Amorphe weichmagnetische CoFeNiSiB-Detektionsschichten in Spinventilen

Abstract

Co-based amorphous magnetic films as detection layer in magnetoresistive systems are an attractive alternative instead of crystalline Ni81Fe19, as they are very soft magnetic, provide a sufficient high saturation magnetization and are zero-magnetostrictive for appropriate chosen stoechiometry.A magnetoresistive spin-valve system consists of two magnetic layers that are separated by a nonmagnetic interlayer. Changing the orientation of the magnetizations to each other by an external magnetic field leads to a change of the electrical resistance due to spin-dependent scattering of the electrons if the interlayer is metallic or due to spin-dependent tunneling contributions of the electrons if the interlayer is insulating. In order to get a high sensitivity at low magnetic field, the detection layer should be very soft magnetic with a high saturation magnetization.Sputtering amorphous Co64Fe5Ni5Si13B13-layers onto SiO2-substrates results in a low coercivity of typical 1-3 Oe and an in-plane anisotropy originating from an oblique-incidence-effect during the fabrication process.Using a Ta-bufferlayer results in a reduced in-plane anisotropy, a lower coercivity in the easy axis but also an enhanced magnetic dead layer thickness compared to samples with SiO2/CoFeNiSiB or Cu/CoFeNiSiB interfaces. In successive thermal annealing the soft magnetic properties of samples with a Cu/CoFeNiSiB interface start to weaken at 200°C, thereas they are stable up to 400°C for layers with SiO2/CoFeNiSiB or Cu/CoFeNiSiB interfaces.By integration of the amorphous CoFeNiSiB-layers in a tunneling magnetoresistance (TMR)-junction a TMR-effect of 12% (15%) was gained for the as-prepared (annealed) sample with a typical switching field of the amorphous layer of 12 Oe. This enhancement in comparison to the coercivity of the single layer is attributed to fringing fields from the hard magnetic layer.Magnetoresistive spin-valve systems CoFeNiSiB/Cu/CoCoOx were investigated regarding the magnetic coupling between the hard and soft magnetic layers. By measuring the surface topology of the amorphous layers with scanning tunneling microscopy, the contribution of the magnetic coupling due to correlated interface roughness is estimated and compared to the observed coupling strenght.