Exchange Coupled Composite Research Articles

The two-spin model with dipolar interactions for the exchange coupled composite media

We study exchange coupled composite (ECC) media where both the hard and soft coupled layers possess perpendicular anisotropy or one of the layers is superparamagnetic. Our model is used to demonstrate the effect of the dipolar interactions on the coercive field. A series of ECC samples with various thicknesses of Ru spacer is manufactured and analyzed. The structure of the samples prepared is CoPtCr-SiO2∕Ru∕CoPtCr-SiO2, with the composition varied around that of (CoPt17Cr10)90-(SiO2)10. The acquired experimental data are used to illustrate the significance of magnetostatic interactions between the two coupled magnetic layers.

Magnetic characterization of exchange-coupled composite media

In this work we compare the magnetization reversal processes in a set of exchange-coupled composite (ECC) media and a similar set of continuous granular coupled (CGC) media. We have studied the reversal processes in both systems as a function of the thickness (ts) of the soft overlayer. We find that the soft overlayer contributes a reversible contribution to the magnetization in both types of media, lowering the coercivity in both cases but that there is an optimum value of ts in terms of the resulting squareness. This has important implications for the output signal in both cases. We have also studied the activation volume of reversal which will ultimately determine the limit to the data density achievable in both types of media. We find that in each case the activation volume increases monotonically with the thickness of the soft layer, indicating that nucleation of reversal in the soft layer will determine the ultimate data density achievable. Although absolute values for the activation volume cannot be determined due to the uncertainty in the value of Ms, our data indicate unexpectedly that an exchange-coupled soft layer, i.e., the ECC media, has smaller activation volumes than is the case for CGC media. However, we cannot discount the possibility that increased anisotropy from the hard layer may contribute to this effect.

Ｒｅｖｅｒｓｅ ＥＣＣ媒体の記録再生及び熱揺らぎ特性

Exchange-coupled composite (ECC) media are candidates for achieving recording densities of over 1 Tbpsi. In this study, we investigated the SNR and thermal stability of ECC media, using a micromagnetic simulator. We found that the vertical-exchange (A⊥) interaction between the soft and hard regions greatly influences both the SNR and the thermal stability, but in opposite ways. In other words, a weak A⊥ results in a high thermal stability, while a strong A⊥ increases the SNR. Thus, it is difficult to simultaneously obtain a high SNR and thermal stability using ECC media. To solve this problem, we propose a reverse ECC (RECC) medium in which the positions of the soft and hard regions are reversed with respect to those in conventional ECC media. Under the condition of a weak A⊥ of 5 pJ/m, which is suitable for achieving good thermal stability, RECC media have SNRs that are 7.5 dB higher than those of ECC media at 2000 kfci. This is because the field gradient of the writing field in the hard region of RECC media is considerably improved by closing to the ABS of the writing head. This study demonstrates that it is difficult for conventional ECC media to achieve 1 Tbpsi recording, and shows that RECC media have the possibility of attaining recording densities of over 1 Tbpsi.

ＥＣＣ媒体の薄膜形状における耐熱特性

Exchange-coupled Composite (ECC) media have attracted a great deal of attention as a potential solution to the tri-lemma between the signal-to-noise (SNR), thermal fluctuations, and the writability of recording media. Several methods have been proposed to systematically design ECC media, taking into account the above-mentioned three issues. A method of evaluating the thermal stability of thin-film media, however, has not yet been established. Monte Carlo methods have been widely used to evaluate to long-term thermal decay, but their application is doubtful to strong exchange-coupled systems such as ECC media. The Langevin method is also limited to very short-term thermal decay. In this paper, we propose a simple method of evaluating thermal decay that combines an analytical method with Langevin calculations.

