Previously, in magnetic recording technology, increasingly higher data densities have been stored on disk drives using what is known as longitudinal recording techniques. As seen in FIG. 3 (Prior Art), in longitudinal recording the magnetization on the recording medium is switched between lying parallel and anti-parallel to the direction in which the write head is moving in relation to the surface. Although increases in areal densities of nearly 100 percent every year or two for the last several years have been achieved with longitudinal techniques, it is believed that the superparamagnetic limit is beginning to limit the capacities of this kind of recording. As a result, perpendicular recording is being adopted by many manufacturers to find ways to further increase areal density. In perpendicular recording, instead of “lying down” as it were, in same plane as the disk or magnetic surface, the bits are recorded “perpendicularly” in the magnetic surface. This enables much higher recording densities.
Perpendicular recording enables the use of media with higher coercivity. The write heads used to record data perpendicularly must generate much stronger magnetic fields as a result. The higher the coercivity, the more thermally stable and therefore, the less likely the medium is to succumb to superparamagnetic effect at higher than present recording densities. However, at the same time, this means that many of the conventional methods of measuring the magnetic properties of recording media may be inadequate for perpendicular recording applications, as they may not have sufficient magnetic field strength to switch the higher coercivity media.
Perpendicular recording media typically consist of at least a perpendicular recording layer and a soft magnetic underlayer (SUL). To switch magnetization in the perpendicular recording layer, a sufficient perpendicular magnetic field has to be applied. As seen in FIG. 5 (Prior Art) in a perpendicular hard disk drive, this is done with a microscopic single pole tip (SPT) write head 30 at near contact distance (of the order of 10 nanometer). In this situation, the SUL 12 functions as a magnetic flux feedback path for the SPT head, practically doubling the field of the head by the magnetic image effect.
For disk testers in line with production machines, a flying SPT head on a spin stand is not practical. At this point one needs to know the macroscopic magnetic properties such as MRT and HR for immediate process feedback. These properties need to be measured with great accuracy and repeatability (both fractions of a percent), over a large area of the disk, and it has to be done quickly (of the order of 1 disk per minute). For this purpose a macroscopic recording head is a good solution. Current art for longitudinal macroscopic disk measurement uses for example macroscopic heads of 2.5 mm wide, positioned at approximately 25-100 micrometer from the disk. The head is not actually flying, it is positioned to a predetermined distance. Typically 3 tracks are measured, an outer diameter (OD), middle diameter (MD) and inner diameter (ID) track.
For a perpendicular tester with a macroscopic SPT write head, the soft underlayer is too thin to function as a magnetic flux feedback. The soft underlayer is typically less than 1 micrometer thick and therefore can not carry the flux from a core that is orders of magnitude larger. Without the flux feedback through the SUL, the perpendicular magnetic field strength of a macroscopic perpendicular SPT head is limited, and may not be enough to even write the perpendicular media. This is particularly true as it is expected that over the coming years, coercivity will creep up in order to enhance thermal stability of ever smaller magnetic bits. Furthermore, for a macroscopic tester to function properly, it has to be able to not only switch the magnetization, but also to drive fields high enough to achieve maximum remanence, typically at least 2.2× larger than coercivity.
The principle of a macroscopic head residing altogether on one side of a disk has worked in the past for longitudinal magnetic inspection and measurement equipment. However for perpendicular testing and inspection equipment this approach yields insufficient magnetic field perpendicular to the disk.