Curie temperatures of CoPt ultrathin continuous films

The effects of layer thickness and thermal annealing on Curie temperature have been studied for CoPt ultrathin continuous layers in AlN/CoPt multilayer structures. It is found that there exists a critical thickness below which Curie temperature rapidly decreases due to the loss of spin-spin interactions in the vicinity of interface. After high temperature annealing, the in-plane lattice constant of CoPt film is increased and the exchange coupling parameter is decreased. Consequently, Curie temperatures decrease for some films with large thickness, compared with as-deposited state. Upon annealing at 600?^°C, CoPt undergoes ordering transformation, which also contributes to the degradation of the Curie temperature. However, when the CoPt film thickness is below 2?nm, the Curie temperature is increased after annealing. Especially for 1?nm thick film, the Curie temperature is strikingly increased from 173?^°C to 343?^°C after annealing at 600?^°C. This effect is attributed to the out-of-plane lattice deformation of CoPt thin layers in AlN/CoPt multilayer structures.     

Microstructures and magnetic properties of CoPt-TiO2 nanocomposite films prepared by annealing CoPt/TiO2 multilayers

CoPt-TiO₂ nanocomposite films were synthesized by rapid thermal annealing of CoPt/TiO₂ multilayers. The effects of annealing temperature, annealing time, Ag addition and TiO₂ volume fraction on the microstructures and magnetic properties of the CoPt-TiO₂ nanocomposite films were studied. Results showed that the ordering degree of CoPt and coercivity of CoPt-TiO₂ nanocomposites increased with annealing temperature. Increasing annealing time and Ag addition were able to increase the ordering degree and coercivity of CoPt. However, complete L1₀-ordering of CoPt at 550 °C annealing was not realized by increasing annealing time up to 30 min and Ag addition up to 30 vol.%. Increasing TiO₂ volume fraction at 700 °C annealing did not lead to the change of ordering of CoPt. However, the grain structure of the films changed slightly when TiO₂ volume fraction was larger than 56%. The coercivity of the film decreased slightly with the addition of TiO₂.     

Realization of a large magnetic moment in the ferromagnetic CoPt bulk phase

Changes in the magnetic moment and other physical properties of a CoPt alloy induced by a new type of ion-beam mixing in an external magnetic field were investigated. This process induces the formation of a metastable phase through extremely rapid quenching from well above the ordering temperature. The measured magnetic moment per Co atom was 2.63 mu(B), larger by 55% and 35% than that of the bulk Co and stable CoPt film, respectively, which is one of the highest values ever observed in the ferromagnetic bulk phase.     

真空退火对L1_0-CoPt薄膜结构、磁性及表面形貌的影响

本文利用磁控溅射的方法制备了L1_0-CoPt薄膜,研究了不同退火条件对薄膜结构、磁性以及表面形貌的影响.通过优化退火温度、保温时间以及升温速率,制备出了具有大矫顽力、高矩形比、粗糙度小的垂直各向异性L1_0-CoPt薄膜.实验发现,较高的退火温度有利于克服CoPt薄膜有序化转变所需要的能量,同时适当延长退火时间可以提高CoPt薄膜的扩散系数,从而促使无序相fcc转化为L1_0有序相fct结构.此外,退火过程中减缓升温速率可以有效减小薄膜粗糙度,从而形成平整连续膜.     

Structural changes in CoPt during ordering

X-ray methods have been applied to CoPt (48. 6 at. Co) during ordering; new data from the superlattice peaks are presented on the shape, size, and orientation of the new phase. The diffraction effects for the main peaks are explained as the result of monoclinic distortion in the cubic matrix. The maximal hardness during ordering is considered to be due to the rotation of blocks produced by the elastic stresses rather than to the stresses themselves.     

Effects of layering and magnetic annealing on the texture of CoPt films

The effect of magnetic field annealing of magnetron sputtered CoPt alloy films and Co/Pt bilayers on the crystallographic texture of the obtained chemically ordered (L1₀) CoPt films is presented. In CoPt alloy films the main effect of the magnetic field is to suppress (1 1 1) growth in the early stages of L1₀ formation whereas the development of (0 0 1) versus (1 0 0) texture is related to chemical ordering strain. A higher degree of (0 0 1) texture is obtained by magnetically annealing Co/Pt bilayers since the initial (1 1 1) texture in the as-sputtered films is avoided and Co-Pt alloying occurs in the presence of the magnetic field.     

