Temperature selectivity for single phase hydrothermal synthesis of PEG-400 coated magnetite nanoparticles.

Herein, we have presented a detailed investigation of the temperature effect on hydrothermal synthesis of Fe3O4 magnetic nanoparticles (MNPs). The appearance of single-phase cubic spinel Fe3O4 at and above critical temperature provides a clear indication that temperature plays a crucial role in the single-phase synthesis of the Fe3O4 MNPs. A detailed investigation of the structural, magnetic and spin dynamic properties of PEG-400 coated Fe3O4 MNPs synthesized by a facile hydrothermal method at different temperatures (120 °C, 140 °C, 160 °C and 180 °C for 16 hours) has been presented. The single-phase cubic magnetite structure with high crystallinity was found in the samples synthesized at 160 and 180 °C and confirmed from XRD results, whereas samples prepared at 120 and 140 °C are of mixed phase (α-Fe2O3 and Fe3O4). The magnetic hysteresis curves reveal that saturation magnetization and coercivity of MNPs enhanced systematically with the increase in the reaction temperature from 120 °C to 180 °C. Maximum saturation magnetization (88.98 emu g-1) and coercivity (134.16 Oe) were found for the sample synthesized at 180 °C. Furthermore, ferromagnetic resonance (FMR) spectra obtained for samples synthesised at higher temperatures indicate a lower value of the line width due to the high magnetic ordering in the samples. Also, the resonance field decreased, and the g-value increased due to enhancement in magnetization for the single-phase samples synthesized at higher reaction temperatures. The spin resonance properties obtained from fitting the FMR data clearly indicate that a large spin-orbit coupling was observed for the single phase Fe3O4 MNPs and excellent magnetic properties were obtained from the static magnetic measurements.

Microwave spin resonance investigation on the effect of the post-processing annealing of CoFe2O4 nanoparticles.

A novel investigation on the finite-size effects on the spin resonance properties of cobalt ferrite (CoFe2O4) nanoparticles has been performed using a room temperature ferromagnetic resonance (FMR) technique. A single broad spectrum was obtained for the CoFe2O4 nanoparticle samples, which indicated that all the samples were showing ferromagnetic characteristics. An asymmetric FMR line shape with a hefty trailing section was obtained due to the high magneto-crystalline anisotropy in CoFe2O4 nanoparticles, which changed with the size distribution. The resonance field for the samples shifted to a higher value due to the increase in the magneto-crystalline anisotropy in the CoFe2O4 nanoparticles with an increase in size. A systematic change in the resonance field and line width was observed with the change in the size distribution of the particles. Initially, it decreased with an increase in the size of the particles and increased after the critical size range. The critical size range is the imprint of the shift of the magnetic domain from a single domain to multi domain. The line width increased at higher annealing temperatures due to the enhancement in the dipole-dipole interaction, which led to a higher spin concentration as well as magneto-crystalline anisotropy. Furthermore, the saturation magnetization (M s) as well as 'M r/M s' increased from 37.7 to 71.4 emu g-1 and 0.06 to 0.31, respectively. The highest coercivity (750.9 Oe) and anisotropy constant (4.62 × 104 erg cm-3) were found for the sample annealed at 700 °C, which can be corroborated by the literature as the critical annealing temperature at which CoFe2O4 nanoparticles shift from single domain nanoparticles to multi-domain nanoparticles. Post-processing annealing is critical in advanced processing techniques and spin dynamics plays a vital role in various interdisciplinary areas of applications.

Dipolar Interaction and Magneto-Viscoelasticity in Nanomagnetic Fluid.

We investigate the effect of dilution on dipolar interaction with linear and non-linear rheological properties of kerosene based magnetic fluid. The steady-state behavior demonstrate a shear thinning behavior and corroborated with a power law, (η = c γ ˙ n + η∞) exponent, n ≤ 1. The shear-induced-breakup (separation) of nanoparticles and the yielding behavior has been explained by Bingham model. Moreover, the magnetoviscous effect showed an initial increase at low shear rate and decrease at higher shear rate. Further, specific viscosity (ηF)-versus-Mason number (Mn) shows a perfect scaling at lower Mn (≤10-4) confirming negligible thermal and colloidal forces. Whereas, at higher Mn (≥10-3) deviation from collapse indicates the dominance of Brownian forces acting on nanofluids. The magnetic field dependent elastic (G') and viscous (G″) modulus reveal a crossover from viscoelastic-to-viscous behavior of nanofluid at critical concentration. Finally, we compare viscoelastic results with De Gans diagonal scaling theory to correlate the functional dependence of storage and loss modules with different particle volume concentration.

Trapping and injecting single domain walls in magnetic wire by local fields.

A single domain wall (DW) moves at linearly increasing velocity under an increasing homogeneous drive magnetic field. Present experiments show that the DW is braked and finally trapped at a given position when an additional antiparallel local magnetic field is applied. That position and its velocity are further controlled by suitable tuning of the local field. In turn, the parallel local field of small amplitude does not significantly affect the effective wall speed at long distance, although it generates tail-to-tail and head-to-head pairs of walls moving along opposite directions when that field is strong enough.

Finite-size scaling in band ferromagnets with non-universal critical behavior.

The ultra-high sensitivity of the ferromagnetic resonance (FMR) technique has been fully exploited to study the finite-size effects in the critical region near the ferromagnetic to paramagnetic phase transition in Cr(75-x)Fe(25+x) (x = 0, 5) thin films of high structural and magnetic quality. Conclusive experimental evidence is provided for the validity of finite-size scaling. Irrespective of the film thickness and composition, the critical exponents ß, γ and ν for spontaneous magnetization, initial magnetic susceptibility and spin-spin correlation length retain their bulk values so that no dimensionality crossover occurs within the film thickness range covered in the FMR experiments. The present results indicate that (i) like Cr(75-x)Fe(25+x), the previously studied Fe, Co, Ni, and CoNi(3) thin films behave as itinerant-electron (band) ferromagnets in which the isotropic long-range interactions between spins decay as J(r)∼r(-(d+σ)) (σ>0), and (ii) the lattice dimensionality d, spin dimensionality m, and range of spin-spin interactions (via the material-specific parameter σ) decide the (non-universal) values of the critical exponents.

Structural and Mössbauer effect investigations of amorphous FeCuNbSiB alloys nanocrystallized (VITROPERM) in magnetic field.

Melt-spun amorphous ribbons of nominal composition Fe73Cu1Nb3Si16B7, annealed at 560-580 degrees C for 1 hour in a magnetic field (H) applied along the width in the ribbon plane, develop uniaxial magnetic anisotropy with easy axis along H and exhibit several novel attributes. The samples labelled as S20 and S150 are nanocomposites consisting of ferromagnetic nanocrystalline grains (volume fraction approximately equal to 84% and 81%) of mean size d = 13(2) nm embedded in a ferromagnetic amorphous matrix and possess a magnetic permeability as large as 20,000 and 150,000, respectively. While nearly 55% of the nanocrystalline grains have a cubic DO3 Fe3Si-like structure with actual Si concentration of about 22 at.%, the remaining 45% nanocrystalline grains have tetragonal Fe3B and hexagonal Fe2Si structure. Since the crystalline volume fraction of Fe3B and Fe2Si nanocrystals is more in the sample S20, this sample exhibits stronger local magnetic anisotropy and hence lower permeability.