HAL Id: jpa-00229174
https://hal.archives-ouvertes.fr/jpa-00229174
Submitted on 1 Jan 1988
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
MICROMAGNETIC STUDIES OF DISCRETE
TRACKS USING LORENTZ MICROSCOPY
T. Nguyen, I. Mcfadyen, C. Hwang, P. Alexopoulos
To cite this version:
JOURNAL DE PHYSIQUE
Cofloque C8, Supplkment au no 12, Tome 49, dkembre 1988
MICROMAGNETIC STUDIES OF DISCRETE TRACKS USING LORENTZ
MICROSCOPY
T. A. Nguyenl, I. R. ~ c ~ a d ~ e n ' , C. Hwang2 and P. S. ~ l e x o ~ o u l o s ~
IBM Research Division, AImaden Research Center, 650 Hany Road, San Jose, California 95120-6099, U.S.A.
Abstract. - Written bits on 0.6 to 10 pm wide discrete tracks have been studied. Lorentz microscopy shows the bits to be well-defined and in saturated remanence state. Ripple structure observed within the bit extends to the track edges. The transition zone is about 3000
A
in width and consists of both ripple and vortex-like structures.Introduction Results and discussion
Recent studies [I, 21 of the recording performance of isolated discrete tracks revealed that the scaling of signal amplitude and media noise to submicron di- mensions can be understood by conventional record- ing theory. These studies also showed that edge noise was substantially reduced in discrete tracks, compared with the considerable edge noise generated in conven- tional media by writing with the fringing field of the thin film head. This investigation
has
been carried out to explain the above phenomena based on observed micromagnetic structures and t o obtain info~mation about the bit definition, transition zone structure and fringing fields as a function of recording frequency and track width. The tracks studied were from 0.6 to 10 pm wide and were recorded a t transition densities of 200, 500 and 1000 fc/mm.Materials and experimental procedures
The two-layer film structures were produced by sputtering the underlayer and Co-alloy targets onto a sputter-cleaned Si disk. The magnetic parameters and thickness of the magnetic layer are: coercive field H, = 950 Oe, remanent magnetization Mr=533 emu/cm3 and media thickness d = 400
A.
The discrete tracks were defined on the disk surface by sputter-etching through a patterned photoresist mask [I, 21. The tracks were well separated radially so that heads wider than the tracks could be used. Thin, but well sup- ported, tracks for transmission electron microscopy (TEM) were obtained by mechanical and ion thinning 3 mm diameter coupons from the disk. The fabri- cation of the materials and TEM sample preparation were described in detail elsewhere [3]. The micromag- netic structure was studied using Lorentz microscopy in Fresnel (phase contrast), Foucault (diffraction con- trast) and differential phase contrast modes.'IBM Magnetic Recording Institute. 2~~~ General Products Division.
Fresnel and Foucault images of a 3 pm wide discrete track with transition density of 200 fc/rnm are shown in figure 1. The arrows indicate the transition zones separating adjacent bits. Between the transition zones are regions that exhibit a remanence state of magne- tization. The Fresnel image (Fig. l a ) displays ripple structure within the bits. This structure is uniform and similar to that observed in a dc erased discrete track (Fig. 1 insert). The difference in major ripple wavelength between figure l a and the insert is proba- bly due to the different demagnetizing fields in the two cases.
Fig. 1. - (a) Fresnel image showing ripple structure within the bit and complex domain structure at the transition zones. (b) Foucault image exhibits good bit definition and magnetic fringing fields in free space adjacent to the track. Insert shows ripple structure in a track at remanence state after magnetization.
There is no observable change in the ripple struc- ture across the track width for all track widths stud- ied. This is despite the fact that the magnetization dispersion should give rise to demagnetizing fields at
the track edge. The uniform magnetic structure a t the
C8 - 1996 JOURNAL DE PHYSIQUE
edge of the track implies that there is no significant component of edge noise. This agrees with recording performance results reported previously [2].
The transition zone exhibits a mixture or ripple structures and distinct vortex-like features in which several domain walls meet at a single point. These transition zone structures were observed for all track widths and for all three transition densities.
The Foucault image clearly shows the individual bits and the magnetic fringing field in the free space ad- jacent t o the track. The magnetization reversal be- tween bits is reflected in the contrast reversal in the fringing fields. Quantification of the fringing fields has been carried out using differential phase contrast (DPC) Lorentz microscopy. This provides a map of the integrated in-plane component of magnetic induc- tion which is shown in figure 2 for the track in figure 1. Non-magnetic contrast from the substrate material, at the edge of the track, prevented the stray fields from being measured closer to the track edge. In this map the direction of the magnetic field is given by the direc- tion of the arrows, and the strength of the field is given by the arrow length. The largest arrows correspond to an integrated magnetic field which is 60 % of the M,d
of the track. The measurable field extends to more than 3 pm from the track edge, at which point the integrated field is less than 15 % of M,d. Comparison of these experimentally measured fringing fields with calculations show that the transition zone has a finite width of the order of 3000
A.
This study is reported in detail elsewhere [4].T R A C K E D G E
C
I
Fig. 2. - Vector map of the fringing fields from the 3 pm
discrete track shown in figure 1.
Summary
Written bits at densities of 200 to 1000 fc/mm on 0.6 t o 10 pm wide discrete tracks have been studied. The written bits are well-defined and regions between the transition zones are in saturated remanence state. Transition zone structure is complex and consists of both ripple and vortex-like structures.
The ripple structure within the bits was uniform all the way to the track edge for all track widths studied. This agrees with the conclusion obtained from record- ing data in previous studies [l, 21 that linear scaling of the recording physics extends t o sub-micron dimen- sions.
In continuous films, considerable magnetic structure is observed a t the track edges due to the fringing field of the write head. These features are absent in discrete tracks since there is no magnetic media at the edges. Instead, stray fields are observed a t the track edges which permit flux closure between transitions. The absence of side writing eliminates a potential limiting factor due t o edge noise in scaling track width to the one-micron range in metal film recording media. The DPC studies of the fringing fields indicate that the transition zone has a finite extent. A detailed study of this is reported elsewhere [4].
Acknowledgements
The authors are grateful to S. E. Lambert, I. L. Sanders, A.
M.
Patlach, M. T. Krounbi and K. Barron, for providing the discrete track media, and for fruitful and interesting discussions.[I] Lambert, S. E., Sanders, I. L., Patlach, A. M. and Krounbi, M. T., IEEE Trans. Magn. MAG-23 (1987) 3690.
[2] Lambert, S.
