Sung Baek Kim

Magnetic properties and the crystallization of amorphous Fe83B9Nb7Cu1

The amorphous state of ferromagnetic Fe83B9Nb7Cu1 and its nanocrystallization have been studied by x‐ray, Mossbauer spectroscopy, and magnetic moment measurements. In the amorphous state at 13 K, the Mossbauer spectrum exhibits an essentially symmetric hyperfine field distribution with a half‐width of 72 kOe. The average hyperfine field Hhf(T) of the amorphous state shows a temperature dependence of [Hhf(T)−Hhf(O)]/Hhf(O)=−0.48(T/TC)3/2 u −0.22(T/TC)5/2 for T/TC<0.7, indicative of spin‐wave excitation. The quadrupole splitting just above the Curie temperature TC is 0.43 mm/s, whereas the average quadrupole shift below TC is zero. The Curie and crystallization temperatures are determined to be TC=393 K and Tx=775 K, respectively, for a heating rate of 5 K/min. The occupied area of the nanocrystalline phase at the optimum annealing temperature is about 33%–71%. It is notable that the magnetization of the amorphous phase decreases more rapidly with reduced temperature than those of nanocrystalline ferromagne...

Mössbauer Studies of Double Perovskite Sr 2 Fe l-x Cr x MoO 6

We investigated the crystallographic and magnetic properties of double perovskite Sr₂Fe_(1-x)Cr_xMoO_6 (x=0.0, 0.01, 0.03, 0.05, and 0.10). Mossbauer spectra of the Sr₂Fe_(0.9)Cr_(.01)MoO_6 have been taken at various temperatures ranging from 15 to 415 K. As the temperature increased towards TC (415 K), the Mossbauer spectra showed line broadening and 1, 6 and 3, 4 line-width differences because of anisotropic hyperfine field fluctuation. The Mossbauer spectra indicated that an anisotropic field fluctuation of +H (P+=0.85) was greater than that of -H (P_= 0.15). We also calculated the field fluctuation frequency factors and the temperature dependence of anisotropy energies from its relaxation rate. We interpreted the effect of Cr (t³_(2g)) doping as a decrease in the anisotropy energy.

Anisotropic hyperfine field fluctuation in Sr2FeMoO6

The double perovskite Sr2FeMoO6 has been studied by the Mossbauer technique, neutron, and x-ray diffraction. The structure is found to be tetragonal with lattice constants a0=5.5729 A and c0=7.9077 A. Mossbauer spectra and neutron diffraction measurements of Sr2FeMoO6 have been taken at various temperatures ranging from 10 to 473 K. The low-field magnetoresistance magnitude (Δρ/ρ0) at 500 Oe was 3.1% and 1.8% at 77 and 300 K, respectively. As the temperature increases toward to the Curie temperature, TC=425 K. Mossbauer spectra show the line broadening and 1, 6 and 3, 4 linewidth difference because of anisotropic hyperfine field fluctuation. The anisotropic field fluctuation of +H(P+=0.85) was great than −H(P−=0.15). We also calculated frequency factor and anisotropy energy with values of 9.8 Γ/ℏ and 149.6 erg/cm3, respectively, using the relatively accurate data for T=260 K which is associated with the large line broadening.

Atomic migration in CoGa/sub x/Fe/sub 2-x/O/sub 4/

Co-Ga ferrite has been studied with X-ray diffraction, vibrating sample magnetometer and Mossbauer spectroscopy. The crystal structure for this system is spinel and lattice constants is in accord with Vegards law. The Neel temperature decreases linearly with Ga concentration suggesting the superexchange interaction Co-O-Fe link is stronger than that for the Ga-O-Fe link. Atomic migration of CoGa/sub 0.1/Fe/sub 1.9/O/sub 4/ starts near 350 K and increases rapidly with increasing temperature to such a degree that 73% of the ferric ions as the A sites have moved to the B site by 700 K. The temperature dependence of the magnetic hyperfine field and magnetization of CoGa/sub 0.1/Fe/sub 1.9/O/sub 4/ is explained by the Neel theory of ferrimagnetism using three superexchange integrals: J/sub A-B/=-11.5 k/sub B/, J/sub A-A/=9.8 k/sub B/, J/sub B-B/=21.3 k/sub B/.

Effects of La/sup 3+/ substitution on the magnetic properties of double perovskites A/sub 2/FeMoO/sub 6/ (A=Ca and Ba)

We have studied effects of the partial substitution of La/sup 3+/ for A/sup 2+/ on the magnetic properties of double perovskites A/sub 2/FeMoO/sub 6/ (A=Ca and Ba). Polycrystalline A/sub 2-x/La/sub x/FeMoO/sub 6/ (0/spl les/x/spl les/0.3) samples have been prepared by the conventional solid-state reaction in a stream of 5% H/sub 2//Ar gas. The X-ray data indicate that A=Ca is monoclinic with the space group P2/sub 1//n and A=Ba is cubic with the space group Fm3m. The substitution of La/sup 3+/ for A/sup 2+/ results in a cell volume increase for A=Ca and a cell volume reduction for A=Ba. Saturation magnetization, which is 3.5 /spl mu//sub B//f.u. for undoped A=Ca (x=0) and 3.9 /spl mu//sub B//f.u. for undoped A=Ba (x=0), decreases with increasing x. This decrease of magnetization arises from the reduction of moment associated with the electron doping and the disorder at the Fe and Mo sites. The partial substitution of La/sup 3+/ for Ba/sup 2+/ considerably enhances the Curie temperature T/sub c/ from 316 K for x=0 to 334 K for x=0.2. However, the T/sub c/ in Ca/sub 2-x/La/sub x/FeMoO/sub 6/ is not enhanced significantly with increasing x.

