interfacial spin structure in epitaxial fe/fesn2 bilayers with exchange bias

phys. stat. sol. (c) 1, No. 12, 3754–3759 (2004) / DOI 10.1002/pssc.200405548

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Interfacial spin structure in epitaxial Fe / FeSn2 bilayers with exchange bias

F. Stromberg*1, V. E. Kuncser2, K. Westerholt3, and W. Keune1

1 Universität Duisburg-Essen (Campus Duisburg), Lotharstrasse 65, D-47048 Duisburg, Germany 2 National Institute for Materials Physics, MG7 76900 Bucharest-Magurele, Romania

3 Ruhr-Universität-Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany

Received 27 June 2004, accepted 14 October 2004 Published online 20 December 2004

PACS 75.70.Cn, 76.80.+y

Fe/FeSn2 structures with epitaxial FeSn2 layers have been grown by MBE (Molecular Beam Epitaxy). Ex-change bias and pinning phenomena were proved by SQUID magnetometry. In order to elucidate the spin structure at the Fe/FeSn2 interface and in some depth of the FeSn2 layer with CEMS (Conversion Electron Mössbauer Spectroscopy), 57FeSn2 tracer layers of approx. 50 Å thickness have been incorpo-rated in the base structure, the only difference being the isotopic enrichment with 57Fe. An ellipsoidal model was ap-plied to represent the spin structure. A strong out-of-plane component of the spin structure at the interface was observed.


1 Introduction

The pinning phenomena at the interface of magnetic bilayer systems with different magnetic anisotropy lead to interesting physical effects, like the exchange bias effect. This effect has important applications in data storage technology and sensors [1, 2]. It relies on the induction of a unidirectional anisotropy at the interface of a ferromagnet (F) and an antiferromagnet (AF) and was first observed by Meiklejohn and Bean in 1956 [3]. The theoretical models of this effect differ depending on the interface spin structure [4, 5]. It is therefore crucial to investigate the interface with experimental methods which deliver micro-scopic information of the spin structure. CEMS (Conversion Electron Mössbauer Spectroscopy) com-bined with the 57Fe tracer layer technique is such a method. One can obtain depth selective information within the resolution of a few monolayers [6]. The present work reports on the magnetic investigations of Fe/FeSn2 bilayer systems with antiferromagnetic, epitaxial FeSn2 phases grown on InSb(001) substrates by CEMS.

2 Experimental

Fe / FeSn2 and FeSn2 structures have been grown on commercial InSb(001) substrates (cubic crystal structure; a = 6.47 Å) by MBE (Molecular Beam Epitaxy). Tracer layers with enriched 57Fe were intro-duced at different positions, and different growth conditions were applied, as described below. The sam-ples consist of the antiferromagnet (AF), namely FeSn2 , with a 50 Å 57FeSn2 tracer layer on top or in the middle of the film, respectively, covered or not covered by a thin Fe layer (see Table 1).

* Corresponding author: e-mail: [email protected], Phone: +49 203 379 2384, Fax: +49 203 379 3601

phys. stat. sol. (c) 1, No. 12 (2004) / www.pss-c.com 3755


Table 1 Sample structure and Néel temperatures for samples fs005, fs004, fs006 and eb3

sample annealing treatment

TN structure

fs005 fs004 fs006 eb3

yes “ “ “

≈ 380 K “ “ “

40 Å Sn/50 Å 57FeSn2/350 Å FeSn2/InSb (001) 40 Å Sn/60 Å Fe/50 Å 57FeSn2/350 Å FeSn2/InSb (001) 40 Å Sn/60 Å Fe/175 Å FeSn2/50 Å 57FeSn2/175 Å FeSn2/InSb (001) 30 Å Sn/48 Å Fe/12 Å 57Fe/50 Å 57FeSn2/200 Å FeSn2/InSb (001)