Effect of short-range order on electronic and magnetic properties of disordered Co-based alloys

We here study electronic structure and magnetic properties of disordered CoPd and CoPt alloys using augmented space recursion technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. Effect of short-range ordering present in disordered phase of alloys on electronic and magnetic properties has been discussed. We present results for magnetic moments, Curie temperatures and electronic band energies with varying degrees of short-range order for different concentrations of Co and try to understand and compare the magnetic properties and ordering phenomena in these systems.     

Recent topics in microstructure control by magnetic field in Ni<Subscript>2</Subscript>MnGa and CoPt and new transformation behavior in Ti-Ni-Fe alloys

Three topics related to solid-solid phase transformations are presented. The first topic is related to ferromagnetic shape memory alloys. The general condition for rearrangement of martensite variants by magnetic field is discussed quantitatively. The second topic is related to microstructure control of CoPt (tetragonal L1₀-type structure) during ordering heat-treatment under a magnetic field. We show that a single variant state is realized by magnetic field, and magnetic field is especially effective at the early stage of ordering. The third topic is related to the so-called precursor phenomena in Ti-Ni-Fe shape memory alloys. In the topic we will show the existence of a commensurate phase, which inherits the microstructure of the incommensurate phase and is probably different from the R-phase.     

Structural origin of magnetic anisotropy in Co-Pt alloy films probed by polarized XAFS

Polarized X-ray absorption fine structure (XAFS) measurements at the Co K and Pt L₃ edges show that the perpendicular magnetic anisotropy found in epitaxial fcc CoPt₃ (111) films stems from the existence of anisotropic local ordering. Such ordering, induced during the codeposition process and dependent on the growth temperature, is characterized by preferential CoCo pairs in the film plane, balanced by preferential CoPt pairs out of the plane, resulting from some Co 2D-segregation. Polarized XAFS at the Pt edge reveals similar anisotropic local ordering in epitaxial hcp Co₃Pt (0001) films exhibiting a larger magnetocrystalline anisotropy compared to that of bulk hcp Co. Besides, a polarization dependence of the Co XANES profile is observed only for the Co₃Pt films exhibiting hcp symmetry. Received: 29 April 1998 / Revised: 27 July 1998 / Accepted: 31 August 1998     

低相变温度垂直取向的CoPtCu/Ag纳米复合膜

采用磁控溅射(Ag/Cu/CoPt)_n多层膜先驱体结合真空退火的方法制备了一系列CoPtCu/Ag纳米复合薄膜，通过优化薄膜中Ag以及Cu的含量，成功制备出了低相变温度垂直取向的CoPtCu/Ag纳米复合膜，该膜在450℃退火即可发生相变，该温度比目前所报导的CoPtAg纳米复合膜的相变温度降低了150℃. 实验结果表明，薄膜中一定含量的Ag元素能够有效诱导薄膜的(001)取向，Cu元素的加入能有效降低薄膜的有序化温度. 对于特定组分为Co₄₀Pt₃₆Cu₈Ag₁₆的薄膜，经500℃退火后已经显示了明显的(001)取向，垂直于膜面方向上的矫顽力为5.0×10⁵A/m，并且薄膜中晶粒尺寸仅为4—5nm，为将来CoPt-L1₀有序相合金薄膜用于超高密度垂直磁记录介质打下了基础. 关键词： 磁记录材料 CoPt 纳米复合膜     

Magnetic optical bifunctional CoPt_3/Co multilayered nanowire arrays

CoPt3/Co multilayered nanowire(NW) arrays are synthesized by pulsed electrodeposition into nanoporous anodic aluminum oxide(AAO) templates. The electrochemistry deposition parameters are determined by cyclic voltammetry to realize the well control of the ratio of Co to Pt and the length of every segment. The x-ray diffraction(XRD) patterns show that both Co and CoPt3 NWs exhibit face-centered cubic( fcc) structures. In the UV-visible absorption spectra,CoPt3/Co NW arrays show a red-shift with respect to pure CoPt3 NWs. Compared with the pure Co nanowire arrays, the CoPt3/Co multilayered nanowire arrays show a weak shape anisotropy and well-modulated magnetic properties. CoPt3/Co multilayered nanowires are highly encouraging that new families of bimetallic nanosystems may be developed to meet the needs of nanomaterials in emerging multifunctional nanotechnologies.     

