Robust, high-performance data storage via magnetic anisotropy

The latest generation of magnetic hard drives is made of thin magnetic films, which are invar materials. They allow for an extremely robust and high data storage density by locally heating ultra-small nanodomains with a laser, called heat-assisted magnetic recording, or HAMR. The volume in such invar materials hardly expands despite heating. A technologically relevant material for such HAMR data memories are thin films of iron and platinum nanograins. An international team led by Prof. Dr. Matias Bargheer’s joint research group at HZB and the University of Potsdam have experimentally observed for the first time how a special spin lattice interaction in these thin iron and platinum films cancels thermal expansion . of the crystal lattice.

In thermal equilibrium, iron-platinum (FePt) belongs to the class of invar materials, which hardly expand when heated. This phenomenon was observed as early as 1897 in the nickel-iron alloy “Invar”, but only in recent years have experts been able to understand what mechanisms are driving it: normally, the heating of solids leads to reticular vibrations that cause expansion because vibrating atoms need more space. Surprisingly, however, heating the spins in FePt leads to the opposite effect: the warmer the spins, the more the material contracts in the direction of magnetization. The result is Invar’s known property: minimal expansion.

A team led by Professor Matias Bargheer has now experimentally compared this fascinating phenomenon for the first time in different thin films of iron and platinum. Bargheer leads a joint research group at Helmholtz-Zentrum Berlin and the University of Potsdam. Together with colleagues from Lyon, Brno and Chemnitz, I wanted to investigate how the behavior of perfectly crystalline FePt layers differs from the FePt thin films used for HAMR memories. These consist of crystalline nanograins in stacked monoatomic layers of iron and platinum embedded in a carbon matrix.

The samples were heated and excited locally with two laser pulses in rapid succession and then measured by X-ray diffraction to determine how strongly the crystal lattice expands or contracts locally.

“We were surprised to find that continuous crystalline layers expand when heated briefly with laser light, while freely arranged nano grains contract in the same crystal orientation,” explains Bargheer. “HAMR data memories, on the other hand, whose nanograins are embedded in a carbon matrix and grow on a substrate, react much weaker to laser excitation: first they contract slightly and then expand slightly.”

Alexander von Reppert, first author of the study and Ph.D. Student in Bargheer’s group says: “Through these experiments with ultrashort X-ray pulses, we have been able to determine how important the morphology of such thin films is.” The secret, he says, is the transverse contraction, also known as the Poisson effect. .

“Everyone who has pressed firmly on an eraser knows it,” says Bargheer. “The rubber becomes thicker in the middle.”

Reppert adds: “Nanoparticles can do that too, whereas in the perfect film there is no room for expansion in the plane, which would have to go along with the contraction driven by the rotation perpendicular to the film.”

So FePt, embedded in a carbon matrix, is a very special material. It not only has exceptionally robust magnetic properties. Its thermomechanical properties also prevent excessive stress from being created when heated, which would destroy the material, and that is important to HAMR!