In-plane uniaxial magnetic anisotropy in metallic multilayers

In-plane uniaxial magnetic anisotropy in metallic multilayers

Journal of Magnetism and Magnetic Materials 242–245 (2002) 601–603 In-plane uniaxial magnetic anisotropy in metallic multilayers M. Carbucicchioa,*, ...

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Journal of Magnetism and Magnetic Materials 242–245 (2002) 601–603

In-plane uniaxial magnetic anisotropy in metallic multilayers M. Carbucicchioa,*, M. Rateoa, G. Ruggieroa, G. Turillib a

Department of Physics, University of Parma, and INFM, Parco Area delle Scienze 7/A, 43100 Parma, Italy b MASPEC Laboratory, CNR, Parma, Italy

Abstract Co films and Co/Fe, Co/Pt, Fe/Al multilayers were e-beam evaporated in uhv. AES and GIXRD allowed to exclude contamination at interfaces. Polar plots of remanence ratio vs. applied field direction were obtained by AGFM. All samples showed an in-plane magnetic anisotropy with 1801 periodicity. Any influence from substrate material, stresses induced by sample cutting, and metal oxides contamination was excluded. r 2002 Elsevier Science B.V. All rights reserved. Keywords: AnisotropyFuniaxial; Thin filmsFmultilayer; Magnetic orientation; Hysteresis loop

Magnetic films and multilayers have attracted growing attention in the last years due to their peculiar physical properties and the wide field of applications, especially in sensor and storage technology [1]. Thin films, non epitaxially grown, may show an in-plane uniaxial magnetic anisotropy and, more rarely, the same phenomenology can be observed also in polycrystalline magnetic multilayers. In these systems such kind of anisotropy is uncommon or have attracted little attention, being this drawn by the more fascinating and extensively studied perpendicular magnetic anisotropy. Nonetheless, uniaxial in-plane magnetic anisotropy occurs also in thin film magnetic multilayers and may have a great technological relevance, e.g. for realizing the magneto-optical transducers used in parallel recording [2]. The phenomenon is still under debate, several hypothesis have been advanced and specific experimental approaches have been performed in order to clarify its origin. The aim of the present work was to shed more light on the origin of the uniaxial magnetic in-plane anisotropy observed in low-dimensional planar systems. Co thin films and Co/Fe, Fe/Al, and Co/Pt multilayers were electron beam evaporated under ultra*Corresponding author. Tel.: +39-0521-905255/905264; fax: +39-0521-905223. E-mail address: carbucicchio@fis.unipr.it (M. Carbucicchio).

high vacuum onto amorphous quartz and Si(1 0 0) single crystal without native oxide removal. The starting vacuum vas B108 Pa and the operating one B106 Pa. The film thickness, measured during deposition by a quartz micro-balance, and the deposition rates are reported in Table 1. The absence of contaminants at the surface of Co, Fe and Al films was proved by in situ auger electron spectroscopy (AES) measurements. Moreover, grazing incidence X-ray diffraction (GIXRD) measurements allowed to exclude the presence of metal oxides at the multilayer interfaces. Magnetic measurements were performed at room temperature by means of an alternating gradient field magnetometer (AGFM) in a maximum magnetic field of 2 T, applied in the film plane at various angles. Fig. 1 shows the hysteresis loops for (a) Co5.0nm/ Fe2.0nm, (b) Co0.6nm/Pt0.2nm and (c) Fe2.0nm/Al2.0nm multilayers. Due to the presence of the uniaxial magnetic anisotropy, the shape of the hysteresis loop is highly squared when the field is applied parallel to the easy axis. On the contrary, applying the field perpendicular to the easy axis, the loop becomes almost a straight line with little hysteresis. The polar plots of the remanence ratio Mr/Ms as a function of the angle a between the applied magnetic field direction lying in the film plane and the easy axis have been obtained for all samples. As an example, in

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 1 0 7 2 - 1

M. Carbucicchio et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 601–603

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Table 1 Growth parameters of thin films and multilayers Sample

Thickness Evaporation rate (nm) (nm/min)

