Presence Of Superparamagnetic Relaxation Research Articles

Evidence of enhanced ferromagnetic nature and hyperfine interaction studies of Ce-Sm doped Co-Ni ferrite nanoparticles for microphone applications

We present the results of the structural, Magnetic and low temperature Mössbauer spectroscopy studies of Co0.5Ni0.5Fe2-(x+y)CexSmyO4 (where x,y = 0.0, 0.01, 0.015 and 0.02) which was prepared by the solution combustion route using mixture of glucose and urea as fuel. The prepared samples were subjected to X-Ray Diffractometer (XRD), Transmission Electron Spectroscopy (TEM), Vibrating Sample Magnetometry (VSM) and Mössbauer Spectroscopy at room temperature and low temperature (15 K) to understand the phase, structure and magnetic behavior of the samples. The results of XRD reveals the formation of a single phase spinel cubic structure with space group Fd3̅m. The crystallite sizes are found to be increasing with increasing Ce3+ and Sm3+ content. This is due to the fact that bigger ionic radii of Ce3+ and Sm3+ replace Fe3+ ions at B site. The lattice parameter is found to be increasing with the increasing Ce3+ and Sm3+ content due to increased internal strain upon doping. The TEM micrographs reveals that the particles almost appear as spherical and agglomerated. Magnetic measurements show that all the obtained samples exhibit higher saturation magnetization (MS), remanence (Mr), coercivity (HC), anisotropy constant (K) and magneton number (ηB). This is due to the increase in A-B super exchange interaction in the samples. Further the increase in HC with the dopant concentration is interpreted as the enhanced magnetic pinning and ferromagnetic behavior in the samples. The room temperature Mössbauer spectra consist of broadened magnetic sextet and paramagnetic quadrupole-split lines. The presence of the magnetic sextet and doublet component on the Mössbauer spectra indicate the presence of superparamagnetic relaxation with a higher TB than the room temperature. The obtained results suggest that the rare earth doped Co-Ni ferrites are potential candidates for microphone applications.

Magnetic Decoupling in Nanocrystalline Systems

Anomalous composition dependences of the Curie temperature of the residual amorphous phase in nanocrystalline Fe 92-x B x Zr 7 Cu 1 (2 < x < 23) alloys were studied by Mossbauer and magnetization measurements. The observed large (almost 300 K) increase with respect to that of the precursor amorphous phase at low B content is attributed to the volume effect. Mossbauer line broadening shows the presence of superparamagnetic relaxation of the bcc magnetization in nanocrystalline Fe 74 B 18 Zr 7 Cu 1 well below the Curie temperature of the residual amorphous phase suggesting that interphase exchange coupling is weaker than the dipolar magnetic fields.

Physical and chemical properties of superparamagnetic iron oxide MR contrast agents: ferumoxides, ferumoxtran, ferumoxsil.

The bulk physiochemical properties of the active ingredients in three AMI colloidal, superparamagnetic iron oxide (SPIO), MR contrast agents are described. Ferrous content and X-Ray diffraction (XRD) of the colloids are consistent with nonstoichiometric magnetite phases in all three active ingredients. No separate maghemite (gamma-Fe2O3) phases were detected by XRD. XRD line-broadening determinations of representative samples of ferumoxides (dextran coated), Ferumoxtran (dextran covered), and ferumoxsil (siloxane coated) yielded mean crystal diameters (volume weighted distribution) of 4.8-5.6, 5.8-6.2, and 7.9-8.8 nm, respectively. Transmission electron microscopy (TEM) showed that the crystal sizes were lognormally distributed with respective mean crystal diameters (number weighted distribution) of 4.3-4.8, 4.3-4.9, and 8.0-9.5 nm, respectively. Consistent with their small crystal sizes, the three SPIO colloids are superparamagnetic with no remanence after saturation at high applied fields (< 1 T), and showed characteristic relaxed Mössbauer spectra. The Mössbauer spectra of ferumoxides and Ferumoxtran were consistent with the presence of superparamagnetic relaxation above a blocking temperature of approximately 60 K. Due to the larger crystal sizes of ferumoxsil, its Mössbauer spectra showed the presence of rapid collective magnetic excitations on the Mössbauer time scale (approximately 1-10 ns). All three colloids showed high MR relaxivities. TEM of the SPIO colloids showed that ferumoxides and ferumoxsil are composed of aggregates of nonstoichiometric magnetite crystals, while Ferumoxtran consists of single crystals of nonstoichiometric magnetite. Dynamic light scattering (PCS) measurements showed that Ferumoxtran particles have average hydrodynamic diameters of approximately 21 nm (number weighted distribution) or 30 nm (volume weighted distribution). The data indicate that Ferumoxtran crystals are coated with an 8-12 nm layer of dextran T-10. Ferumoxides aggregates have average particle sizes of approximately 35 nm (number average distribution; TEM and PCS), or approximately 50 nm (volume weighted distribution; PCS). Mean sizes of ferumoxsil aggregates are approximately 300 nm (intensity weighted distribution). A discussion of the various particle size distributions is presented.