Synthesis and multifunctional properties of Ni-Co Co-doped lithium ferrites for structural, electrical, and dielectric performance in device applications

Abstract In this work, we synthesized ultra-fine particles of lithium ferrites co-substituted with nickel (Ni) and cobalt (Co), following the formula Li 0.5-x/2 (Ni 0.7 Co 0.3 ) x Fe 2.5-x/2 O 4 (0.0 ≤ x ≤ 0.8), using the low-temperature sol–gel auto-combustion method, to enhance electromagnetic interference (EMI) shielding effectiveness (SE) to address the growing concerns regarding EMI pollution in both technology and human health. XRD investigations demonstrated the formation of a single-phase spinel structure, with crystallite size “D” in the range of 136.26–100.40 nm. A reduction in “D” and lattice constant “a” was observed with the inclusion of Ni-Co, accompanied by an increase in porosity, strain, and dislocation density. DC electrical measurements revealed that direct current (DC) electrical conductivity “σ dc ” of spinel ferrites at room temperature diminishes up to x ≤ 0.4 with the lowest value (1.71 × 10 −10 mho/cm) and then enhances with the escalation of doping content (x). As per Maxwell–Wagner’s model, the dielectric parameters, including the dielectric coefficient, dielectric loss, and tangent of loss, reduce with frequency escalation. The alternating current (AC) electrical conductivity “σ ac ” revealed a direct proportionality with the Ni-Co content because of the enhancement of some free charge carriers and improved charge hopping mechanism. The frequency exponent ( n ) value increases from 0.65 to 0.78 by Jonscher’s Power law. Magnetic parameters such as saturation magnetization (M s ), remanence (M r ), coercivity (H c ), and squareness ratio (SR) have been measured from hysteresis loops. The observed highest value of M s was 7.13 emu/g for the prepared ferrite with x = 0.8. The H c value falls within a few hundred Oersted range, which is crucial for electromagnetic materials. The magnetoresistance was greater with applying a magnetic field than without it. The magnetic susceptibility “ X m ” of all the synthesized spinel ferrites depicted a progressive mitigation as the temperature enhanced due to the disturbance of spin orientation at the Curie temperature (T c ). The noted T c value of all samples was 413–353 K. This research highlights Ni-Co co-doped lithium ferrites ascribed with multifaceted functionalities, making them auspicious candidates for specific applications such as in high-frequency electromagnetic devices, RF devices, and microwave absorption.