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The present study explores the effect of Dy3+ rare-earth ion substitution on the crystal structure, morphology, and magnetic properties of magnetostrictive Co0.7Mn0.3Fe2O4 spinel ferrite and demonstrates their potential applications in magnetomechanical sensors. The intrinsic CoFe2O4 and Dy-substituted Co0.7Mn0.3Fe2−xDyxO4 (x = 0.0–0.1) were prepared by the standard solid-state chemical reaction method. X-ray diffraction studies along with the Rietveld refinement confirm that all the samples exhibit single-phase cubic spinel structure with space group Fd 3 ¯ m. Raman and Mössbauer data analyses reveal that the cation redistribution with Mn and Dy cosubstitution in CoFe2O4 and confirm the presence of a mixed spinel structure. Electron microscopy analysis indicates the significant effect of Mn, Dy cosubstitution on the microstructure of CoFe2O4. All the samples exhibit the magnetic hysteresis (M-H) loops at 5 K and 300 K. Saturation magnetization (Ms) and the cubic anisotropy constant (K1) values increase with Mn substitution, while with Dy substitution, Ms reduces due to the decrease of magnetic interactions with Dy substitution. However, the coercive field decreases with Mn and increases with Dy substitution. Higher values of magnetostriction coefficients (λ11 = −95 ppm and λ12 = 52 ppm) and the strain derivative (d λ / d H = 0.075 ppm / Oe at 600 Oe) observed make Co0.7Mn0.3Fe1.95Dy0.05O4 a suitable candidate for designing torque/stress sensors and a magnetostrictive phase for making magnetoelectric composite. Chemical composition optimization yields higher values Ms (89 emu/g, i.e., 3.73 μB) at lower coercivity (Hc = 241 Oe) for Co0.7Mn0.3Fe2O4 and higher values of λ11, λ12, and d λ / d H at a lower magnetic field (below 800 Oe) for Co0.7Mn0.3Fe1.95Dy0.05O4. The results suggest and demonstrate that Co0.7Mn0.3Fe2O4 and Co0.7Mn0.3Fe1.95Dy0.05O4 are the potential candidates for designing magnetomechanical sensor applications. [ABSTRACT FROM AUTHOR]

We report on the enhanced magnetostrictive properties of nanocrystalline Dysprosium (Dy3+) substituted iron-rich cobalt ferrites (Co0.8Fe(2.2-x)DyxO4, referred to as CFDO). The CFDO samples with a variable Dy concentration (x = 0.000–0.075) were synthesized by the sol-gel auto-combustion method. The phase purity and crystal structure were confirmed from X-ray diffraction analyses coupled with Rietveld refinement. Surface morphology analysis using scanning electron microscopy imaging indicates the agglomerated magnetic particles with a non-uniform particle size distribution, which is desirable to transfer the strain. The magnetostriction coefficient (λ11) measurements indicate that the CFDO with Dy concentration x = 0.025 exhibits the highest strain sensitivity, (dλ/dH) ∼1.432 nm/A (for H ≤ 1000 Oe). On the other hand, the magnetostriction coefficient (λ12) measurements indicate that the Dy concentration x = 0.075 exhibits the larger (dλ/dH) ∼ 0.615 nm/A (for H ≤ 1000 Oe). The maximum λ11value of 166 ppm (at H = 3300 Oe) was observed for a compound with Dy concentration x = 0.050. Magnetization measurements indicate that the saturation magnetization and coercivity of CFDO samples are dependent on the Dy3+content; the highest value of squareness ratio of 0.424 was observed for x = 0.050. The interplay between strain sensitivity (dλ/dH) and instantaneous susceptibility (dM/dH), as derived from magnetostriction and magnetization results, demonstrates that these CFDO materials may be useful for developing torque/stress sensors, as a constituent magnetostrictive phase for making the magnetoelectric composite materials and thus suitable for magnetoelectric sensor applications. [ABSTRACT FROM AUTHOR]

Gadolinium (Gd) substituted cobalt ferrites (CoFe2-xGdxO4, referred to CFGO) with variable Gd content (x=0.0-0.4) have been synthesized by solid state ceramic method. The crystal structure and impedance properties of CFGO compounds have been evaluated. X-ray diffraction measurements indicate that CFGO crystallize in the inverse spinel phase. The CFGO compounds exhibit lattice expansion due to substitution of larger Gd ions into the crystal lattice. Impedance spectroscopy analysis was performed under a wide range of frequency (f=20 Hz-1 MHz) and temperature (T=303-573 K). Electrical properties of Gd incorporated Co ferrite ceramics are enhanced compared to pure CoFe2O4 due to the lattice distortion. Impedance spectroscopic analysis illustrates the variation of bulk grain and grain-boundary contributions towards the electrical resistance and capacitance of CFGO materials with temperature. A two-layer heterogeneous model consisting of moderately conducting grain interior (ferrite-phase) regions separated by insulating grain boundaries (resistive-phase) accurately account for the observed temperature and frequency dependent electrical characteristic of CFGO ceramics. [ABSTRACT FROM AUTHOR]

