Search

SrFe 12 O 19 is widely used due to its low cost, stability and corrosion resistance. In this study, Sm-doped porous SrFe 12 O 19 powder with high coercivity was synthesized using KCl- assisted spray pyrolysis. The Sm doping, microstructure and magnetic properties were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). The results showed that when Sm-doping amount was 0.2, the ferrite powder prepared by KCl-assisted spray pyrolysis had high coercivity of 7.286 kOe and saturation magnetization of 56.422 emu/g. The coercivity of the Sm-doped samples increased by 39.23 % compared with that of the undoped samples. The significant increase in coercivity was associated with the improvement of super-exchange interaction between Fe3+ particles caused by Sm doping, and the size decrease of the hexagonal plate Sr-ferrite particle caused by KCl action. The influencing mechanism of coercivity was discussed in detail based on the analysis of crystal structure and microstructure observation. [ABSTRACT FROM AUTHOR]

Nanoparticles of Co-doped copper ferrite, Cu 0.75 Co 0.25 Fe 2 O 4 , were successfully synthesized by hydrothermal method. The preparation conditions were optimized to produce small nanoparticles with crystallite size of 20 nm that fall into the single-domain regime. The influence of Co-doping on the structure and magnetic properties of pure copper ferrite, CuFe 2 O 4 , was investigated. The prepared ferrite nanoparticles were found to be in a single structural phase with a spinel-type structure, according to the XRD and FT-IR measurements. When compared to pure Cu ferrite, the addition of Co increased the lattice constant and decreased the density. The TEM results confirmed the spherical morphology of the prepared ferrite nanoparticles. For the entire temperature range of the ferrite nanoparticles, the magnetization measurements showed a single ferrimagnetic phase. It was observed that the coercivity and remanent magnetization increased with decreasing temperature. Magnetic anisotropy was found to increase with Co-doping in comparison to pure Cu ferrite. The ZFC–FC magnetization curves showed that the blocking temperature ( T B ) of the prepared nanoparticles is above room temperature, demonstrating that they are ferrimagnetic at room temperature and below. Additionally, it was found that decreasing the magnetic field lowers T B . The FC curves below T B were observed to be nearly flat, indicating spin-glass behavior that might be attributed to nanoparticle interactions and/or surface effects such as spin canting and spin disorder. [ABSTRACT FROM AUTHOR]

Nanoparticles of Co-doped copper ferrite, Cu0.75Co0.25Fe2O4, were successfully synthesized by hydrothermal method. The preparation conditions were optimized to produce small nanoparticles with crystallite size of 20 nm that fall into the single-domain regime. The influence of Co-doping on the structure and magnetic properties of pure copper ferrite, CuFe2O4, was investigated. The prepared ferrite nanoparticles were found to be in a single structural phase with a spinel-type structure, according to the XRD and FT-IR measurements. When compared to pure Cu ferrite, the addition of Co increased the lattice constant and decreased the density. The TEM results confirmed the spherical morphology of the prepared ferrite nanoparticles. For the entire temperature range of the ferrite nanoparticles, the magnetization measurements showed a single ferrimagnetic phase. It was observed that the coercivity and remanent magnetization increased with decreasing temperature. Magnetic anisotropy was found to increase with Co-doping in comparison to pure Cu ferrite. The ZFC–FC magnetization curves showed that the blocking temperature (TB) of the prepared nanoparticles is above room temperature, demonstrating that they are ferrimagnetic at room temperature and below. Additionally, it was found that decreasing the magnetic field lowers TB. The FC curves below TB were observed to be nearly flat, indicating spin-glass behavior that might be attributed to nanoparticle interactions and/or surface effects such as spin canting and spin disorder.

