Jae-Gwang Lee

Magnetic properties of CoFe2O4 thin films prepared by a sol-gel method

Thin films with cobalt ferrite layers on thermally oxidized silicon wafers were fabricated by a sol-gel method. Magnetic and structural properties of the films were investigated with an x-ray diffractometer, a vibrating sample magnetometer and atomic force microscopy. The crystallization temperature for Co ferrite thin films was determined by using Mossbauer spectroscopy. Co ferrite films annealed at and above 450 °C have only a single phase spinel structure without any preferred crystallite orientation. Their rms surface roughness is less than 3 nm and the size of grains is about 30 nm for annealing temperatures greater than 650 °C. Films fired at and above 550 °C have moderate saturation magnetization and there is no significant difference of their magnetic properties for external fields applied parallel and perpendicular to their planes. The coercivity shows a strong dependence on the annealing temperature.

Growth of ultra-fine cobalt ferrite particles by a sol-gel method and their magnetic properties

Ultra-fine CoFe2O4 particles are fabricated by a sol–gel method and magnetic and structural properties of powders are investigated. Cobalt ferrite powders fired at and above 450 °C have only a single-phase spinel structure and behave ferrimagnetically. Powders annealed at 350 °C have a typical spinel structure and are of the paramagnetic and ferrimagnetic nature, simultaneously. With X-ray diffraction and Mossbauer spectroscopy measurements, the formation of nano-crystallized particles is confirmed when cobalt ferrite is annealed at 200 °C. In addition, the transition from the paramagnetic to the ferrimagnetic state is observed in samples fired at 200 °C as the measuring temperature decreases from the room to liquid nitrogen temperature. The magnetic behaviour of CoFe2O4 powders fired at and above 350 °C shows that an increase of the annealing temperature yields a decrease in the coercivity and, in contrast, an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of cobalt ferrite powders prepared by the sol–gel method are 2020 Oe and 76.5 e.m.u. g−1, respectively.

Growth of ultrafine Co–Mn ferrite and magnetic properties by a sol–gel method

Ultrafine Co0.9Mn0.1Fe2O4 particles are fabricated by a sol–gel method. Magnetic and structural properties of powders are investigated with x-ray diffraction, vibrating samples magnetometer, and Mossbauer spectroscopy. Co–Mn ferrite powders which were fired at and above 673 K have only a single phase spinel structure and behave ferrimagnetically. Powders annealed at 673 K have a typical spinel structure and the paramagnetic and ferrimagnetic nature, simultaneously. The formation of nanocrystallized particles is confirmed when Co–Mn ferrite is annealed at 623 K. The magnetic behavior of Co–Mn ferrite powders fired at and above 823 K shows that an increase of the annealing temperature yields a decrease of the coercivity and an increase of the saturation magnetization. The maximum coercivity and the saturation magnetization of Co–Mn ferrite powders are 1523 Oe and 66.7 emu/g, respectively. 57Fe Mossbauer spectra of Co–Mn ferrite have been taken at various temperatures from 13 to 850 K. The isomer shifts indi...

Magnetic properties of CoFe2O4 powders and thin films grown by a sol-gel method

Abstract Ultra-fine CoFe 2 O 4 particles and thin films were fabricated by a sol-gel method and their magnetic and structural properties were investigated. Co-ferrite powders fired at and above 450°C had only a single phase spinel structure and became ferrimagnetic. Powders annealed at 200°C and 350°C had a typical spinel structure, however, they showed a transition from the paramagnetic to ferrimagnetic state as the measuring temperature decreased from the room to liquid nitrogen temperature. Co-ferrite films annealed at 650°C had a single phase spinel structure without any preferred crystallite orientation and the in-plane and perpendicular coercivity. The maximum value of the coercivity was 2550 Oe for the thin film fired at 850°C.

Magnetic properties of Cr/sup 3+/ substituted BaFe/sub 12/O/sub 19/ powders grown by a sol-gel method

Cr/sup 3+/ substituted Ba-hexaferrite was fabricated by a sol-gel method. The crystallographic and magnetic properties of BaFe/sub 12-x/Cr/sub x/O/sub 19/ (0/spl les/x/spl les/7) were investigated XRD, Rutherford backscattering spectrometry, vibrating sample magnetometry and Mossbauer spectroscopy. The crystal structure was found to be magnetoplumbite, typical of M-type hexagonal ferrite. By substituting Fe/sup 3+/ in BaFe/sub 12/O/sub 19/ by Cr/sup 3+/, we have been able to attribute the Mossbauer parameters to the 5 crystallographic sites of the structure. Only the octahedral sublattices were occupied by Cr ions. The isomer shifts indicate that the valence state of the Fe ions was Fe/sup 3+/. The Curie temperatures of BaFe/sub 12-x/Cr/sub x/O/sub 19/ decreased linearly increasing Cr-substitution, at a rate of 55 K/Cr atom.

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.

