Synthetic Ferrimagnetic Media Research Articles

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.

Percolation Studies in Synthetic Ferrimagnetic Recording Media

We have studied the percolation limit of synthetic ferrimagnetic media (SFM) by the measurement of noise power and third harmonic ratio and by imaging with a high-resolution magnetic force microscope (MFM). The noise power spectrum, which helps to find the onset of percolation, is significantly different in SFM structures compared to the conventional recording media. Properties of the stabilization layers, such as switching field distribution (SFD), are highly influential in deciding the integrated noise power spectrum. With suitable selection of high moment stabilization layers, the noise at higher densities, within the percolation density limit, can be substantially reduced. Third harmonic ratio studies indicate that the partial erasure occurs above /spl sim/900 kfci. Using a high-resolution high-Q MFM, it is possible to obtain stable and nonpercolating high density patterns up to 1000 kfci on media with tBr=0.22 memu/cm/sup 2/ and Hc/spl sim/4500 Oe. This results in practical designs of media for recording densities >150 Gbits/in/sup 2/.

Ｒｕ／Ｃｏ−Ｃｒ／Ｒｕ交換結合層で分断したＣｏ−Ｃｒ−Ｐｔ−Ｂ面内記録媒体のノイズ特性

An exchange-coupling layer consisting of a Co-Cr ferromagnetic layer laminated with a thin Ru layer on both sides was employed as a layer separating the recording layers in longitudinal HDD media. The Co-Cr-Pt-B recording layers, which are divided into sections of equal thickness, are ferromagnetically coupled via the antiferromagnetically coupled Co-Cr layer by a function of the thin Ru layers. Because of the separation of the recording layers, a reduction of about 30% in media noise was observed at 400 kFCI, and the thermal stability was the same as or better than that of the conventional type of synthetic ferrimagnetic media (SFM).

交換結合媒体のオーバーライトと熱ゆらぎ耐性改善

We found that overwrite could be improved in synthetic ferrimagnetic media (SFM) where the stabilizing layer has an anisotropy field with a higher value than that of the recording layer by a simple analysis of transition shift. An exchange field acting on the recording layer facilitates the reversal of the magnetization of the recording layer, because the higher anisotropy field of the stabilizing layer has the effect of reversing the magnetization in the recording layer earlier than that in the stabilizing layer. In an experiment, we not only found an improvement in overwrite characteristics, but also excellent thermal robustness in areas where a stabilizing layer with higher anisotropy was employed.

R/W properties and thermal stability of synthetic ferrimagnetic media

The thermal stability and read-write properties of synthetic ferrimagnetic media (SFM) are compared with conventional media (CM) of the same stability factor KuV/kT and Mr/spl delta/ (remanent magnetization and thickness product). SFM are found to be more stable than the equivalent CM, suggesting that the thermal stability advantage does not arise simply from an increased effective volume. The overwrite properties (OW) and nonlinear transition shift (NLTS) are better for CM. However, for media with the same signal decay rate, the OW is similar over a wide head field range and the NLTS match at larger fields.

Signal-to-noise ratio and thermal stability issues in extending synthetic ferrimagnetic media technology over 100 Gb/in/sub 2/

Signal-to-noise ratio (SNR) and thermal stability issues are addressed to evaluate synthetic ferrimagnetic media (SFM) for achieving areal densities >100 Gb/in/sup 2/. Thermally stable SFM is obtained at M/sub r//spl delta/ values as low as 0.15 memu/cm/sup 2/, utilizing a higher magnetic anisotropy for the stabilization magnetic layer. The SNR relations for SFM are decided by the effective M/sub r//spl delta/ and coercivity, similar to conventional media. The recording characteristics of a low M/sub r//spl delta/ medium was compared with a higher M/sub r//spl delta/ medium used for the 106-Gb/in/sup 2/ demonstration. Lower medium noise and higher signal resolution of the lower M/sub r//spl delta/ medium indicate an areal density extension to beyond /spl sim/150 Gb/in/sup 2/.

Synthetic ferrimagnetic media for over 100 Gb/in 2 longitudinal magnetic recording

Synthetic ferrimagnetic media (SFM) technology to achieve longitudinal magnetic recording over 100 Gb/in 2 is reported. SFM is made using an initial or stabilization layer and a top magnetic layer separated by a thin Ru spacer layer. The magnetic anisotropy ( K U) of the stabilization layer is found to be one of the key parameters for extending this technology to higher densities. A stabilization layer with K U<0.5×10 6 erg/cm 3 can be used for reducing the M r δ (remanent magnetization and thickness product), but with a minimal contribution to the thermal stabilization of SFM. A higher K U of the stabilization layer contributes substantially to the thermal stability of SFM, with the energy barrier being increased by approximately 55% of the anisotropy energy of the stabilization layer. This enables utilization of thinner top magnetic layers or smaller grain sizes, which leads to better signal-to-noise ratios. A recording density of 106 Gb/in 2 is demonstrated using an SFM with a coercivity of 3900 Oe, M r δ of 0.37 memu/cm 2, and a thermal stability performance better than –0.01 dB/decade was observed at 400 kfci. Furthermore, an SFM with good thermal stability could be made at very low M r δ of 0.15 memu/cm 2 and schemes for further advances are suggested.

