TM-TbCo/RE-TbCo/Glass雙層熱寫磁讀薄膜與FePt-SiNx/Glass顆粒狀薄膜的磁性質及微結構研究

Abstract

在TM-rich TbCo/RE-rich TbCo雙層薄膜方面，是以磁控濺鍍的方式，於康寧基板上鍍製Tb17.5Co82.5 / Tb29.5Co70.5雙層薄膜。首先，固定TM-rich之Tb17.5Co82.5薄膜之厚度，分別為20 - 90 nm，並針對每個固定厚度的Tb17.5Co82.5薄膜而改變Tb29.5Co70.5薄膜的厚度(20 - 90 nm)。反之，固定RE-rich之Tb29.5Co70.5薄膜之厚度，分別為20 - 90 nm，並針對每個固定厚度的Tb29.5Co70.5薄膜改變Tb17.5Co82.5薄膜的厚度(20 - 90 nm)。藉由改變Tb17.5Co82.5 及Tb29.5Co70.5磁性層厚度來探討此疊合後的雙層薄膜之磁性質變化。 藉由TM-rich TbCo/RE-rich TbCo雙層薄膜間的交換耦合效應，將使得讀出層(TM-rich)的矯頑磁力大幅提昇。當讀出層厚度固定且記錄層(RE-rich)厚度大於50 nm時，各雙層結構才會有明顯的exchange coupling現象產生。當讀出層固定在70 nm之Tb17.5Co82.5 (90 nm)/ Tb29.5Co70.5 (90 nm)雙層薄膜，其讀出層的矯頑磁力由單層Tb17.5Co82.5的325 Oe上升到12 kOe，此時可得到最大的interface wall energy約5 erg/cm2。反之，當記錄層厚度固定在50 nm以上時，各雙層結構才會有明顯的exchange coupling現象產生。當記錄層厚度固定在90 nm時之Tb17.5Co82.5 (20 - 90 nm)/ Tb29.5Co70.5 (90 nm)雙層薄膜，其interface energy皆維持在約6 erg/cm2。此外，當Tb29.5Co70.5薄膜厚度固定為90 nm時並改變Tb17.5Co82.5薄膜厚度(50 - 90 nm)時，其雙層膜之Hc⊥值會隨著溫度的上升由25 ℃的約7 kOe下降到350 ℃的低於500 Oe，此矯頑磁力適合於磁頭的寫入。此外，Tb17.5Co82.5 (20 - 90 nm)/ Tb29.5Co70.5 (20 - 90 nm)雙層膜之飽和磁化量皆大於200 emu/cm3，此值有利於巨磁阻 (GMR)磁頭讀取。 在FePt薄膜方面，是以磁控濺鍍的方式，於康寧基板上鍍製FePt薄膜。藉由改變不同FePt厚度、退火溫度及時間，來獲得FePt垂直膜面異向性。 當FePt薄膜濺鍍速率為0.33 nm/min並於700 oC退火30分鐘後，可得到較強的fct-FePt (001)及fct-FePt (002)序化相繞射峰。當FePt薄膜為10 nm時並於700 oC退火30分鐘後，可得到最佳之垂直膜面角型比約為1，且在此條件下之FePt薄膜，有最大之垂直膜面矯頑磁力約為20 kOe 。所以我們將以10 nm 之FePt薄膜作為後續添加SiNx之FePt- SiNx granular 薄膜為基礎。 在(FePt)1-y(SiNx)y顆粒狀薄膜方面，是以磁控濺鍍的方式於康寧玻璃基板上將Fe、Pt及SiNx靶分別至於同心圓上進行共鍍，其中Fe與Pt元素是以直流控制器(dc)鍍製，SiNx元素是以交流控制器(rf)鍍製。藉由改變不同SiNx含量及退火時間，來獲得分佈均勻的FePt-SiNx顆粒狀薄膜。 (FePt)1-y(SiNx)y薄膜經700 oC退火30分鐘後，當SiNx含量由0 vol.%增加到63 vol.%時，(FePt)1-y(SiNx)y薄膜的行為可分成三個階段。首先，當SiNx含量小於15 %時，薄膜在真空中退火時進入薄膜內的熱能比較不容易在冷卻時以傳導方式由薄膜內部快速散出，使得所形成的fct-FePt相比較完美而造成矯頑磁力的下降。在此階段微結構型態的改變皆是由island growth轉變成continuous growth的趨勢，磁化反轉機制將由domain rotation轉變成domain wall motion，使得矯頑磁力降低。第二階段：當SiNx含量大於15 vol.％時，由於基底的SiNx含量已大幅增加造成domain wall移動的阻礙增加(pinning sites)，且在此階段pinning sites效應大於完美fct-FePt相的形成而造成矯頑磁力大幅度的上升。另一方面，在此階段FePt晶粒已經開始被SiNx較均勻的分離，這也造成FePt晶粒開始縮小而有較多的單磁區粒子產生，使得(FePt)1-y(SiNx)y薄膜隨著SiNx含量增加而有較高的矯頑磁力。第三階段矯頑磁力的下降：當SiNx含量大於39 vol.％時，完美fct-FePt相將較SiNx含量為15 vol.％時大幅度增加，使得矯頑磁力開始下降。另一方面，此時部份的FePt晶粒已小於超順磁粒子尺寸，因此當SiNx含量為46 vol.％時其矯頑磁力會大幅度降低。