Crystallographic and Magnetic Properties of Nickel Substituted Manganese Ferrites Synthesized by Sol-gel Method

Nickel substituted manganese ferrites, Mn 1-x Ni x Fe₂O₄ (0.0 ≤ x ≤ 0.6), were fabricated by sol-gel method. The effects of sintering and substitution on their crystallographic and magnetic properties were studied. X-ray diffractometry of Mn 0.6 Ni 0.4 Fe₂O₄ ferrite sintered above 523 K indicated a spinel structure; particles increased in size with hotter sintering. The Mossbauer spectrum of this ferrite sintered at 523 K could be fitted as a single quadrupole doublet, indicative of a superparamagnetic phase. Sintering at 573 K led to spectrum fitted as the superposition of two Zeeman sextets and a single quadrupole doublet, indicating both ferrimagnetic and paramagnetic phase. Sintering at 673 K and at 773 K led to spectra fitted as two Zeeman sextets due to a ferrimagnetic phase. The saturation magnetization and the coercivity of Mn 0.6 Ni 0.4 Fe₂O₄ ferrite sintered at 773 K were 53.05 emu/g and 142.08 Oe. In Mn 1-x Ni x Fe₂O₄ (0.0 ≤ x ≤ 0.6) ferrites, sintering of any composition at 773 K led to a single spinel structure. Increased Ni substitution decreased the ferrites’ lattice constants and increased their particle sizes. The Mossbauer spectra could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and the octahedral sites of the Fe³? ions. The variations of saturation magnetization and coercivity with changing Ni content could be explained using the changes of particle size.

Crystallographic and Magnetic Properties of Nano-sized Nickel Substituted Cobalt Ferrites Synthesized by the Sol-gel Method

Nano-sized nickel substituted cobalt ferrite powders, Ni x Co 1-x Fe₂O₄ (0.0 ≤ x ≤ 1.0), were fabricated by the sol-gel method, and their crystallographic and magnetic properties were studied. All the ferrite powders showed a single spinel structure, and behaved ferrimagnetically. When the nickel substitution was increased, the lattice constants and the sizes of particles of the ferrite powders decreased. The Mossbauer absorption spectra of NixCo1-xFe2O4 ferrite powders could be fitted with two six-line subspectra, which were assigned to a tetrahedral A-site and octahedral B-sites of a typical spinel crystal structure. The increase in values of the magnetic hyperfine fields indicated that the superexchange interaction was stronger, with the increased nickel concentration in Ni x Co 1-x Fe₂O₄. This could be explained using the cation distribution, which can be written as, (Co 0.28-0.28x Fe 0.72+0.28x )[Ni x Co 0.72-0.72x Fe 1.28-0.28x ]O 4 . The two values of the saturation magnetization and the coercivity decreased, as the rate of nickel substitution was increased. These decreases could be explained using the cation distribution, the magnetic moment, and the magneto crystalline anisotropy constant of the substituted ions.

Crystallographic and Magnetic Properties of Mn x Fe 3-x O 4 Powders

Mn x Fe 3-x O₄ powders have been fabricated by using sol-gel methods; their crystallographic and magnetic properties were investigated by using X-ray diffraction, scanning electron microscopy, Mossbauer spectroscopy, and vibrating sample magnetometer. The Mn x Fe 3-x O₄ ferrite powders annealed at 500 ℃ had a single spinel structure regardless of the Mn²?-doping amount and their lattice constants became larger as the Mn²? concentration was increased. Their Mossbauer spectra measured at room temperature were fitted with 2 Zeeman sextets due to the tetrahedral and octahedral sites of Fe ions, which made them ferrimagnetic. The magnetic behavior of Mn x Fe 3-x O₄ powders showed that the Mn²?-doping amount made their saturation magnetization increase, but there were no severe effects on their coercivities. The saturation magnetization of the Mn x Fe 3-x O₄ powder varied from 38 emu/g to 70.0 emu/g and their minimum coercivity was 111.1 Oe.

Synthesis and Magnetic Properties of Nano-sized Mn Ferrite Powder and Film

Nano-sized manganese ferrite powders and films, MnFe₂O₄, were fabricated by the sol-gel method, and the effects of annealing temperature on the crystallographic and magnetic properties were studied by using X-ray diffractometry, field emission scanning electron microscopy, Mossbauer spectroscopy, and vibrating sample magnetometry. X-ray diffraction spectroscopy of powder samples annealed above 523 K indicated the presence of spinel structure, and the film samples annealed above 773 K also had spinel structure. The particle size increased with the annealing temperature. For the powder samples, the Mossbauer spectra annealed above 573 K could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of Fe³? ions. Using the Mossbauer subspectrum area ratio the cation distribution could be written as (Mn 0.52 Fe 0.48 ) [Mn 0.48 Fe 1.52 ] O₄. However the spectrum annealed at 523 K only showed as a doublet due to a superparamagnetic phase. As the annealing temperature was increased, the saturation magnetization and the corecivity of the powder samples increased, as did the coercivity of film samples.