Effect of process parameters on the properties of compositionally graded FeCoNi films: A High Throughput Strategy

Abstract

Fe-Co-Ni magnetic thin films with continuously varying compositions have been fabricated by the magnetron co-sputtering method. The films were characterized by high-throughput techniques. The results reveal the correlation between a range of compositions and the electrical properties of the films fabricated at various process parameters.Introduction: Soft magnetic materials are essential elements of electro-magnetic energy transformation technologies and they are widely used in various distribution, conversion, generation devices, such as transformers, motors, converters, generators, actuators, sensors etc. However, it usually takes long time for a new material before its acceptance for commercial application. Therefore, it is very crucial to speed up the discovery of new materials and incorporate them into various systems. High-throughput methods are efficient research methods [1-3] to explore this immense search space to identify new or optimized materials. Fe-Co based alloys received great attention due to high saturation magnetization. And what makes them even more fascinating is that their magnetic and electric properties can be tuned by controlling the alloy composition and microstructure. Among these compositions, Fe-Co-Ni films revealed significant induced magnetic anisotropy, low coercivity and high anisotropy of magnetoresistivity, what makes them a good candidate for technical applications. The resistivity is an important parameter to determine power loss in many soft magnetic applications. In this work we studied the effect of process parameters on the structure and resistivity of compositionally graded Fe-Co-Ni films to find process conditions at which optimum properties are obtained.Experimental details: Fe-Co-Ni films with continuously varying alloy compositions were fabricated by the magnetron sputtering method. Two experimental procedures were designed: a) Fe-Co-Ni films were deposited on SiO2/Si substrates by co-sputtering of permalloy and Co target materials using DC sources at different substrate temperatures (Ts): in particular, ambient temperature, 300 °C and 500 °C; b) the film was deposited at ambient temperature and annealed for 2 hours in vacuum at 500 °C. The base pressure in the chamber was controlled at 4 x 10 - 4 Pa. Depositions were performed in argon gas environment while the pressure was kept approximately at 0.3 Pa. For compositional, structural and electrical properties high-throughput Electron Probe Micro-Analysis (EPMA), X-ray Diffraction, four-point probe measurements were performed to determine chemical compositions, structure and resistivities of the films. Results and discussion: Fe-Co-Ni films with continuously varying alloy compositions were fabricated by magnetron sputtering method. The results of high-throughput Electron Probe Micro-Analysis measurements across Fe-Co-Ni films (Fig.1) revealed that atomic composition of Ni was highest, while Co was lowest in the film fabricated at Ts = 300 °C. The ratios of Co and Ni atomic compositions in deposited graded films were calculated as well. We found that Fe-Co-Ni film fabricated at Ts = 300 °C shows the lowest ratio as compared to other films. XRD measurements and analysis of multiple selected areas on Fe-Co-Ni film fabricated at ambient substrate temperature exhibited amorphous structure. As the substrate temperature increased diffraction peaks appeared, showing a clear change in structure. The diffraction patterns showed that only one FCC phase appeared. The peak intensity increased with temperature and the grain sizes became larger. The increase of peak intensity and average grain size indicated that increasing the substrate temperature from room temperature to 500 °C enhances the crystallinity of Fe-Co-Ni films. As for the annealed samples the intensity of the peak was low, and peak shifted towards lower angles indicating crystal lattice expansion. In addition, oxide peaks have been observed. Fig. 2 shows the measured resistivities as a function of composition of various process parameters. Resistivities of the graded film deposited at Ts =300 °C exhibited higher values than resistivities of the films deposited at ambient, 500 °C, and at ambient temperature followed by annealing at 500 °C. These results indicate that Ni plays important role in tuning resistivities of the films.Conclusion: Fe-Co-Ni films with continuously varying compositions have been fabricated by magnetron sputtering and characterized by high-throughput experimentation methods. The results revealed the correlation between continuously varying alloy compositions and selected properties of the films fabricated at various process parameters. Acknowledgement: This work is supported by the AME Programmatic Fund by the Agency for Science, Technology and Research, Singapore under Grant No. A1898b0043 **