Electricity: measuring and testing – Magnetic – Magnetic information storage element testing
Reexamination Certificate
1999-08-20
2001-03-13
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic information storage element testing
C324S212000, C324S238000
Reexamination Certificate
active
06201390
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method of analyzing defects in a magnetic thin film, for example to determine relative levels of defects, the predominant type of defects, or defect distribution; and particularly, but not exclusively, to analyzing nano-sized defects in a magnetic thin film.
BACKGROUND OF INVENTION
Defects in magnetic thin films occur at the surface or within the film (in-depth). As the size of modern magnetic devices (such as magnetic read and write heads, recording thin film media and other magnetic sensors) is reduced, the thickness of magnetic thin films is also reduced. Indeed, the thickness of some of the magnetic thin films is approaching the nanoscale range. With increased miniaturization, the influence of nanoscale defects such as voids or dislocations in the magnetic thin films cannot be neglected. Taking the magnetic recording hard disk thin film media as an example, it is known that one of the most important reasons for high error rate of hard disk drives is due to contamination and defects on and in the thin film media. Furthermore, as the thickness of the magnetic layer for the hard disk thin film media decreases to around 10 nm and the average size of magnetic grains decreases to around 7.5 nm, and with an expected increase of the recording areal density of commercial recording disk media up to 10 Gbit/in
2
(1.6 Gbit/cm
2
) within five years, the problem of the nanoscale defects within the thin film media will become increasingly significant.
With equipment like the optical microscope and the atomic force microscope (AFM), it is possible to observe and characterize certain surface defects. However, there is still very little development of methods to observe and characterize nanoscale in-depth defects such as voids and dislocations in magnetic thin film, particularly in a fast, easy and non-destructive way. Although the high resolution electron microscope (HRTEM) may be used to analyze the nanoscale structure in thin films and to detect the nanoscale defects in them, sample preparation is tedious and time consuming. Thus, the HRTEM has only limited application to defect detection in magnetic thin film devices manufactured on an industrial production line. Ferrofluid detection may only by used to localize and characterize micron size defects in magnetic thin films. Although, magnetic force microscopy may be used to localize and characterize submicron sized defects in magnetic thin films, the approximate location of each defect must be known beforehand. A media tester may be used together with surface observation equipment to localize submicron indepth defects in the magnetic recording film media. However, this requires a sample which includes a protection overcoat and lubricant layer. In addition, it is difficult to give meaningful information based on the average result over a big area in a short time.
An object of the invention as to provide a way of analyzing defects, especially nano-sized defects, in magnetic thin films which overcomes at least some of the limitations of existing techniques to enable various information about the defects to be gathered efficiently.
SUMMARY OF INVENTION
According to a first aspect of the present invention, there is provided a method of analyzing defects in a magnetic thin film, comprising: applying a magnetic field to the magnetic thin film; measuring the magnetizations of the magnetic thin film over a range of different field strengths; calculating a value representative of a magnetic hardness coefficient for the magnetic thin film from the magnetizations measured; comparing the calculated value with a reference value; and determining defect information in dependence upon the comparison made.
The present invention is particularly useful for analyzing nano-sized defects in magnetic thin films of nano-sized thickness because the magnetic hardness coefficient is very sensitive to defect variations. This is due to the fact that the defect size is comparable to the thickness of the magnetic thin film. The magnetic hardness coefficient may be calculated from certain magnetization readings which are substantially inversely proportional to the applied field. Usually, the magnetization becomes inversely proportional to the applied field as the magnetic thin film approaches saturation.
It is known that the magnetization of a ferromagnetic body when subjected to an ever increasing magnetic field H, tends to a limit. This high field behavior has been modeled by the Law of Approach to Saturation (LATS) as given by Becker and Doring [Ferromagnetismus, Julius Springer, Berlin, 1939, pages 154-167] which is expressed in the form of a series:
M=M
s
(1
−a/H−b/H
2
−) +kH
where:
M is the magnetization of a body;
H is the applied field;
M
s
is the saturation magnetization;
a is the magnetic hardness coefficient
b relates to the magnetocrystalline anistropy constant; and
kH represents the forced magnetization.
The forced magnetization is the field induced increase in spontaneous magnetization, which is a very small contribution except at high field. Typically H has to be of the order of 10
5
or 10
6
Oe (10
7
or 10
8
amperes per meter) before this last term becomes significant.
Néel [C. R. Acade. Sci., 220: 738(1945)] proved by experiment and theoretical analysis that the above equation may be used to characterise the saturation magnetization process for specimens with different densities made from iron powder. He found that the iron specimens become more difficult to magnetize as porosity increased, giving rise to larger magnetic hardness coefficients, a. Brown [Phys. Rev. Vol. 60:139(1941)] also showed that certain deformations, arising from dislocations of the lattice would give rise to an approach law varying as 1/H for the magnetization. The deformations result from the decrease (negative dislocation) or increase (positive dislocation) by one unit of the number of atoms which constitute successive rows of the crystal.
The present applicants have appreciated that the pioneering theoretical and experimental work carried out by Becker and Doring, Néel and Brown may be applied to the analysis of defects in magnetic thin film. The calculated value may correspond to M
s
a (that is the magnetic hardness coefficient multiplied by factor M
s
) or it may in fact be the magnetic hardness coefficient. The type of defect information determined, in accordance with the present invention depends on the nature of the reference value.
The reference value may be representative of a magnetic hardness coefficient of a control specimen. The control specimen may be a comparable magnetic thin film having a predetermined defect level or concentration. Obviously, to give a meaningful result, the calculated value and reference value should be capable of comparison. For example, if the calculated value corresponds to M
s
a, the reference value must correspond to M
s
a
1
, where a
1
represents the magnetic hardness coefficient of the control specimen.
Suppose it is necessary to ascertain the suitability of a magnetic thin film sample for a particular role. There may be a threshold defect level which, if exceeded, will suggest that the magnetic thin film sample is not suitable for the intended role. In which case, the reference value is determined by calculating the magnetic hardness coefficient for a control specimen of magnetic thin film having the threshold defect level. If the magnetic hardness coefficient for the sample is greater than that of the control specimen, the sample may be deemed unsuitable for the intended role.
Alternatively, the reference value may be representative of a magnetic hardness coefficient calculated for the magnetic thin film when the magnetic field is applied in a different direction. The magnetic field directions for the two magnetic hardness coefficient calculations are preferably non-parallel, and may be perpendicular. The present applicants have found that the magnetic hardness coefficient may vary with the direction in which the fiel
Liew Thomas Yun Fook
Tan Lea Peng
Wang Jian-Ping
Andersen Henry A.
Data Storage Institute
Nixon & Vanderhye P.C.
Oda Christine
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