Tuning interfacial domain walls in GdCo/Gd/GdCo' spring magnets

Resumen del póster presentado a la XXXV Reunión Bienal de la Real Sociedad Española de Física y al 25º Encuentro Ibérico de Enseñanza de la Física, celebrados en Gijón (España) del 13 al 17 de julio de 2015.-- et al.Exchange coupled multicomponent ―spring-magnet” systems that combine ferrimagnetic and ferromagnetic materials are the focus of a renewed interest due to their potential as recording media with tailored magnetic behavior. Different geometries such as thin film heterostructures, coreshell nanoparticles or hard/soft lateral nanocomposites are employed in order to optimize system properties for specific applications. In particular, amorphous ferrimagnetic Rare Earth (RE) - Transition Metal (TM) alloys, with their strongly temperature dependent magnetic properties allow to tune important parameters such as spin polarization and magnetic moments and multilayers based in these ferrimagnetic alloys have been used to produce different magnetic behaviors such as macroscopic ferrimagnet states, macroscopic spin-flip like metamagnetic states or giant exchange bias. In these systems, exchange coupling gives rise to interfacial domain walls (DW) that govern the full magnetic behavior of the multicomponent system. GdxCo1-x amorphous alloys are RE-TM ferrimagnetic materials where the Gd and Co moments are antiferromagnetically coupled and collinear. Depending on the composition and temperature, the net magnetization in these alloys can be aligned either with the Co or the Gd moments. Therefore, it is possible to prepare exchange coupled GdxCo1-x/GdyCo1-y bilayers in which the net magnetization of each of the layers is dominated by a different kind of magnetic moments (i.e., by the Gd or Co moments). Interfacial exchange coupling between Co atoms at each layer results in the antiparallel alignment of the magnetization in each of them at remanence. Then, in the presence of an in-plane magnetic field of increasing magnitude, the system must switch to a parallel magnetization configuration and a Bloch wall across the thickness is created at the interface. The extension, position and symmetry of these walls respect to the interface plane, and their field and temperature dependence control the characteristic switching fields in the hysteresis loop. In this work, we have prepared GdxCo1-x/Gd/GdyCo1-y trilayers by sputtering in which a Gd layer with variable thickness is introduced at the interface in order to modulate the strength of the ―spring magnet” effect in the system. As Gd thickness increases from 0 to 30 nm, coupling between the upper GdxCo1-x and lower GdyCo1-y layers weakens due to the much smaller Gd-Gd exchange interactions in comparison with Co-Co exchange. Magnetooptical Kerr effect hysteresis loops (sensitive to Co magnetization at the upper layer) show that, as a consequence, the characteristic fields for nucleation of DWs across the thickness are reduced and the ―spring-magnet” effect becomes restricted to a lower temperature range.X-ray resonant magnetic scattering (XRMS) measurements at APS Argonne NL have been used to determine the depth profile of Gd magnetic moments configuration as a function of applied magnetic field. Figure 1 shows XRMS data at 50 K for a 50 nm Gd35Co65/8 nm Gd/50 nm Gd11Co89 trilayer at different applied fields (either with or without interfacial DW). Fitted magnetization depth profiles of XRMS reflectivity data in a 50 nm Gd35Co65/50 nm Gd11Co89 bilayer show clear asymmetries in DW width at each side of the interface (9.4 nm at Gd11Co89 vs. 2.2 nm at Gd35Co65), in agreement with the different saturation magnetization in each layer. In the trilayers, this asymmetry is enhanced and the DW becomes localized at the Gd/Gd11Co89 interface in which Gd-Gd exchange interactions determine the strength of the effective ―spring-magnet”.Work supported by Spanish MINECO under grants FIS2008-06249 and FIS2013-45469. Work at Argonne was supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.Peer Reviewe