Room temperature dielectric and magnetic properties of BiFeO3 samples, co-doped with magnetic Gd and non-magnetic Ti in place of Bi and Fe, respectively, were reported. The nominal compositions of Bi0.9Gd0.1Fe1−xTixO3 (x = 0.00-0.25) ceramics were synthesized by conventional solid state reaction technique. X-ray diffraction patterns revealed that the substitution of Fe by Ti induces a phase transition from rhombohedral to orthorhombic at x > 0.20. Morphological studies demonstrated that the average grain size was reduced from ∼ 1.5 µm to ∼ 200 nm with the increase in Ti content. Due to Ti substitution, the dielectric constant was stable over a wide range of high frequencies (30 kHz to 20 MHz) by suppressing the dispersion at low frequencies. The dielectric properties of the compounds are associated with their improved morphologies and reduced leakage current densities probably due to the lower concentration of oxygen vacancies in the compositions. Magnetic properties of Bi0.9Gd0.1Fe1−xTixO3 (x = 0.00-0.25) ceramics measured at room temperature were enhanced with Ti substitution up to 20 % compared to that of pure BiFeO3 and Ti undoped Bi0.9Gd0.1FeO3 samples. The enhanced magnetic properties might be attributed to the substitution induced suppression of spiral spin structure of BiFeO3. An asymmetric shifts both in the field and magnetization axes of magnetization versus magnetic field (M-H) curves was observed. This indicates the presence of exchange bias effect in these compounds notably at room temperature.
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.
A trilayer structure, which has weak exchange coupling and high active current, has been optimized emphasizing for high field-sensitivity planar Hall effect (PHE) sensor. To illustrate the high field sensitivity of the PHE sensor, three different structures are fabricated: a bilayer thin film Ta(3)/NiFe(10)/IrMn(10)/Ta(3) (nm), a spin-valve thin film Ta(3)/NiFe(10)/Cu(1.2)/NiFe(2)/IrMn(10)/Ta(3) (nm), and a trilayer thin film Ta(3)/NiFe(10)/Cu(0.12)/IrMn(10)/Ta(3) (nm). The characterized results reveal that the field sensitivity of PHE sensor based on trilayer thin film is about one order larger than that of bilayer and is about twice larger than that of spin-valve thin film. Moreover, in trilayer structure, the thinner spacer layer gives the better performance. When the nominal thickness of spacer Cu layer is the smallest, the PHE sensor exhibits the best performance, i.e., in this experiment, it is about 0.12 nm.
We present a simple technique to synthesize ultrafine nanoparticles directly from bulk multiferroic perovskite powder. The starting materials, which were ceramic pellets of the nominal compositions of Bi0.9Gd0.1Fe1−xTixO3 (x = 0.00-0.20), were prepared initially by a solid state reaction technique, then ground into micrometer-sized powders and mixed with isopropanol or water in an ultrasonic bath. The particle size was studied as a function of sonication time with transmission electron microscopic imaging and electron diffraction that confirmed the formation of a large fraction of single-crystalline nanoparticles with a mean size of 11-13 nm. A significant improvement in the magnetic behavior of Bi0.9Gd0.1Fe1−xTixO3 nanoparticles compared to their bulk counterparts was observed at room temperature. This sonication technique may be considered as a simple and promising route to prepare ultrafine nanoparticles for functional applications.
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