不同厚度下NiFe/CoFe兩個鐵磁共振態的變化

Abstract

根據我們的實驗結果，NiFe/CoFe的雙層膜在總厚度≧48.5 nm，頻 率>8 GHz的情況下可以觀察到兩個鐵磁共振(ferromagnetic resonance)的吸收峰，分別稱作光學模式(optical mode)和聲學模式(acoustic mode)。其中的光學模式的交換耦合磁場(exchange coupling field,H_ex)與NiFe和CoFe厚度的關係是，當NiFe厚度固定在43.5 nm並且CoFe厚度有改變時，H_ex ∝ t_CoFe的-0.6次方;而當CoFe厚度固定在30 nm並且NiFe厚度有改變時，H_ex ∝ t_NiFe的-1.88次方。藉由分析交換耦合磁場和有效磁化強度(effective magnetization, M_eff )，發現光學模式主要是CoFe磁矩進動(precession)的貢獻，而巨大的H_ex可用自旋駐波(standing spin wave)來解釋。 聲學模式的FMR訊號在NiFe厚度大於28.5 nm時M_eff幾乎是由NiFe的 磁矩貢獻；當NiFe厚度小於28.5 nm時，M_eff有NiFe和CoFe的貢獻。H_ex是由與磁矩同方向的，由兩種材料介面的磁矩釘作用(magnetization pinning)產生的有效磁場所造成的[1]。聲學模式的H_ex會隨CoFe和NiFe厚度增加而增大。兩個吸收峰大小隨著頻率變動的方式印證了過去其他人的理論預測，並可由微波在導體中的集膚效應(skin effect)來解釋[2]。量測磁滯曲線(hysteresis)的結果，飽和磁化強度(saturation magnetization)和矯頑力(coercivity, Hc)隨厚度的變化與理論值並不吻合，但有著相似的趨勢。

In our findings, when total thickness of NiFe/CoFe bilayer is ≧48.5 nm and frequency > 8 GHz, two ferromagnetic resonance peaks, which are called optical mode and acoustic mode, can be observed. The relation of exchange coupling field (H_ex) of optical mode and thickness of CoFe and NiFe is that when NiFe thickness is fixed at 43.5 nm and we vary CoFe thickness, H_ex ∝ t_CoFe to the power of -0.6; when CoFe thickness is fixed at 30 nm and we vary NiFe thickness, H_ex ∝ t_NiFe to the power of -1.88. By analyzing H_ex and effective magnetization (M_eff), it is found that in optical mode, FMR signal is mainly contributed by CoFe, and the large H_ex can be explained by standing spin wave. In acoustic mode, when NiFe is thicker than 28.5 nm, FMR M_eff is mainly contributed by NiFe; when NiFe is thinner than 28.5 nm, FMR M_eff is contributed by NiFe and CoFe. H_ex is attributed to interface magnetization pinning, which provides a field that is parallel to the orientation of magnetization[1]. H_ex of acoustic mode increases with CoFe and NiFe thickness. The change of absorption peak amplitude with frequency meets previous theoretical calculation, and this behavior can be explained by skin effect of microwave in conductor[2]. The results of hysteresis loops show that the saturation magnetization Ms and coercivity Hc of different thickness do not fit well with the theory, but follow similar trends.