While modern computers are already very fast, they also consume large amounts of electricity. In recent years, there has been a lot of talk about a new technology which, although still in its infancy, could one day revolutionize computing: spintronics. The word is a portmanteau for “spin” and “electronics”, because with these components the electrons no longer flow through the computer chips, but the spin of the electrons serves as an information carrier. A team of researchers with staff from Goethe University Frankfurt have now identified materials that have surprisingly fast properties for spintronics. The results were published in the specialist magazine “Natural materials.”
“You have to imagine the spins of electrons as if they were tiny magnetic needles attached to the atoms of a crystal lattice and which communicate with each other,” explains Cornelius Krellner, professor of experimental physics at Goethe University Frankfurt. How these magnetic needles react with each other is fundamentally dependent on the properties of the material. To date, ferromagnetic materials have mainly been studied in spintronics; with these materials – as with iron magnets – the magnetic needles prefer to point in one direction. In recent years, however, more emphasis has been placed on so-called antiferromagnets, as these materials are said to provide even faster and more efficient switching than other spintronic materials.
With antiferromagnets, neighboring magnetic needles always point in opposite directions. If an atomic magnetic needle is pushed in one direction, the neighboring needle rotates to face the opposite direction. This in turn causes the penultimate neighbor to again point in the same direction as the first needle. “As this interaction takes place very quickly and with virtually no pressure drop, it offers tremendous potential for entirely new forms of electronic components,” says Krellner.
Above all, crystals with rare-earth group atoms are considered attractive candidates for spintronics because these relatively heavy atoms have strong magnetic moments — chemists call the corresponding electron states 4F orbitals. Among the rare earth metals – some of which are neither rare nor expensive – are elements such as praseodymium and neodymium, which are also used in magnet technology. The research team has now studied seven materials with different rare earth atoms in total, from praseodymium to holmium.
The problem in the development of spintronic materials is that perfectly designed crystals are needed for such components, because the smallest deviations immediately have a negative impact on the overall magnetic order in the material. This is where the know-how from Frankfurt came into play. “The rare earths melt at around 1000 degrees Celsius, but the rhodium which is also needed for the crystal does not melt until around 2000 degrees Celsius”, explains Krellner. “That’s why the usual crystallization methods don’t work here.”
Instead, the scientists used hot indium as the solvent. The rare earths, as well as the necessary rhodium and silicon, dissolve in it at around 1500 degrees Celsius. The graphite crucible was held at this temperature for about a week and then gently cooled. As a result, the desired crystals grew in the form of thin discs with an edge length of two to three millimeters. These were then studied by the team using X-rays produced on the Berlin synchrotron BESSY II and on the Swiss light source at the Paul Scherrer Institute in Switzerland.
“The most important finding is that in the crystals we have grown, rare-earth atoms react magnetically with each other very quickly, and the strength of these reactions can be specifically tuned by the choice of atoms,” Krellner explains. . This paves the way for further optimization – ultimately spintronics is still purely basic research and years away from commercial component production.
However, there are still many issues to be resolved on the road to market maturity. Thus, the crystals – which are produced in blazing heat – only offer convincing magnetic properties at temperatures below minus 170 degrees Celsius. “We suspect that operating temperatures can be increased significantly by adding iron atoms or similar elements,” Krellner says. “But it remains to be seen whether the magnetic properties are then equally positive.” Thanks to the new results, the researchers now have a better idea of where it makes sense to change the settings.