In TM-rich TbCo / RE-rich TbCo bi-layer films, Tb17.5Co82.5 / Tb29.5Co70.5 bi-layer films were deposited on the Corning glass substrate. For type I bi-layer films, the thickness of Tb17.5Co82.5 layer was fixed at 20-90 nm and the thickness of Tb29.5Co70.5 layer was changed from 20 to 90 nm for every Tb17.5Co82.5 thickness. For type II bi-layer films, the thickness of Tb29.5Co70.5 layer was fixed at 20-90 nm and the thickness of Tb17.5Co82.5 layer was changed from 20 to 90 nm for every Tb29.5Co70.5 thickness. Effects of the films thickness on the magnetic properties of Tb17.5Co82.5 (20-90 nm) / Tb29.5Co70.5 (20-90 nm) bi-layer films were investigated. The coercivity of readout layer Tb17.5Co82.5 was largely increased by the exchange coupling effect in the interface of Tb17.5Co82.5 / Tb29.5Co70.5 bi-layer films. In type I bi-layer films, the exchange coupling effects were observed in the M-H loops when the thickness of Tb29.5Co70.5 film was larger than 50 nm. The largest interface energy (5 erg/cm2) and readout layer coercivity (12 kOe) could be obtained when the thickness of Tb29.5Co70.5 film was 90 nm and the thickness of Tb17.5Co82.5 was 70 nm. In type II bi-layer films, the exchange coupling effects were observed in the M-H loops when the thickness of Tb29.5Co70.5 layer was larger than 50 nm. The large interface energy (~ 6 erg/cm2) could be obtained when the Tb29.5Co70.5 film was fixed at 90 nm for Tb17.5Co82.5 (50 - 90 nm)/ Tb29.5Co70.5 (90 nm) bi-layer films. For the Tb17.5Co82.5 (50 - 90 nm)/ Tb29.5Co70.5 (90 nm) bi-layer films, the out of plane coercivities (Hc⊥) of these films decreased form about 7 kOe to about 500 Oe as the temperature increased from 25 ℃ to 350 ℃. Moreover, the saturation magnetization (Ms) of type I and type II bi-layer films are all larger than 200 emu/cm3. These films have potential to be applied on Heat Assisted Magnetic Recording media. In FePt films, different thicknesses of FePt films were deposited on the Corning glass substrates. In order to obtain the perpendicular magnetic anisotropy of FePt films, the thickness, annealing temperature and annealing time of the FePt films were varied. It is found that the stronger diffraction peaks of fct-FePt (001) and fct-FePt (002) in X-ray diffraction patterns could be obtained when the sputtering rate of FePt films is 0.33 nm/min and the FePt films are annealed at 700 oC for 30 minutes. The largest out-of-plane squareness (S┴) (about 1) and out-of-plane coercivity (Hc⊥) value (about 20 kOe) could be obtained as the 10 nm FePt film annealed at 700 oC for 30 minutes. This 10 nm FePt film will be added SiNx to form the FePt- SiNx granular films in the further. In (FePt)1-y(SiNx)y films, (FePt)1-y(SiNx)y (y=0 ~ 63 vol.%) nanocomposite thin films were fabricated by dc and rf magnetron co-sputtering of Fe, Pt and SiNx targets, then annealed at different temperatures and times. As the SiNx content increases from 0 vol.% to 63 vol.%, the behavior of the (FePt)1-y-(SiNx)y films could be separated into three sections. First, as the SiNx content is smaller than about 15 vol.%, the FePt particles are surrounded by the insulator SiNx, which is a poor heat conductor. Therefore, the partial of fcc-FePt phase of as-deposited film will transform to perfect fct-FePt phase after annealing and leads to the decrease of the coercivity. From a field emission gun high resolution transmission electron microscope (FEG-TEM) images show that the growth mode of the film tends to change from isolate to continuous films, then the magnetization mechanism may transfer from domain rotation to domain wall motion, hence the coercivity decrease as SiNx content is increased. Second, as the SiNx content increases from 15 vol.% to about 39 vol., the SiNx will impede the reverse of the spin moments of FePt. Therefore, the high Hc┴ and in-plane coercivity (Hc//) values are obtained due to pinning sites effect of SiNx. As the SiNx content increases from 39 vol.% to 46 vol.%, the Hc┴ value decreases from 21.2 to 16.3 kOe. The fcc-FePt phase of as-deposited film will transform to more perfect fct-FePt phase than that of 15 vol.% SiNx after annealing and leads to the decrease of the coercivity. As the SiNx content is higher than 46 vol.%, the Hc┴ and Hc// of (FePt)1-y-(SiNx)y films decrease rapidly as the SiNx content is increased. From the FEG-TEM image of (FePt)45-(SiNx)55 and (FePt)37-(SiNx)63 films, it is found that some particles are smaller than 5 nm which are smaller than Dp (minimal stable particle diameter). Therefore, the thermal agitation effect will cause the Hc┴ and Hc// of (FePt)1-y-(SiNx)y films decrease rapidly. The Hc┴, Hc//, S┴, and in-plane squareness (S//) values of the (FePt)37-(SiNx)63 film are 6.5 kOe, 6.3 kOe, 0.75, and 0.45 respectively. And a uniform particle size distribution granular (FePt)37-(SiNx)63 film with an average particle size about 5 nm was obtained. This (FePt)37- (SiNx)63 film is a good candidate for application on high density perpendicular magnetic recording media.