So-called “zero-point energy” is a term familiar to some cinema lovers or series fans; in the fictional world of animated films such as “The Incredibles” or the TV series “Stargate Atlantis”, it denotes a powerful and virtually inexhaustible energy source. Whether it could ever be used as such is arguable. Scientists at Jülich have now found out that it plays an important role in the stability of nanomagnets. These are of great technical interest for the magnetic storage of data, but so far have never been sufficiently stable. Researchers are now pointing the way to making it possible to produce nanomagnets with low zero-point energy and thus a higher degree of stability.
At about a nanometer, a billionth of a meter in size, quantum effects come into play. They make it harder, for example, to stabilize magnetic moments. Researchers worldwide are looking for suitable materials for magnetically stable nanomagnets so that data can be stored safely in the smallest of spaces.
Stable means that the magnetic moments point consistently in one of two preassigned directions. The direction then codes the bit. However, the magnetic moments of atoms are always in motion. The trigger here is the so-called zero-point energy, the energy that a quantum mechanical system possesses in its ground state at absolute zero temperature. “It makes the magnetic moments of atoms fluctuate even at the lowest of temperatures and thus works against the stability of the magnetic moments”, explains Dr. Julen Ibañez-Azpiroz, from the Helmholtz Young Investigators Group “Functional Nanoscale Structure Probe and Simulation Laboratory” at the Peter Grünberg Institute and at the Institute for Advanced Simulation. When too much energy exists within the system, the magnetic moments turn over and the saved information is lost.
“Our calculations show that the zero-point magnetic fluctuations can even reach the same order of magnitude as the magnetic moment itself”, reports Ibañez-Azpiroz. “This explains why the search for stable nanomagnets is so difficult”. There is, however, also a counterpart to this, in the form of an energy barrier, which the moment must overcome as it rotates. The height of the barrier depends on the material it is made from.
The Jülich researchers investigated how quantum effects influence magnetic stability in detail using particularly promising materials from the class of transition metals. From their results they have established guidelines for the development of stable nanomagnets with low levels of quantum fluctuations. Their chart showing the suitability of different elements should serve as a construction kit for combining complex nanomagnets made from several different atoms.
Artistic depiction of the magnetic fluctuations (blue arrows) of a single atom (red ball) lying on a surface (gray balls).
Copyright: Reprinted with permission from Nano Lett., DOI: 10.1021/acs.nanolett.6b01344. Copyright 2016. American Chemical Society
Stabilizing the magnetic signal of single adatoms is a crucial step toward their successful usage in widespread technological applications such as high-density magnetic data storage devices. The quantum mechanical nature of these tiny objects, however, introduces intrinsic zero-point spin-fluctuations that tend to destabilize the local magnetic moment of interest by dwindling the magnetic anisotropy potential barrier even at absolute zero temperature. Here, we elucidate the origins and quantify the effect of the fundamental ingredients determining the magnitude of the fluctuations, namely, the (i) local magnetic moment, (ii) spin–orbit coupling, and (iii) electron–hole Stoner excitations. Based on a systematic first-principles study of 3d and 4d adatoms, we demonstrate that the transverse contribution of the fluctuations is comparable in size to the magnetic moment itself, leading to a remarkable over 50% reduction of the magnetic anisotropy energy. Our analysis gives rise to a comprehensible diagram relating the fluctuation magnitude to characteristic features of adatoms, providing practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations.
SOURCES – Forschungszentrum Jülich, Nanoletters