Amorphous ZnFe 2 O 4 Thin Films: Unexpected Magnetism and Spin Dynamics

Amorphous magnetic oxides provide an ideal platform to investigate spin dynamics governed by short‐range exchange interactions in the absence of long‐range crystallinity. Here, static magnetism and spin dynamics are studied in amorphous ZnFe 2 O 4 thin films deposited on fused quartz by magnetron sputtering in oxygen, with substrate temperatures ranging from room temperature to 800°C. X‐ray and electron diffraction confirm the amorphous structure, while Raman and X‐ray spectroscopies reveal short‐range spinel‐like Fe–O and Zn–O coordination. Magnetization measurements show a low‐temperature cluster‐glass transition ( T G ≈ 20–220 K) coexisting with ferromagnetic–paramagnetic transition, with effective magnetic ordering temperature, T C ≈ 450 K. Field‐cooled hysteresis loops below T G exhibit finite coercivity (up to ∼2.2 kOe) and small exchange‐bias shifts (≈50–175 Oe), indicating glassy freezing driven by disorder and competing antiferromagnetic–ferromagnetic interactions. Broadband ferromagnetic resonance (2–40 GHz) demonstrates coherent spin dynamics in the amorphous state, with linewidths as low as ∼100–200 Oe and tunable effective magnetization up to ∼3.8 kG for films grown at 500°C. These results show that magnetic order and low‐loss spin dynamics in amorphous ZnFe 2 O 4 are governed by local exchange interactions and oxygen stoichiometry rather than crystallinity, highlighting ZnFe 2 O 4 as a promising platform for ferrite‐based microwave and spintronic applications.