Spin Polarization in Heusler Alloy Films

Abstract

Spin polarization measurements are essential for searching new alloys with high spin polarization. While tunneling magnetoresistance (TMR) measurements have been used to estimate the spin polarization of ferromagnetic (FM) electrodes of magnetic tunneling junctions (MTJs), it requires thin film stack and tedious microfabrication processes to prepare TMR pillars. On the other hand, point contact Andreev reflection (PCAR) can estimate current spin polarizations of bulk alloys relatively easily. Low temperature spin polarization of a series of quaternary substitutional alloys, Co\(_{2}\)Y(Z\(_{1-x}\)Z\(^\prime _{x})\), designed based on the Fermi level tuning, have been investigated using PCAR. Promising alloys were screened and they were evaluated as ferromagnetic (FM) layers of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) pseudo spin valves. Some quaternary Heusler alloys , such as Co\(_{2}\)Fe(Al\(_{0.5}\)Si\(_{0.5})\), Co\(_{2}\)Mn(Ge\(_{0.75}\)Ga\(_{0.25})\) and Co\(_{2}\)Fe(Ge\(_{0.5}\)Ga\(_{0.5})\) were found to be excellent FM layer for CPP-GMR. Recent theoretical and experimental investigations on the correlation between anisotropy magnetoresistance (AMR) and MR outputs suggest that AMR measurements will be more effective way for screening alloys for high spin polarization, as it can estimate spin polarization at finite temperatures.