Investigation of L1_0 FePt-based soft/hard composite bit-patterned media by micromagnetic simulation

The soft/hard composite patterned media have potential to be the next generation of magnetic recording,but the composing modes of soft and hard materials have not been investigated systematically.L10 Fe Pt-based soft/hard composite patterned media with an anisotropic constant distribution are studied by micromagnetic simulation.Square arrays and hexagonal arrays with various pitch sizes are simulated for two composing types:the soft layer that encloses the hard dots and the soft layer that covers the whole surface.It is found that the soft material can reduce the switching fields of bits effectively for all models.Compared with the first type,the second type of models possess low switching fields,narrow switching field distributions,and high gain factors due to the introduction of inter-bit exchange coupling.Furthermore,the readout waveforms of the second type are not deteriorated by the inter-bit soft layers.Since the recording density of hexagonal arrays are higher than that of square arrays with the same center-to-center distances,the readout waveforms of hexagonal arrays are a little worse,although other simulation results are similar for these two arrays.     

Magnetization reversal in a preferred oriented (111) L1(0) FePt grown on a soft magnetic metallic glass for tilted magnetic recording

L1(0) FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L1(0) FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L1(0) FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L1(0) FePt/glassy Fe(71)Nb(4)Hf(3)Y(2)B(20)/SiO(2)/Si) in which the magnetic easy axis of L1(0) FePt is tilted by ~36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L1(0) FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L1(0) FePt with the soft magnetic Fe(71)Nb(4)Hf(3)Y(2)B(20) glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ~10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L1(0) FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.     

Perpendicular Exchange Coupling in TbFeCo/FePt Bilayer Films

We investigate the structure and exchange coupling in TbFeCo/FePt bilayer films. It is found that FePt has the L10 structure and the easy axis of the FePt film is perpendicular to the film plane. Results of the vibrating sample magnetometer and the magneto-optical Kerr effect 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 switching field dependence on the exchange coupling has been modelled by micromagnetic simulation and the interlayer coupling constant is about -0.9 erg/cm^2 according to this simulation.     

Critical Fe thickness for effective coercivity reduction in FePt/Fe exchange-coupled bilayer

FePt/Fe perpendicular exchange-coupled bilayers with different Fe thicknesses were prepared to study the exchange coupling effect and the magnetization switching mechanism. An Fe thickness of 3 nm was found to be the critical point where the coercivity reduction became saturated and had the largest thermal stability gain factor of 2.25. This thickness was close to the exchange length between the magnetically hard and soft layers. Within the exchange length the soft phase strongly coupled to the hard phase and the magnetization of the bilayer processed single switching; beyond the exchange length reversible magnetization increased with the Fe thickness and exchange spring effect was found. Our simulation results also revealed that the exchange length was the critical Fe thickness for effective coercivity reduction and for maintaining high remanence.     

(0 0 1) Exchange-coupled FCC/L10 FePt bilayers

Exchange coupling between hard and soft magnetic materials has implications for both permanent magnet and magnetic recording technologies. This paper looks at exchange coupling of FCC FePt epitaxially grown onto (0 0 1) oriented L1₀ FePt deposited on (0 0 1) MgO substrates at elevated temperature. By varying the thickness of the FCC layer there is a relaxation of the single crystal FCC layer that produces a polycrystalline microstructure as evidenced by the development of rings in the electron diffraction pattern. A concurrent decoupling of the layers is apparent from magnetization curves with two distinct switching fields as the FCC layer thickness is increased above 20 nm. The results shown here confirm the importance of the epitaxial relationship between materials of disparate anisotropies in maintaining strong exchange coupling.     

On the phenomena occurring at the interface between iron and iron–platinum thin films

FePt films were rf sputtered at room temperature and 550?C on MgO monocrystalline substrates. Room temperature films were post-annealed at 550?C. The high temperature growth induces a tetragonal distortion of the structure with the formation of L1₀-FePt phase. Soft iron layers, with different thickness, were e-beam deposited on these hard films. The phenomena occurring at the interface have been analysed and connected with the final magnetic behaviour of the system. It has been found that a strong exchange coupling between soft and hard layers has been established through the formation of an interfacial layer constituted by FePt small particles.     

Morphological and microstructural study of L10-ordered FePt and L10-FePt/Fe ultrathin films grown by UHV e-beam evaporation technique

Ultrathin films of L1₀-ordered FePt alloy with different thickness were grown by UHV e-beam evaporation technique. In this ultrathin regime, the increase of the thickness induces a strong improvement of the magnetic properties followed by a decrease of the grain size. Starting from these hard layers, FePt/Fe/FePt trilayers with different thickness of the outermost layer have been grown. The samples show a single-phase magnetic behaviour and a strong perpendicular anisotropy thanks to the exchange-coupling that established at the soft/hard interfaces. By increasing the thickness of the outer layer, a strong reduction of the switching field distribution has been obtained with the appearance of a predominant exchange-type interaction among the magnetic grains. The morphology is characterized by well separated and elongated islands, while the magnetic pattern shows finely dispersed bubble domains.     

