A temperature difference of just 50°C can determine whether an ultrathin magnetic film stays smooth or begins breaking apart, according to new research that could help scientists design better materials for future data storage.
Magnetic materials are essential for technologies such as hard disk drives and magnetic random-access memory, or MRAM.
These devices store information using magnetic states that represent digital data. For reliable storage, those magnetic states need to remain stable rather than accidentally changing because of heat or other disturbances.
Researchers at the University of Toyama in Japan studied an ultrathin magnetic material made from iron and palladium, known as L10-FePd. The films were only about five nanometers thick.
FePd is particularly promising for MRAM because its magnetization can be switched using relatively little energy.
However, producing films that are both extremely smooth and highly ordered at the atomic level has been difficult.
Greater atomic order generally improves magnetic stability. Heating the material can encourage its iron and palladium atoms to arrange themselves into a highly ordered structure, but high temperatures can also make the surface rough and cause the film to break apart.
In the study, published in the Journal of Alloys and Compounds, the researchers investigated whether a two-stage heating process could provide better control.
They first deposited iron and palladium atoms onto a strontium titanate surface at relatively low temperatures. The samples were then cooled to room temperature before being heated again to 600°C. This second heating stage gave the atoms enough energy to rearrange themselves into the desired crystal structure.
Surprisingly, the temperature used during the first stage made an enormous difference.
When the film was initially grown at 150°C, it remained smooth and continuous after later heating. This produced a flat magnetic film with properties that could be particularly useful for MRAM.
Increasing the initial temperature by only 50°C, to 200°C, produced a dramatically different result. At this temperature, atoms could move around more easily, triggering a process known as solid-state dewetting.
During dewetting, an ultrathin film begins pulling apart rather than remaining as a continuous layer. In the experiments, the FePd developed square-shaped holes. At the same time, however, it achieved an almost perfectly ordered atomic structure.
When the initial temperature was raised further to 300°C, the film became much rougher and separated into island-like structures.
Computer calculations showed that differences in surface energy between the magnetic film and the material beneath it help drive this behavior. However, defects created during the first heating stage appear to determine when the breakup process actually begins.
The discovery means scientists could use temperature as a relatively simple tool for producing different magnetic structures for different technologies.
Smooth films produced at 150°C could be well suited to energy-efficient MRAM. Meanwhile, carefully controlled dewetting at around 200°C could potentially create self-organized nanoscale magnetic structures useful for extremely high-density data storage.
By learning how to control instability rather than simply trying to prevent it, researchers may eventually develop magnetic memory that stores more information while consuming less energy.