The 57FeSn2 tracer layers were all structurally identical to the base phase, the only difference being the isotopical enrichment with 57Fe. The epitaxial FeSn2 layers were prepared in ultrahigh vacuum (UHV) by co-evaporation of Fe and Sn from two effusion cells. The Fe layers (bcc-Fe, a = 2.87 Å) are all polycrys-talline due to the strong mismatch between the lattice dimensions of FeSn2 and Fe and were approxi-mately 60 Å thick. The purity of the materials was 99.9985 at.% for natural Fe and 99.995 at.% for Sn. The 57Fe for the tracer layers was enriched to 95 % with 57Fe. The substrate temperature during the evapo-ration of FeSn2 was 250 °C [7]. An additional in-situ annealing in UHV (3 h at 350 °C) of the FeSn2 lay-ers was performed, in order to increase the Néel temperature (TN) of the AF up to the bulk value of 380 K [8]. For the samples with Fe films on top of the FeSn2 layers the substrate temperature was 50 °C during evaporation of Fe. The structure of all films was verified by RHEED (Reflection High Energy Electron Diffraction), LEED (Low Energy Electron Diffraction), Auger spectroscopy and X-ray diffraction. Mag-netic measurements were performed by SQUID magnetometry. 57Fe CEMS was performed in perpen-dicular and 45° geometry, after field cooling (FC) from above TN. FC signifies that an in-plane magnetic field of 500 Oe directed along the [110]-direction of the AF was maintained while cooling through TN. Due to the change of the spin structure of the AF with temperature [8] it was expected that the direction of the cooling field has a significant influence on the onset and the magnitude of the exchange bias field (He) and the coercivity (Hc). But to our surprise they turned out the be nearly independent of the FC field direction [9].

3 Results and discussion

The SQUID measurements indicate strong pinning effects at the F/AF interface (for samples with the F/AF structure) which show the distinct behavior connected with the exchange bias phenomenon. Vari-ous hysteresis curves at different temperatures were collected on most of the samples with the F/AF structure. A correlation and trends between the magnitudes of the exchange field (He) and the coercive field (Hc) and different preparation conditions of the AF, producing either polycrystalline FeSn2 or an-nealed and non-annealed epitaxial FeSn2, is shown elsewhere [10]. Here we present a typical hysteresis curve of sample eb3 for different temperatures (Fig. 1). One observes that with rising temperature the exchange field, He , decreases sharply from a value of about 139 Oe at 5 K. It is expected that the macroscopic parameters He and Hc depend strongly on the spin structure, phase mixture and roughness of the F/AF interface. One aspect, namely the spin structure of the AF, will be considered in this presentation. Mössbauer spectroscopy is applied to enlighten this aspect. The main information can be inferred from the R23 ratio (ratio of the second to the third line in-tensity) or, equivalently, the R54 ratio, which is defined as 2 2

234 sin /(1 cos )R θ θ= ⋅ + , where θ is the angle

between the iron spin direction and the γ-ray direction. For the 57FeSn2 tracer layers the ellipsoid model was applied [11]. This model describes the spin orientations by an ellipsoidal function of probability, whose axes are proportional to the so called Nx, Ny and Nz populations. At least two measurements with different incidence angles of the γ-ray are necessary for the determination of Nx and Nz or Ny and Nz (depending on the polar angle of the spin projection relative to the z-axis). The last coefficient is then determined by the condition Nx+Ny+Nz = 1. The x-axis of the ellipsoid was chosen to coincide with the cooling field direction. CEMS measurements were performed in zero external magnetic field and at room temperature, RT.

3756 F. Stromberg et al.: Interfacial spin structure in epitaxial Fe / FeSn2 bilayers


The incidence of the γ-ray was either perpendicular to the sample plane, i.e. Φ = 90°, or at an angle of Φ = 45° to the sample plane. All spectra were fitted with the NORMOS program of R.A. Brand [12]. The maximum number of subspectra was three, the minimum number was two. The common feature of all spectra is a doublet, which represents a disordered FeSn2 phase (presumably non-stoichiometric and/or inverse occupation between Fe and Sn sites).

Fig. 2 CEMS spectra of sample fs005 (only AF phase), taken at perpendicular incidence of the γ-ray (top) and for 45° incidence (bottom). Both spectra were taken at RT.

For the sample without an Fe layer (Fig. 2), one sextet and one doublet was used for the fitting procedure, the sextet representing the AF stochiometric FeSn2 phase. The four outer lines visible in the spectra of fs004 and fs006 (Fig. 3) are from the Fe top layer and are of minor importance because of the small resonant area. The Mössbauer R23 ratios of the 57FeSn2 layer and the results deduced from the ellipsoidal spin distribution model can be found in Table 2.

