping of Cu significantly changes the morphology and provide connections between the Fe nanoparticles.

As shown in the studies on 1–2 ML Co/ Cu共100兲,²¹ the annealing-induced percolation of two-dimensional islands drastically produces a T_Cjump of 100 K and enhance the M_R to the level of thin films. In fact, after annealing the 6.7 ML Fe nanoparticle assembly capped with 6 ML Cu to more than 450 K, the remanent 共MR兲 and saturation 共MS兲 Kerr signal drastically increase to the same level as that of 6.7 ML Fe thin film due to the percolation between nanoparticles. It also evidences that the reduction of magnetization in nanoparticle assembly, as compared with the thin film system, indeed originates from the finite size effect in the isolated nanoparticles, not from such extrinsic reasons such as oxidation, etc.

Similar phenomenon of the T_Cenhancement by capping Cu layer is also observed in the 8 ML Co nanoparticle assembly. Figure 4共a兲 shows that the TCof uncapped sample is

FIG. 1.共Color online兲 STM images of 3–13 ML Fe nanoparticle assembly on Al₂O₃/ NiAl共100兲. The particle size increases with coverage, and the shape of particles sustains up to 13 ML.

FIG. 2. The in-plane MOKE hysteresis loops of 3–13 ML Fe nanoparticles on Al₂O₃/ NiAl共100兲 measured at 150 K. After 5 ML, the onset of remanent magnetization is observed.

FIG. 3.共Color online兲共a兲 The Kerr remanence 共MR兲 shown as a function of temperature for 6.7 ML Fe with 0, 2, and 6 ML Cu capping layers. Signifi- cant TC enhancement is observed. For comparison, MR of 6.7 ML Fe/ NiAl共100兲 thin film grown and measured at RT is exhibited. 共b兲 The corresponding STM images and MOKE hysteresis loops.

153117-2 Lin et al. Appl. Phys. Lett. 88, 153117共2006兲

(3)

investigated to be⬃286 K. After capping 4 ML Cu, both the magnetic Kerr signal and the TC are enhanced. The STM images in Figs. 4共b兲 and 4共c兲 also indicate that the Cu capping layer covers the assembly and provides connections between the Co nanoparticles. The repeat in both Co and Fe nanoparticle assembly demonstrates that the effect is univer- sal in various magnetic nanoparticles.

The critical exponent␤ is also estimated by fitting the curves for M_R as a function of temperature: 关MR= M₀共1

− T / T_C兲^␤兴共Refs. 20 and 22兲共Figs. 3 and 4兲. It ranges from 0.31 to 0.42, which is very similar to the results of the in situ measurements for Fe dots/Cu共111兲 prepared by BLAG.⁹The fitted␤ values are quite close to the prediction in three-dimensional共3D兲 Heisenberg 共␤= 0.365兲 and Ising 共␤= 0.325兲 models,^20,22 and far away from that of two- dimensional systems^20,22 共␤= 0.125– 0.24 in experimental and theoretical reports兲. The tendency of ␤ values toward 3D models also informs the intrinsic magnetism in nanoparticles.

Besides, the Cu capping effect on 9–11 ML Fe nanoparticle assembly is also investigated. The reduction of remanent and saturated Kerr signal for samples with 6 ML Cu capping layer can be observed, just the same as the reports on thin film systems. However, from the data in the previous sections, when the magnetic nanoparticles are still small and close to each other, the connection provided by the Cu capping layer dominates over the usual reducing effect caused by the reduction of surface anisotropy, strengthens the magnetic coupling between the nanoparticles, and eventually re- sults in the enhancement of TC.

In summary, Fe nanoparticle assembly is shown to sustain the particle shape up to 13 ML coverage and the ferromagnetism is observed above 150 K after 5 ML. Cap- ping of Cu overlayer on Fe and Co nanoparticle assembly

induces significant TC enhancement. The Cu capping layer connects the nanoparticles, providing additional channels for the magnetic coupling between the individual nanoparticles and resulting in the increase of TC. Such a pronounced capping effect may be avoided by using insulating layers.²³ Since the protective capping layer is always needed for the application of magnetic nanodevices, this effect might be- come a crucial issue in the near future for modulating their magnetic properties.

This work was supported by the National Science Coun- cil of Taiwan under Grant Nos. NSC 94-2112-M-002-005 and NSC 94-2112-M-001-045.

