Magnetic Anisotropy Used for Robust and High Performance Data StorageJuly 22, 2020 by Luke James
A joint research group has observed for the first time how special spin-lattice interaction in iron-platinum thin films cancels out the thermal expansion of its crystal lattice.
Today’s latest magnetic hard disk drives are made of magnetic thin films, which are invar materials (a nickel-iron alloy known for its low coefficient of thermal expansion with temperature changes). These thin films enable very high data storage density through the local heating of ultra-small nano-domains using a laser. This is known as heat-assisted magnetic recording or HAMR.
In addition to a nickel-iron alloy, another technologically relevant material for HAMR data memories are thin films of iron-platinum nanograins. Now, for the first time ever in research published in Science Advances, a joint team led by Professor Matias Bargheer at the University of Potsdam claims that they have observed how a special spin-lattice interaction in these thin films cancels out the thermal expansion of the crystal lattice.
Investigating Crystalline Iron-Platinum Layers
Indeed, it was this crystalline lattice that Bargheer’s team set out to investigate. In this research, samples were locally heated and ‘excited’ by two laser pulses in quick succession. The samples were then measured by x-ray diffraction to figure out how strongly the crystal lattice locally expands or contracts.
"We were surprised to find that the continuous crystalline layers expand when heated briefly with laser light, while loosely arranged nanograins contract in the same crystal orientation," explains Bargheer. "HAMR data memories, on the other hand, whose nano-grains are embedded in a carbon matrix and grown on a substrate react much weaker to laser excitation: They first contract slightly and then expand slightly."
Two laser pulses hit the thin film of iron-platinum nanoparticles at short intervals: The first laser pulse destroys the spin order, while the second laser pulse excites the now unmagnetized sample. An x-ray pulse is then used to determine how the lattice has expanded or contracted. Image credited to the University of Potsdam
The Poisson Effect
"Through these experiments with ultrashort X-ray pulses, we have been able to determine how important the morphology of such thin films is," says Alexander von Reppert, first author of the study and PhD student in Bargheer's group.
The secret, according to the researchers, is transverse contraction, also known as the ‘Poisson effect’. Reppert uses the example of pressing firmly onto a rubber eraser to illustrate how it works. The rubber gets thicker in the middle, and the nanoparticles can do that too, whereas in the perfect film there isn’t any room for expansion in the plane, which would need to go along with the spin-driven contraction perpendicular to the film.
This makes iron-platinum, embedded in a carton matrix, a special material. It not only possesses very robust magnetic properties, but its thermomechanical properties also stop excessive tension from being created when heated, which is important for HAMR because the material would otherwise be destroyed.