• Journal of Magnetics
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• 제8권4호
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• pp.157-159
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• 2003

The strengths of the inter-grain exchange interaction were evaluated for nanocrystalline $Nd_{2.33}Fe_{14}B_{1.06}Si_{0.21}$ magnets of different grain size by comparing the $_{i}H_{c}$ calculated by micromagnetics with the experiments. With increase of the grain boundary thickness to that of the magnet of grain diameter 12.4, 24.8, 37.2 and 49.6 nm, the strengh of the interaction in reference to that without the grain boundary phase decreases to 83％, 69％, 54％ and 42％.

• 한국자기학회지
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• 제5권5호
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• pp.631-635
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• 1995

Longitudinal magnetic recording process of thin film media was simulated by the 2-D finite element method (FEM). To describe precisely the hysteresis behavior of thin film media, scalar Preisach model was used. In this paper, we discussed the formation of bit patterns and importance of modeling of minor loops in high density recording. The effects of the media coercivity and film thickness on the remanent magnetization and transition shape were investigated.

• 한국자기학회지
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• 제5권5호
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• pp.421-426
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• 1995

The magnetic properties of the hot-pressed magnets made from the Fe-Nd-B alloys, mechanically ground and subsequently blended with binary additives such as Al-Cu and Ag-Zn before hot pressing, were investigated. The coercivities of the magnets increased as the concentration of Al-Cu increased up to 1 wt.% or up to 3 wt.% in the case of Ag-Zn. At higher concentrations the coercivities decreased markedly. The maximum gain in coercivity by the addition was about 20 %. typical values of $_{i}H_{c}$ and $B_{r}$ of a hotpressed magnet containing 1 wt.% Al-Cu were 18 kOe and 7 kG, respectively. It was found that Cu, Ag, and Zn, which diffused into the magnet during hot pressing, were mostly concentrated on the Nd-rich grain boundary phase whereas Al was present not only in the grain boundary region but also in the matrix grains.

• Journal of Magnetics
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• 제18권1호
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• pp.1-4
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• 2013

Structural and magnetic properties of $Fe_{50}Mn_{50}$ nanocrystalline alloys prepared by the mechanical alloying technique (using commercial Fe and Mn powders as the precursors) are studied as a function of milling time, 1 hr to 48 hrs. The nano-crystallite size and shape are examined by using scanning electron microscopy (SEM). The effect of milling time on structural characterization was investigated using X-ray diffractometer (XRD) and extended X-ray absorption fine structure spectroscopy (EXAFS). Both XRD and EXAFS studies showed that the alloying process should be completed after 36 hrs milling. Concerning the magnetic behavior, the data obtained from superconducting quantum interference devices (SQUID) exhibited both magnetic saturation ($M_s$) and coercivity ($H_c$) depend strongly on the milling time, which are related to the changes in the crystallite size and magnetic dilution.

• 한국재료학회지
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• 제9권8호
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• pp.831-836
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• 1999

RF 스퍼터링법으로 제작한 비정질 (Fe(sub)1-xCo(sub)x)(sub)89Zr11 자성박막의 자기특성을 Co농도에 따라 조사하였다. 130 Oe의 인가자기장중 190~20$0^{\circ}C$ 에서 10분간 2단 열처리를 행하였을때, 시편중 x=0.4의 박막은 높은 자왜를 나타냄에도 불구하고 0.25 Oe의 낮은 보자력 H(sub)c과, 8.7 MHz, 10 mOe의 여기자기장에서 측정된 미분투자율 $\mu$(sub)d는 280 정도로 가장 양호한 값을 나타내었다. 이와 같은 거동은 박막내 최적의 압축응력상태에서 야기된 자기이방성의 변화에 기인한다.

• 한국결정성장학회지
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• 제8권4호
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• pp.655-659
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• 1998

과냉각된 melt-spun $Nd_{14.73}Fe_{78.67}B_{6.60}$리본의 급속응고속도 및 열처리에 따른 보자력의 변화를 조사하였으며, 이 결과를 분석하기 위하여 비정질로 부터의 결정화 거동을 연구하였다. 리본 제조시 급속응고속도가 증가함에 따라 결정화 후의 고유 보자력은 감소하였다. $Nd_{14.73}Fe_{78.67}B_{6.60}$ 비정질은 안정한 $Nd_2Fe_{14}B$ 상으로 결정화하는 과정에서 $\alpha$-Fe와 $Fe_3B$ 상이 생성된다. $Fe_3B$ 상은 $Nd_2Fe_{14}B$ 상이 결정화됨과 동시에 사라지나 $\alpha$-Fe 상은 완전히 결정화된 이후에도 기지상인 $Nd_2Fe_{14}B$ 상과 함께 잔류한다. 열처리된 리본내에 잔류하는 연자성상인 $\alpha$-Fe에 의해 고유 보자력의 특성이 저하하였다.

• 한국자기학회지
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• 제12권2호
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• pp.57-63
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• 2002

Nd-Fe-B계 소결자석의 특성을 향상시키기 위해서는 Nd-Fe-B계 합금의 조성 및 제조공정을 조절하여 자성분말의 입도 및 입도분포, 강자성상인 N $d_2$F $e_{14}$B상의 분율, 자성분말의 배향도, 산소 함량, grain size 등과 같은 factor들을 최적화 하여야 한다. 본 연구에서는 실험실 규모로 Nd-Fe-B계 합금 조성 및 공정 조절을 통하여 Nd-Fe-B계 소결자석을 제조하는 연구를 수행하였으며, 분쇄매체 분쇄시간 및 ball size에 따른 Nd-Fe-B계 소결자석의 자기적특성을 분석하여 최적의 분쇄조건을 조사하였다. 또한 분쇄공정 중 FeGa합금을 첨가하여 잔류자속밀도의 감소없이 Nd-Fe-B계 소결자석의 보자력을 향상시킬 수 있었다. 이와 같은 분쇄 조건의 연구, FeGa 합금에 의한 보자력 향상, 건식분쇄 방법 및 powder blending 공정을 적용하여 잔류자속밀도( $B_{r}$,) : 14.4kG, 보자력($_{i}$ $H_{c}$) : 9.4kOe, 최대자기에너지적((BH)$_{\max}$) : 47 MGOe의 자기적 특성을 갖는 Nd-Fe-B계 소결자석을 제조하였다.

