# Synopsis: Pushing the superparamagnetic limit

#### High magnetic moments and anisotropies for FexCo1−x monolayers on Pt(111)

G. Moulas, A. Lehnert, S. Rusponi, J. Zabloudil, C. Etz, S. Ouazi, M. Etzkorn, P. Bencok, P. Gambardella, P. Weinberger, and H. Brune

Published December 17, 2008

The miniaturization of magnetic recording devices, which store information in nanosized magnetic grains or “bits,” is constrained by the so-called superparamagnetic limit: when grains are too small, thermal fluctuations can easily flip the direction of magnetization in each bit, causing permanent loss of information.

The design of a practically useful bit therefore requires finding materials with a high magnetic anisotropy, which is the energy required to flip the spin of a single atom. However, the magnetic field needed to polarize (i.e., “write”) the bit increases with the magnetic anisotropy, unless the magnetization per bit grows as well.

Writing in Physical Review B, Géraud Moulas and co-workers at the EPFL in Switzerland and collaborators in Austria, France, and Spain have made significant progress in achieving high magnetic anisotropies while maintaining acceptable writing fields using monolayer-thick films of ${\text{Fe}}_{x}{\text{Co}}_{1-x}$. They find that the magnetic anisotropy is enhanced by more than two orders of magnitude compared to what is found in bulk $\text{FeCo}$ alloys. At the same time, the average saturation magnetization is found to grow monotonically from $2{\mu }_{\text{B}}$ to $3{\mu }_{\text{B}}$ per atom for the case where the alloy is pure $\text{Co}$ to the case where it is pure $\text{Fe}$, a striking manifestation of reduced dimensionality. – Ashot Melikyan