Thickness Dependence of Degree of B2 Order of Polycrystalline Co <sub>2</sub>(Mn <sub>0.6</sub>Fe <sub>0.4</sub>)Ge Heusler Alloy Films Measured by Anomalous X-Ray Diffraction and its Impacts on Current-Perpendicular-To-Plane Giant Magnetoresistance Properties

Abstract

We investigated the degree of B2 order (SB2) of 4-10 nm-thick polycrystalline Co2(Mn0.6Fe0.4)Ge (CMFG) Heusler alloy films using anomalous X-ray diffraction (AXRD) and its correlation to the magnitude of current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) effect. We found that SB2 of the CMFG films is degraded for the film thickness below 6 nm, which is an issue for practical applications of CPP-GMR devices such as read head sensors. Since SB2 has a thickness-dependence, the bulk spin polarization (β) of the CMFG layers is also expected to vary depending on the thickness; thus, the deduction of β using the Valet-Fert model, where β is assumed to be thickness-independent, is not valid for the CPP-GMR devices with polycrystalline CMFG layers.