Probing the energy barrier distribution in arrays of patterned magnetic nano islands

Abstract

This work reports on the successful examination of the switching probability and switching probability distribution of magnetic nano islands of a bit patterned recording medium with an areal density of 100 Gbit/in^2 and 200 Gbit/in^2. For the experiments a SiNx film is prepatterned using standard electron beam lithography prior to the blanket deposition of a [Co/Pd][Co/Ni] multilayer based exchange coupled composite recording layer with perpendicular anisotropy. A static read/write tester, in which a conventional recording head is in physical contact with the sample, is used to investigate the magnetization reversal of individual bit islands. The switching probability of the magnetic islands is recorded for different write field exposure times ranging from 0.1us up to 1000us. As a function of the field exposure time it can be described by an extended Arrhenius-Néel model for the magnetization reversal in an applied field. The extended model assumes a normal distribution of the energy barriers for the magnetization reversal in the medium. The mean value and variance of the energy distribution are extracted from fitting the analytical expression that describes the model to the experimentally observed time-dependent switching probability data. Measurements carried out at different thermal fly-height control power values show that the switching probability scales with the used power. This is explained in terms of the increasing write pole protrusion towards the sample with increasing thermal fly-height control power. The increased protrusion directly transfers into a higher write field acting on the medium which lowers the energy barrier for the magnetization reversal. Due to the statistical nature of the thermally assisted magnetization reversal the lower energy barrier is equivalent to a higher switching probability. The fit of the analytical model to the experimental data yields that the mean of the energy barrier correspondingly scales inversely with the thermal fly-height control power. The field dependence of the energy barrier also explains the variation of the switching probability with the position of the island relative to the recording head. It is found that the remaining energy barrier is lowest for bits close to the trailing edge of the write pole and increases with increasing down-track distance from the trailing edge. Further measurements confirm that the remaining energy barrier also increases with increasing cross-track offset. This is in good qualitative agreement with the head field profile in the sample plane which is calculated from finite element simulations. Taking the calculated head field profile and the field dependence of the remaining energy barrier into account allows for the extrapolation of the islands’ thermal stability at zero field. The mean energy barrier at zero field is estimated to be on the order of 75 kBT to 85 kBT. Hence, the activation volume in which the initial critical reversal takes place is on the order of 700 nm^3 to 800 nm^3. This is equivalent to a cylindrical grain of 10 nm to 11nm in diameter which is well within the usually observed range for similar films. However, the activation volume, and thus the thermally stable unit, is much smaller than the island volume. This leaves room for improvement of the storage medium and is of technological relevance for future patterned media design. In order to maximize the stability of the medium against thermal fluctuations, the activation volume has to be increased. From a materials science point of view this means that the grain size and the intergranular exchange coupling have to be increased in order to end up with a thermally stable unit of the size of the complete island.