Arguably the smallest bar magnets in the world

Most of us probably know magnets as the colourful badges that we stick on our fridge door, as the buttons on our handbag, or as moveable com-pass needles. However, numerous electrical devices—from the telephone to the particle accelerator—only function thanks to their electromagnetic components. Chemists at the University of Jena are investigating a special type of magnets—they are meticulously working on magnetic polymers that could be used to store large volumes of data.

Dr Michael Böhme is using computa­tional chemistry to work on molecular chains that behave like tiny magnets. The chemist describes the object of his research: »These are polymers in which a large amount of magnetic metal ions, such as cobalt, are lined up like a pearl necklace«. The individual metal cen­tres form their own magnetic domains, which can store magnetic information.

If, at some point in the future, we want to use these magnetic molecules as data storage systems, we have to exactly un­derstand and predict their properties, which is technically infeasible at the moment. »These systems are highly complex,« explains Böhme. The chains are not infinitely long in reality, which means their ends also affect the prop­erties. »And the metal centres are not identically structured. The order in which they are arranged also has an effect on the magnetism that we can ultimately observe in the experiment«. This puts to the test all previous the­oretical models that researchers have used to interpret and predict the prop­erties of the tiny »bar magnets«

Therefore, Böhme is simplifying his calculations by firstly looking at mo­lecular rings of various sizes instead of an endless chain of molecules. He has been working with Prof. Dr Winfried Plass from the Institute of Inorganic and Analytical Chemistry at the Uni­versity of Jena to develop a computer model that can be used to better inter­pret the experimental data of the real molecules and predict their magnetic properties with greater accuracy. 

A 100-year-old model is still used for research today

Until then, however, there is another problem to be solved: »The comput­ers at our disposal are not powerful enough to calculate the properties of long chains. They need around one week to carry out ab initio calculations for one single metal centre. It simply isn’t feasible to calculate a complete do­main from several centres with the cur­rent technology,« explains Prof. Plass.

The »Ising model« was developed way back in the 1920s to study magnetic molecular chains in a highly simplified manner. »The Ising model has essen­tially been used for a hundred years,« says Plass.

Michael Böhme has now developed a less idealized model on the basis of ab initio calculations, which is closer to re­ality than the Ising model. »In addition to the actual metal centres, the links that facilitate interaction between the magnetic centres are also important,« explains Böhme. »We can obtain this information by adapting the theoret­ical model to the actual measurement data, which ultimately enables us to calculate the domain properties. This also allows us to make predictions as to the behaviour of previously unknown sin gle­chain magnets«.

Instead of calculating an endless chain, Böhme has applied his model to rings with three, six, nine, and twelve mem­bers. »Twelve is the highest possible amount for us, because there are 4,096 possible states that have to be calculat­ed,« explains Michael Böhme. »How­ever, we can then extrapolate this data to accurately predict the properties of longer chains«. Winfried Plass highlights some of the potential future applications: »Mag­netic materials are highly suitable for storing information. Individual mag­netic molecules can store much more information than the current storage media, where individual areas are mag netized«.

By Marco Körner