Towards Terabit/in2 Magnetic Storage Media

AbstractIn modern society there is an almost insatiable demand for ever increasing storage capacities in computers and consumer electronics. Magnetic recording is the dominant storage technology and the hard disk, due to its versatility, is becoming a pervasive device in various applications. The soaring demand arises from data-intensive computer applications, including graphics, animation, multimedia and desktop publishing, to which can be added a growing market for non-PC consumer devices such as set-top boxes, cameras, mobile phones, laser printers and satellite navigation systems. In response to this demand the hard disk drive manufacturers have come forward with spectacular increase in storage capacities and densities over the last decade. It is currently projected that the evolution of conventional perpendicular recording storage density in the hard disk industry will reach a limit of 500 Gbit/in2, while further progress will require major breakthroughs and alternative technologies. In this presentation we will review the state-of-the-art in magnetic recording media and we will discuss the future approaches to reach densities in excess of 1 Tbit/in2 densities along the three axes : a) self-assembled coercive nanoparticles, b) exchange spring media and percolated media, and c) bit-patterned media (nanoscale patterning). This entails resolving several conflicting requirements with regard to signal to noise ratio (SNR) , writability and thermal stability of these new promising systems.

Exchange coupling effects in CoCrPt–SiO2/FeCoTaCr composite media for perpendicular recording

The exchange coupled composite (ECC) media consisting of a magnetically hard CoCrPt–SiO2 layer and a magnetically soft FeCoTaCr layer were fabricated as an approach to improve writability. A nonmagnetic CoCr layer with different thickness was inserted between the hard and soft layers to adjust the exchange coupling strength. With proper CoCr thickness, a decrease of coercivity was observed. Transmission electron microscopy (TEM) revealed that the soft/hard stacked grains were magnetically isolated by SiO2 along grain boundaries. Magnetic properties were characterized by vibrating sample magnetometer (VSM). Angle-dependent coercivity indicated that the stacked magnetic grains switched magnetically in a coherent mode. Angle-dependent switching field demonstrated that ECC media had a better tolerance of the angle dispersion than perpendicular media. The switching field decreased with the increase in thickness of the soft magnetic layer. However, the thermal stability factor was kept almost constant (150) until the soft layer was over 3.3 nm.

Nanocomposite magnetic films for high-density perpendicular magnetic recording media

Multi-layer nanocomposite structures of Ta/Ru/CoCr 1/FeCoTaCr(soft magnetic layer)/CoCr 2/CoCrPt–SiO 2(hard magnetic layer or recording layer)/C and Ta/Ru/CoCr 1/CoCrPt–SiO 2/CoCr 2/FeCoTaCr/C were proposed. This exchange coupled composite (ECC) media consisting of hard/soft stacked magnetic layers were promising in improving the writability of perpendicular magnetic recording media. A small CoCrPt c-axis orientation dispersion of about 3° was achieved with the optimized sputter conditions. The CoCrPt–SiO 2 grains were well segregated by SiO 2 at grain boundaries. The macro-magnetic properties showed that the stacked magnetic grains switched in a coherent mode and that switching field decreased with increasing the thickness of the soft magnetic layer.

Comparison of Recording Head Designs for Perpendicular and Exchange-Coupled Composite Media

Exchange-coupled composite (ECC) media consisting of hard and soft coupled regions has been predicted to exhibit attempt frequencies that are much larger than conventional perpendicular media due to the presence of two degrees of freedom. As a result, adjacent track erasure (ATE) and archivability are adversely affected, whereas writability is significantly enhanced due to thermal fluctuation assisted recording. Recording heads for ECC and perpendicular media, designed dimensionally for recording at 250 Gbits/In <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , are evaluated within this context. In comparison, recording on ECC media is shown to yield about a factor 2 advantage in areal density

A Comparative Study of Perpendicular Media

Three types of perpendicular recording media were compared using simulations. Exchange coupled composite (ECC) type media could support minimum track pitches up to 18 nm less than a single layer medium, allowing significantly higher areal recording densities. The recording performance of all media was highly susceptible to grain-to-grain variations of uniaxial anisotropy Ku and, in the case of ECC media, to variations in the interlayer coupling strength. Areal densities of around 500 Gb/in2 should be achievable using current technologies, higher densities require improved manufacturing tolerances

