Deposition temperature dependence of interface magnetism of Co2FeGe-Heusler-alloy/Ag films studied with 57Fe Mössbauer spectroscopy

We investigated local magnetism at the interface between Co2FeGe Heusler alloy and Ag by 57Fe Mössbauer spectroscopy. Interface-sensitive samples, where the 57Fe isotope was used only for several interfacial atomic layers of the Co2FeGe layer on the Ag layer, were prepared using atomically controlled alternate deposition at various deposition temperatures. The 57Fe Mössbauer spectra measured at room temperature indicated that the interface region of the Co2FeGe layers for the samples deposited below 500 °C retains the ferromagnetic B2 structure. On the other hand, interdiffusion between the Co2FeGe and Ag layers occurs and a nonmagnetic phase grows for the samples deposited at the substrate temperature higher than 550 °C. We observed magnetoresistance (MR) ratio of 3.6% at room temperature for Co2FeGe/Ag/Fe current-perpendicular-to-plane giant-magnetoresistive (CPP-GMR) structures grown at 300 °C, whereas we could not observe distinct MR effect for samples grown at 600 °C. These results support that the existence of interfacial nonmagnetic phase due to the interdiffusion between Heusler alloy and nonmagnetic-metal layers severely reduces the performance of CPP-GMR devices.