Micro-indentation and scanning probe microscopy to assess multilayer magnetic film damage

Abstract

We have combined several techniques to evaluate the effect of mechanical deformation on the topography and magnetics of an underlying thin magnetic film. Micro-indentation techniques are used to introduce tailored deformations to magnetic recording disks. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) then map out the topography of the indentations and the effect on the underlying magnetic film. Indentation morphology includes pyramidal and conical indents, linear scratches, and reciprocating wear tracks. The extent of topographical damage assessed by AFM correlates with the carefully controlled normal and tangential forces of the micro-indenter. Sub-surface `damage' is assessed by MFM, monitoring for loss of resolution in pre-written magnetic data tracks. Mechanical tests are conducted at very low speeds and loading rates; hence, frictional heating or other thermo-mechanical factors that might cause ambiguities in data interpretation can be safely ruled out. By using the combination of micro-mechanical testing, Raman spectroscopy, and AFM/MFM techniques, we have found that mechanically introduced bit contrast degradation is attributed to physical modification of the magnetic coating and protective overlayer. The indentation and scratch tests both confirm that static pressure alone is not sufficient to cause the observed magnetic degradation; other mechanical factors must be invoked, including alteration of the geometry of the magnetic coating. Microwear is capable of causing magnetic damage at much lower pressures than indentations or scratches. From wear track data, the degree of disk surface damage is correlated with the degree of magnetic bit contrast degradation. From wear track data on DC-erased samples it is concluded that mechanical deformation can induce magnetic contrast via concerted reorientation of grain c-axis and, hence, reorientation of magnetic moments.