Effect of the soft layer thickness on magnetization reversal process of exchange-spring nanomagnet patterns

A systematic study of the magnetization reversal behavior in the regular arrangement of L1₀-FePt based exchange-spring nanomagnets with different thicknesses of the Co soft magnetic layer is presented. The magnetic property of the hard magnet is compared to two tuned exchange-spring magnets: its systems of 20 nm L1₀-FePt/3 nm, and 7 nm Co. In particular, we focus on the switching field distribution. The exchange coupling showed narrower SFD, in spite of the decoupled part switches earlier. The magnetization switching mechanism of exchange-spring nanomagnets patterns has been revealed with a first-order reversal curves technique and the switching field distribution. Further, the microscopic results using magnetic force microscopy show that the spin rotation of the non-interacting part in the thicker soft layered exchange-spring magnet. The part influences the magnetization reversal process. According to the experimental results, exchange coupling strength can be tuned by the thickness of the soft magnetic layer.     

Effect of exchange coupling on magnetic property in Sm–Co/α-Fe layered system

The hysteresis loops as well as the spin distributions of Sm–Co/α-Fe bilayers have been investigated by both threedimensional(3D) and one-dimensional(1D) micromagnetic calculations, focusing on the effect of the interface exchange coupling under various soft layer thicknesses t~s. The exchange coupling coefficient Ahsbetween the hard and soft layers varies from 1.8 × 10~(-6)erg/cm to 0.45 × 10~(-6)erg/cm, while the soft layer thickness increases from 2 nm to 10 nm. As the exchange coupling decreases, the squareness of the loop gradually deteriorates, both pinning and coercive fields rise up monotonically, and the nucleation field goes down. On the other hand, an increment of the soft layer thickness leads to a significant drop of the nucleation field, the deterioration of the hysteresis loop squareness, and an increase of the remanence. The simulated loops based on the 3D and 1D methods are consistent with each other and in good agreement with the measured loops for Sm–Co/α-Fe multilayers.     

An improved broad-spectrum room acoustics model including diffuse reflections

More and more literature has paid attention to the diffuse reflections in enclosed space during the past few years. In this paper, the current computer models including diffuse reflections have been reviewed briefly at first. Then, to realize the broad-spectrum simulation for enclosed sound fields including diffuse reflections, an improved ray-tracing algorithm, which combines the splitting coefficient diffusion model and a dynamic sound ray receiving method, has been given. The algorithm can deal with broad frequency bands simultaneously by using the frequency independent splitting coefficient. To test the algorithm and also to investigate the significance of the diffuse reflections in enclosed sound fields, experiments have been made in three spaces including a virtual room and two real rooms. The results and discussions have validated the applicability of the improved algorithm and they have also shown that diffuse reflections can improve room acoustic prediction, although it not always promote a sound field to be more diffused.     

氧化物隔离对Si基片上生长L1_0相FePt薄膜磁性的影响

用MgO和SiO_2两种氧化物将FePt薄膜与Si(100)基片隔离,分析隔离层在FePt层发生A1→L1_0转变过程中的作用,寻找用Si母材涂敷L1_0-FePt磁性层来提高磁力显微镜针尖矫顽力的合理方案.采用磁控溅射法在400?C沉积Fe Pt薄膜,在不同温度进行2 h的真空热处理,分析晶体结构和磁性的变化.结果表明:没有隔离层,Si基片表层容易发生扩散,50 nm厚FePt薄膜的矫顽力最大只有5kOe(1 Oe=10~3/(4π)A·m~(-1));而插入隔离层,矫顽力可以超过10 kOe;MgO在Si基片上容易碎裂,热处理温度不能高于600?C,用作隔离层,FePt的最大矫顽力为12.4 kOe;SiO_2与Si基片的晶格匹配更好,热膨胀系数差较小,能承受的最高热处理温度可以超过800?C,使得Fe Pt的矫顽力可以在5 kOe到15 kOe范围内调控,更适合用于制作矫顽力高并可控的磁力显微镜针尖.     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

Hard–soft composite magnets

A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasis on multilayers constituted by hard–soft exchange-coupled phases. The developed one—dimensional micromagnetic model leads to a complete magnetic phase diagram in terms of layer thicknesses. Both perpendicular and parallel configurations are considered. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. The model has been applied to a variety of hard magnetic phases (e.g., FePt, CoPt, SmCo and NdFeB), coupled to different soft materials (e.g., Fe, FeCo, FeRh or permalloy) both in the form of bilayers and multilayers. The most significant results will be highlighted.     

Understanding magnetic properties of arrays of small FePt dots with perpendicular anisotropy

FePt dot arrays with dot size down to 15 nm are fabricated by film annealing and patterning. The array coercivity shows an increase with dot size decreasing from 100 to 30 nm, and a slight reduction for the 15 nm dot sample. Annealing these dot arrays at higher temperatures results in large enhancements in the coercivities, except the 15 nm dot array where the coercivity increases a little. Micromagnetic models of a 15 nm FePt dot with uniform and nonuniform edges of soft magnetic defects and with inside defects are calculated to reveal the microstructure origins of the dot magnetic properties. It is found that the volume fraction of the L1₀-phase FePt with perpendicular c-axis orientation is about 50% in the dot and the switching field distribution of the dot array can be influenced significantly by the defect arrangement in the dots.     