Fig. 1 Hysteresis curves of sample eb3, taken at 100, 70, 30 and 5 K. Before each cycle the sample was field cooled in 500 Oe.



Fig. 3 Room temperature CEMS spectra of samples fs004 (top) and fs006 (bottom) taken at perpendicular incidence of the γ-ray and for 45° incidence.

Table 2 Sample code, R23 ratios and populations Nx , Ny , Nz according to the ellipsoidal spin-distribution model

sample R23 90° 45°

Nx Ny Nz

fs005 3.9 2.71 0.61 0.36 0.03

fs004 1.3 2.45 0.17 0.22 0.61

fs006 2.5 2.6 0.36 0.3 0.34

The corresponding graphical representations are given in Fig. 4. The main features are the remarkable out-of-plane spin component (characterized by the magnitude of Nz) in the interfacial region of the AF for sample fs004 compared to the “bare” FeSn2 sample fs005,

3758 F. Stromberg et al.: Interfacial spin structure in epitaxial Fe / FeSn2 bilayers


Fig. 4 Graphical representations of the spin orientation distribution through the ellipsoidal model for samples fs005 (top, left) fs004 (top, right) and fs006 (bottom, left) at RT after field cooling. The field cooling direc-tion was along [110] which is also parallel to Nx, as indicated. The upper ellipsoids show a top view, the lower ones a side view referring to the film plane.

which has nearly zero Nz. This behavior may be attributed to the strain induced by the top iron layer due to the strong lattice mismatch between Fe and FeSn2 in sample fs400. Sample fs006 (tracer layer in the middle of the AF) shows a lower out-of-plane spin component than sample fs004, but higher than for fs005. The Fe spins in the middle of the AF layer form a quasi-spherical distribution of orientations, with a slightly pronounced component in the cooling field direction. The interfacial Fe spins in the “bare” FeSn2 sample fs005 show the largest component in the cooling field direction (Nx is very large in fs005 as compared to fs004, fs006). It will be shown elsewhere that the out of plane component influences the magnitude of the exchange bias field [13]. But for the overall exchange bias behavior, domain formation through defects in the AF has to be taken into account. How the contributions of Nx and Ny relate to the magnitude of the exchange bias is still unclear and under investigation. Acknowledgements We appreciate valuable technical assistance by U. v. Hörsten. This work was sponsored by DFG (SFB491) and CERES/89/2003. The Financial support by the Alexander von Hum-boldt Stiftung is gratefully acknowledged (V.K.)

References

[1] S. Tumanski, Thin Film Magnetoresistive Sensors (Institute of Physics, Bristol and Philadelphia, 2001). [2] J. C. S. Kools, IEEE Trans. Magn. 32, 3165 (1996). [3] W. H. Meiklejohn and C. P. Bean, Phys. Rev. 105, 904 (1957). [4] T. C. Schulthess and W. H. Butler, J. Appl. Phys. 85, 5510 (1999). [5] U. Nowak et al., Phys. Rev. B 66, 24400 (2002). [6] T. Shinjo and W. Keune, J. Magn. Magn. Mater. 200, 598 (1999). [7] V. Kuncser, M. Doi, B. Sahoo, F. Stromberg, and W. Keune, J. Appl. Phys. 94, 3573 (2003).



[8] G. LeCaër, G. Malaman, B. Venturini, and G. Fruchart, Phys. Rev. B 35, 738 (1987). [9] F. Stromberg, Struktur und Magnetismus von Fe/FeSn2(001)/InSb(001)-Heterostrukturen mit Austausch- anisotropie, Diploma Thesis, Universität Duisburg-Essen, Campus Duisburg, 2003. [10] V. E. Kuncser, F. Stromberg, M. Acet, and W. Keune, J. Appl. Phys. (submitted). [11] J. M. Greneche and F. Varret, J. Phys. C 15, 5333 (1982). [12] R. A. Brand, Nucl. Instrum. Methods B 28, 398 (1987). [13] F. Stromberg, V. E. Kuncser, W. Keune, and K. Westerholt, to be published.

interfacial spin structure in epitaxial fe/fesn2 bilayers with exchange bias

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