Seong-Gi Min

Analysis of magnetization and magnetocaloric effect in amorphous FeZrMn ribbons

The magnetization behaviors have been measured for amorphous Fe90−xMnxZr10 (x=8 and 10) alloys. The Curie temperature is decreased from 210Kto185K with increasing Mn concentration (x=8 to x=10). The magnetization measurements were conducted at temperatures above the Curie temperature in the paramagnetic region. In both samples, the magnetic properties showed superparamagnetic behavior above Tc where the mean magnetic moment of the superparamagnetic spin clusters decreased with increasing temperature. A large magnetic entropy change ΔSM, which is calculated from H vs M curves associated with the ferromagnetic-paramagnetic transitions in amorphous, has been observed. The maximum ΔSM of Fe82Mn8Zr10 is 2.87J∕kgK at 210K for an applied field of 5T. The peak of magnetic entropy change was observed at the Curie temperature. The ΔSM decreases with increasing Mn concentration to 2.33J∕kgK.

Electron paramagnetic resonance study of La0.7Ca0.3−xBaxMnO3 lanthanum manganites

Electron paramagnetic resonance (ESR) study of La0.7Ca0.3−xBaxMnO3 manganites (x=0; 0.15; 0.3) is presented. Experimentally observed exponential decreasing of line intensity on temperature is in agreement with the deduced one and indicates the spin-lattice relaxation in manganites near the Curie temperature Tc. The obtained dependence of ESR linewidth on the x value shows the decreasing of strength of spin-lattice interaction with the increasing of Tc (caused by the increasing of the x value).

Annealing Temperature Dependence of Exchange Bias Effect in Short Time Annealed NiFe / NiMn Bilayer Thin Film by FMR Measurement

The NiMn/NiFe bilayer structure which was short time annealed in order to induce unidirectional anisotropy were studied as a function of annealing temperature. The maximum exchange bias field of NiMn/NiFe bilayer was presented at 250℃ after short time annealing process with no external field. The appearance of exchange bias was due to phase transformation of NiMn layer. In plane angular dependence of a resonance field distribution which measured by FMR was analysed as a combined effect of unidirectional anisotropy and uniaxial anisotropy. The resonance field and the line width from FMR measurement were also analysed with annealing temperature.

Phase control of nonadiabatic parametric amplification of spin wave packets

The influence of the phase shift between the signal and pumping wave on the process of parametric amplification of microwave spin wave pulses has been examined. The experiment was specifically made in the nonadiabatic regime, where the pumping field is strongly localized in a region on the order of the spin wave carrier wavelength. In this regime all previous experiments exhibited a strong random modulation (beating) of the amplified output signal. For a fixed phase shift between the signal and the pumping carrier waves the output signal was found to be stable, and no beating was observed. The amplitude of the amplified output pulse varies from the maximum value (corresponding to the maximum gain factor of 15 dB) to the minimum value (corresponding to almost no gain) when the phase shift between signal and pumping wave is changed. The measured phase interval between two consecutive minima in the phase dependence of the output power is equal to π.

Effect of CuO Addition on the Magnetic and Microwave Properties of YIG

The structure, magnetic and microwave properties of polycrystalline YIG prepared by a conventional ceramic method with the addition of a small quantity of CuO as a sintering agent, were studied using X-ray diffractometry, magnetometry and ferromagnetic resonance experiments. Samples sintered with the sintering agent consisted of mainly a cubic YIG structure. Small amounts of Cu ferrite and grain boundary phase were formed in specimens with CuO content above 1.5 wt% during sintering. Saturation magnetization and Curie temperature of specimens were almost independent of the content of CuO. Sintering temperature for YIG can be reduced by about 100 K by adding 0.5-1.0 wt% CuO without any change in saturation magnetization and Curie temperature. The specimen prepared with CuO showed improved microwave property.

