Seung Wha Lee

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 ...

Mössbauer studies of BaFe11.9Mn0.1O19 by a sol–gel method

BaFe11.9Mn0.1O19 powders were prepared by a sol–gel method. Magnetic and structural properties of the powders were characterized by Mossbauer spectroscopy, x-ray diffractometry, and vibrating sample magnetometry. X-ray diffraction and Mossbauer measurements showed that the BaFe11.9Mn0.1O19 had an M-type hexagonal structure with a0=5.900 A and c0=23.219 A. Mossbauer spectroscopy was performed at various temperatures ranging from 13 to 800 K, and each spectrum for a temperature below the Curie temperature (TC=775±5 K) was fitted with five subspectra of Fe sites in the structure (4fVI, 2a, 4fIV, 12k, and 2b). The area fractions of the subspectra at 13 K were 18.0%, 10.2%, 17.5%, 46.1%, and 8.2%, respectively. The 2b site had a very large quadrupole splitting. The isomer shifts indicated that the valence state of the Fe ions was ferric (Fe3+). The saturation magnetization Ms was 58 emu/g, and coercivity Hc was 5141 Oe at room temperature under an applied field of 15 kOe.

Superparamagnetic Properties of Nanoparticles Ni 0.9 Zn 0.1 Fe 2 O 4 for Biomedical Applications

Nanoparticles Ni_(0.9)Zn_(0.1)F₂O₄ is fabricated by a sol-gel method. The magnetic and structural properties of powders were investigated with XRD, SEM, Mossbauer spectroscopy, and VSM. Ni_(0.9)Zn_(0.1)F₂O₄ powders annealed at 300 ℃ have a spinel structure and behaved superparamagnetically. The estimated size of Ni_(0.9)Zn_(0.1)F₂O₄ nanoparticle is about 10 ㎚. The hyperfine fields at 13 K for the A and B patterns are found to be 533 and 507 kOe, respectively. The ZFC curves are rounded at the blocking temperature (T_B) and show a paramagnetic-like behavior above T_B. T_B of Ni_(0.9)Zn_(0.1)F₂O₄ nanoparticle is about 250 K. Nanoparticles Ni_(0.9)Zn_(0.1)F₂O₄ annealed at 400 and 500 ℃ have a typical spinel structure and is ferrimagnetic in nature. The isomer shifts indicate that the iron ions were ferric at the tetrahedral (A) and the octahedral (B). The saturation magnetization of nanoparticles Ni_(0.9)Zn_(0.1)F₂O₄ annealed at 400 and 500 ℃ are 40 and 43 emu/g, respectively. The magnetic anisotropy constant of Ni_(0.9)Zn_(0.1)F₂O₄ annealed at 300 ℃ were calculated to be 1.6 × 10^6 ergs/㎤.

Charge ordering and Mössbauer studies of single crystal R1/3Sr2/3FeO3 (R=Pr, Sm, and Nd)

Single crystals of R1/3Sr2/3FeO3 (R=Pr, Nd, and Sm) were synthesized by the floating zone method and their magnetic properties and charge ordering (CO) transition related to lattice dynamics were systematically investigated. Mossbauer spectra of R1/3Sr2/3FeO3 were taken at various temperatures ranging from 12 K to room temperature. The charge disproportionation in Pr1/3Sr2/3FeO3 was detected below 190 K, in which two kinds of iron with valence states Fe3+ and Fe5+ were found with ratio of 2:1. The iron with valence state Fe4+ in Pr1/3Sr2/3FeO3 coexists at and above 150 K, and its ratio increased from 13% to 66% as the temperature rose. The (Nd1−ySmy)1/3Sr2/3FeO3 (y=0.0, 0.2, 0.4, 0.6, and 0.8) with least lattice distortion underwent a CO phase transition at and below TCO=163 K and accompanying the charge disproportionation into nominally Fe3+ and Fe5+ sites as well as a canted antiferromagnetic spin ordering. In this charge ordering state, a sequence of Fe+3Fe+3Fe+5Fe+3Fe+3Fe+5 exists aligned along the [1...

Crystallization and Mössbauer studies of melt-spun NdFe10.7TiB0.3Nδ alloys

Magnetic properties of melt-spun NdFe10.7TiB0.3Nδ ribbons have been investigated as functions of quenching rate and nitriding period. NdFe10.7TiB0.3 were prepared with substrate velocity vs⩽18 m/s and were nitrogenated at 500 °C for 15 min. The NdFe10.7TiB0.3Nδ retains the ThMn12-type tetragonal structure with lattice constants a0=8.640 A and c0=4.811 A, but with an increase in the unit cell volume. The NdFe10.7TiB0.3Nδ was confirmed to have uniaxial anisotropy by x-ray diffraction. Mossbauer spectra were taken at various temperatures ranging from 13 to 855 K. The Curie and Debye temperatures are determined to be Tc=833 K and Θ=390 K, respectively. Each spectrum below Tc was fitted with six subspectra of Fe sites (8i1, 8i2, 8j2, 8j1, 8f, and α-Fe). The area fraction of the subspectra at 13 K are 10.2%, 8.2%, 16.5%, 17.5%, 44.3%, and 3.3%, respectively. The magnetic hyperfine fields for the Fe sites decrease in the order, Hhf(8i)>Hhf(8j)>Hhf(8f).