A comparison of the thermal stability factors in single layer and exchange-coupled longitudinal media

We have measured the time-dependent remanent coercivity in a series of films to determine the thermal stability of antiferromagnetically exchange coupled multilayers [synthetic ferrimagnetic media (SFM)]. Samples were deposited by dc magnetron sputtering on glass substrates. Antiferromagnetic coupling between the magnetic layers was induced by a 0.7-nm-thick Ru spacer. The measurements were done using an alternating gradient magnetometer and in the series of samples the upper layer thickness was changed. The thermal stability factor (KV/kT) and the intrinsic switching field (H0) were extracted from the fit of the experimental data points to Sharrock’s formula. Single layer samples showed a linear growth of H0 with the magnetic layer thickness with some increase in KV/kT. In contrast, no substantial change in H0 with thickness was observed in the SFM. However, a significant increase in the stability factor of 30% and 44% was observed in SFM samples with effective thicknesses of 7 and 12 nm, respectively, compared to single layer media samples with comparable thicknesses. Data confirm the strong dependence of the stability factor on the properties of the top layer.

Advanced synthetic ferrimagnetic media (invited)

The exchange coupling field (Hex) in two-layer synthetic ferrimagnetic media (SFM) is enhanced to extend the areal density capability of earlier structures. The thermal stability of SFM is improved by employing high magnetic anisotropy stabilizing layers without significantly affecting the required write fields. However, this necessitates larger Hex values than were earlier realized, to preserve the antiparallel layer magnetic configuration at remanence. Hex is improved by inserting thin high Co content hcp layers, called “E-layers,” between the Ru layer and the magnetic layers. Large Hex values from 1 to 4 kOe are obtained depending on the E-layer composition and thickness. As the intergranular coupling is affected by the Co-rich E-layers, a method is provided that improves Hex to values &amp;gt;1 kOe without degrading read-write properties.

Improved thermal stability of synthetic ferrimagnetic media with enhanced exchange coupling strength

The effect of the antiferromagnetic interlayer exchange coupling strength J between the initial or stabilization magnetic layer L1 and the top magnetic layer L2 on the thermal stability of synthetic ferrimagnetic media (SFM) is investigated. Enhanced J is achieved by the insertion of a thin Co-rich hexagonal close-packed layer between the Ru layer and L1, and such a structure did not change the read/write properties significantly. The contribution of the anisotropy energy of L1 to the thermal stability of SFM is improved by increasing J. However, a significant increase in the contribution of L1 to the thermal stability is achieved at relatively low J values (0.14 erg/cm2) which translate into only a 3–4% increase in the switching field H0.

Computational study in synthetic ferrimagnetic coupled media

A simulation study has been performed to understand the parameters that affect the magnetization reversal process of synthetic ferrimagnetic (SF) media. It is found that to optimize an SF media, it is necessary to realize sufficiently high antiferromagnetic coupling energy, and there is an upper limit for the anisotropy energy and the thickness of the stabilizing layer.

Exchange coupling strength in synthetic ferrimagnetic media

The antiferromagnetic exchange coupling strength (H/sub ex/) between two CoCrPtB magnetic layers in bilayer synthetic ferrimagnetic media (SFM) is investigated. H/sub ex/ is found to be inversely proportional to the thickness and magnetization of the initial magnetic layer (L1). H/sub ex/ also shows a decrease with increasing temperature. An exchange coupling strength of J/spl ap/0.05 erg/cm/sup 2/ was obtained from H/sub ex/. For further improvement of thermal stability at higher recording densities, the thickness or magnetic anisotropy of L1 may be increased. On the other hand, for an effective SFM, the coercivity of L1 must be lower than H/sub ex/. Therefore, a large H/sub ex/ is critical for better thermal stabilization.

Synthetic ferrimagnetic media

The thermal stability and read-write performance of longitudinal magnetic recording media with antiferromagnetically coupled layers are investigated. The thermal stability is strongly dependent on the interaction between layers and the anisotropy energy of the stabilizing layer or layers. For media with two coupled layers, the energy barrier is increased by approximately 45% of the anisotropy energy of the stabilizing layer. Although the total thickness is larger compared to a conventional medium with the same remanence magnetization and thickness product Mr/spl delta/, the resolution is higher and depends on a thickness proportional to the magnetization difference between the layers. With these "synthetic ferrimagnetic" structures, thermally stable, low M/sub r//spl delta/ and high resolution media are made possible.