Highlights • Ba-ferrite nanoparticles with good properties are synthesized by optimized process. • These nanoparticles show high coercivity and good thermal stability. • Excessive Ba ions is necessary for the synthesis of single phase particles. • The particle sizes are well controlled in the single domain size range. Abstract Nano-powders with controllable particle size, excellent magnetic properties and thermal stability of barium hexaferrite (BaFe 12 O 19) have been synthesized via a co-precipitation/calcination technique. The phase composition, morphology and magnetic/thermal properties of the products were systematically studied. XRD patterns reveal that a long co-precipitation reaction time (5 h) and high calcination temperature (1100 °C) are beneficial for the formation of BaFe 12 O 19 phase and decreasing the tendency to agglomeration. SEM micrographs show that the products show a hexagonal flake-like particle shape and the size are well controlled and maintained at single-domain particle size range area (<460 nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. The products with j H c of 5934 Oe, temperature coefficient of remanence (α Br) of −0.176% K−1 and temperature coefficient of coercivity (β jHc) of 0.0427% K−1 were obtained when co-precipitated for 5 h and calcined at 900 °C for 2 h. A high saturation magnetization of 66.9 emu/g was obtained when co-precipitated for 5 h and calcined at 1100 °C for 2 h, approaching the theoretical saturation magnetization (72 emu/g). [ABSTRACT FROM AUTHOR]

Abstract: The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker–Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is demonstrated that for a nonzero bias field as the magnitude of the ac field increases the reversal time first increases and having attained its maximum at some critical value of the ac field, decreases exponentially. [Copyright &y& Elsevier]

Abstract: Energy absorption produced by non-interacting single-domain ferromagnetic particles suspended in a liquid matrix is studied. The particles are assumed to be rigid dipoles so that all the dissipation in the system is due to the viscous flow induced in the matrix by orientational motion of the particles in response to a linearly polarized uniform AC field. The kinetic approach employed is valid for arbitrary values of the field frequency and amplitude. Comparison of the results obtained against those of the conventional phenomenological description reveals that the amplitude–frequency dependencies delivered by the latter are limited to weak field regimes. [Copyright &y& Elsevier]

The observation by magnetic force microscopy (MFM) of Sm2Fe17N3 (SFN) particles 2 to 4 µm in diameter (D), about 15 particles in all, revealed that the particles show a single-domain state and/or more than three domain states. The magnetic and domain wall energies of the single-domain particles (SDP) and those of two- and three-domain particles (2DP, 3DP) were calculated from the measured saturated polarization (Js) and the domain wall energy (γ). The calculation results are as follows: (1) the energy difference between 2DP and 3DP is less than 6% for total energy, in the range of ESDP > E3DP > E3DP; (2) 3DP becomes stable, E2DP > E3DP, in particles with D > 3.0 µm, assuming that the critical diameter is 2 µm. A local demagnetizing field and the presence of macroscopic defects in a particle strongly influence the nucleation of domain wall formation, and it is obvious that 2DP has a higher static magnetic energy than 3DP. Therefore, it can be considered that the morphological complexity of real particles results in greater stability of 3DP than 2DP in the sample particles. On the other hand, the discrepancy between the calculated critical diameter of SDP (dc ≈ 0.18 µm) and the measured value (dc ≈ 2 µm) can be explained by the temperature dependence of the relative ratio of the crystal magnetic anisotropy and the saturation polarization in the SFN particles, that is, K = K(T)/KR.T.) versus $J = J_{s}(T)/J_{s}^{\rm R.T.}$. It is revealed that the decrease in the anisotropy is more pronounced in the higher temperature region near the Curie temperature than the saturation polarization $[K_{1} \propto J_{s}^{n} (n > 4)]$. This is probably the reason that the measured SDP size is greater than the critical radius theoretically calculated using K1 and Js at room temperature. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(1): 1–8, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20212 [ABSTRACT FROM AUTHOR]

Patterned media shows great potential for future ultrahigh-density magnetic recording. The thermal stability is determined by the relevant transition states. We apply the nudged elastic band method to calculate energy barriers and thermal reversal modes as a function of the island size and intergrain exchange coupling. Nonuniform reversal modes and transition states are found even in single-domain islands. The resulting energy barriers are significantly lower than those estimated from simplified reversal modes. [ABSTRACT FROM AUTHOR]