Synthesis and Magnetic Properties of Zn, Co and Ni Substituted Manganese Ferrite Powders by Sol-gel Method

The Zn, Co and Ni substituted manganese ferrite powders, Mn1-x(Zn, Co, Ni) x Fe₂O₄, were fabricated by the solgel method, and their crystallographic and magnetic properties were studied. The Zn substituted manganese ferrite, Zn 0.2 Mn 0.8 Fe₂O₄, had a single spinel structure above 400 ℃, and the size of the particles of the ferrite powder increased when the annealing temperature was increased. Above 500 ℃, all the Mn 1-x (Zn, Co, Ni) x Fe₂O₄ ferrite had a single spinel structure and the lattice constants decreased with an increasing substitution of Zn, Co, and Ni in Mn 1-x (Zn, Co, Ni) x Fe₂O₄. The Mossbauer spectra of Mn 1-x Zn x Fe₂O₄ (0.0≤x≤0.4) could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the Fe 3+ ions. For x = 0.6 and 0.8 they showed two Zeeman sextets and a single quadrupole doublet, which indicated they were ferrimagnetic and paramagnetic. And for x = 1.0 spectrum showed a doublet due to a paramagnetic phase. For the Co and Ni substituted manganese ferrite powders, all the Mossbauer spectra could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the Fe 3+ ions. The variation of the Mossbauer parameters are also discussed with substituted Zn, Co and Ni ions. The increment of the saturation magnetization up to x = 0.6 in Mn 1-x Co x Fe₂O₄ could be qualitatively explained using the site distribution and the spin magnetic moment of substituted ions. The saturation magnetization and coercivity of the Mn 1-x (Zn, Co, Ni) x Fe₂O₄ (x = 0.4) ferrite powders were also compared with pure MnFe₂O₄.

Superparamagnetic Properties of Ni0.7Zn0.3Fe₂O₄ Nanoparticles

Nanoparticles Ni_(0.7)Zn_(0.3)Fe₂O₄ is fabricated by a sol-gel method. The magnetic and structural properties of powders were investigated with XRD, SEM, Mossbauer spectroscopy, and VSM. Ni_(0.7)Zn_(0.3)Fe₂O₄ powders annealed at 300 ℃ have a spinel structure and behaved superparamagnetically. The estimated size of Ni_(0.7)Zn_(0.3)Fe₂O₄ nanoparticle is about 11 ㎚. Ni_(0.7)Zn_(0.3)Fe₂O₄ annealed at 400 and 500 ℃ has a typical spinel structure and is ferrimagnetic in nature. The isomer shifts indicate that the iron ions were ferric at the tetrahedral (A) and the octahedral (B). Blocking temperature (T_B) of Ni_(0.7)Zn_(0.3)Fe₂O₄ nanoparticle is about 260 K. The magnetic anisotropy constant of Ni_(0.7)Zn_(0.3)Fe₂O₄ annealed 300 ℃ were calculated to be 1.7 × 10^6 ergs/㎤. Also, temperature of the sample increased up to 43 ℃ within 7 minutes under AC magnetic field of 7 ㎒.

Atomic migration and superexchange interaction in Cu/sub 0.9/Ni/sub 0.1/Fe/sub 2/O/sub 4/

Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/ was studied with X-ray diffraction and Mossbauer spectroscopy. The crystal structure was found to be a cubic spinel with the lattice constant a/sub 0/=8.386/spl plusmn/0.005 /spl Aring/. The Neel temperature was determined to be T/sub N/=755 K for a heating rate of 5 K/min. The Mossbauer spectra consisted of two six-line patterns corresponding to Fe/sup 3+/ at the tetrahedral (A) and octahedral (B) sites. Debye temperatures for A and B sites were found to be 568/spl plusmn/5 K and 194/spl plusmn/5 K, respectively. Atomic migration of Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/ starts near 350 K and increases rapidly with increasing temperature to such a degree that 71% of the ferric ions from the A sites moved to the B sites at 550 K. The temperature dependence of the magnetic hyperfine field of Ni/sub 0.1/Cu/sub 0.9/Fe/sub 2/O/sub 4/ was explained by the Neel theory of ferrimagnetism using three superexchange integrals: J/sub A-B/=-29.2 k/sub B/, J/sub A-A/=-21.9 k/sub B/, J/sub B-B/=0.5 kg.

Crystallographic and magnetic properties of NdFe10.7TiM0.3(M=B, Ti)

NdFe10.7TiM0.3(M=B, Ti) has been studied with x‐ray diffraction, Mossbauer spectroscopy, and a vibrating sample magnetometer. The alloys were prepared by arc‐melting under an argon atmosphere. The NdFe10.7TiB0.3 exhibits a pure single phase, whereas the NdFe10.7Ti1.3 contains some α‐Fe, from x‐ray and Mossbauer measurements. The NdFe10.7TiB0.3 has the ThMn12‐type tetragonal structure with a0=8.587 A and c0=4.788 A. The Curie temperature (TC) is 570 K from Mossbauer spectroscopy performed at various temperatures ranging from 13 to 770 K. Each spectrum below TC was fitted with five subspectra of Fe sites in the structure (8i1, 8i2, 8j1, 8j2, and 8f ). The area fraction of the subspectra at room temperature are 16.4%, 8.2%, 14.8%, 21.3%, and 39.3%, respectively. Magnetic hyperfine fields for the Fe sites decrease on the order of Hhf(8i)≳Hhf(8j)≳Hhf(8f ). The average hyperfine field Hhf(T) of the NdFe10.7TiB0.3 shows a temperature dependence of [Hhf(T)−Hhf(O)]/Hhf(O)=−0.39(T/TC)3/2 & −0.17(T/TC)5/2 for T/TC<0...

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...