• 한국자기학회지
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• 제5권1호
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• pp.38-41
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• 1995

$\alpha$-Fe를 주상으로 하는 새로운 Nd-Fe-B계 합금을 개발하기 위하여 Nd 함유량을 4at.%로 고정시킨 Nd-Fe-B 초미세결정립합금의 제조 및 자기특성이 조사되었다. 급속응고법으로 제조된 $Nd_{4}Fe_{85.5}B_{10.5}$ 비정질합금은 결정화하여 $\alpha$-Fe 기지상에 $Nd_{2}Fe_{14}B$이 형성되나 자기특성${_{i}H_{c}=95.5kA/m(1.2kOe),\;Br=1.2T}$은 열화된다. Nb 및 Cu를 첨가한 $Nd_{4}Fe_{82}B_{10}Nb_{3}Cu_{1}$ 합금은 $\alpha$-Fe 결정립미세화(<30nm)로 보자력이 207kA/m(2.6 kOe)로 증가하나 잔류자화는 개선되지 않았다. 이 합금조성에 8at.% Co 첨가는 결정립을 더욱 미세화시키며 자기특성을 개선 시킨다. 최적열처리조건에서 $Nd_{4}Fe_{74}Co_{8}B_{10}Nb_{3}Cu_{1}$ 합금의 잔류자화, 보자력 및 최대에너지적이 각각 1.34 T, 219 kA/m (2.75kOe) 및 $95.5kJ/m^{3}$(12MGOe) 이다.

• 한국분말재료학회지
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• 제9권5호
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• pp.359-364
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• 2002

We report the structure, thermal and magnetic properties of a non-equilibrium $Al_{0.6}(Fe_{50}Cu_{50})_{0.4}$ alloy powder produced by rod milling and chemical leaching. An X-ray diffractometry(XRD), a transmission electron microscope(TEM), a differential scanning calorimeter(DSC), a vibrating sample magnetometer(VSM), and superconducting quantum interference device(SQUID) were utilized to characterize the as-milled and leaching specimens. The crystallite size reached a value of about 8.82 nm. In the DSC experiment, the peak temperatures and crystallization temperatures decreased with increasing milling time. The activation energy of crystallization is 200.5 kJ/mole for as-milled alloy powder. The intensities of the XRD peaks of as-milled powders associated with the bcc type $Al_{0.5}Fe_{0.5}$ structure formative at $350^{\circ}C$ sharply increase with increasing annealing temperature. Above $400^{\circ}C$, peaks alloted to $Al_{0.5}Fe_{0.5}$ and $Al_{5}Fe_{2}$ are observed. After annealing at $600^{\circ}C$ for 1h, the leached Ll specimen transformed into bcc $\alpha$-Fe and fcc Cu phases, accompanied by a change in the structural and magnetic properties. The saturation magnetization decreased with increasing milling time, and a value of about 8.42 emu/g was reached at 500 h of milling. The coercivity reached a maximum value of about 142.7 Oe after 500 h of milling. The magnetization of leached specimens as function of fields were higher at 5 K, and increased more sharply at 5 K than at 100 K.

• 한국자기학회지
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• 제5권5호
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• pp.366-371
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• 1995

High performance magnetic materials are characterized by the combination of outstanding magnetic properties and optimized microstructures, e.g., nanocrystalline composites of multilayers and small particle systems. The characteristic parameters of the hysteresis loops of these materials vary over more than a factor of $10^{6}$ with optimum values for the coercive field of several Tesla and permeabilities of $10^{6}$. Within the framework of the computational micromagnetism (nanomagnetism) using the finite element method the upper and lower bounds of the coercive field of different types of grain ensembles and multilayers have been determined. For the case of nanocrystalline composites the role of grain size, exchange and dipolar coupling between grains and the degree of grain alignment will be discusses in detail. It is shown that the largest coercivities are obtained for exchange decoupled grains, whereas remanence enhancing requires exchange coupled grains below 20 nm. For composite permanent magnets based on $Nd_{2}Fe_{14}B$ with an amount of ~ 50% soft $\alpha$-Fe-phase coercivities of ${\mu}_{0}H_{c}=0.75\;T$, a remanence of 1.5 T and an energy product of $400\;kJ/m^{3}$ is expected. In nanocrystalline systems the temperature dependence of the coercivity is well described by the relation ${\mu}_{0}H_{c}=(2\;K_{1}/M_{s}){\alpha}-N_{eff}{\mu}_{0}M_{s}$, where the microstructural parameters $\alpha$ and $N_{eff}$ take care of the short-range perturbations of the anisotropy and $N_{eff}$ is related to the long-range dipolar interactions. $N_{eff}$ is found to follow a logarithmic grain size size dependence ${\mu}_{0}H_{c}=(2\;K_{1}/M_{s}){\alpha}-N_{eff}(\beta1nD){\mu}_{0}M_{s}$. Several trends how to achieve the ideal situation $\alpha$->1 and $N_{eff}$->1->0 will be discussed.