Write Head Design for Exchange Coupled Composite Media

A writer is designed for 10-nm-thick exchange coupled composite (ECC) media. At the air bearing surface (ABS), the writer is 60 nmtimes60 nm. Down track notches, cross track flares, and side shields are employed. A recording density of 1.18 Tb/in2 is achieved based on the write field profile. The bit aspect ratio (BAR) is estimated to be 8.9

Domain Wall Switching: Optimizing the Energy Landscape

It has recently been suggested that exchange spring media offer a way to increase media density without causing thermal instability (superparamagnetism), by using a hard and a soft layer coupled by exchange. Victora has suggested a figure of merit xi=2E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> m <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sw</sub> , the ratio of the energy barrier to that of a Stoner-Wohlfarth system with the same switching field, which is 1 for a Stoner-Wohlfarth (coherently switching) particle and 2 for an optimal two-layer composite medium. A number of theoretical approaches have been used for this problem (e.g., various numbers of coupled Stoner-Wohlfarth layers and continuum micromagnetics). In this paper we show that many of these approaches can be regarded as special cases or approximations to a variational formulation of the problem, in which the energy is minimized for fixed magnetization. The results can be easily visualized in terms of a plot of the energy E as a function of magnetic moment m <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> , in which both the switching field [the maximum slope of E(m <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> )] and the stability (determined by the energy barrier DeltaE) are geometrically visible. In this formulation we can prove a rigorous limit on the figure of merit xi, which can be no higher than 4. We also show that a quadratic anistropy suggested by Suess comes very close to this limit.

Synthetic antiferromagnet for hard layer of exchange coupled composite media

It has been shown that exchange coupled composite (ECC) media exhibit a superior thermal stability to switching field ratio compared to conventional perpendicular media. However, optimal media designs have employed a low magnetization hard layer. A more physically achievable design may consist of replacing the hard layer with a synthetic antiferromagnet consisting of two ferromagnetic hard layers coupled by an antiferromagnetic exchange interaction. This approach is shown to decrease the switching field and increases the thermal stability ratio when compared to ECC media with a single hard layer.

Fabrication and Characterization of Exchange Coupled Composite Media

Magnetic hard and soft phases CoCrPt-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films were developed to fabricate exchange coupled composite (ECC) media. Domain wall nucleation and propagation from the soft regions to the hard regions in the composite grains was found to be the switching mechanism in the ECC media. ECC media on the disk substrate with soft underlayer was fabricated. Interlayer thickness dependence of saturation field and domain wall length in CoCrPt-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> soft layer suggested that domain wall nucleation and propagation in ECC media. Spin-stand testing showed more than a six-times reduction of saturation writing current and more than a 10 dB increase of total signal-to-noise ratio (SNR) for ECC media. Time decay results of readback signals indicated that there is no thermal stability problem for ECC media. The roll-off curve of ECC media showed the same SNR level as a state-of-art reference perpendicular media targeted around 200 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . With further optimizations, areal densities beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> seem achievable for ECC media

Issues in Recording Exchange Coupled Composite Media

Exchange coupled composite (ECC) media exhibits a lower switching field than conventional perpendicular media at the same level of thermal stability, and thus is able to achieve higher areal density. The inhomogeneity of ECC media is predicted to enhance the thermal attempt frequency: this increases both writability and adjacent track erasure (ATE). The adverse change in ATE is, however, countered by the angular dependence of the ECC media switching field. This angular dependence is further exploited with a head design featuring a larger pole width than track width and the introduction of notches in the down track direction. This reduces the effect of head skew and increases the write field. An optimized head is found to be consistent with terabit/in2 recording

Simulations of Perpendicular Recording Media for 600 Gb/in2

Four types of perpendicular magnetic recording media were compared using simulations to determine the most suitable candidate for use in future, high density storage products. Assuming a 75 nm track pitch, the minimum Ku values for no adjacent track erasure were determined. Roll-off curves provided an estimate of the maximum linear density. Exchange-coupled composite media appeared to be the most promising candidates, due to their high resolution and SNR and, if produced under well controlled conditions, could support areal densities of 600 Gb/in2.

