Tuning Ferrimagnetic Response in Compositionally Spread Gd-Fe-B Amorphous Thin Films

As global electric power consumption rises, reducing energy loss in power usage has become a crucial environmental challenge. Soft magnetic materials account for about 30% of total energy loss in electrical devices, underscoring the need for high-performance materials. Nanocrystalline amorphous alloys are the best soft magnetic materials available now, but there is still room for development. In this study, we propose a novel soft magnetic composite alloy consisting of an amorphous Fe-based nanocrystalline material doped with the rare earth element Gd. The combination aims to leverage the high saturation magnetization from ferromagnetic Fe nanocrystals and antiferromagnetic exchange coupling from ferrimagnetic residual amorphous Gd-Fe-B matrix. This unique structure is expected to result in a ferrimagnetic material with a rapid response to external magnetic fields. Our objective is to identify the optimal conditions for achieving superior soft magnetic properties in the proposed Gd-doped Fe-based amorphous/nanocrystalline alloys. To achieve this, we employ combinatorial synthesis and high-throughput measurements to analyze the ferrimagnetic properties and underlying mechanisms.<br/>Gd<i>_x</i>Fe_85-<i>x</i>B₁₃Nb₁Cu₁ (<i>x</i> = 0-25 at%) were deposited as composition-graded thin film on Si (100) substrate via RF magnetron sputtering. A 2 nm Ta layer was applied to prevent diffusion and oxidation. Post-annealing for nanocrystallization was performed at various temperatures ranging from 250^oC to 650^oC using an infrared lamp heating system. The fabricated samples were characterized by out-of-plane X-ray diffraction (XRD) analysis magneto-optical Kerr effect (MOKE) loop measurement. Additionally, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements were conducted at the BL25SU beamline of SPring-8, Japan, to analyze chemical bonding state magnetic moments.<br/>Crystal structure analysis revealed that samples with low Gd concentration exhibited a nanocrystallization/amorphous composite structure after annealing at 250-650^oC, while those with 25 at% Gd showed an impurity peak of the Gd-Fe-B compound after annealing at 650^oC. The XAS spectrum confirmed the presence of Fe⁺²/Fe⁺³ peaks alongside Fe metal peaks, attributed to the B-bonding following Gd addition. Magnetic moment analysis using XMCD revealed the sublattice magnetic structure unique to ferrimagnetism in the proposed material. MOKE measurements of pre-annealed samples (amorphous precursors) with varying Gd concentrations revealed that the MOKE signal decreased with increasing Gd content, ultimately resulting in a reversal of the magnetic hysteresis, which is characteristic of ferrimagnetism. This indicates that a Gd concentration of ~25 at% represents the magnetic compensation point at room temperature. Additionally, the slope of hysteresis (magnetic susceptibility) improved with the addition of small amounts of Gd, suggesting an enhancement in rapid magnetic response due to ferrimagnetic exchange coupling between Fe and Gd.