Room-Temperature Exchange-Coupled Nanometer-Thick ZnFe ₂ O ₄ and SrFe ₁₂ O ₁₉ Bilayers: Implications for Spintronics and Exchange-Spring Magnets

Exchange-spring ZnFe2O4/SrFe12O19 (ZNF/SFO) bilayers were investigated to explore interfacial exchange coupling and dynamic magnetic response by varying the nanometer-scale thickness of the top ZNF layer. Room-temperature M–H loops exhibit two-step reversal (HCsoft ≈ 65 Oe and HChard ≈ 5.6 kOe) and magnetic stiffening (Hk = 7.5 kOe) with a 59/41 hard/soft phase ratio, confirming strong exchange coupling. X-band FMR (9.5 GHz) shows a reduced resonance field (Hr = 2456 Oe) and doubled effective magnetization (4πMeff ≈ 1.9 kG) compared to single ZNF films. Micromagnetic simulations using optimized parameters (MS = 473 kA/m, A = 1.2 × 10–11 J/m, K = 0.53 kJ/m3) reproduce experimental trends, revealing efficient torque transfer and coherent interfacial coupling. These results establish that MS, A, and K, respectively, govern coupling strength, torque transmission, and coercivity, positioning ZNF/SFO bilayers as promising rare-earth-free materials for high-energy-density magnetic and spintronic devices.