By Josh Perry, Editor
[email protected]

Scientists from the Helmholtz-Zentrum Berlin (HZB) research group and the University of Potsdam in Germany have investigated heat transport processes at the nanoscale using metallic-magnetic model systems, which could potentially lead to next-generation, high-efficiency data storage devices that are locally heated and rewritten by laser pulses (heat-assisted magnetic recording).

The laser pulse (red) generates heat in the thin-film system. The physical mechanisms by which the heat is distributed can be analyzed by temporally resolved X-ray diffraction experiments. (HZB/University Potsdam)

According to a report from HZB, the measurements showed that heat is distributed more than 100 times slower than expected in the model system, which is comprised of nanometer-thick metallic and magnetic layers.

“The model system consists of a nanometer-thin ferromagnetic nickel layer (12.4 nm) applied to a magnesium oxide substrate, with an even thinner layer of gold (5.6 nm) deposited over the nickel,” the article explained. “Using an ultra-short laser pulse (50 femtoseconds), the physicists introduced heat locally into the model system, then with extremely short X-ray pulses (200 femtoseconds), determined how the heat was distributed in the two nanolayers over time.”

Researchers discovered that the system took far longer to reach thermal equilibrium than expected. Rather than one picosecond, it took 100 times longer.

The reason behind this time lapse was that the energy from the laser was transferred to the nickel electrons instead of the gold crystal lattice that it hit first.

“Because the electron system in nickel is much more strongly coupled to the nickel crystal lattice vibrations than in the case of gold, the nickel crystal lattice absorbs the heat from the nickel electrons faster and the nickel electrons initially cool,” the article continued. “However, since the heat conduction through the now warmer but poorly conducting nickel crystal lattice directly to the cooler gold crystal lattice is very low, the thermal energy finds another pathway from the warmer nickel lattice to the cooler gold lattice.”

This forces the thermal energy to flow back from the nickel lattice to the gold electrons, which in turn excite the gold lattice vibrations.

“Future data memories based on what are referred to as heat-assisted magnetic recording techniques (HAMR) can be locally heated and overwritten with laser pulses, the article concluded. “With a deeper understanding of the transport processes, such systems might be able to be developed in such a way that they can manage with minimal input energy.”

The research was recently published in Nature Communications. The abstract read:

“Ultrafast heat transport in nanoscale metal multilayers is of great interest in the context of optically induced demagnetization, remagnetization and switching. If the penetration depth of light exceeds the bilayer thickness, layer-specific information is unavailable from optical probes. Femtosecond diffraction experiments provide unique experimental access to heat transport over single digit nanometer distances.

“Here, we investigate the structural response and the energy flow in the ultrathin double-layer system: gold on ferromagnetic nickel. Even though the excitation pulse is incident from the Au side, we observe a very rapid heating of the Ni lattice, whereas the Au lattice initially remains cold.

“The subsequent heat transfer from Ni to the Au lattice is found to be two orders of magnitude slower than predicted by the conventional heat equation and much slower than electron–phonon coupling times in Au. We present a simplified model calculation highlighting the relevant thermophysical quantities.”