【作者】 查超麟；

【导师】 干福熹； 金庆原；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2006， 博士

【摘要】 信息时代的今天，磁记录是主要的存储手段。人们在追求大容量硬盘的同时对记录密度提出了更高的要求。业界只有通过利用新材料、新的磁学现象来开发新的记录方式来满足大众的需求。希捷与日立目前走在前列，这两家公司所提出的热辅助磁记录（Heat Assisted Magnetic Recording，HAMR）技术有望成为垂直记录的取代者，并让每平方英寸1Tbit的存储密度成为可能。本论文就是在这种背景下展开研究的，对热辅助磁存储的介质材料制备、性能和复合介质结构方面作一个深入的研究，提出了基于L1₀ FePt纳米颗粒膜的复合结构介质膜。本论文主要包含以下几个内容：第一，我们在实验上探索了用非外延的方式来获得c轴垂直取向的L1₀ FePt薄膜。在Si衬底上覆盖一层超薄的1 nm厚的FeO_x层，然后将6 nm的FePt沉积在它上面。通过控制退火条件，可以得到c轴垂直取向的高度有序化的L1₀ FePt膜。通过HRTEM，我们发现室温下生长得到的FeO_x层其结晶不好，而退火之后转变为晶格相，结晶变好，而室温下沉积得到的FePt是多晶的（111）织构，在600℃下10min退火后转变为（001）织构，同时完成了从A1相到L1₀相的有序化过程。通过1 nm的FeO_x和Fe作为底层的对比研究发现，沉积态下他们都是（111）织构，而600℃退火后，他们都获得了完全有序化的L1₀相，可是只有FeO_x才能使得FePt完全（001）织构，而Fe层上的FePt除了获得部分（001）织构外，还有（111）织构。从HRTEM的剖面图知道，这个差别应该起源于退火过程中FeO_x层和FePt层互溶很小，始终保持清晰的界面，而Fe层在退火过程中和FePt互溶形成合金态，界面不清晰。而后来的实验表明FeO_x或是Fe层在室温下是（110）织构，而经过600℃退火后转变为（200）织构。从而说明在退火过程中，FePt在FeO_x层和Fe层上的c轴取向差别主要是由于受底层影响的程度不同而导致的。FeO_x层在转变为晶态的过程中促进了FePt的取向，Fe层由于互溶而没有使得FePt完全（001）织构。同时，我们研究了FePt室温下沉积在不同的衬底：MgO（200），SrTiO₃（200），Si（100）上，其中Si上覆盖了一层1 nm厚的FeO_x。在完全相同的沉积条件和退火条件下，由XRD测试发现，相同温度下的有序度在SrTiO₃上最低，FeO_x层上最大，MgO处于中间；然而FePt c轴方向晶格参数刚好和有序度相反，在SrTiO₃上最大，而FeO_x层上最小，MgO处于中间。通过研究发现这是由于应力不同导致这一结果；第二，为了改善FePt高矫顽力的写入能力，我们试着制备了FePt的c轴和法线有一定夹角的薄膜。实验上使用经化学各向异性腐蚀的Si片作为衬底（其表面呈金字塔织构），研究发现L1₀ FePt的c轴由不同取向组成，主要是垂直取向，而其他的取向倾斜于表面。当外磁场沿着垂直方向磁化时，易轴倾斜的晶粒先翻转，然后带动整个薄膜磁化翻转。在金字塔Si片上获得了小矫顽力，合适的矫顽力斜率α和高矩形比的L1₀ FePt，而且矫顽力在倾斜的方向上增加；第三，改善FePt高矫顽力的另一种方式就是热辅助磁记录，为了满足记录要求的温度特性，因此我们设计了一种新的交换耦合的复合结构介质薄膜：TbFeCo-FePt双层膜。由实验测试表明，双层薄膜的矫顽力随温度变化趋势可以满足热辅助记录的要求，同时我们对这种新结构的交换耦合（亚铁磁—硬磁的垂直交换耦合）进行了研究，发现他们之间存在着强耦合，同时，通过调节界面耦合强度可以控制双层膜的翻转场；最后，我们对交换耦合的复合介质（ECC）进行了研究，选择了Fe（NiFe,CoFe）—FePt双层膜作为研究的体系，他们之间是平行易轴的软磁和垂直易轴的硬磁之间交换耦合。在我们选择的软磁厚度里（0～3 nm），FePt与Fe（NiFe,CoFe）之间表现出整体的磁化行为，随着软磁层厚度的增加，矫顽力和矩形度都降低。在FePt和CoFe之间插入一个非磁的Cu层之后，随Cu层厚度的增加，其矫顽力先降低再增加，这表明随着非磁厚度增加，FePt和CoFe之间耦合程度降低。当为4 nm厚时，其矫顽力和单层FePt相当，这说明他们之间已经解耦合。更多还原

