ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2008, Vol. 72, No. 10, pp. 1379–1381. © Allerton Press, Inc., 2008. Original Russian Text © E.A. Gan’shina, N.S. Perov, S. Phonghirun, V.E. Migunov, Yu.E. Kalinin, A.V. Sitnikov, 2008, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2008, Vol. 72, No. 10, pp. 1455–1457.
Enhancement of Magneto-Optical Response in Nanocomposite–Hydrogenated Amorphous Silicon Multilayers E. A. Gan’shinaa, N. S. Perova, S. Phonghiruna, V. E. Migunova, Yu. E. Kalininb, and A. V. Sitnikovb a
b
Moscow State University, Moscow, 119992 Russia Voronezh State Technical University, Moskovskii pr. 14, Voronezh, 394026 Russia e-mail:
[email protected]
Abstract—The dependence of the magnetic and magneto-optical properties on the semiconductor layer thickness has been studied for a [(Co45Fe45Zr10)35(Al2O3)65(X)/α-Si:H(Y)]30 multilayer. It is found that an increase in the Si layer thickness to 1.3–1.7 nm leads to an increase in the transverse Kerr effect, magnetization, and coercive force. The changes in the properties of the nanomultilayer system are related to the percolation transition between CoFeZr granules through Si streaks. This percolation leads to effective exchange interaction between isolated ferromagnetic granules of Co45Fe45Zr10 alloy and increase in magneto-optical response. DOI: 10.3103/S1062873808100201
In recent years, great interest has been shown in ferromagnetic metal–semiconductor multilayers in view of the found dependence of the magnetic interaction between metal layers on the semiconductor interlayer thickness [1–4]. The physical properties of nanomultilayer systems, with layer thicknesses of about few nanometers, are determined in many respects by the structure and interface phenomena between different phases. The main difficulties in interpreting the results obtained for such structures is their very high sensitivity to the quality of the substrate surface and microdefects during preparation (a factors that may cause a direct contact between metal layers) and the diffusion processes at the interface between dissimilar phases, leading to the formation of metal–semiconductor compounds. One of the ways for solving these problems is the use of a metal–insulator composite with a composition below the percolation threshold as a ferromagnetic layer. In the prepercolation state, nanogranules, randomly distributed in the insulating matrix, are out of direct contact with each other. Such a structure makes it possible, on the one hand, to reduce to minimum the effect of contacts of granules from the neighboring layers on the macroscopic properties of the system. On the other hand, the presence of a barrier insulating layer hinders the formation of compounds at the composite– semiconductor interface. In this paper, we report the results of studying the magnetic and magneto-optical properties in complex nanomultilayer systems [(Co45Fe45Zr10)35(Al2O3)65(X)/αSi:H(Y)]30 with an amorphous structure, obtained by
ion-beam sputtering. The semiconductor layer thickness Y was continuously changed from 0.45 to 5.5 nm and the magnetic layer thickness X was varied from 1.08 to 1.88 nm. The magnetic layers were a nanocomposite below the percolation threshold, in which magnetic nanogranules of Co45Fe45Zr10 alloy were randomly distributed in an insulating Al2O3 matrix. Magnetostatic measurements were performed on a vibrational magnetic anisometer. The magneto-optical properties were studied in the transverse Kerr effect (TKE) geometry in the energy range from 0.5 to 4.2 eV. Figure 1 shows the spectral and field dependences of the TKE of the [(Co45Fe45Zr10)35(Al2O3)65/α-Si:H]30 nanomultilayer system. Analysis of these curves shows that the general view of the TKE spectra of multilayer films is similar to that of the spectrum for granular (Co45Fe45Zr10)35(Al2O3)65 alloy. All multilayer films exhibit a linear dependence of the TKE on the magnetic field magnitude. Figure 2 shows the concentration dependence of the multilayer TKE on the semiconductor interlayer thickness hSi. It can be seen that the TKE value significantly increases, with an increase in hSi to 1.3–1.7 nm. A further increase in the semiconductor interlayer thickness leads to a decrease in the TKE magnitude. Investigation of the magnetostatic hysteresis loops, with application of a magnetic field in the film plane and perpendicularly to it showed that a change in the semiconductor interlayer thickness leads to a change in the magnetization and coercive force, which reach maxima at hSi of about 1.5 nm. The multilayers with hSi = 1.3 nm exhibited perpendicular magnetic anisotropy; a
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GAN’SHINA et al. TKE, δ × 103
(a)
3 2 (CoFeZr)35(Al2O3)65 1 0 Y(nm)/X(nm) 1.08/0.67 –1 1.08/1.06 1.16/1.34 –2 1.26/1.67 1.37/2.05 –3 1.49/2.34 1.68/3.0 1.83/4.3 –4 1.83/5.3 –5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 E, eV TKE, δ × 103 (b) 0 –1
X(nm)/Y(nm) 1.08/0.67 1.08/1.06 1.16/1.34 1.26/1.67 1.37/2.05 1.49/2.34 1.68/3.0 1.83/4.3 1.83/5.3
–2 –3 –4
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 H, kOe Fig. 1. (a) Spectral and (b) field dependences of the TKE of[(Co45Fe45Zr10)35(Al2O3)65/α-Si:H]30 multilayer nanostructures.