Mössbauer studies of iron-doped La0.67Sr0.33Mn0.9957Fe0.01O3

The iron-doped perovskite La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 compound has been studied by X-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometry. The single phase of the polycrystalline La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 powder has been prepared by a water-based sol-gel method. Crystalline La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 was a rombohedral structure with lattice parameters a 0 = 5.480 A, α = 60.259°. Mossbauer spectra of La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 have been taken at various temperatures ranging from 20 to 400 K. Analysis of 57 Fe Mossbauer spectrum has considered anisotropic hyperfine field fluctuation. Temperature dependence of anisotropy energy is calculated from the relaxation rate.

Crystallographic and magnetic properties of nanocrystalline Fe78Al4Nb5B12Cu1 alloys

The effects of crystallographic change induced during the annealing process upon the magnetic properties of nanocrystalline Fe78Al4Nb5B12Cu1 alloy were investigated by using x-ray diffraction, Mossbauer spectroscopy, and macroscopic magnetometry. Special focus was concentrated on the structures of interfacial layer, which is a region between nanocrystallite and amorphous matrix. To examine the differences in the crystallographic and the magnetic properties associated with the thermal treatments, amorphous samples were annealed in two different ways, i.e., flash annealing and conventional annealing. The Mossbauer spectra were analyzed to decompose into three or four components, and revised Vincze method was used to extract the distributions of hyperfine parameters. Interfacial layers were found to have a considerable crystalline order. Flash annealing advances the crystallization process earlier than the conventional annealing. As a result, the fraction of the crystalline phase produced by flash annealing ...

Mossbauer studies of nanocrystalline Fe/sub 83/B/sub 9/Nb/sub 7/Cu/sub 1/ alloy by flash annealing

Melt-spun Fe/sub 83/B/sub 9/Nb/sub 7/Cu/sub 1/ alloy with ultrathin ribbon has been studied with Mossbauer spectroscopy and X-ray diffraction. The enhanced magnetic property of the flash-annealed alloy was attributed to the reduced /spl alpha/-Fe phase grain size to 6 nm and the higher effective permeability and smaller magnetic core loss at 1 MHz than conventional annealed alloys. The occupied area of the nanocrystalline phase at the optimum 773 K is about 73% whereas that for conventional annealing temperature at 893 K is about 71%. The flash annealing technique was effective in improving the high-frequency soft magnetic property of nanocrystalline Fe/sub 83/B/sub 9/Nb/sub 7/Cu/sub 1/ alloy.

Color Theorems, Chiral Domain Topology, and Magnetic Properties of FexTaS2

Common mathematical theories can have profound applications in understanding real materials. The intrinsic connection between aperiodic orders observed in the Fibonacci sequence, Penrose tiling, and quasicrystals is a well-known example. Another example is the self-similarity in fractals and dendrites. From transmission electron microscopy experiments, we found that FexTaS2 crystals with x = 1/4 and 1/3 exhibit complicated antiphase and chiral domain structures related to ordering of intercalated Fe ions with 2a × 2a and √3a × √3a superstructures, respectively. These complex domain patterns are found to be deeply related with the four color theorem, stating that four colors are sufficient to identify the countries on a planar map with proper coloring and its variations for two-step proper coloring. Furthermore, the domain topology is closely relevant to their magnetic properties. Our discovery unveils the importance of understanding the global topology of domain configurations in functional materials.

Mössbauer studies of multiferroic spinel CoCr1.98Fe0.0257O4

In order to elucidate the role of Cr ions in CoCr2O4 exhibiting multiferroic property, we have substituted a small amount of Fe ions for Cr sites and investigated the magnetic behavior of Fe ions on atomic scale, using Mossbauer measurement. Polycrystalline CoCr1.98Fe0.0257O4 compound was prepared by wet-chemical process. The crystal structure was found to be a single-phase cubic spinel with space group of Fd-3m. The lattice constant (a0) and the internal structural parameter (x) of the oxygen were determined to be 8.340 and 0.264A, respectively. Mossbauer absorption spectra at 4.2K show that the well developed two sextets are superposed with small difference in hyperfine field. Isomer shift values (δ) of the two sextets are found to be 0.34 and 0.35mm∕s relative to the Fe metal, which are consistent with the high spin Fe3+ charge state. With increasing temperature, the sextets gradually split into two subspectra, and then around 28K the absorption line broadening of outer sextet appears rapidly. Above th...