AVS1999 Session MI+VM+AS-MoA: Magnetic Recording: Media and Heads

Monday, October 25, 1999 2:00 PM in Room 618/619

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	MI+VM+AS-MoA-1 Spectro-Microscopy of Magnetic Materials Using Polarized Soft X-Rays J. Stohr (IBM Almaden Research Center) The talk discusses the motivation for and challenges of obtaining magnetic information for ferromagnetic and antiferromagnetic systems on a length scale below 100nm. It reviews the principles of linear and circular x-ray magnetic dichroism spectro-microscopy and presents state-of-the-art results (20nm resolution) obtained with a dedicated soft x-ray photoelectron emission microscope (PEEM) installed on the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. Results include studies of the antiferromagnetic domain structure at the surface of LaFeO3(100), NiO(100) and polycrystalline NiO and the ferromagnetic domain structure in hard/soft magnetic tunnel junctions. Future plans to reach a spatial resolution below 10nm will also be discussed.
2:40 PM	Invited	MI+VM+AS-MoA-3 Micromagnetic Properties and Recording Performance in High Density Magnetic Recording Media T. Suzuki, G. Lauhoff (Toyota Technological Institute, Japan) In order to increase areal recording density, much effort has been made to improve magnetic charactreristics and microstructure in magnetic recording media. In order for decreasing noise and for realizing a sharp transition between written bits, an inter-granular exchange coupling must be lowered, which enhances a thermal instability of magnetization. Given this conditions, it is vitally important to understand the role of magnetic activation volume or magnetic coupled region in conjunction with micromagnetics, which plays a key role in noise mechanisms. The present study is to discuss activation volume in longitudinal magnetic recording media of various types including CoCrPtTa thin-films and granular-type recording media. The activation volume is evaluated based on the time decay of magnetization at a certain field. A novel method to define the activation volume is proposed. The Barkhausen volume is estimated through the field-sweep-rate dependence of coercivity. It is found that the activation volume, which is of the order of 10^-18 cm^-3, decreases with applied field, then levels off, and starts increases at fields beyond coercivity. This trend is found for all the high density recording media of CoCrPtTa and granular-type media under consideration. The recording noise is found to be closely related to activation volume. Measurments of δM curves suggest that lesser the granular-exchange-coupling is, the smaller the activation volume becomes.
3:20 PM	Invited	MI+VM+AS-MoA-5 Sub 50 nm Planar Magnetic Nanostructures Fabricated by Ion Irradiation T. Devolder, C. Chappert (IEF/Université Paris Sud, France); Y. Chen (L2M Bagneux/CNRS, France); H. Bernas (CSNSM/Université Paris Sud, France); J.-P. Jamet, J. Ferré (LPS/Université Paris Sud, France); E. Cambril (L2M Bagneux/CNRS, France) Areal density enhancement is a major challenge in magnetic recording. Near field magneto-optical techniques are one fast-developing attempt to respond. At bit density values above 65 Gbits/in², a most drastic requirement will be to write stable bits with nanometer wall jaggedness, at very precise locations on the disk. Patterned media could be a promising response to this problem.¹ However, surface roughness, and polarization dependent effects due to abrupt changes in optical index, will likely deteriorate the signal to noise ratio. Through interface mixing, light ion (He+) irradiation can modify in a precisely controlled way the magnetic properties of multilayers, with negligible change of surface roughness and optical indices. In (Co/Pt) multilayers with perpendicular easy magnetization axis, the anisotropy decreases with irradiation, which first reduces the coercive force, then induces in-plane magnetization. Patterning only the magnetic properties can then be obtained by irradiation through a lithographic PMMA resist mask.² Using SiO₂ masks, we have fabricated regular arrays of such irradiation-patterned nanostructures with sizes down to 30 nm. Different configurations such as hard (resp. soft) nanostructures in soft (resp. hard) media have been obtained and characterized using far field magneto-optical microscopy. Special attention has been devoted to the study of the transition zone between irradiated and protected areas, and its effect on magnetization reversal. The technique may be a powerful tool for ultrahigh density magnetic recording applications. ¹ ¹ S. Chou et al., Data Storage 35 (1995). ² C.Chappert et al., Science 280, 1919 (1998).
4:00 PM	Invited	MI+VM+AS-MoA-7 Crystallographic Texture and Stress in Co-Based Magnetic Recording Media and Underlayers B.M. Clemens, G. Khanna (Stanford University) The magnetic and recording properties of Co-based media are a strong function of crystallographic texture and stress in the media layers, which are in turn strongly influenced by the texture and stress of the Cr underlayer. The anisotropy in strain and the distribution of c-axes in the hcp Co alloy media can determine the magnetic hysteresis anisotropy through magnetostrictive and magnetocrystalline effects. The processing conditions and mechanical texture grooves in the NiP/Al substrate strongly influence the microstructural and mechanical properties of the Cr and Co films. We report x-ray studies of the texture and strain in Cr and Co-based films sputter deposited over a range of temperatures and substrate biases on smooth and mechanically textured substrates. All films showed an in-plane compressive stress. The magnitude of the stress in Cr and Co films grown without a substrate bias decreases with decreasing growth temperature, consistent with thermal mismatch stress. The stress in the Cr films grown with a -300V bias was greater than that of unbiased films, and independent of growth temperature. This suggests that the bias-induced stress is close to the yield stress of the film. However, the biased Co films show a decrease in compressive stress with decreasing temperature. For mechanically textured substrates, the stress measured in the direction perpendicular to the texture grooves was less compressive relative to the parallel direction in both Cr and Co. However, the stress anisotropy vanishes in smooth substrates. A simple strain relaxation model is used to explain the observed Cr stress values in the textured disks. The anisotropic strain in the Cr may also account for the observed preferential alignment of Co c-axes along the grooves. Calculation of the magnetoelastic and magnetocrystalline energies predicts that Co stress anisotropy and the preferential alignment of Co c-axes along the grooves both contribute to the observed hysteresis anisotropy.
4:40 PM	Invited	MI+VM+AS-MoA-9 Noise in GMR Recording Heads H.T. Hardner, M.B. Hurben (Seagate Technology) Magnetoresistive sensors exploit a close coupling between magnetization and resistance to convert changes in magnetic field to an electrical signal. Thus, the enhanced sensitivity of the giant magnetoresistive (GMR) materials to magnetic field is accompanied by larger electrical noise due to magnetization noise. A magnetic contribution to 1/f noise originates in thermal fluctuations in magnetization. This is a concern for sensors intended for low frequency applications rather than for magnetic recording heads due to the very high frequencies at which the heads operate. However, discrete steps in the resistance can also appear due to large discrete changes in domain structure. When these steps occur as a continuous magnetic signal is applied to the device they are called Barkhausen noise. In small enough devices discrete magnetoresistive steps can be observed even at fixed magnetic field. While the sensitivity of the resistance to changes in magnetization is proportional to the magnetoresistance, the propensity for complex domain structure with fluctuations between multiple metastable states varies by material, processing, and design. The suppression of these fluctuations to create a device with a single stable magnetization state is a key goal in the design and manufacture of magnetoresistive recording heads. This talk will provide some brief background on how recording heads are stabilized as well as an overview of resistance noise data from GMR devices including comparisons of different materials and the use of resistance noise measurements to study domain structure. Recent results on resistance noise in spin-valve recording heads both at the finished stage and during wafer processing will be presented along with discussion of how a detailed study of electrical noise can help identify stability problems. Analysis of both time and frequency domain data will be considered.