Spark-based improved Basin-Hopping Monte Carlo algorithm for structural optimization of alloy clusters

CoPt alloy clusters are promising candidates for catalysts in fuel-cell applications because of their enhanced catalytic performances compared with pure Pt clusters. In this article, an improved Basin-Hopping Monte Carlo (BHMC) algorithm is proposed to optimize the stable structures of CoPt clusters with different sizes and compositions, and the Spark parallel framework is employed to accelerate the structural optimizations. The results show that the improved BHMC algorithm has higher probability to find the lowest-energy structure and more rapid convergence than the traditional BHMC one. Through the comparison of different threads during Spark parallel computing, it is found that the acceleration ratio increases with the thread number while decreases for large cluster due to the limitation of the resource allocation. By investigating the optimized structures of CoPt clusters, it is revealed that for small CoPt clusters, the most stable structures exhibit a wide variety at different Co/Pt ratios; for large CoPt clusters, however, their structures are independent of the Co/Pt ratios and close to the results of their monometallic counterparts. Besides, for all the CoPt clusters, Pt atoms tend to distribute near the surface and Co atoms are generally located in the interior.     

Structure and magnetic properties of CoPtCu:Ag nanocomposite films with (001) texture

Nanocomposite (001) textured CoPtCu:Ag films consisting of well separated L1₀ structure CoPtCu nanoparticles have been prepared using a CoPt/Cu/Ag multilayer precursor on SiO₂/Si(100) substrate by magnetron sputtering and subsequent vacuum thermal annealing. With a Cu concentration of 6–10% and Ag of 10–20% in atomic ratio, the films start ordering at an annealing temperature of 450 °C, which is roughly lower by 150 °C than that needed for most CoPt-based films especially with (001) texture. The perpendicular coercivities for the film are in the range from 5 kOe to 7 kOe after annealing at 500 °C in vacuum. The (001) texture for the film is partially due to the formation of an Ag underlayer after annealing, the decreasing of the ordering temperature is most-likely related to the formation of CoPtCu alloy nanoparticles in the film. PACS 75.30.Gw; 75.50.Kj; 75.70.Ak     

CoPt纳米棒异常红外光学性能研究

采用原位模板结合化学还原法制备了一维CoPt纳米棒,扫描电子显微镜(SEM)结果表明CoPt纳米棒平均直径为166.3 nm,长度在1.0~5.0μm之间。以CO为探针分子,在固/气界面研究了CoPt纳米棒的红外光学性质,并同本体Pt电极进行对比分析。原位固/气红外光谱结果表明,吸附态CO(COad)在本体Pt上为正常红外吸收,而CoPt纳米棒在GC基底上表现出异常红外效应(AIREs),当CoPt纳米棒负载到高反射率的金基底上可以得到相同的结论,说明CoPt纳米棒表现出异常红外效应是有材料本身的性质,与反射基底无关。论文从固/气界面进一步验证了一维纳米材料的异常红外效应,有助于加深对低维纳米材料异常红外光学性能的理解。     

Texture development and magnetic properties of [ZrO2/CoPt]n/Ag nanocomposite films

The L1₀ CoPt films with (0 0 1) preferred orientation are achieved by fabricating on the glass substrates and post annealing at 600° C for 30 min. The preferred orientation of [ZrO₂/CoPt]_n/Ag films dependence of the Ag underlayer thickness, ZrO₂ and CoPt interlayer thickness is investigated. A large perpendicular magnetic anisotropy and a nearly perfect L1₀ CoPt (0 0 1) texture are obtained in the [ZrO₂ (3 nm)/CoPt (5 nm)]₃/Ag (10 nm) film. The existence of ZrO₂ plays an important role in reducing the intergranular interactions and in determining the size of CoPt grains. Magnetic reversal in textured CoPt films are close to a Stoner-Wolfarth rotation.     

