Effect of Dy3+ and Tb3+ Rare-Earth Cation Co-Substitution on the Structure, Magnetic, and Magnetostrictive Properties of Ni-Co-Ferrites

The design and development of electromagnetic and magnetoelectric materials with enhanced properties and performance are desirable for numerous technologies, which are based on integrated electromagnetic materials and components. Nevertheless, engineering the crystalline materials with multi-complex chemistry and multiple cations is challenging. In this context, herein, we report on the effect of rare-earth (RE) cations, namely, Dy3+ and Tb3+, co-substituted into the Co-Ni-mixed ferrite materials for applications in stress/torque sensors. The RE-cations that co-substituted Co-Ni-ferrite materials with a composition of Ni0.8Co0.2Fe2-x(Dy1-yTby)xO4 (x = 0-0.1, y = 0.3; NCFDT) were prepared by the high-temperature solid-state chemical reaction method. The effect of variable composition (x) on the structure, morphology, chemical bonding, and magnetic properties of NCFDT materials is investigated in detail, and the structure-property optimization enabled realizing magnetostrictive NCFDT for sensor applications. X-ray diffraction analysis coupled with Rietveld refinement confirms the face-centered cubic crystal structure. Chemical bonding analysis made using Raman spectroscopic and Fourier transform infrared spectroscopic measurements validates the active modes corresponding to the spinel ferrite structure. The effect of Dy3+ and Tb3+ substitution is primarily seen in the grain size (range of 5-15 μm), as evident from the scanning electron microscopy patterns. Energy-dispersive spectroscopy confirms the presence of all constituent elements with expected composition and without any impurities. The magnetic property measurements indicate that the remnant magnetization (Mr) increases from 0.06 to 0.17 μB/f.u. with the rare-earth (Dy and Tb) substitution and has achieved the maximum squareness ratio (Mr/Ms) = 0.097 at x = 0.10. To validate their application potential in magneto-mechanical sensors, we have measured the magnetostriction coefficients (λ11 and λ12), which demonstrate high values of λ11 = -92 ppm (along the parallel direction) and λ12 = 66 ppm (along the perpendicular direction) for NCFDT with x = 0.05 at H = 7000 Oe. In addition, the maximum value of strain sensitivity is observed, particularly dλ11dH = -0.764 nm/A whereas dλ12dH = 0.361 nm/A. The correlation between strain sensitivity (dλ/dH) and susceptibility (dM/dH), as derived from magnetostriction and magnetization measurements, respectively, is established. The outcomes of this study indicate that Ni-Co-ferrites with Dy3+ and Tb3+ substitution are suitable for stress/torque sensors. These NCFDT ferrites may also be useful as a necessary constitutive phase for the manufacture of magnetoelectric composite materials, making them appropriate for magnetic field sensors and energy harvesting applications.