TKE, δ × 103 (a)
0 –1 –2
E, eV 0.97 1.60 2.50
–3 –4 0
1
2
3
4
5 6 hα-Si, nm
further increase in hSi resulted in deviation of the sample magnetization vector from the vertical direction. The saturation fields in the film plane were in the range 5–6 kOe. The obtained results of the magnetic and magnetooptical investigations correlate with the data on the transport and high-frequency magnetic properties of these systems [5]. Figure 3 shows the concentration dependence of the resistivity ρ of the multilayer under consideration on the semiconductor interlayer thickness. It can be seen that the multilayer resistivity depends very strongly on the thickness of the α-Si:H layer. At 1.5 < hSi < 2 nm, ρ sharply decreases by three orders of magnitudes. A further increase in the semiconductor interlayer thickness does not lead to significant changes in the multilayer resistivity. The established behavior of ρ(hSi) is due to the structural features of the growth of the α-Si:H semiconductor interlayer on the (Co45Fe45Zr10)35(Al2O3)65 composition layer. Using the principle of minimization of the structure surface energy during growth, we can suggest the semiconductor film nuclei on the composite surface to be concentrated on metallic granules, since the surface energies γ of Co45Fe45Zr10 alloy, Al2O3, and Si are, respectively, 2.8, 1.4, and 1.2 J m–2. The formation of α-Si:H semiconductor on a metal granule will be of the layer-by-layer type. The first atomic layer of the semiconductor can form with a high probability a silicide with the metal and then grow on the surface in the form of islands (Fig. 4a). This island structure (Fig. 4b) of the α-Si:H layer will only slightly affect the resistivity, up to thickness corresponding to the formation of a grid of infinite granule–semiconductor–granule channels (Fig. 4c). It follows from the dependence ρ(h) that the thickness at which infinite conduction channels Co45Fe45Zr10–α-Si:H– Co45Fe45Zr10 begin to form is 1.5 nm, and, at thicknesses exceeding 2 nm, the formation of a continuous semiconductor film is completed. ρ, Ω cm 1
Magnetization, G 200 (b) 160
H, Oe 40
10–2
120
20
10–3
80
0
10–1
1
40 0
2
3 hSi, nm
2
10–4 10–5
1
2
3
4
5 6 hSi, nm
Fig. 2. Dependences of the (a) TKE and (b) magnetization of the [(Co45Fe45Zr10)35(Al2O3)65 /α-Si:H]30 multilayer on the semiconductor layer thickness.
0
1
2 3 hα-Si:H, nm
Fig. 3. Dependence of the [(Co45Fe45Zr10)35(Al2O3)65 /αSi:H]30 multilayer resistivity on the semiconductor layer thickness (curve 2). The straight line (1) shows ρ of the (Co45Fe45Zr10)35(Al2O3)65 composite.
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ENHANCEMENT OF MAGNETO-OPTICAL RESPONSE α-Si:H
Al2O3
(a) Metal silicide
(b)
Co45Fe45Zr10
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tilayer system leads to strong effective exchange interaction between isolated granules of ferromagnetic Co45Fe45Zr10 alloy and to a transition from the superparamagnetic behavior to the ferromagnetic one. It is difficult to answer the question about the mechanism of the exchange interaction enhancement: either due to the Si electrons tunneling or as a result of the formation of silicides at the ferromagnet/Si interface. This question requires further study.
(c)
ACKNOWLEDGMENTS Fig. 4. Model of growth of the α-Si:H interlayer on the composite surface.
Thus, the change in the properties of a nanomultilayer system in the range of thicknesses 1–2 nm can be related to the formation of a Si streak at ferromagnetic CoFeZr granules. The concentration dependences ρ(hSi) suggest that percolation in the multilayer composite–semiconductor system occurs in the range of thicknesses hSi of about 1.5 nm. In this case, percolation begins in the direction perpendicular to the film plane (between composite layers), which leads to perpendicular magnetic anisotropy at thicknesses hSi = 1.3 nm. The observed enhancement of the magneto-optical response in this system can also be related to the percolation threshold, which was also repeatedly observed for other nanocomposites [6]. The increase in the magnetization and coercive force with an increase in the Si interlayer thickness proves that the formation of an α-Si:H streak in a mul-
This study was supported by the Russian Foundation for Basic Research, project nos. 06-02-16604a and 07-02-91583. REFERENCES 1. Strijkers, G.J., Kohlhepp, J.T., Swagten, H.J.M., and de Jonge, W.J.M., Phys. Rev. Lett., 2000, vol. 84, p. 1812. 2. Gareev, R.R., Burgler, D.E., Buchmeier, M., et al., Phys. Rev. Lett., 2001, vol. 87, 157 202. 3. Gareev, R.R., Burgler, D.E., Buchmeier, M., et al., J. Magn. Magn. Mater., 2002, vol. 240, p. 235. 4. Burgler, D.E., Buchmeier, M., Cramm, S., et al., J. Phys.: Condens. Matter, 2003, vol. 15, p. S443. 5. Belousov, V.A., Kalinin, Yu.E., Korolev, K.G., et al., Vestn. VGTU, 2006, vol. 2, no. 11, p. 34. 6. Gan’shina, E.A., Vashuk, M.V., Vinogradov, A.N., et al., Zh. Eksp. Teor. Fiz., 2004, vol. 125, no. 5, p. 1172 [JETP (Engl. Transl.), vol. 98, no. 5, p. 1027].
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