多層交換結合複合粒子の磁化反転の解析

摘要

2-4層構造ECC粒子の磁化反転磁界を単純スピンモデルを用いて解析した．2層構造では，ソフト層の異方性磁界をゼロとすることで最小の規格化スイッチング磁界が得られた．また，ソフト層の飽和磁化と体積を大きくすることでより小さなスイッチング磁界とできるが，規格値の極限でも0．5であった．3層構造では第2層の規格化異方性磁界k_(h2)と飽和磁化k_(m2)の積を0．2とすることで規格化スイッチング磁界は極小値を示し，最小値0．46が得られた．このとき，スイッチング磁界の印加磁界角度依存性も最小となった．4層構造でも中間層の異方性を小さくすることでスイッチング磁界を大きく低減でき，ソフト層と隣接する層の異方性が重要であることがわかった．しかし，3層構造に比べて4層化の効果は7％ほどと小さかった．また，スイッチング磁界の印加磁界角度依存性も中間層の異方性磁界の選択で小さくできるが，非常にクリティカルであった．4層ECCのメリットはあまり大きくなく，中間層の異方性を小さくした3層構造が実際的にもより適していると結論された．%Switching field reduction and applied field dependence of the switching field of 2 to 4-layer exchange coupled composite (ECC) dots were studied. The analysis was performed based on simple spin-model representation. For 2-layer ECC dots, minimum switching field was obtained for zero anisotropy for the soft layer. The switching field was decreased with increasing saturation magnetization of the soft layer. For 3-layer ECC dots, smaller values of normalized switching field, h*, were obtained for a small H_k of the second layer, and minimum h~* of 0.46 was obtained. Smaller applied field angle dependence of the switching field was also obtained for the smallest normalized switching field structure. For 4-layer ECC dots, smaller h* values were obtained for a small H_k of the third layer regardless of the second layer H_k values. This indicated that the third layer which contacts to the soft layer plays an important role for minimizing the switching field. The smallest h* value of 0.42 was obtained which is smaller by only 7 % from that for 3-layer ECC dots. The change in the switching field with the applied field angle could be minimized similar to 3-layer ECC dots, but the choice of the anisotropy was critical. It was concluded that 3-layer ECC dots with a small H_k for the second layer is the best choice for practical applications.