Laser ablation synthesis of monodispersed magnetic alloy nanoparticles

Monodispersed CoPt alloy nanoparticles were synthesized by a pulsed laser ablation (PLA) technique coupled with a low-pressure operating differential mobility analyzer (LP-DMA). The CoPt alloy nanoparticles were generated by laser ablating a solid Co–Pt target. In CoPt alloy nanoparticles synthesized from a target with a Co composition of 75 at%, the nanoparticle surfaces were covered by an oxide layer and exhibited a core-shell structure. In contrast, no shell was observed in particles generated from a target with a Co:Pt ratio of 50:50 at%. According to an EDX analysis, the compositions of the individual nanoparticles were almost the same as that of the target material. Finally, the magnetic hysteresis loops of the CoPt alloy nanoparticles exhibited ferromagnetism.     

Magneto-optical Static Recording Performances of Sputtered CoPt Alloy Films

CoPt alloy films with good magnetic and mageto-optical properties were prepared on Pt buffer layer or directly on glass subst rate by sputtering a composite target. Their static recording properties were investigated. The results showed that static signals could be easily written into the CoPt alloy films without Pt buffer layers using moderate recording power. The introduction of Pt buffer layer in CoPt alloy films not only led to a smaller Kerr rotation but also greatly increased the recording power.     

AB合金有序化过程的动力学(Ⅰ)

由原子在点阵中扩散的反应率过程理论,在最近邻近似下,计算了AB合金的长程有序化动力学及长程序为零时的短程有序化动力学。所得结果与前人的实验结论及计算结果相比定性地符合,定量地接近。长程有序化动力学的计算表明,有序化过程存在有孕育期,有c-曲线形式的转变曲线,s近似与exp{-αt²}正比,算得的平衡有序度与前人计算结果相近。长程序为零时的短程有序化动力学的计算表明,σ近似与exp{-βt}正比,此时有序化速率甚快,以致在最快的淬火速率下,样品中也必然存在有一定的短程序,所得的平衡短程序值也符合前人的计算结果。本文是作为继续计算短程有序化动力学的准备及基础。     

Magnetic reversal mechanism in strong perpendicular magnetic anisotropic CoPt/AlN multilayer film

Magnetic reversal mechanism of the Sub/AlN5 nm/[CoPt2 nm/AlN5 nm]₅ nano multilayer film, which shows strong perpendicular magnetic anisotropy (K_u=6.7×10⁶ erg/cm³), has been studied. The angle-dependent magnetic hysteresis loops of this highly perpendicular anisotropic CoPt/AlN multilayer film were measured in the present work, applying a magnetic field along different angles φ with respect to the film normal. It demonstrates that the magnetic reversal of the CoPt ultrathin layers in the CoPt/AlN multilayer film is occurred by the reversible magnetization rotation and the irreversible displacement of domain walls. The φ-dependent part of coercive field is resulted from the internal stress according to the Kondorsky and Kersten model. The φ-independent part of coercive field implies some random and isotropy pinning centers (e.g., vacancies, dislocations, grain boundaries) in the ultrathin CoPt layers. Our work is useful for coercivity control of metal/ceramics layered structures, in particular the perpendicular magnetic tunneling junctions.     

Effect of C layer on the structures and magnetic properties of (0 0 1)-oriented [C/CoPt]n/Ag films

CoPt/Ag and [C/CoPt]n/Ag thin films have been prepared onto the glass substrates by magnetron sputtering. We investigated the evolution of texture and magnetic properties of CoPt/Ag and [C/CoPt]n/Ag films. The results show that C-doping plays an important role in improving (0 0 1) texture, improving the order parameter S, reducing the intergrain interactions, and making the magnetization reversal mechanism more close to Stoner-Wolfarth rotational mechanism. The growth mechanism of (0 0 1) texture also seems to be related strongly to the films thickness. Our results show that the highly (0 0 1)-oriented films with ordered fct phase have a significant potential for the perpendicular media of extremely high-density recording.