【Abstract】 Nowadays, magnetic recording, mainly in hard disk drive （HDD）, becomes the most important way to store information. The increasing need in data storage has pushed the HDD into a rapid increase both in areal density and recording speed. Though the ultra-high-density storage with the areal density larger than 100 Gbit/in² can be achieved （134 Gbit/in² has already been achieved in product） by the perpendicular magnetic recording （PMR）, the superparamagnetism, like that in longitudinal magnetic recording （LMR）, will again limit the density increase seriously when the areal density goes to larger than 500 Gbit/in . One of the new recording methods, so-called heat-assisted magnetic recording （HAMR）, has been proposed by Seagate and Hitachi, etc. companies to realize an even higher areal density beyond 1 Tbit/in² in the next 5 or 10 years. This thesis will study a candidate media material based on L1₀ FePt thin films for HAMR, and mainly includes the following contents.Firstly, microstructural and magnetic properties of co-sputtered L1₀ FePt films with different ultra-thin underlayers have been investigated. Two 1 nm thick Fe films are at first deposited on Si substrates with a natural oxidized SiO_x layer, and then one is oxidized by the oxygen plasma. The FePt films of 6 nm nominal thickness are deposited on their top （with Fe and FeO_x underlayers, respectively）, using dc magnetron co-sputtering at room temperature. After post-annealed in high vacuum environment at 600 ℃, the fully ordered L1₀ FePt with c-axis perpendicular orientation is obtained for FeO_x underlayer while the c-axis is partially perpendicular oriented for Fe underlayer with a larger coercivity. According to the high resolution Transmission Electron Microscopy （HRTEM）, Fe coalesces with FePt film for Fe underlayered sample, while in FeO_x underlayered sample FeO_x is isolated from FePt film after post-annealing. This indicates that the different orientation evolution of FePt films with FeO_x and Fe underlayers under annealing, results from the different thermal history of eutectic reaction between FePt film and underlayers.Secondly, FePt thin films with thickness of 6 nm are deposited onto three kinds of （100）-oriented single crystalline substrates: SrTiO₃, MgO and Si. For Si substrate, there is a 1-nm-thick FeO_x underlayer which is fabricated from sputtered Fe film by plasma oxidation. The annealed FePt films at 600 ℃ are all with （001） preferred orientation, and their magnetic anisotropies are perpendicular to the film plane. The L1₀ ordering process has been studied as a function of annealing temperature. It isfound that the ordering temperature of L1₀ FePt films onto Si with FeO_x underlayer is lower by 100～150 K than those films onto MgO and SrTiO₃ substrates. The stresses resulting from the lattice mismatch, difference of thermal expansion coefficients and residual depositing stress, are responsible to the ordering kinetic difference. High chemical ordered L1₀ FePt with c-axis perpendicular to the film plane has been obtained at lower annealing temperature for the film deposited on Si substrate with FeO_x underlayer.Thirdly, tetragonal L1₀ FePt films are deposited on the pyramid-type Si substrates fabricated by chemical etching process from （100） Si wafers. The films are composed of FePt grains with different easy axis orientations, most of which along substrate normal while the others tilted from the normal direction. The easy-axis tilted grains reverse their magnetization first under an out-of-plane external magnetic field, and then the whole film magnetization reverses for higher field. Small coercivity, suitable α and high squareness are obtained. And we also find that the coercivity increases when the applied field is tilted from the normal direction.Fourthly, the structure and exchange coupling in TbFeCo-FePt bilayer film have been investigated. It is found that FePt has an L1₀ structure and that the easy axis of the FePt film is perpendicular to the film plane. Results of vibrating sample magnetometer （VSM） and magneto-optical Kerr effect （MOKE） measurements show a strong perpendicular exchange coupling between the ferrimagnetic TbFeCo layer and the hard ferromagnetic FePt layer. The magnetization direction of each layer and the process of magnetization reversal are discussed in detail. The swiching field dependence on the exchange coupling has been modeled by micromagnetic simulation.Finally, we have investigated the exchange coupling between soft magnetic Fe （CoFe, NiFe） thin films and hard magnetic L1₀ FePt films, which is the coupling between in-plane and out-of-plane magnetizations. With the increasing of soft magnetic thickness, the coercivity and squareness of the bilayer decrease. And there is a critical thickness between rigid exchange coupling and spring exchange coupling. When a Cu thin film is inserted between CoFe and FePt bilayer, the coercivity is modulated by the Cu thickness, and they are decoupled when the Cu thickness is larger than a critical thickness.更多还原