Unveiling High Spin Polarization in Mn-Rich Co-Mn-Ge Heusler Alloys via High-Throughput XMCD Analysis

The rapid growth of digital data has necessitated higher recording densities in HDDs. Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices have attracted significant attention as next-generation HDD read heads, potentially replacing tunnel magnetoresistance (TMR) devices. However, conventional ferromagnetic materials have inadequate magnetoresistance (MR) ratios, highlighting the need for materials with higher spin polarization to achieve significant MR ratio enhancements[1]. This study focuses on the half-metallic Heusler alloy Co₂MnGe, based on predictions from machine learning-driven virtual material discoveries. Specifically, the goal is to identify with higher spin polarization in Mn-rich regions by fabricating compositional-spread thin films and performing high-throughput soft X-ray magnetic circular dichroism (XMCD) measurement.<br/><br/>Co-Mn-Ge compositional-spread thin films were deposited on MgO substrates using a magnetron sputtering system. X-ray structural analysis confirmed the L2₁ ordered structure across a wide compositional range. XMCD measurements at the Co and Mn L_2,3absorption edges were performed at the BL25SU beamline of SPring-8, Japan to evaluate the magnetic moments of the respective elements. Anisotropic magnetoresistance (AMR) measurements were conducted post-device fabrication to assess the AMR ratios for various compositions. First-principles calculations were employed to analyze the electronic density of states and spin polarization.<br/><br/>The results from XMCD and AMR measurements suggest a correlation between the increased spin magnetic moment of Co in Co₂MnGe alloys and the enhancement of the AMR ratio. Notably, the high negative AMR ratios observed in Mn-rich regions are attributed to the high saturation magnetization of Co₂MnGe alloys with an L2₁ ordered structure. The significant negative AMR ratios indicate high spin polarization in Mn-rich regions. These experimental results are consistent with first-principles calculations, which also suggest high spin polarization in Mn-rich Co₂MnGe alloys. Therefore, it is evident that the manifestation of high spin polarization is both experimentally and theoretically predictable. This study highlights the potential of Mn-rich Co₂MnGe alloys as high-performance materials for CPP-GMR devices, contributing to the advancement of next-generation HDD read heads with significantly enhanced MR ratios. The combination of experimental and theoretical approaches provides a robust framework for predicting and validating high spin polarization in new materials.