Attaining high magnetostrictive strain sensitivity (dλ/dH) with high magnetostriction strain (λ) is desirable for sintered polycrystalline cobalt ferrite for various applications. It is shown that substitution of a small amount of Fe(3+) by Mg(2+) in CoMgxFe2-xO4 (x < 0.1) gives a comparable maximum magnetostriction coefficient to that of the unsubstituted counterpart, with large improvement in the strain sensitivity at relatively low magnetic fields. A large increase in the magnetostriction coefficient is obtained at low magnetic fields for the substituted compositions. The magnetostriction parameters are further enhanced by magnetic field annealing of the sintered products. The results are analyzed based on powder XRD, Raman spectroscopy, XPS and magnetic measurements and based on the results from these studies, the changes in the magnetostriction parameters are correlated with the changes in the cation distribution, magnetic anisotropy and microstructure.

Structural, magnetic and magnetostrictive properties of sintered aluminium-substituted cobalt ferrite, CoAl x Fe 2-x O 4 (x = 0.0, 0.1, 0.20, 0.30), derived from nanosized powders synthesized by a citrate-gel method, have been investigated. The sample with x = 0.1 is found to exhibit higher maximum magnetostriction strain at relatively lower magnetic fields (230 ppm at 286 kA/m) than that obtained for the unsubstituted cobalt ferrite (217 ppm, at 446 kA/m). All the Al-substituted compositions show larger strain sensitivity (dk/dH) at low magnetic fields compared to that for the unsubstituted cobalt ferrite. The variation of the magnetostriction coefficient as well as the strain sensitivity with Al content is likely to be due to the changes in the cation distribution in the tetrahedral and octahedral sites of the spinel lattice along with the associated changes in the magnetocrystalline anisotropy. The magnetostriction coefficient of x = 0.1 could be further enhanced to 306 ppm (at 220 kA/m) after a magnetic field annealing at 300°C. A very high strain sensitivity of 4.5 9 10 -9 m/A is obtained for the magnetically annealed sample, larger than that reported for any substituted cobalt ferrite samples. The combination of high magnetostriction coefficient and strain sensitivity is suitable for device applications.

Comparative studies have been made to understand the role of different crystallographic site preferences of the substituted non-magnetic divalent metal ions in the magnetostrictive properties of cobalt ferrite, by substitution of Zn2+ and/or Mg2+ for Fe3+ in CoMgxFe2−xO4, CoZnxFe2−xO4, and CoMgx/2Znx/2Fe2−xO4 (0.0 ≤ x ≤ 0.2). Detailed Raman spectral and magnetic characterizations are made to extract the information on the tetrahedral/octahedral site preferences of Zn and Mg in the spinel lattice of cobalt ferrite. The structural, microstructural, magnetic, Raman spectral, and magnetostrictive parameters of the studied compositions show distinguishable variations for x &lt; 0.1 and x ≥ 0.1. Co-substitution of a small amount of Mg and Zn for Fe in CoMgx/2Znx/2Fe2−xO4 (x &lt; 0.1) showed relatively larger strain sensitivity, [dλ/dH]max (−2.6 × 10−9 mA−1 for x = 0.05), higher than that for the Mg-substituted samples (−2.05 × 10−9 mA−1 for x = 0.05) and comparable to that for the Zn-substituted samples (−2.47 × 10−9 mA−1 for x = 0.05), without much drop in the maximum value of magnetostriction, λmax (−189 ppm for x = 0.05) compared to that for the unsubstituted counterpart (−221 ppm). The results show that it is possible to obtain high strain sensitivity (at fields &lt;50 kA/m), along with high magnetostriction strain at low magnetic fields (∼250 kA/m), by tuning the distribution of the substituted cations in the tetrahedral and octahedral sites of the cobalt ferrite lattice.

The effect of Mg2+ and In3+ substitution on the structural, magnetic, magnetostrictive and dielectric properties of NiFe2O4 samples derived through sintering of nanocrystalline ceramic powders is investigated.