Ferromagnetic interlayer coupling and switching process of exchange coupled composite media

The ferromagnetic coupling effects in the exchange coupled composite (ECC) magnetic recording media were studied by varying the interlayer thickness and the saturation magnetization (Ms) of the soft layer. A minimum coercivity was observed at a certain interlayer thickness for the ECC media with a low Ms soft layer. When the Ms of the soft layer was increased (700emu∕cm3 in this work), no interlayer was needed to get the lowest coercivity for the ECC media, which makes the ECC media practical for a manufacturing process. A method to characterize the ferromagnetic coupling strength for the ECC media was quantitatively proposed. The switching process of the ECC media was observed to consist of the reversible switching of the soft layer and the irreversible switching of the hard layer.

Preparation for exchange-coupled permanent magnetic composite between α-Fe (soft) and Nd2Fe14B (hard)

Abstract Soft magnetic α-Fe nanoparticles were prepared by a coprecipitation route and hard magnetic Nd 15 Fe 77 B 8 nanoparticles were prepared by ball milling for 20 h by using a shaker mill. A mechanical ball-mill technique was applied to build up exchange-coupled nanoparticles. A mixture of Nd 2 Fe 14 B and α-Fe nanoparticles in a stainless steel boat was milled for 2 h and annealed in a vacuum furnace under vacuum (∼10 −5 Torr) at 650 °C for 30 min. The crystal structure of the nanoparticles was confirmed by using X-ray powder diffraction (XRD). The surface morphology was identified by FE-SEM. The magnetization curve was measured with a vibrating-sample magnetometer (VSM). Thermogravimetry using a microbalance with magnetic field gradient positioned below the sample was used for the measurement of a thermomagnetic analysis (TMA) curve showing the downward magnetic force versus temperature.

Multilayer exchange spring media for magnetic recording

The concept of exchange spring media is extended from two layers to N layers. The coercive field of the multilayer structure decreases with 1∕N while the energy barrier is only determined by the magnetic properties of the hardest layer. A 25nm thick trilayer has a 7.5 times smaller coercive field (1.4T) as a single layer at the same thermal stability and for the same value of the magnetization. A continuous variation of the anisotropy in the recording media reduces the coercive field by a factor of 10 for a layer thickness of 25nm.

Micromagnetics of exchange spring media: Optimization and limits

The magnetic properties of exchange spring media are compared with perpendicular recording media using simple analytical estimates and using micromagnetic simulations. For a hard layer exchange coupled to a perfectly soft layer, the theory predicts a coercive field of the entire system of 1 4 of the anisotropy field of the hard layer in the limit of infinite layer thicknesses. A value of the coercive field close to the maximum reduction can be achieved for soft layers that are thicker than l h where l h 2 = 2 π 2 A s / K h , A s is the exchange constant in the soft layer and K h the anisotropy constant of the hard layer. For multilayer structures with gradually decreasing anistropy in each layer the coercive field can be decreased even further. A genetic optimization algorithm is used to design a bilayer structure as close as possible to the theoretical limit. The optimization shows that a gain in energy barrier of about 70% can be expected practically in exchange spring media with two layers and zero anisotropy in the soft layer. The switching time of exchange spring media for external fields applied 10° off the easy axis is about 0.4 ns which is comparable to single-phase media.

Effect of intragranular exchange on exchange-coupled composite media

A micromagnetic approach was adopted to study the effect of the intragranular exchange parameter, A(ergs∕cm), on the switching of exchange-coupled composite (ECC) media. A single grain of ECC media was simulated by subdividing it into small cubes and introducing appropriate exchange within different regions of the grain. We find full coupling between the hard and soft regions to be necessary to facilitate switching of a hard region with very high anisotropy, Ku(∼2×107ergs∕cm3). This method also allows for rotation mechanism analysis within the ECC grain. We report nonuniform rotation that is observed as a domain nucleating in the bottom region of the soft part and moving its way up to and through the hard/soft layer interface, if the intragranular exchange coupling is appropriate. Field sweep-time analysis revealed that ECC media was capable of following the rapidly changing field more efficiently than regular perpendicular media.