Ferromagnetism occurs when the magnetization of a material, the average orientation of its microscopic magnetic moments, is preserved along a specific direction without having to apply an external magnetic field. It is a common feature of permanent magnets and, for example, it is the way we use to keep encoded our data in a spinning hard-disk. Conceiving a single atomic layer system that preserves a net magnetization at relatively high temperatures is a challenging result. Even more difficult is to realize a system formed by an alloy of magnetic and non-magnetic atoms, such as Gd and Ag, that show such property in monolayer thickness and that is also stable over structural and chemical disturbances.
A group of researchers from the Applied Physics Department of the University of the Basque Country, led by Enrique Ortega, in collaboration with scientists from the Liège University and the Catalan Institute of Nanoscience and Nanotechnology (ICN2), has demonstrated that a single layer GdAg2 alloy has a remarkable high Curie temperature of 85K, it is structurally stable and may offer a template for the ordered growth of other materials, as for example organic molecules or metal clusters, without losing its magnetic properties. By using a combination of techniques, including ALBA's X-rays, the researchers have been able to demonstrate the origin of the surprising phenomenon.
Combining different research techniques
The researchers have examined their samples structure with scanning tunneling microscopy and electron diffraction, finding that the GdAg2 alloy at single layer coverage forms a structurally perfect atomic lattice modulated by a long-range "moiré", as shown in Fig 1a.
Then they replicated the preparation at the BOREAS beamline in order to investigate the magnetic properties of such GdAg2 monolayer as a function of temperature with the XMCD technique (Fig 1b), obtaining a Curie temperature of about 75K, in agreement with magneto-optic Kerr effect experiments (85 K).
Finally, they investigated the GdAg2 valence band electronic structure close to the Fermi level, both experimentally, by means of angular resolved photoemission (ARPES), and theoretically, with DFT calculations, finding that the Gd-Gd magnetic coupling in the alloy is mediated by hybrid Ag-Gd states as efficiently as in a pure metal ferromagnet.
Figure 1: (from left to right: the STM and electron diffraction (LEED) of the GdAg2 monolayer showing the moiré modulation; center) Gd XMCD spectra collected on a GdAg2 monolayer at BOREAS beamline at 5K of temperature and 6T of applied field; right) temperature dependent magnetization curves (Arrot plot) collected at BOREAS showing (inset) the decreasing of the remanent magnetization as a function of temperature indicating a Curie temperature of 75K.
