[14a-501-1~11] 10.3 Spin devices, magnetic memories and storages

▲ [14a-501-2] Enhanced CPP-GMR effect by improved B2 order of Co₂(Mn_0.6Fe_0.4)Ge Heusler layer deposited on amorphous CoFeBTa underlayer: a quantitative estimation of site-disorder by anomalous x-ray diffraction

〇(PC)Songtian Li¹, Tomoya Nakatani¹, Yuya Sakuraba¹, Hiroo Tajini², Takao Furubayashi¹, Kazuhiro Hono¹ (1.National Institute for Materials Science, 2.Japan Synchrotron Radiation Research Institute/SPring-8)

Keywords:CPP-GMR, chemical order of Heusler alloy, anomalous x-ray diffraction

Improving the chemical order of Heusler alloy films under relatively low annealing temperatures (<400 °C) is important for achieving large output in current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) polycrystalline devices for practical sensor applications such as HDD read sensors. We studied the effect of amorphous CoFeBTa (CFBT) underlayer on the site-disorder in Co₂(Mn_0.6Fe_0.4)Ge (CMFG) Heusler alloy films, by which an enhanced CPP-GMR effect was recently reported [Choi et al. Appl. Phys. Express 10, 013006, (2017)]. Polycrystalline pseudo spin-valve films with the stacking structure of thermally oxidized Si_substrate /Ta/Cu/Ta bottom electrode /Ru (2 nm)/CoFe (1 nm)/CFBT (t nm)/CMFG (5 nm)/CoFe (0.4 nm)/AgSn (4 nm)/ CoFe (0.4 nm)/CMFG (5 nm)/ CoFe (1 nm) /Ru cap were prepared by magnetron sputtering, and annealed at 300 °C for 3 h. The CPP-GMR devices with various thickness of CFBT insertion show a clear enchantment of ΔRA with increasing CFBT thickness. A large MR ratio of 25% was observed at room temperature. We performed anomalous x-ray diffraction study in the synchrotron facility of Spring-8 to investigate the site disorder in CMFG. By comparing the experimental result to the simulation result, we can estimate the quantity of the Co-Mn or Co-Fe site-disorders, and thus lead to a quantitatively estimation of B2 order in the CMFG layers as a function of thickness of the CFBT seed layer. The degree of B2 order in the CMFG layers increased from ~47% to ~76% after inserting a 1.2 nm-thick CFBT underlayer, which is the reason for the enhancement of ΔRA in CPP-GMR device after inserting CFBT underlayer below CMFG layers. Our results clearly showed that thin (~1 nm) amorphous underlayer plays a significant effect on the improvement of the B2 order of Heusler alloy films, which is an effective way to fabricate highly spin-polarized Heusler alloy films at a relatively low annealing temperature of 300 °C.