Transition metals are 38 elements in the periodic table that are all conductive. What makes transition metals different from the other elements is that they have valence electrons (electrons that combine with other elements) in both their inner shell and their outer shell. This means that when they are combined with oxygen, an oxidation state arises that can give these metals unique properties.
Now researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) in Germany have taken this oxidation process for four transition metals (manganese, iron, cobalt and nickel) to a new level. For the first time, they have successfully created a one-dimensional magnetic atom chain of these metals by using oxygen. The researchers claim that this feat will prove to have a fundamental impact on magnetic data storage as well as chemistry in general.
In experiments described in the journal Physical Review Letters, the researchers grew these transition metals on an iridium surface. In a departure from the usual results of separation from a substrate, the researchers used the oxygen to separate the metals from the substrate while maintaining their magnetic properties. The entire process occurs through a process of self-assembly in which the atoms arrange themselves into these one-dimensional chains.
“Evaporating metals onto a metallic surface in a vacuum is a common procedure,” explained Alexander Schneider, a professor at FAU who led the research, in a press release. “However, this often produces a two-dimensional layer of metal. For the first time, with the help of oxygen, we have managed to produce atom chains that cover the entire iridium surface, are arranged with a regular distance of 0.8 nanometres between each atom and can be up to 500 atoms long, without a single structural fault.”
The researchers observed that the oxygen serves as a kind of lifting mechanism to separate the transition metals from the substrate. This lifting mechanism involves the bonding geometry that dictates the position of the atoms in that system.
“Essentially the oxygen clings to the magnetic atom (binds strongly and sucks away electrons) which in turn looses interest (electrons) in binding to the iridium,” Schneider told IEEE Spectrum in an e-mail interview. “As a consequence the position of the magnetic atom is determined by the bond angles to the oxygen which are least bent if the magnetic atom is lifted from the iridium.”
In their one-dimensional form, these transitional metals take on slightly different properties: nickel takes on a non-magnetic state, cobalt maintains its ferromagnetic properties, while iron and manganese become antiferromagnetic. It is the alternating state of magnetism that each link possesses that is a unusual feature of the chain.
“This means that we can create mixed systems in which ferromagnetic sections of chains can be separated from antiferromagnetic or non-magnetic sections, for example,” said Schneider.
The restriction of the dimensionality of a material produces much of its unique properties. For instance, a two-dimensional material like graphene, has very different properties from its carbon cousin, the one-dimensional carbyne. Now that it’s possible to remove a one-dimensional atom chain from a thin film substrate that is in two dimensions, it’s possible to foresee more research into the potential of one-dimensional systems.
For instance, the researchers expect that this research will lead to further experiments in which a number of different pieces of chains with different lengths and magnetic properties can be tested to see how they can lead to further miniaturization in data storage.
Schneider added: “We will look for other similar exciting configurations in other nanostructured oxides and also investigate molecular systems for which we may use the found system as a substrate.”