Modifications of the spin density wave of Cr in Fe/Cr multilayers by insertion of Sn studied by neutron diffraction

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Abstract

Recently, the magnetic properties of chromium in Fe/Cr multilayers were studied with Mössbauer spectroscopy by inserting a tin monolayer in the chromium. Complementary neutron-diffraction studies on Fe/Cr(t) and Fe/Cr(t/2)/Sn(2 Å)/Cr(t/2) multilayers with t=80 and 160 Å are presented here. The Fe/Cr multilayers behave as previously observed, however the samples with tin deviate from these trends. These deviations are more prominent in the Fe/Cr/Sn/Cr multilayer with t=80 Å. Instead of an incommensurate to commensurate spin-density-wave (SDW) transition with increasing temperature (T) as seen without tin, a dominant commensurate component is observed at low T transforming to an incommensurate phase at higher T. The general SDW behaviour of chromium in Fe/Cr multilayers seems to be changed significantly by the insertion of Sn, allowing the tailoring of magnetic properties of Fe/Cr systems in a new way.

Introduction

Magnetic thin film research has been one of the most exciting and innovative fields in magnetism over the last 15 years. Fe/Cr multilayers have played an important role in the field as many of the important effects, like antiferromagnetic exchange coupling between the ferromagnetic layers [1], the giant magnetoresistance effect [2], and non-collinear coupling [3], were discovered using this system.

From the beginning it has been asked what role the intrinsic antiferromagnetism of bulk Cr plays in these thin film structures. In pursuit of an answer, this problem has been investigated with perturbed angular correlation spectroscopy [4], neutron scattering [5], [6], magnetic circular dichroism [7], and more recently, Mössbauer spectroscopy [8]. The results so far have been somewhat contradictory, even when the same experimental method was used [5], [6]. These discrepancies have partially been attributed to differences in sample preparation leading to variations in the structure of the Fe/Cr interface (see for example Ref. [9]).

Recently, Mössbauer spectroscopy studies were carried out on Fe/Cr superlattices implanted with thin layers of Sn within the Cr. The 119Sn sites act as a probe where information is obtained from the local hyperfine field established by the surrounding magnetic Cr atoms. Using this approach, a reduction of the magnetic moment has been observed for thin Cr layers [8], [10].

However, in order to investigate the sensitivity of the magnetic structure of Cr to the presence of the Sn, a technique which does not depend on any probe atom is required. Neutron scattering is ideally suited for this and it is used in this study for this purpose.

Section snippets

Experimental

Two sets of Fe(10 Å)/Cr(t) and Fe(10 Å)/Cr(t/2)/Sn(2 Å)/Cr(t/2) multilayers with t=80 Å (60 repeats) and t=160 Å (50 repeats) were grown on MgO(0 0 1) at 200°C using electron beam epitaxy. The samples were characterized by X-ray reflectivity and diffraction at GKSS. The samples show good epitaxy of Cr in the (0 0 1) direction and interface r.m.s. roughnesses between 6 and 11 Å.

Neutron-diffraction experiments were carried out at the ILL on the cold neutron triple-axis spectrometer IN12. This instrument is

Neutron diffraction

Unpolarized neutron-diffraction scans along L around the Cr (0 0 1) position for samples without Sn are shown in Fig. 2. In this short contribution we can only focus on the most prominent features of the data. Both samples with different Cr-layer thicknesses have a similar SDW behaviour, with an ISDW at low temperatures transforming into a CSDW at higher temperatures. The resulting period length of the ISDW is plotted against temperature in Fig. 3. Both samples show bulk-like behaviour, indicated

Conclusion and outlook

The results from the neutron diffraction experiments clearly show that the SDW behaviour in the Fe/Cr multilayers changes dramatically with the insertion of Sn. Polarized neutron diffraction studies were also carried out and will be presented elsewhere.

In order to validate the models suggested here, structure calculations of the SDW in Cr for the samples with Sn insertion are needed for quantitative analysis. Also, polarized neutron reflectivity studies are in progress to examine the coupling

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