Role of Ag addition in L10 ordering of FePt-based nanocomposite magnets

The FePt system has important perspectives as high-temperature corrosion-resistant magnets. In the form of rapidly solidified melt-spun ribbons, FePt-based magnets may exhibit in certain cases a two-phase hard–soft magnetic behaviour. The present paper deals with a microstructural and magnetic study of FePtAgB alloys with increasing Ag content. The aim is to identify and confirm the effect of Ag addition in decreasing the temperature of the FePt disorder–order structural phase transformation. A detailed high-resolution transmission electron microscopy study is employed, and the alternative disposal of hard and soft regions within the two-phase microstructure is observed and interpreted with respect to the X-ray diffraction results. In the as-cast Ag-containing samples, it is shown that there is an optimum of the Ag content for which best magnetic properties are obtained. Ag addition creates a nonlinear behaviour of the coercive field and the ordering parameter, similar to the RKKY interaction-induced interlayer exchange coupling (IEC) observed in magnetic layers separated by non-magnetic spacer layers. Direct formation of the L1₀ phase from the as-cast state in the FePtAgB alloys is reported with magnetic parameters compatible to other exchange spring permanent nanomagnets. These findings open novel perspectives into utilization of such alloys in applications requiring magnets operating in high-temperature industrial environments.     

Magnetic reversal processes and critical thickness in FePt/α-Fe/FePt trilayers

Magnetic reversal processes of a FePt/α-Fe/FePt trilayer system with in-plane easy axes have been investigated within a micromagnetic approach. It is found that the magnetic reversal process consists of three steps: nucleation of a prototype of domain wall in the soft phase, the evolution as well as the motion of the domain wall from the soft to the hard phase and finally, the magnetic reversal of the hard phase. For small soft layer thickness L^s, the three steps are reduced to one single step, where the magnetizations in the two phases reverses simultaneously and the hysteresis loops are square with nucleation as the coercivity mechanism. As L^s increases, both nucleation and pinning fields decrease. In the meantime, the single-step reversal expands to a standard three-step one and the coercivity mechanism changes from nucleation to pinning. The critical thickness where the coercivity mechanism alters, could be derived analytically, which is found to be inversely proportional to the square root of the crystalline anisotropy of the hard phase. Such a scaling law might provide an easy way to test the present theory. Further increase of L^s leads to the change of the coercivity mechanism from pinning to nucleation.     

Dependence of microstructure and magnetic properties of FePt/B nanocomposite films on boron content

Compared with previous work, FePt/B nanocomposite films with larger B content in a wide range were prepared by magnetron sputtering plus subsequent annealing on Si (1 0 0) substrates. When the volume percentage is no less than 12%, both interstitial incorporation and intergranular segregation of the B are found, resulting an enlargement of the FePt lattice and a decrease of the FePt grain size. Effect of B content on magnetic properties, such as magnetization, switching field and exchange coupling of the FePt based films, is also discussed.     

Magnetization reversal in CoPt-based hard–soft homocomposites

CoPt-based hard–soft sputtered bilayers with (1 1 1) texture have been produced by appropriate heat treatment of the bottom layer. Two samples with different degrees of chemical ordering of the hard layer are compared. The anisotropy of the hard layer determines its robustness against destabilization from the soft one. Detailed measurements of the soft layer minor hysteresis loop features as a function of the magnetic state of the hard layer are proposed as a means to study the nature of interfacial exchange interactions and the mechanism of magnetization reversal. When hard layer anisotropy is not robust enough, the reversed soft layer can induce irreversible changes to the magnetic structure at the interface leading to a decoupling of exchange field from the magnetic state of the hard layer.     

正交各向异性双层交换弹簧薄膜的磁矩分布

本文以Pt_(84)Co_(16)/TbFeCo双层交换弹簧体系为研究对象,利用微磁学连续模型,研究了软/硬磁层易轴方向相互垂直的新型体系中磁矩的分布特征.研究结果表明,磁矩偏离薄膜法线方向的角度在软磁层中沿膜厚方向的变化速率比硬磁层中的快.通过调节软磁层参数来增加软/硬磁的各向异性常数比、交换能常数比、饱和磁化强度比或外磁场强度,都可有效改变磁矩偏角在软/硬磁层中的变化速率.特别是当软/硬磁各向异性常数比值和交换能常数比值同时增大时,可以使得磁矩在硬磁层中的变化速率快于软磁层中的.而饱和磁化强度比值对磁矩变化速率的影响源于饱和磁化强度的变化会相应地改变各向异性常数,进而改变磁矩在软/硬磁层中磁矩方向变化速率的比值.此体系的磁滞回线显示磁性参数的改变可以显著改变体系的剩磁及饱和磁场.软磁层中的退磁场能及体系的正交各向异性可导致负的成核场.