Jin Han-Min

Effect of Fe doping in La1−xSrxMnO3

The effects of Fe doping on the magnetic, transport, and magnetoresistance properties are studied for La0.7Sr0.3Mn1−yFeyO3 (y=0–0.18) and La1−xSrxMn0.88Fe0.12O3 (x=0.1–0.50). For increasing y, a peak in the thermal dependence of the resistivity appears and develops at the temperature TPl lower than TC, while the another peak near TC becomes smaller and then disappears. At 78 K the La1−xSrxMn0.88Fe0.12O3 oxides are insulators for x 0.5 and only the lower-temperature metal–insulator transition peak appears for 0.25<x<0.45. With increase of x from 0.25, TPl increases, becomes maximum at x=0.4 and then decreases. The temperature dependence of resistivity for the doped samples is quite different from that of the undoped La1−xSrxMnO3 series.

Analysis of the magnetic properties of R2Fe17 (R identical to Gd, Tb, Dy, Ho, Er or Tm)

The values of the R-Fe exchange field and the crystalline-electric-field parameters are evaluated by fitting the calculations to the experiments for R2Fe17 (R identical to Gd, Tb, Dy, Ho, Er or Tm). The experiments include the magnetization curves along the crystal axes at 4.2 K for the Gd, Tb, Dy, Ho and Er compounds, the energy differences between the energy levels for the Ho ion in Ho2Fe17, and the spin reorientation temperature TSR and the easy-magnetization direction above and below TSR for Tm2Fe17.

Magnetization process related to the singular point detection signal observed at high fields on Pr2Fe14B and Nd2Fe14B

Abstract The singular point detection signal, d 2 M /d t 2 , observed on Pr 2 Fe 14 B and Nd 2 Fe 14 B at high fields and low temperatures is explained on the basis of one sublattice crystal field model. The signal is attributed to a steeper reversible magnetization process and a first order magnetization process for Pr 2 Fe 14 B and Nd 2 Fe 14 B respectively.

Demagnetization curves at different temperatures for single-phase nanocrystalline Nd-Fe-B magnets studied micromagnetically

The demagnetization curves at different temperatures between 4 and 250?K were calculated for single-phase nanocrystalline Nd-Fe-B magnets by using the micromagnetic finite-element technique. The calculations were carried out for a model magnet comprised of 216 grains of dimension 20?nm, taking into account the anisotropic characteristics of the curve with respect to the field direction. The calculations simulate the experimental curves for the nanocrystalline Nd13Fe77B10 magnet fairly well. The inter-grain exchange interaction in the magnet is estimated to be about 75% of the intra-grain exchange interaction.

Evaluation of exchange and crystalline field parameters in R2Fe14B compounds (R = Tb, Dy, Ho, Er and Tm)

Abstract The values of the R—Fe exchange field H ex and averaged over f and g sites CEF parameters A nm in R 2 Fe 14 B compounds (R = Tb, Dy, Ho, Er and Tm) are evaluated by fit of calculation with a series of experiments. These experiments include temperature dependences of the spontaneous magnetization M ( T ) of the compounds, magnetization curves of the compounds along [100], [110] and [001] axes at 4.2 K and at higher temperatures, the canting angle of the Ho compound, and spin reorientation temperatures T SR of Ho, Er and Tm compounds. The value of H ex of the Gd compound is also evaluated from the M ( T ) curve. The fit was restricted by the condition that the parameters vary monotonically across the R. The fitted H ex varies nearly proportional to the average of the 2nd power of the orbital radius of 4f electrons 〈 r 2 〉, and A nm nearly proportional to 〈 r n 〉 across the R. The calculation reproduces the experiments well.

Hysteresis loops at different temperatures for melt-spun

Hysteresis loops of melt-spun were measured at different temperatures between 4.2 K and 250 K. The loops were analysed on the basis of the micromagnetism using the finite element technique. By fitting the calculated with the experiment, the value of the exchange constant across the inter-grain boundary was estimated to be about 90% of that across the inner-grain boundary. increases to a maximum at the spin reorientation temperature and then decreases with increasing temperature. With decrease of the exchange interaction across the grain boundary, decreases below and increases above , and the Stoner-Wohlfarth model provides the theoretical lower and upper limits of below and above , respectively. The calculations reproduce the hysteresis loops fairly well. The magnetization proceeds by non-coherent rotation, and the spin distribution becomes quite non-uniform in some grains when H decreases to near .

Anisotropic characteristics of demagnetization curve for nanocrystalline Nd-Fe-B magnet calculated by micromagnetics

The demagnetization curves for nanocrystalline Nd-Fe-B magnets of a stoichiometric composition were calculated by making use of the finite element technique of micromagnetics. The curve, especially iHc, varies in a wide range with the direction of applied field if the grain number N is taken to be small. With the increase of N, the range becomes smaller and the average of iHc decreases and approaches a limit iHc(N = ∞). iHc for finite N is larger than, or at least equal to, iHc(N = ∞). Jr/Js is weakly affected by N and the field direction. Jr/Js(N = ∞) decreases with the increase of grain size L. These are larger than the experimental values for the Nd-rich Nd2.33Fe14B1.06Si0.21 magnets by ~0.05. iHc(N = ∞) increases with the increase of L and is close to or somewhat smaller than the experimental values of the Nd-rich magnet, as would be expected. In contrast, the curve calculated for the non-interacting grain system (Stoner-Wohlfarth model) of N ≥ 30 depends neither on the field direction nor on N.

Exchange and crystalline electric fields and magnetization processes in Pr2Co14B and Nd2Co14B

The values of R-Co exchange field and crystalline electric field parameters Anm in the compounds Pr2Co14B and Nd2Co14B are evaluated by fitting calculations to experiments. The experiments include the magnetization curves along the crystal axes at 4.2 and 290 K and the spin reorientation temperatures for Pr2Co14B and Nd2Co14B, and also the magnetization curves along the (100) axis at a series of temperatures between 77 and 190 K for Nd2Co14B. Amm is nearly proportional to (rn), and the sign of A40 is opposite to that for R2Fe14B. A20 decreases smoothly with increase in temperature in a similar way for Pr2Co14B and Nd2Co14B. The magnetization processes of the rare-earth and the Co sublattices are analysed.

Domain patterns of sintered NdFeB permanent magnet at different magnetized states

Abstract Domain patterns of a sintered NdFeB permanent magnet with different magnetic states in a zero applied field are observed on the pole and side surfaces of the sample using Kerr effect and Bitter technique, respectively.

The anisotropy of Gd?Fe exchange interaction for Gd2Fe17 and Gd2Fe17H3

The magnetization curves along the crystal axes for Gd2Fe17 and Gd2Fe17H3 were analysed based on the single-ion model. If the Gd–Fe exchange interaction has been taken as isotropic as usual, the fitted values of magneto-crystalline anisotropy of the Fe sublattices in Gd2Fe17 and Gd2Fe17H3 would become unreasonably different from those of the corresponding Y or Lu compounds. It was shown that the large difference is caused by the neglect of the anisotropy of the Gd–Fe exchange interaction.