Scanning electron micrograph (SEM) images of the BaFe12O19 particles prior to irradiation (a), (b) Aldrich, taken on a Zeiss UltraPlus Field Emission SEM, (c), (d) AFT, taken on a JEOL 5900 tungsten hairpin filament SEM.
(Color online) Temperature dependent DC specific magnetization (σ) and AC susceptibility (χ′) for two types of BaFe12O19 powders before irradiation. (a) Aldrich DC, (b) AFT DC, (c) Aldrich AC, (d) AFT AC. Legend for the magnetic history is shown in the upper right graph (c) and explained in the text.
(Color online) Frequency dependence of the real part of the AC susceptibility (χ′) as a function of temperature for the two nonirradiated powders. Spectra shown were taken on demagnetized powders during warming with no applied DC field after cooling in zero DC field (ZFCW, demag, H dc = 0). Spectra of powders in the remnant state are similar but with a reduced susceptibility. (a) AFT, (b) Aldrich.
(Color online) Hysteresis up to 50 kOe at 300 K for Aldrich and AFT particles before and after irradiation. Irradiated materials shown in dotted lines and nonirradiated materials in solid lines.
(Color online) Mössbauer results for Aldrich and AFT powders. (a) Experimental data (dots), fitted data (solid line), individual fitted sites (dashed lines) including 12 k octahedral, 4f2 octahedral, 4f1 tetrahedral and 2a octahedral, 2b five-fold, unidentified with fit parameters as in Table IV: (b) comparison of nonirradiated and irradiated samples (c) hyperfine field distributions by Fe site for nonirradiated (solid lines) and irradiated (dashed lines) samples for both Aldrich and AFT powders.
(Color online) Structural view of BaFe12O19 showing the (100) plane (left) and (110) plane (right). The largest atoms are barium, the next largest are oxygen, and the smallest are iron. Iron sites are labeled with their Wyckoff symbols corresponding to the five different sites investigated with the Mössbauer data. The first number in each symbol is the number of these sites per unit cell. Spin up and spin down are indicated by half colored (spin up) and solid (spin down) iron atoms. The letters to the right of the figure illustrate the different structural blocks that makeup an M-type hexaferrite: S (spinel), R (rhombohedral), with S* and R* being identical to S and R but with a rotation of 180° about the c axis (vertical in this figure).
(Color online) Comparison of irradiation effects on the DC magnetization (σ) and AC magnetic susceptibility (χ′) vs temperature for micrometer-size (AFT) and nano-size (Aldrich) particles. DC specific magnetization taken as ZFCW, remnant, H dc = 0. AC susceptibility taken as ZFCW, demagnetized, H dc = 0, H dc = 5 Oe. (a) σ for both particles, before and after irradiation; (b) % change of σ after 1 MGy from nonirradiated value; (c) χ′ at 10 kHz for both particles, before and after irradiation; (d) % change of χ′ after 1 MGy from nonirradiated value.
Physical parameters of the investigated particles compared to similar materials in the literature. S is specific surface are, ρ is the x-ray density, d is the x-ray crystallite size, and D is the average characteristic size (diameter). Subscripts on D indicate the method of determination (SEM: scanning electron microscope, PSD: particle size distribution from light scattering) whether the sizes are for primary particles (pp) or agglomerates (agl).
Summary hysteresis properties before and after irradiation and comparison to literature.
AC magnetic susceptibility results (units of emu/g/Oe). Data shown are the maximum 10 kHz AC susceptibility and the temperatures (Kelvin) where maxima in the susceptibility occur in parenthesis. See text for interpretation of measurement conditions.
Mössbauer fitted parameters for 300 K spectra under zero applied fields. Shown are average center shift (〈δ〉), average quadrupole shift (〈ɛ〉), Lorentzian full-width half-maximum linewidth (Γ), average hyperfine field (〈|H hf|〉), standard deviation of the hyperfine field distribution (stdev(|H hf|) > fractional area of fitted sites (A), and reduced chi squared parameter for goodness-of-fit (χ2, red).
Estimates of the magnetic dead layer thickness λ and equivalent dead volume V at 300 K assuming spherical particles with a shell of magnetically dead material. Values used in the calculation are in Table I (S and ρ), Table II (σS), or in the text (D and σ0). The ratio R of magnetic fraction to total particle is also shown. Both V and R assume values of λ according to Eq. (3).
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