利用電壓及氫化反應可逆地控制材料磁性

Abstract

在我們的電壓控磁的研究中顯示了，在鐵/氧化鋅結構中，可以藉由適當的電壓促使鐵的矯頑場減小15%–20%，而且只要一移除電壓，矯頑場就會回復至原本的大小。另外，如果施加過量的電壓，則可以導致鐵/氧化鋅介面上的鐵氧化成氧化鐵、三氧化二鐵或四氧化三鐵等不同的氧化態，並且使樣品的矯頑場上升。各種氧化態的比例會取決於鐵和氧化鋅的厚度比例，且一旦氧化後是不可回復的。但就算發生了介面氧化，還是可以藉由適當的電壓來調降矯頑場的大小。 在氫化控磁的研究中，我們觀察了鈷-30%鈀-70%的合金在不同氫氣氣壓下的磁性變化。原本無法觀察到磁域的樣品，在足夠的氫氣壓力下(0.2 bar)，會在樣品磁性翻轉時觀察到磁域，同時磁致曲線也會變得方正。我們依據在不同的固定外加磁場和氫氣濃度下所畫出的磁化翻轉曲線，可以推論出最小的翻轉單位Barkhausen volume 會隨著氫氣壓力上升而變小。同時，氫氣壓力上升也會使得樣品的脫釘磁場變大，導致磁致曲線的矯頑場在0.2 bar以上的氫氣壓力會逐漸增加。 此外，我們也研究了氫氣對於多層膜磁性耦合的影響。在Pd/Fe/Pd/Fe/MgO(001) 中，也許是基板表面傾斜的緣故，第一層的鐵只有單軸磁異相性，到了第二層的鐵才出現雙軸磁異相性。當外加磁場和第一層的磁易軸的夾角接近90度時，兩個鐵磁層之間的耦合力會和外加場競爭。當外加場不夠強時，上層的鐵磁會被下層的鐵磁吸引並翻轉致同磁化方向。而且當樣品吸附氫氣後，鐵磁耦合會增強，需要更大的外加場才能打破耦合。因此當外加場在6 Oe時，氫氣的脫吸附能使上層的鐵磁在外加場和底層鐵磁之間扭轉。

In our study, the magnetic coercivity (Hc) of Fe/ZnO heterostructure was significantly enhanced by 2–3 times after applying a suitable current. This Hc enhancement originates from the Fe-oxidation at the Fe/ZnO interface induced by direct current heating. Depth-profiling X-ray photoemission spectroscopy analysis confirmed the formation of FeO, Fe3O4, and Fe2O3 close to the interface region, depending on the Fe thickness. Furthermore, the magnetic coercivity of Fe/ZnO heterostructure monotonically decreased as a relatively small voltage was applied. The reversibility of this effect was demonstrated by cyclically changing the bias voltage from 0 to 6–9 V; the Hc decreased 15%–20%. As thick Fe-oxide gradually formed at the interface by using a larger direct current heating, the Hc increased and the Fe/ZnO heterostructure still demonstrated a similar voltage-induced the reduction of Hc. In part two, the hydrogenation effect on the magnetic domain formation and domain wall velocity of 25 nm Co30Pd70 alloy thin films grown on SiO2/Si(100) substrates was investigated using magneto-optical Kerr microscopy. There was no domain wall motion observed in vacuum, but the nucleation and domain wall motion was formed and intensified with hydrogen pressure increased. Not only the domain formation but also the reversal motion was changed. Finally, series of reversal time constant τ, Barkhausen volume V, and depinning field under various magnetic field and hydrogen pressure were deduced from the reversal curve fitting and domain wall velocity. In part three, we deposited Pd/Fe/Pd/Fe multilayer film on MgO(001) substrate. The double loop was due to the pinning of bottom Fe uniaxial easy axis which was 90 degrees to the magnetic field instant of the antiferromagnetic coupling of two Fe layer. The hydrogenation would enhance the magnetic coupling of two Fe layers. If we set the magnetic field at appropriate magnitude, the magnetism of top Fe would be switched between extra field and bottom during the absorption or desorption of hydrogen.