Magnetic thin films are used to make the most recent class of magnetic hard drives. These films are made of invar materials and enable highly robust and high data storage density through the local heating of ultrasmall nano-domains using a laser.
This method is termed the so-called heat-assisted magnetic recording (HAMR). In such invar materials, the volume does not expand much despite heating. Thin films made of iron-platinum nanograins constitute a technologically relevant material for such HAMR data memories.
Under the guidance of the collaborative research group of Prof. Dr Matias Bargheer at HZB and the University of Potsdam, an international team has now achieved, for the first time, experimental observation of how the thermal expansion of the crystal lattice is canceled out by a unique spin-lattice interaction in such iron-platinum thin films.
Iron-platinum (FePt) falls into the family of invar materials in thermal equilibrium. These materials hardly expand upon heating. Although experts observed this phenomenon in the nickel-iron alloy “Invar” as early as 1897, only recently have they been able to understand the mechanism that drives it: In general, when solids are heated, lattice vibrations occur that result in expansion since the vibrating atoms need more space.
However, astonishingly in FePt, heating of the spins results in the opposite effect: the contraction of material contracts along the direction of magnetization is directly proportional to how warm it is. The outcome is the property characteristic of Invar—minimal expansion.
Led by Prof. Matias Bargheer, a team of researchers has now performed, for the first time, an experimental comparison of this exciting phenomenon on various iron-platinum thin films. Bargheer leads a collaborative research group at Helmholtz-Zentrum Berlin and the University of Potsdam.
In collaboration with researchers from Lyon, Brno, and Chemnitz, he intended to analyze how the behavior of perfectly crystalline FePt layers varies from that of the FePt thin films employed for HAMR memories. These include crystalline nanograins of stacked monatomic layers of platinum and iron interspersed in a carbon matrix.
The researchers locally heated and excited the samples using two laser pulses in quick succession and performed quantification through X-ray diffraction to identify the extent to which the crystal lattice expands or contracts locally.
We were surprised to find that the continuous crystalline layers expand when heated briefly with laser light, while loosely arranged nano grains contract in the same crystal orientation. 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.
Dr Matias Bargheer, Professor, Helmholtz-Zentrum Berlin
“Through these experiments with ultrashort X-ray pulses, we have been able to determine how important the morphology of such thin films is,” noted Alexander von Reppert, PhD student in Bargheer’s group and first author of the study. The key is a transverse contraction, also called the Poisson effect.
Bargheer added, “Everyone who has ever pressed firmly on an eraser knows this. The rubber gets thicker in the middle.”
The 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 spin driven contraction perpendicular to the film.
Alexander von Reppert, Study First Author and PhD Student, Helmholtz-Zentrum Berlin
Thus, FePt integrated into a carbon matrix is a highly unique material. Apart from exhibiting exceptionally strong magnetic properties, its thermomechanical properties also inhibit the creation of excessive tension upon heating, which would damage the material—and that is crucial for HAMR.
Von Reppert, A., et al. (2020) Spin stress contribution to the lattice dynamics of FePt. Science Advances. doi.org/10.1126/sciadv.aba1142.