Cr-substituted Co-Zn ferrite nanoparticles for magnetic hyperthermia: tuning magnetic anisotropy and induction heating

Magnetic hyperthermia requires nanoparticles whose heat output can be tuned while the material remains biocompatible. Here, Cr-substituted Co–Zn ferrite nanoparticles, Co 0.6 Zn 0.4 Cr x Fe 2−x O 4 (x = 0, 0.1, 0.2, 0.3), were synthesised by auto-combustion and calcined at 500 ο C for 2 h. X-ray diffraction confirmed a single-phase cubic spinel, with the lattice constant and crystallite size decreasing from 8.4039 Å and 34 nm to 8.3947 Å and 25 nm as the smaller Cr 3+ ions replaced Fe 3+ , and electron-microscopy particle diameters decreased from 38.55 to 31.96 nm. Cr substitution reduced the saturation magnetisation from 58.73 to 36.13 A m 2 kg −1 and the effective anisotropy constant from 134.5 to 82.4 kJ m −3 , lowering the anisotropy energy barrier to 0.35 of its unsubstituted value, with the decreases in particle volume and anisotropy contributing about equally. Under a field of 14.92 kA m −1 at 337 kHz, the specific absorption rate (SAR) at 0.5 mg mL −1 decreased from 342 to 251 W g −1 and the intrinsic loss power from 4.57 to 3.34 nH m 2 kg −1 . SAR was higher at 0.5 than at 1 mg mL −1 for every composition, consistent with interparticle interactions. After a 600 s exposure, the unsubstituted sample exceeded 47 ο C at both concentrations, whereas the x = 0.3 sample at 0.5 mg mL −1 reached 46.47 ο C, within the 42–47 ο C therapeutic range, with the temperature still rising at the end of the exposure. In vitro assays showed less than 4% haemolysis for all samples and cell viabilities above 90% for the higher-Cr samples. Cr substitution therefore offers a compositional route to lower, tunable heating output in Co–Zn ferrite nanoparticles while retaining appreciable heating efficiency.