The influence of size and crystallographic site preference of three non-magnetic isovalent metal ions of larger (In 3+ ), comparable (Ga 3+ ) and smaller (Al 3+ ) sizes, substituted for Fe 3+ in the spinel lattice of CoFe 2 O 4 on its magnetostrictive properties is compared. For the different compositions in CoFe 2−x M x O 4 (M = In 3+ , Ga 3+ , Al 3+ and 0 ⩽ x ⩽ 0.3), significant changes in the structural and magnetic parameters are observed with the degree of substitution, due to the size and site preferences. Magnetic and Raman spectral studies revealed that Al 3+ is substituted for Fe 3+ at both octahedral and tetrahedral sites for all compositions, whereas In 3+ and Ga 3+ are substituted for Fe 3+ at the tetrahedral site only for x ⩽ 0.2 and partly at the octahedral site for x > 0.2. Regardless of the differences in the ionic size, site preference and the magnetic properties, compositions in all three series with x = 0.1 showed almost equal magnitude of maximum magnetostriction (λ max = ~230 ppm), marginally higher than that of x = 0 (217 ppm). However, at higher substituted compositions, λ max is decreased with x, but the decrease is much faster for the Al-substituted compositions. The maximum strain sensitivity, [dλ/dH] max , is also found to be comparable for all three compositions. The comparable magnetostriction characteristics and high strain at low magnetic fields for different substituted compositions at low levels of substitution are attributed to the local structural distortions associated with the inhomogeneous distribution of the substituted ions in the spinel ferrite lattice. The studies suggest ways to optimise the magnetostriction properties of properly substituted sintered cobalt ferrite for applications in sensors and actuators.

We report on the tunable dielectric properties achieved in cation-substituted nickel ferrite (NiFe 2 O 4 ; NFO) by selectively engineering the crystallographic site occupation of the dopant cation in the NFO ceramics. The NFO, Mg-substituted NFO (NiMg 0.2 Fe 1.8 O 4 ; NMFO), and In-substituted NFO (NiIn 0.2 Fe 1.8 O 4 ; NIFO) nanocrystals were synthesized by employing a tartrate-gel chemical route, followed by calcination at 500 °C. High-resolution transmission electron microscopy (HRTEM) analyses indicate the crystal quality of NFO, NMFO, and NIFO nanomaterials. The HRTEM data revealed that all of the ferrite materials were nanocrystalline with sizes in the range of 15−25 nm. Coupled with HRTEM analyses, X-ray diffraction analyses indicate the formation of single-phase spinel-structured NFO, NMFO, and NIFO materials without any detectable impurities. Chemical analysis performed using Mossbauer spectroscopy indicates that Mg 2+ occupies the octahedral site, while In 3+ preferably occupies the tetrahedral site of the spinel-structured NFO. The Fourier transform infrared (FTIR) absorption bands corresponding to metal−oxygen (M−O) intrinsic stretching vibration in the octahedral unit (MO 6 ) and the tetrahedral unit (MO 4 ), respectively, were noted in all of the samples. However, the trend in the frequency shift of these bands in Mg-and In-substituted NFO materials is different due to the occupation of different sites of Mg and In as confirmed by Mossbauer studies. The electronic structure and chemical valence state analysis using X-ray photoelectron spectroscopy (XPS) corroborates with chemical bonding analyses performed by FTIR and cation distribution evaluation carried out by Mossbauer spectroscopy. The energy-dispersive X-ray spectrometry (EDS) data, in addition to XPS and FTIR analyses, further validate the formation of uniform and chemically homogeneous samples. The corresponding cation distribution revealed from Mossbauer studies correlates with the variation of dielectric properties of the doped NFO samples with respect to intrinsic NFO. At room temperature, the dielectric constant (ε) of NFO and NMFO is found to be nearly the same and constant over a wide range of frequencies. However, in NIFO, ε decreases with the applied frequency. The differences are attributed to the size effect and site preference of dopants (Mg 2+ and In 3+ ). Irrespective of the dopants, an increase in the temperature enhances the dielectric constant, which is due to an increase in the number of free charge carriers. A thermally activated electrical conduction mechanism was operative in NFO, NMFO, and NIFO materials. The activation energies for NFO, NFMO, and NIFO were 18.84, 34.48, and 57.14 meV, respectively. The enhanced dipolar effect of In 3+ at the tetrahedral site compared to Mg 2+ at the octahedral site may be the origin of the relatively higher value of activation energy in NIFO compared to intrinsic and Mg-substituted NFO.

No abstract

No abstract