Tuning magnetostrictive strain sensitivity through controlled spin–orbit and superexchange interactions: nonmagnetic Zn and Mg cation substitution in CoFe2O4†

We report an economical method to enhance magnetostrictive strain sensitivity in cobalt ferrite, CoFe₂O₄ (CFO), through controlled tetrahedral (A) and octahedral (B) superexchange interactions by substituting nonmagnetic divalent cations, Zn²⁺ or Mg²⁺, at the A and B sites, respectively. Two series of samples, Co_1−xZn_xFe₂O₄ (CZFO) and Co_1−xMg_xFe₂O₄ (CMFO), with x = 0, 0.1, and 0.2, were synthesized via a single-step autocombustion method. According to Mössbauer spectroscopy results, for Zn-substituted samples, the occupancy of Zn²⁺ ions at the tetrahedral site favored the migration of many Fe³⁺ ions from the A-sites to the B-sites and many Co²⁺ ions from the B-sites to the A-sites. This led to a remarkable reduction in superexchange interaction and a drastic decrease in the coercivity of the samples with an increase in Zn substitution. However, for Mg-substituted samples, Mg²⁺ ions replaced the Co²⁺ ions at the B-sites. This did not lead to a significant reduction in the superexchange interaction strength and the coercivity of the samples with an increase in Mg²⁺ substitution. As the spin–orbit interaction of Co²⁺ ions at the octahedral sites led to magnetostriction behavior in CoFe₂O₄, the observed reduction in the occupancy of Co²⁺ ions through the substitution of Zn or Mg resulted in a decrease in the maximum magnetostriction (λ_max) value with increasing ‘x’ for both series. Simultaneously, the decrease in A–B superexchange interaction led to a sharp decrease in magnetic coercivity and a sharp increase in magnetostriction strain (λ) at lower applied magnetic fields, which drastically enhanced magnetostrictive strain sensitivity (dλ/dH). Herein, all the Zn-substituted CFO samples exhibited higher maximum strain sensitivity (dλ/dH)_max at lower magnetic fields compared with the Mg-substituted CFO samples. Among both the series of samples, the Co_0.8Zn_0.2Fe₂O₄ sample exhibited the highest (dλ/dH)_max value: −3.8 × 10⁻⁹ m A⁻¹. This (dλ/dH)_max value is nearly 200% higher than that of the pristine CFO sample, even at a modest magnetic field of ∼26 kA m⁻¹. Our findings demonstrate a suitable way to select appropriate substituents for developing cost-effective and highly sensitive Co-ferrite-based magnetostrictive strain sensors for various applications.