Co5.0nm Co10.0nm Co20.0nm Co5:0nm =Fe0:5nm Co5:0nm =Fe2:0nm Co0.6nm/Pt0.2nm Fe1.5nm/Al2.0nm Fe2.0nm/Al2.0nm Fe3.0nm/Al2.0nm

5.0 10.0 20.0 5:0=0:5 5:0=2:0 0.6/0.2 1.5/2.0 2.0/2.0 3.0/2.0

90 120

60

0.9

No. of bilayers 0.6 150

30

F

0.7

0.3

0.4/0.03

20 10 6

0.5/2.0

10

0.7/0.5

0.9

0.6

0.3

180

0.0 0.0

0

0.0 0.0

0.3

0.6

0.9

0.3 330

210 0.6

1e-3

(a)

0.9

240

5e-4

300

270

0

Fig. 2. Polar plot of Mr =Ms as a function of applied field angle a in the film plane with respect to easy axis for Fe2.0nm/Al2.0nm multilayer.

-5e-4 -1e-3

9

(b)

8

1e-5

a) b) c) d)

7

0

6

f (α)

Magnetization (emu)

10

2e-5

-1e-5

5

-2e-5

4

1e-4

3 2

(c)

5e-5

1 0

0

0

π/6

-5e-5 -1e-4 -100

π/3

π/2

Angle α

-50

0

50

100

Applied Magnetic Field (Oe) Fig. 1. AGFM hysteresis loops measured at different applied magnetic field angle in the film plane for (a) Co5.0nm/Fe2.0nm, (b) Co0.6nm/Pt0.2nm, and (c) Al2.0nm/Fe2.0nm multilayers.

Fig. 2 the polar plot for Fe2.0nm/Al2.0nm multilayer is reported. The magnetic texture of the polycrystalline films and multilayers have been obtained by applying to the above data the original theory of Shtrikman et al. [3], adapted by el-Hilo et al. [4] for two dimensional systems. In such

Fig. 3. Easy axis distribution function f ðaÞ for the easy axis distribution function evaluated, with n o2, for: (a) Al2.0nm/ Fe2.0nm, (b) Co5.0nm/Fe2.0nm, (c) Co20nm and (d) Co0.6nm/Pt0.2nm samples.

a framework, the distribution function of the easy magnetic axis direction is given by N X f ðaÞ ¼ A0 þ A2n cosð2naÞ; n¼1

where Z A0 ¼

p=2

Mr ðbÞ db 0

M. Carbucicchio et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 601–603

and ð2n  1Þð2n þ 1Þ An ¼ 2 ð1Þnþ1

Z

p=2

Mr ðbÞcosð2nbÞ db: 0

f(a)da is proportional to the number of particles having the easy axis at angle a7da with respect to easy magnetization direction, which is assumed as sample symmetry direction. The obtained distribution functions are reported in Fig. 3. A 1801 periodicity is present for all the samples and the larger anisotropy in the distribution function was found for Fe2.0nm/Al2.0nm multilayer. In order to understand the origin of such in-plane anisotropy, the influence of some growth parameters has been evaluated. The effect induced by various substrate materials was studied by growing multilayers both on amorphous quartz and on silicon single crystal. It has been found that the substrate material does not influence the arising and the degree of such an anisotropy. The hypothesis that stresses induced by sample cutting for

603

measurement purpose can influence the anisotropy has also been excluded by performing hysteresis loops on samples deposited on previously cut substrates. On the other hand, precipitation of metal monoxides during the growth and the consequent orientation of the impinging ions to form the film is to be excluded because of the adopted ultra-high vacuum conditions. Work is in progress to verify if the presence of environmental magnetic field during growth can justify the arising of this phenomenon.

References [1] S. Yamashita, J. Yamasaki, M. Ikeda, N. Iwabuchi, J. Appl. Phys. 70 (1991) 6627. [2] K. Postava, et al., J. Magn. Magn. Mater. 163 (1996) 8. [3] S. Shtrikman, D. Treves, J. Appl. Phys. 31 (1960) 58S. [4] M. el-Hilo, P.E. Kelly, K. O’Grady, J. Popplewell, IEEE Trans. Magn. 26 (1990) 210.