This work reports on the effect of substituting a low-anisotropic and low-magnetic cation (Ni 2+ , 2μ B ) by a high-anisotropic and high-magnetic cation (Co 2+ , 3μ B ) on the crystal structure, phase, microstructure, magnetic properties, and magnetostrictive properties of NiFe 2 O 4 (NFO). Co-substituted NFO (Ni 1−x Co x Fe 2 O 4 , NCFO, 0 ≤ x ≤ 1) nanomaterials were synthesized using glycine-nitrate autocombustion followed by postsynthesis annealing at 1200 °C. The X-ray diffraction measurements coupled with Rietveld refinement analyses indicate the significant effect of Co-substitution for Ni, where the lattice constant (a) exhibits a functional dependence on composition (x). The a-value increases from 8.3268 to 8.3751 Å (±0.0002 Å) with increasing the "x" value from 0 to 1 in NCFO. The a−x functional dependence is derived from the ionic-size difference between Co 2+ and Ni 2+ , which also induces grain agglomeration, as evidenced in electron microscopy imaging. The chemical bonding of NCFO, as probed by Raman spectroscopy, reveals that Co(x)-substitution induced a red shift of the T 2g (2) and A 1g (1) modes, and it is attributed to the changes in the metal−oxygen bond length in the octahedral and tetrahedral sites in NCFO. X-ray photoelectron spectroscopy confirms the presence of Co 2+ , Ni 2+ , and Fe 3+ chemical states in addition to the cation distribution upon Co-substitution in NFO. Chemical homogeneity and uniform distribution of Co, Ni, Fe, and O are confirmed by EDS. The magnetic parameters, saturation magnetization (M S ), remnant magnetization (M r ), coercivity (H C ), and anisotropy constant (K 1 ) increased with increasing Co-content "x" in NCFO. The magnetostriction (λ) also follows a similar behavior and almost linearly varies from −33 ppm (x = 0) to −227 ppm (x = 1), which is primarily due to the high magnetocrystalline anisotropy contribution from Co 2+ ions at the octahedral sites. The magnetic and magnetostriction measurements and analyses indicate the potential of NCFO for torque sensor applications. Efforts to optimize materials for sensor applications indicate that, among all of the NCFO materials, Co-substitution with x = 0.5 demonstrates high strain sensitivity (−2.3 × 10 −9 m/A), which is nearly 2.5 times higher than that obtained for their intrinsic counterparts, namely, NiFe 2 O 4 (x = 0) and CoFe 2 O 4 (x = 1).

We demonstrate an approach based on substituting a magnetic cation with a carefully chosen isovalent non-magnetic cation to derive catalytic activity from otherwise catalytically inactive magnetic materials. Using the model system considered, the results illustratively present that the catalytically inactive but highly magnetic strontium hexaferrite (SrFe 12 O 19 ; SFO) system can be transformed into a catalytically active system by simply replacing some of the magnetic cation Fe 3+ by a non-magnetic cation Al 3+ in the octahedral coordination environment in the SFO nanocrystals. The intrinsic SFO and Al-doped SrFe 12 O 19 (SrFe 11.5 Al 0.5 O 19 ; Al–SFO) nanomaterials were synthesized using a simple, eco-friendly tartrate-gel technique, followed by thermal annealing at 850 °C for 2 h. The SFO and Al–SFO were thoroughly characterized for their structure, phase, morphology, chemical bonding, and magnetic characteristics using X-ray diffraction, Fourier-transform infrared spectroscopy, and vibrating sample magnetometry techniques. Catalytic performance evaluated toward 4-nitrophenol, which is the toxic contaminant at pharmaceutical industries, reduction reaction using NaBH 4 (mild reducing agent), the Al-doped SFO samples exhibit a reasonably good performance compared to intrinsic SFO. The results indicate that the catalytic activity of Al–SFO is due to Al-ions occupying the octahedral sites of the hexaferrite lattice; as these sites are on the surface of the catalyst, they facilitate electron transfer. Furthermore, surface/interface characteristics of nanocrystalline Al–SFO coupled with magnetic properties facilitate the catalyst recovery by simple, inexpensive methods while readily allowing the reusability. Moreover, the activity remains the same even after five successive cycles of experiments. Deriving the catalytic activity from otherwise inactive compounds as demonstrated in the optimized, engineered nanoarchitecture of Al-doped-Sr-hexaferrite may be useful in adopting the approach in exploring further options and designing inexpensive and recyclable catalytic materials for future energy and environmental technologies.

We report on the influence of Al and Zn co-substitution on the structural, magnetic, and magnetostrictive properties of cobalt ferrite (Co1–x Zn x Fe2–x Al x O4, 0 ≤ x ≤ 0.15; CZFAO) materials, which were made using a glycine-nitrate autocombustion process. The cubic spinel structure is present in both the as-synthesized and the sintered CZFAO samples, where it was evident that the crystallite size and lattice parameter reduced with increasing x due to the incorporation of smaller Al3+ ions in place of larger Fe3+ ions in spinel ferrite. Co-substitution induced changes in the metal–oxygen bond lengths, causing both the tetrahedral and octahedral infrared and Raman spectroscopic bands to shift toward higher wavenumbers. The electron microscopy analyses indicate that the Al–Zn substitution induces grain growth, leading to a dense, interconnected grain morphology. Mossbauer analyses indicate that the Al3+ occupies the octahedral site, whereas Zn2+ is substituted at the tetrahedral site. Due to equal magnetic dilution of both tetrahedral and octahedral sites caused by the presence of Zn and Al at the appropriate sites, the saturation magnetization M S of CZFAO is the same as that of pure CFO. All other magnetic parameters (coercivity H C, magnetocrystalline anisotropy constant K 1, and Curie temperature T C) decrease with increasing x, where the decline observed is mostly due to superexchange interaction (A-O-B) reduction caused by the substituents’ non-magnetic character. CZFAO materials exhibit higher magnetostriction strain λ and strain sensitivity dλ/dH at relatively low magnetic fields; among all the samples, x = 0.15 demonstrates a maximum strain sensitivity of −2.53 × 10–9 m/A at 23 kA/m magnetic field. The composition-tuned CZFAO materials with desirable properties are suitable for application in torque sensors.

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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, Dy 3+ and Tb 3+ , cosubstituted into the Co-Ni-mixed ferrite materials for applications in stress/torque sensors. The RE-cations that co-substituted Co-Niferrite materials with a composition of Ni 0.8 Co 0.2 Fe 2−x (Dy 1−y Tb y ) x O 4 (x = 0−0.1, y = 0.3; NCFDT) were prepared by the hightemperature 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 Dy 3+ and Tb 3+ 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 (M r ) increases from 0.06 to 0.17 μ B /f.u. with the rare-earth (Dy and Tb) substitution and has achieved the maximum squareness ratio (M r /M s ) = 0.097 at x = 0.10. To validate their application potential in magneto-mechanical sensors, we have measured the magnetostriction coefficients (λ 11 and λ 12 ), which demonstrate high values of λ 11 = −92 ppm (along the parallel direction) and λ 12 = 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 H d d 11 = −0.764 nm/A whereas H d d 12 = 0.361 nm/A. The correlationbetween 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 Dy 3+ and Tb 3+ 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.