1R. Vollmer, S. van Dijken, M. Schleberger, and J. Kirschner, Phys. Rev. B 61, 1303共2000兲.

2C. M. Schneider, P. Bressler, P. Schuster, J. Kirschner, J. J. de Miguel, and R. Miranda, Phys. Rev. Lett. 64, 1059共1990兲.

3F. Wilhelm, U. Bovensiepen, A. Scherz, P. Poulopoulos, A. Ney, H.

Wende, G. Ceballos, and K. Baberschke, J. Magn. Magn. Mater. 222, 163 共2000兲.

4M. E. Buckley, F. O. Schumann, and J. A. C. Bland, Phys. Rev. B 52, 6596共1995兲.

5D. S. Wang, Ru-qian Wu, and A. J. Freeman, J. Magn. Magn. Mater. 129, 237共1994兲.

6Y. Huttel, G. van der Laan, T. K. Johal, N. D. Telling, and P. Bencok, Phys. Rev. B 68, 174405共2003兲.

7W. L. OBrien, T. Droubay, and B. P. Tonner, Phys. Rev. B 54, 9297 共1996兲.

8R. Pentcheva and M. Scheffler, Phys. Rev. B 61, 2211共2000兲.

9J. P. Pierce, M. A. Torija, Z. Gai, Junren Shi, T. C. Schulthess, G. A.

Farnan, J. F. Wendelken, E. W. Plummer, and J. Shen, Phys. Rev. Lett. 92, 237201共2004兲.

10Z. Gai, B. Wu, J. P. Pierce, G. A. Farnan, D. Shu, M. Wang, Z. Zhang, and J. Shen, Phys. Rev. Lett. 89, 235502共2002兲.

11J. Bansmann, S. H. Baker, C. Binns, J. A. Blackman, J.-P. Bucher, J.

Dorantes-Dávila, V. Dupuis, L. Favre, D. Kechrakos, A. Kleibert, K.-H.

Meiwes-Broer, G. M. Pastor, A. Perez, O. Toulemonde, K. N. Trohidou, J.

Tuaillon, and Y. Xie, Surf. Sci. Rep. 56, 189共2005兲.

12M. Valden, X. Lai, and D. W. Goodman, Science 281, 1647共1998兲.

13S. Gwo, C.-P. Chou, C.-L. Wu, Y.-J. Ye, S.-J. Tsai, W.-C. Lin, and M.-T.

Lin, Phys. Rev. Lett. 90, 185506共2003兲.

14H.-J. Freund, Surf. Sci. 500, 271共2002兲.

15T. Hill, S. Stempel, T. Risse, M. Baumer, and H.-J. Freund, J. Magn.

Magn. Mater. 198/199, 354共1999兲.

16T. Hill, M. Mozaffari-Afshar, J. Schmidt, T. Risse, and H.-J. Freund, Surf.

Sci. 429, 246共1999兲.

17H. H. Chang, M. Y. Lai, J. H. Wei, C. M. Wei, and Y. L. Wang, Phys. Rev.

Lett. 92, 066103共2004兲.

18M. H. Pan, H. Liu, J. Z. Wang, J. F. Jia, Q. K. Xue, J. L. Li, S. Qin, U. M.

Mirsaidov, X. R. Wang, J. T. Market, Z. Zhang, and C. K. Shih, Nano Lett.

5, 87共2005兲.

19W. C. Lin, C. C. Kuo, M.-F. Luo, Ker-Jar Song, Minn-Tsong Lin, Appl.

Phys. Lett. 86, 043105共2005兲.

20C. C. Kuo, C. L. Chiu, W. C. Lin, and Minn-Tsong Lin, Surf. Sci. 520, 121共2002兲.

21P. Poulopoulos, P. J. Jensen, A. Ney, J. Lindner, and K. Baberschke, Phys.

Rev. B 65, 064431共2002兲.

22Yi Li and K. Baberschke, Phys. Rev. Lett. 68, 1208共1992兲.

23Z. Gai, J. Y. Howe, J. Guo, D. A. Blom, E. W. Plummer, and J. Shen, Appl. Phys. Lett. 86, 23107共2005兲.

FIG. 4. 共Color online兲共a兲 The Kerr remanence shown as a function of temperature for the coverage of 8 ML Co before and after capping of 4 ML Cu. Significant TCenhancement is observed.共b兲 and 共c兲 are the STM images before and after capping of 4 ML Cu.

153117-3 Lin et al. Appl. Phys. Lett. 88, 153117共2006兲