Novel Methods to Provide New Insights Into Crystallography

With the help of POLI, a single-crystal diffractometer developed by RWTH Aachen and jointly operated with Forschungszentrum Jülich, Dr. Vladimir Hutanu succeeded in measuring the magnetic parameters of the  multiferroic compound Ba₂CoGe₂O₇ by using a new method. The investigations conducted at the Heinz-Maier-Leibnitz-Zentrum in Garching led to valuable results which are expected to contribute to the development of novel data storage solutions.

 

Ba₂CoGe₂O₇ is a dark blue crystal developed in the lab by a team of Japanese researchers. It is not the color that is of interest to researchers, but the multiferroic compund’s electric and magnetic properties and effects. A special characteristic of multiferroics is that external electric fields can change the magnetic orientation of the material. Importantly, such materials can be used for the development of cost-efficient data storage devices.

Magneto-electric materials have been known for over a hundred years, but their potential to provide new storage technologies was discovered only in 2003. Subsequently researchers set out to discover new multiferroics and developed various theories to better understand the interaction between electricity and magnetism in these materials and thus to optimize them for potential applications. However, the validity of the competing theoretical models can only be tested using highly complex experimental measuring methods.

3D Polarization Analysis Using Hot Neutrons

An international research group led by Dr. Vladimir Hutanu now succeeded in measuring the magnetic parameters of the Ba₂CoGe₂O₇ compound much more precisely than hitherto possible, with the help of the single-crystal diffractometer “POLI.” In order to be able to exploit the unique characteristics of neutron beams for research purposes, the RWTH Institute of Crystallography, in cooperation with Forschungszentrum Jülich, is conducting research at the Heinz Maier Leibnitz Center in Garching near Munich, a facility for cutting-edge research with neutrons and positrons that operates a research reactor and provides a state-of-the art neutron source. There, Hutanu conducted a 3D polarization analysis using hot neutrons – a method that can only be applied at two research centers worldwide, in Garching and at the Institute Laue-Langevin, Grenoble, France.

In a first step, in order to determine the exact alignment of atoms in the crystal, Hutanu used non-polarized neutrons. Contrary to several existing models, he found that the crystal has more of an orthorhombic than a tetragonal structure. Based on this result he was able to analyze the crystal using polarized neutrons and gain information on the alignment and size of the elementary magnetic moments of individual unpaired electrons within the crystal lattice. In order to obtain this information, it was necessary to decipher the domain pattern beforehand.

Many crystals consist of identically structured areas or domains which are differently oriented towards each other and thus minimize the total energy of the crystal. The Ba₂CoGe₂O₇ crystal features domains whose magnetic moments are perpendicular to each other. For the first time, it was possible to investigate and determine the proportion of each domain type in the crystal and their change under the influence of magnetic and electric fields. Without the influence of a magnetic field, all domain types occur in equal proportions, which results in a total magnetic moment of zero. This result contradicts a theory which predicted a positive, albeit small, total magnetic moment.

Another theorist calculated that it requires an extremely strong magnetic field of about one tesla – about 30000 times higher than the earth’s magnetic field – to push only one of these small domains across the entire crystal. The experiment, however, had a surprise in store for the researchers: The crystal reacted very quickly to a change in magnetic field, and only one tenth of the theoretically assumed field strength was sufficient to realize a single domain structure. With the help of these unique experiments, the research group succeeded in demonstrating experimentally that only one of the several competing theories is supported by the measurement results.

Measurements Under Extreme Conditions

The complex experiment required a large number of measurements under extreme conditions. For example, the observed effect only occurs at very low temperatures, in this case 266.5 °C. In order to protect the very sensitive measurements, the experiment had to be shielded from the earth’s magnetic field using niobium shields, which had to be constantly cooled down below -264 °C during the experiments – a complex process which requires a very accurate monitoring of several parameters. Working under such difficult conditions, however, made it possible to shed light on the interaction mechanism between magnetism and electricity at the microscopic level of a crystal such as Ba₂CoGe₂O₇.

The result of the study were published in the high-profile journal Physical Review B. The researchers plan to apply the successful measuring method and their experience to other compounds, so as to be able to achieve groundbreaking results at room temperature.