Manganese (Mn) substituted cobalt ferrites (CoFe2-xMnxO4, referred to CFMO) have been synthesized by the solid state reaction method and their dielectric properties and ac conductivity have been evaluated as a function of applied frequency and temperature. X-ray diffraction measurements indicate that CFMO crystallize in the inverse cubic spinel phase with a lattice constant ∼8.3 8Å. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with relaxation time of 10-4 s and spreading factor of 0.35(±0.05). The frequency (20 Hz-1MHz) and temperature (T=300-900 K) dependent dielectric constant analyses indicate that CFMO exhibit two dielectric relaxations at lower frequencies (1-10 kHz), while completely single dielectric relaxation for higher frequencies (100 kHz-1 MHz). The dielectric constant of CFMO is T-independent up to ∼400 K, at which point increasing trend prevails. The dielectric constant increase with T>400K is explained through impedance spectroscopy assuming a two-layer model, where low-resistive grains separated from each other by high-resistive grain boundaries. Following this model, the two electrical responses in impedance formalism are attributed to the grain and grain-boundary effects, respectively, which also satisfactorily accounts for the two dielectric relaxations. The capacitance of the bulk of the grain determined from impedance analyses is ∼10 pF, which remains constant with T, while the grain-boundary capacitance increases up to ∼3.5 nF with increasing T. The tan δ (loss tangent)-T also reveals the typical behavior of relaxation losses in CFMO. [ABSTRACT FROM AUTHOR]

Manganese (Mn) substituted cobalt ferrites (CoFe2-xMnxO4, referred to CFMO) were synthesized and their structural, magnetic, and dielectric properties were evaluated. X-ray diffraction measurements coupled with Rietveld refinement indicate that the CFMO materials crystallize in the inverse cubic spinel phase. Temperature (T = 300 K and 10 K) dependent magnetization (M(H)) measurements indicate the long range ferromagnetic ordering in CoFe2-xMnxO4 (x = 0.00-0.15) ferrites. The cubic anisotropy constant (K1(T)) and saturation magnetization (Ms(T)) were derived by using the 'law of approach' to saturation that describes the field dependence of M(H) for magnetic fields much higher than the coercive field (Hc). Saturation magnetization (Ms), obtained from the model, decreases with increasing temperature. For CoFe2O4, Ms decreases from 3.63 μB per formula unit (f.u.) to 3.47 μB/f.u. with increasing temperature from 10 to 300 K. CFMO (0.00-0.15) exhibit the similar trend while the magnitude of Ms is dependent on Mn-concentration. Ms-T functional relationship obeys the Bloch's law. The lattice parameter and magnetic moment calculated for CFMO reveals that Mn ions occupying the Fe and Co position at the octahedral site in the inverse cubic spinel phase. The structure and magnetism in CFMO are further corroborated by bond length and bond angle calculations. The dielectric constant dispersion of CFMO in the frequency range of 20 Hz-1 MHz fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived from modeling the experimental data are ∼10-4 s and ∼0.35(±0.05), respectively. [ABSTRACT FROM AUTHOR]

We report on the frequency (f=20Hz-1MHz) and temperature (T=300-600K) dependent electrical properties of gadolinium (Gd) substituted cobalt ferrite (CoF2-x Gd x O4 (CFGO); x=0.00-0.40). Gd-substituted cobalt ferrites crystallize in cubic inverse spinel phase. CFGO materials exhibit lattice parameter expansion from 8.373Å (x=0.0) to 8.422Å (x=0.4) due to the larger ionic radius of Gd3+ compared to Fe3+. The effect of Gd-substitution is significant on the electrical properties; electrical conductivity decreases with increasing Gd-concentration. A two-layer model consisting of semiconducting-CFGO-grains separated by insulating grain-boundaries was able to account for the observed temperature and frequency dependent electrical properties. [ABSTRACT FROM AUTHOR]

We report on the impedance spectroscopic characterization of Sm and Ho doped Ni ferrite materials, namely NiO.Fe1.925Sm0.075O3 and NiO.Fe1.925Ho0.075O3, to demonstrate their improved electrical properties compared to pure NiO.Fe2O3. Sm and Ho doped Ni ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO3 and HoFeO3 as the additional phase, respectively. Atomic force microscopy measurements indicate that the bulk grains are approximately 2-5 μm in size while the grain boundaries are thin compared to bulk grains. Frequency variation of the dielectric constant shows the dispersion that can be modeled with a modified Debye's function, which considers the possibility of more than one ion, contributing to the relaxation. The resistivity values (at 3.5 KHz) of NiO.Fe2O3, NiO.Fe1.925Sm0.075O3, and NiO.Fe1.925Ho0.075O3 compounds are found to be 0.1×104 Ω m, 0.5×104, Ω m and 0.8×104 Ω m, respectively. Impedance spectroscopic analysis indicates the different relaxation mechanisms and their variation with temperature, bulk grain and grain-boundary contributions to the electrical conductivity (Rg), and capacitance (Cg) of these materials. While the conductivity in pure NiFeO4 is predominantly due to intrinsic bulk contribution (Rg=213 kΩ and Cg=4.5×10-8 F), NiO.Fe1.925R0.075O3 (R=Sm,Ho) exhibits distinct grain and grain-boundary contributions to the conductivity. [ABSTRACT FROM AUTHOR]

We report on the enhanced dielectric constant and electrical resistivity of the Co-ferrite (CoO. Fe2O3) by partially substituting Fe with La. Structural characteristics of La-doped Co ferrite namely CoO.Fe1.925La0.075O3 indicate the cubic inverse spinel phase with a small amount of LaFeO3 additional phase. The lattice parameter obtained is 8.401 Å (±0.001 Å), which is higher than that reported for Co ferrite (8.387 Å, ±0.001 Å). The dielectric constant and electrical resistivity of CoO. Fe1.925La0.075O3 are higher compared with pure Co ferrite. The dielectric constant dispersion of CoO. Fe1.925La0.075O3 in the frequency range of 100 Hz to 1 MHz fits to the modified Debye's function with more than one ion contributing to the relaxation. Temperature-dependent electrical resistivity curves exhibit two distinct regions indicative of two different types of conduction mechanisms. Analysis of the data indicates that the small polaron and variable-range hopping mechanisms are operative in the 220 to 300 K and 160 to 220 K temperature regions, respectively. [ABSTRACT FROM AUTHOR]