$L1_0$ FePt thin films have been subject to intensive studies in an effort to develop high density recording media. However, the magnetic domain structure of the FePt thin films, using Lorentz transmission electron microscopy(LTEM), has rarely been studied despite of its importance in controlling the media noise. Most of LTEM works were confined to soft magnetic thin films. This is partly because that it is usually difficult to interpret the complex LTEM image of a magnetic thin film media which is lack of information on the magnetization distribution. In this work, micromagnetics simulations were performed using the package, OOMMF(the object oriented micromagnetic framework), to calculate the Fresnel contrast during the magnetization reversal process. In addition, calculation results strongly suggested that the magnetization reversal in ordered FePt:C thin film occurs through a local magnetization rotation to form a series of small local vortices at the domain boundaries. The propagation of reversed domains occurred not by the movement of vortices but by the annihilation of old vortices and the formation of new local vortices at the domain boundaries. Calculated contrast results of magnetic domain are in good agreement with the experimental results. This consistency shows that domain structure of FePt:C thin film can be understood by interacting Stoner-Wolfarth model.
The ordered $L1_0$ FePt granular film is a potential candidate for ultra high density recording media because of its high magneto-crystalline anisotropy constant. This film is, however, known to be prone to the occurrence of excessive grain growth during order-annealing and to the high transition noise because of lack of magnetic granular isolation. To solve this problem, 3rd non-magnetic elements(C, Ag and Oxides) are added in FePt thin film. The addition of 3rd elements greatly reduced both the grain growth and ordering kinetics. In order to understand the role of carbon in the grain growth and ordering kinetics, a modified Monte-Carlo model, which was optimized for grain growth with ordering, was applied to calculation. The Monte Carlo simulation study indicated that the decrease of grain growth and ordering kinetics is primarily due to a continuous decrease of the mobility of order-disorder inter-phase with the progress of ordering reaction. A stable 2-phase structure was established in the case of low inter-phase mobility. This was because the grains of one phase come to be in contact with the grains of the other phase and become inter-locked by the low mobility interfaces. This is responsible for the formation of a fine grain size distribution with the carbon addition. Experimental results show good agreements with this model.
The main advantage of the $L1_0$ FePt granular film is its extremely high-anisotropy-field about 11 T at fully ordered phase. However, this main advantage is also a crucial technical problem. Because there is no practical transducer can produce such a high field. To be utilized in recording applications, the $L1_0$ FePt films must be partially ordered, that is two phase structure. From the Fresnel Lorentz Contrast calculation study, the effects of 2-phase structure on micromagnetic structure were investigated to know both the type of magnetic structure and the size of magnetic domain(or cluster). The disordered particles in ordered system act as nucleation sites for reversed domain and produce induced reversed-ordered-domain at the phase-boundary by magnetostatic field. Therefore, the width of reversed domain was correlated with both the width of average soft particles and the number of soft particles. The results suggest that the cross-track magnetic cluster size can be controlled by optimizing the amount of disordered phase and the size of disordered phase. Small amount of disordered phase, less than 10 vol.%, can make narrow domain structure and ring-type dot structure without significant reduction of coercivity squreness (~ 0.8).
The cross-track magnetic cluster size has been known as an important factor of the media noise. A new method was developed based on the cross-track magnetization correlation coefficient to calculate the magnetic cluster size. The cluster size from this method is more realistic than that of previous reports. This determination clearly assists to interpret Lorentz calculation image because it is extremely difficult to judge the domain wall contrast from experimental Fresnel Lorentz image in the high $K_u$ thin film. In this method, the relationship between magnetic cluster size and various parameters, such as Ku, A, $D_{grain}$, was discussed both in longitudinal media and in perpendicular media.
All these subjects and results are useful to understand intrinsic micromagnetic structure of FePt thin film, its Lorentz contrast. and magnetization fluctuations with cross-track magnetic cluster.
본 논문은 FePt 자성박막의 미세조직 및 자화조직에 대한 전산모사 연구이다. FePt 자성박막의 미세조직 연구를 위해 Monte-Carlo 전산모사법을 사용하였다. 결정립성장과 상변화를 동시에 다룰 수 있도록 수정된 model을 사용하였으며 이 때, 사용된 기본가정은 carbon의 결정립계 편석에 의한 상계면의 이동도가 제한된다는 것이다. 실험적으로 관찰된 300℃에서 증착된 $FePt(20nm)/C(4nm)_n(n=0,1,4)$ 자성박막의 열처리에 따른 결정립크기, 결정립크기분포, 결정립성장거동과 일치하였다. n이 증가할수록, 최종결정립크기는 감소하고 결정립크기분포가 좁아질 수 있으며 결정립성장이 크게 억제되어 최종적으로 안정한 2상 구조를 형성하는 것이다. 이로부터 FePt 자성박막에 carbon이 첨가되는 경우, 안정한 2상구조의 형성이 가능하다는 것을 규명하였으며 이러한 영향은 carbon의 상계면으로의 편석현상으로 설명될 수 있음을 보였다.
FePt 자성박막의 자화조직을 전산모사하기 위해 micromagnetics 결과를 이용하여 Fresnel contrast를 계산하는 방식을 사용하였다. 이 방식을 사용하는 경우, 결정립에 의한 contrast가 없는 ideal한 domain wall을 가시화할 수 있다. 계산결과, 자화상태에 따라 극명하게 다른 자화조직을 나타내었으며 ordering 정도에 따라 자화조직은 미세화됨과 동시에 천이되었다. 특히, dc-demag. state에서 다른 자화상태에 비해 가장 조대화된 자화조직을 나타내었으며 $K_{eff}$값에 따라 vortex-antivortex network 타입의 자화조직에서 elongated reversed domain 구조를 거쳐 ring-type dot 구조로 천이된다. vortex-antivortex network 타입의 자구조직은 자화반전거동시‘S'-shape의 ripple로부터 형성된 vortex와 antivortex의 이동에 의해 나타났으며 elongated reversed domain구조는 국부적으로 반전된 자화벡터에 의한 일방향으로의 계속되는 국부적인 vortex의 생성과 소멸과정으로 나타나는 것이었음을 규명하였다. 40vol.% 첨가된 FePt:C박막의 증착온도에 따른 자화조직과 대부분 일치하는 모습을 보였으며 이는 2가지를 의미한다. 첫째 FePt:C 박막의 자화조직은 interacting Stoner-Wohlfarth model로서 이해될 수 있다는 점. 둘째 FePt:C 박막에서 실제로 존재하게 될 2상구조의 자화조직은 effective $K_u$값을 갖는 단상구조의 자화조직으로 표현될 수 있다는 점이다.
FePt 박막의 자화조직과 media noise관점에서 magnetic cluster size는 그것의 기본을 이루는 단위이다. 따라서 이것의 변화를 감지하는 것은 자화조직과 media noise를 이해하는 데에 큰 도움을 준다. 본 연구에서는 cross-track correlation coefficient값을 이용하여 실제적으로 관찰되는 magnetic cluster size를 측정할 수 있는 방법을 개발하였다. magentic cluster size는 $K_{eff}$값이 증가할수록, A값이 감소할수록, 특정 범위의 결정립크기 $(const. A > 0.5\times10^{-6}erg/cm)$에서 최소화됨을 보였다. 한편, 10 vol.% 이하의 최소화된 fcc particle이 존재하는 경우, 보자력이방성의 큰 감소없이 magnetic cluster size는 급격히 미세화됨을 알 수 있었다.
