Seung Iel Park

Atomic migration in Ni–Co ferrite

Ni–Co ferrite has been studied with Mossbauer spectroscopy and x‐ray diffraction. The crystal structure for this system is spinel, and the lattice constant is in accord with Vegard’s law. The Mossbauer spectra consist of two six‐line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Neel temperature increases linearly with Ni concentration, suggesting the superexchange interacion for the Ni–O–Fe link is stronger than that for the Co–O–Fe link. It is found that Debye temperatures for the A and B sites of CoFe2O4 and NiFe2O4 are found to be θA=734 K, θB=248 K, and θA=378 K, θB=357 K, respectively. The intensity ratio of the A to B patterns is found to increase at low temperatures with increasing temperature due to the large difference of Debye temperatures of the two sites and to decrease at high temperatures due to migration of Fe3+ ions from A to B sites. Atomic migration of CoFe2O4 starts near 400 K and increases rapidly with increasing temperature to such a degree that ...

Structure and magnetic properties of Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta//

The perovskite type Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// (x=0.0, 0.25, 0.5, 0.75 and 1.0) system has been studied by X-rays, Mohrs salt analysis, and Mossbauer spectroscopy. From the results of X-ray diffraction measurement the structure of the Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// system is orthorhombic with x values of 0.0, 0.25 and 0.5, and cubic with the x values of 0.75 and 1.0. Mossbauer spectra of Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// have been taken at various temperatures ranging from 13 to 800 K. Analysis of Mossbauer spectra for this system demonstrated the existence of the mixed valence states of iron and the coordination state of Fe/sup 3+/ and Fe/sup 4+/ ions. The Neel temperature decreases linearly with Sr concentration, suggesting that the superexchange interaction for Gd-O-Fe link is stronger than that for Sr-O-Fe link. >

Effects of additives on magnetic properties of sheet Sr‐Ba ferrite magnets

Sr0.75Ba0.25Fe12O19 hexagonal ferrite has attracted much attention due to its large (BH)MAX values and workability. We have prepared sheet magnets by the Dr. Blade method. To examine the effects of additives, such as SiO2, TiO2, Al2O3, and Cr2O3, on magnetic properties of sheet magnets, we used VSM magnetometer, x‐ray diffraction, and Mossbauer spectrometer. The crystal structure is found to be a magnetoplumbite of typical M‐type hexagonal ferrite, but the α‐Fe2O3 phase develops with increasing additives concentration. Using our refined computer program, we have analyzed the Mossbauer spectra in the temperature range from 13 to 800 K. The Mossbauer spectra indicate that the line intensity for the 12k site is reduced with increasing SiO2 concentration, which is different from the reports of Fe‐substituted Ba ferrite. This suggests that the developing α‐Fe2O3 phase is related to 12k sites. The isomer shifts show the charge states of Fe ions is ferric. When the additives concentrations increase, the Curie te...

Mössbauer studies of NdFe10.7Ti1.3Nδ

NdFe 10.7 Ti 1.3 N δ has been studied with X-ray diffraction, Mossbauer spectroscopy and vibrating-sample magnetometer (VSM). The alloys were prepared by arc melting under an argon atmosphere. The NdFe 10.7 Ti 1.3 N δ contains some a-Fe, this was confirmed with X-ray diffraction and Mossbauer measurements. The NdFe 10.7 Ti 1.3 N δ has the ThMn 12 -type tetragonal structure with lattice constants a 0 = 8.638 A and c 0 = 4.819 A and its Curie temperature T c is 743 ± 3 K. Mossbauer spectroscopy was performed at various temperatures ranging from 13 to 800 K. Each spectrum below T c was fitted with six subspectra of Fe sites in the structure (8i 1 , 8i 2 , 8j 2 , 8j 1 , 8f and α-Fe). The area fractions of the subspectra at room temperature are 16.3, 13.3, 11.8, 20.5, 31.9 and 6.2%, respectively. Magnetic hyperfine fields for the Fe sites decrease in the order, H hf (8i) > H hf (8j) > H hf (8f). The average hyperfine field H hf (T) of the NdFe 10.7 Ti 1.3 N δ shows a temperature dependence of [H hf (T)-H hf (0)]/H hf (0) = -0.41(T/T c ) 3/2 -0.14(T/T c ) 5/2 for T/T c < 0.7, indicative of spin-wave excitation.