The Large Magnetocaloric Effect in Amorphous Fe80-xMnxZr10 (x = 4,6,8,10) Alloys

The Magnetization behaviour has been measured for amorphous Fe_(90-x)Mn_xZr_(10) (x = 4,6,8,10) alloys. The Curie temperature decreased from 236 K to 195 K with increasing Mn concentration (x = 4 to x = 10). The magnetization measurements were conducted at temperatures above the Curie temperature in the paramagnetic region. In all samples, the magnetic properties showed superparamagnetic behavior above Tc where the mean magnetic moment of the superparamagnetic spin clusters decreased with increasing temperature. A large magnetic entropy change, ΔS_M, which is calculated from H vs M curves associated with the ferromagnetic-paramagnetic transitions in amorphous, has been observed. With Mn concentration increasing, ΔS_M decreases 1.04, 0.95, 0.87 J/㎏ K at 222, 210, 195 K (the Curie temperature), respectively.

The Annealing Effect on Magnetocaloric Properties of Fe 91-x Y x Zr₉ Alloys

We have carried out the study of magnetocaloric effect for as-quenched and annealed Fe 91-x Y x Zr? alloys. Samples were prepared by arc melting the high-purity elemental constituents under argon gas atmosphere and by single roller melt spinning. These alloys were annealed one hour at 773 K in vacuum chamber. The magnetization behaviours of the samples were measured by vibrating sample magnetometer. The Curie temperature increases with increasing Y concentration (x = 0 to 8). Temperature dependence of the entropy variation ΔS M was found to appear in the vicinity of the Curie temperature. The results show that annealed Fe 86 Y?Zr? and Fe 83 Y?Zr? alloys a bigger magnetocaloric effect than that those in as-quenched alloys. The value is 1.23 J/㎏ K for annealed Fe 86 Y?Zr? alloy and 0.89 J/㎏ K for as-quenched alloy, respectively. In addition, the values of ΔS M for Fe 83 Y?Zr? alloy is 0.72 J/㎏ K for as-quenched and 1.09 J/㎏ K for annealed alloy, respectively.

Ferromagnetic resonance of Fe–Sm–O thin films

In order to investigate the temperature and the annealing conditions dependence of magnetic properties in Fe72.1Sm5.2O22.7(at.u200a%) thin films, such as the effective magnetization Meff, the spectroscopic splitting factor g, and the exchange stiffness constant A, we carried out ferromagnetic resonance (FMR) experiments. Fe–Sm–O thin films were deposited in an argon and oxygen mixed atmosphere using a rf magnetron sputtering apparatus at room temperature. The g is almost constant in the temperature range from 298 to 238 K, but increases with decreasing temperature in the range below 238 K. The A increases slightly with decreasing temperature to 238 K, and then increases linearly below 238 K. When the annealing time increases up to 6u2009h the Meff is almost constant, but the A decreases slightly with increasing annealing time.

Relaxation properties of patterned circular Co dots

Co thin films have been fabricated using the magnetron sputtering method, and Co dot arrays on Si(002) wafer were patterned using e-beam lithography and etching techniques. The dots have a diameter of 500 nm and the distance of 500, 800, and 1000 nm between the dots. The shape of dots was circular, but the sizes of all dots were not exactly the same. To investigate the interaction between dots, ferromagnetic resonance (FMR) experiments were carried out using the in-plane geometry with a varying angle at room temperature and 9.43 GHz. In contrast to plain ferromagnetic films, the spectra of the arrays show an additional resonance peak, whose position depends strongly on the orientation of the external magnetic field and the interparticle interaction.

The Large Magnetocaloric Effect in Amorphous Fe 80-x Mn x Zr 10 (x = 4,6,8,10) Alloys

The Magnetization behaviour has been measured for amorphous Fe_(90-x)Mn_xZr_(10) (x = 4,6,8,10) alloys. The Curie temperature decreased from 236 K to 195 K with increasing Mn concentration (x = 4 to x = 10). The magnetization measurements were conducted at temperatures above the Curie temperature in the paramagnetic region. In all samples, the magnetic properties showed superparamagnetic behavior above Tc where the mean magnetic moment of the superparamagnetic spin clusters decreased with increasing temperature. A large magnetic entropy change, ΔS_M, which is calculated from H vs M curves associated with the ferromagnetic-paramagnetic transitions in amorphous, has been observed. With Mn concentration increasing, ΔS_M decreases 1.04, 0.95, 0.87 J/㎏ K at 222, 210, 195 K (the Curie temperature), respectively.