Energy barriers in patterned media

Abstract

The key to successful data storage is ensuring that the information is not lost. For thousands of years humankind has stored information, from engravings in marble to magnetic data storage in current times. To ensure the stability of the stored data, an energy barrier is required which separates information from non-information. In marble the energy barrier is caused by the attractive force between atoms and in magnetism it is due to the magnetic anisotropy. Current hard disk technology has the capability of storing huge amounts of information. A future replacement to the current hard disk is bit patterned media which will make even larger storage capacities possible. In bit patterned media a bit consists of a magnetic island, separated by non-magnetic material. When a magnetic field is applied, which is sufficiently large to overcome the energy barrier, the magnetisation direction will reverse and the bit is written. Because of differences in these energy barriers, there is a variation between islands in the strength of the magnetic field required to reverse the magnetisation direction. This phenomenon is known as the switching field distribution. It is possible that there are multiple energy barriers which prevent the complete reversal of the magnetisation. In chapter 2 a situation where multiple energy barriers exist is investigated by applying a magnetic field under an angle with the magnetic easy axis. Due to the fact that thermal activation aids in overcoming the energy barrier, the required field for reversal varies from instance to instance for the same island. This thermally induced switching field distribution can be used to determine the difference in energy barrier of magneticallyweak and strong islands. In chapter 3 an alloy aswell as twomultilayer structures are compared using this method. In chapter 4 the temperature dependence of the thermally induced switching field distribution is investigated. For archival purposes it is not the data capacity but the longevity of the data which is most relevant. When we want store information which will outlast the human race, the medium will have different requirements than a mediumused for everyday information storage. For a hard disk, aminimumrequired storage time of 10 years is acceptable, but for such an archival storage system, 1 million to 1 billion years is the relevant timescale. In chapter 5 a storage system has been investigated with high energy barriers which should make data storage for such timescales possible.