Magnetic profiles and coupling in Fe/Cr(110) superlattices

In epitaxial Fe/Cr superlattices the coupling between the Fe layers oscillates between antiferromagnetic (AFM) and ferromagnetic as a function of the Cr layer thickness tCr. The period of the oscillation is the same for superlattices grown with (211) and (100) orientations. We measured the coupling of Fe/Cr(110) superlattices consisting of three identical Fe layers. The magnetization curves were characterized by two to four levels, M/Ms =±1 or ± 3 . Polarized neutron reflectometry identified the direction of the magnetization of each Fe layer and showed that the levels M/Ms =± 3 were not always due to AFM alignment of the central layer, but rather it was found that, in spite of the structural similarity, the bottom Fe layer had a different coupling strength from the top Fe layer. The magnetic coupling must be sensitive to small structural differences at the interface. Furthermore, the measurements indicate an unexpected periodicity for Fe/Cr(110) superlattices. PACS: 61.12.He; 75.70.-i; 75.30.Et The Fe/Cr system is one of the most thoroughly studied magnetic multilayer systems. A short and a long period of oscillatory coupling [1] are present between the Fe layers, depending on the Cr layer thickness. Recent experiments on Fe/Cr/Fe(001) trilayers [2, 3] show a 2 monolayer (ML) period modulated by long (∼ 18 Å) period oscillations. Multilayers are known to exhibit just long period oscillations, which are similar for epitaxial Fe/Cr(100), (211) [4], and (110)-textured polycrystalline [5] films, although highly strained Fe/Cr(110) [6] shows some differences. The origin of the long period in Fe/Cr is still not well understood, especially its apparent independence on growth orientation [7]. Therefore, we chose to investigate Fe/Cr(110), to now the least investigated due to technical difficulties in growing untwined, unfaceted, films [8]. Epitaxial Fe/Cr(110) films were grown by molecular beam epitaxy (MBE) on single crystal Al2O3(110) substrates. ∗Corresponding author. (Fax: +1-630/252-7777, E-mail: [email protected]) The Nb seed layers were grown at 500 ◦C, followed by the Cr buffer layers at 300 ◦C. Then three Fe layers, spaced by Cr layers, were deposited and protected by Cr/Nb double capping layers. In situ structural characterization using RHEED and LEED was performed at every stage of the growth. In these films, there is little evidence for twined or faceted growth in contrast to earlier publications [8]. All samples have a Fe layer thickness of 30 Å. More details on the structural characteristics will be discussed in [9]. Samples with different Cr layer thicknesses (tCr) were investigated by SQUID magnetometry at 10K with the magnetic field H applied along the in plane [001] easy axis. Representative M-H loops are given in Fig. 1. The magnetization curves of the samples with tCr = 12.3 Å and 17.6 Å are similar in shape. In both cases, the total magnetization decreases from M/Ms =+1 to 3 to − 3 to −1, for the descending field curve. Since the first step takes place at a positive field antiferromagnetic (AFM) coupling between the three Fe layers Fig. 1a–d. Magnetization curves, a tCr = 12.3 Å, b tCr = 17.6 Å, c tCr = 23 Å, d tCr = 31 Å. Arrows indicate the direction of the three Fe layer magnetizations as measured (•) or inferred (◦) by symmetry at different fields by PNR. For (a), (c), and (d) the saturation configuration was measured at