Mechanistic insights into anisotropy and magnetoresistance control in cobalt ferrite thin films by swift heavy ion irradiation

Abstract

Swift heavy ion irradiation (SHI) has emerged as a powerful tool for defect engineering and property optimization in magnetic thin films, with prior studies showing that low-fluence irradiation can enhance magnetic anisotropy through domain wall pinning and defect-mediated effects. The sensitivity of cobalt ferrite (CoFe2O4) magnetization and anisotropy to cation redistribution and defect formation is well established, with recent work highlighting site-specific distortions and cationic re-ordering as key mechanisms for tuning magnetic behavior. In this study, we investigate the structural and magnetic properties of CoFe2O4 thin films (∼75 nm) subjected to SHI using 200 MeV Ag12+ and 75 MeV O7+ ions at fluences of 5 × 1011 and 5 × 1012 ions/cm2. The overall magnetic and local structural properties of the magnetic thin films were studied using bulk (vibrating samples magnetometry, VSM) and element-specific (near edge X-ray absorption fine structure, NEXAFS, and X-ray magnetic circular dichroism, XMCD) magnetometry techniques. Emphasis is placed on tailoring magnetic anisotropy by probing irradiation-induced lattice disorder, strain modulation, and defect engineering which are studied through local electronic and coordination changes revealed by NEXAFS. XMCD at the Fe L-edge reveals a notable increase in dichroic intensity (from 0.18 to 0.204) for Ag-irradiated samples at 5 × 1011 ions/cm2, indicating enhanced Fe3+ orbital moment localization. By contrast, higher fluences and O-ion irradiation lead to an overall suppression of dichroism, consistent with defect-induced spin disorder. XMCD at the Co L3,2-edge shows reduced intensity across all irradiated samples, attributable to irradiation-driven distortions and cation redistribution at Co2+ sites. Temperature-dependent magnetization measurements indicate the highest magnetic moment at 70 K, with decreasing values at 150 K and 300 K. Notably, the Ag5 × 1011 ions/cm2 sample demonstrates pronounced anisotropy at 300 K, correlating with the Fe-site XMCD enhancement and confirming irradiation-driven anisotropy engineering. These findings demonstrate that SHI provides a scalable route to engineer anisotropy in oxide spintronic systems, offering pathways for designing high-performance magnetoresistive devices for memory and sensor applications.