Rare-earth crystals offer hope for spintronics

Computer chips and storage elements are expected to function as quickly as possible and to be energy-saving at the same time; innovative spintronic modules are at an advantage here thanks to their high speed and efficiency. European scientists have now found promising properties in crystals grown from rare-earth atoms, which offer hope on the long path towards usage as spintronic components.

While modern computers are already very fast, they also consume vast amounts of electricity. For some years now a new technology has been much talked about, which could one day revolutionise computer technology: spintronics.

With these components, electrons no longer flow through computer chips; rather, the spin of the electrons serves as the information carrier. As explained by Professor Cornelius Krellner from Goethe University Frankfurt, “You have to imagine the electron spins as if they were tiny magnetic needles which are attached to the atoms of a crystal lattice and which communicate with one another.” How these magnetic needles react with one another fundamentally depends on the properties of the material.

To date, ferromagnetic materials have been examined in spintronics above all; with these materials — similarly to iron magnets — the magnetic needles prefer to point in one direction. In recent years, however, the focus has been placed on so-called antiferromagnets to a greater degree, because these materials are said to allow for even faster and more efficient switchability than other spintronic materials.

With antiferromagnets the neighbouring magnetic needles always point in opposite directions. If an atomic magnetic needle is pushed in one direction, the neighbouring needle turns to face in the opposite direction. This in turn causes the next but one neighbour to point in the same direction as the first needle again.

“As this interplay takes place very quickly and with virtually no loss, it offers considerable potential for entirely new forms of electronic componentry,” Prof Krellner said.

Above all, crystals with rare-earth atoms are regarded as interesting candidates for spintronics as these comparatively heavy atoms have strong magnetic moments — chemists call the corresponding states of the electrons 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. Prof Krellner and his colleagues have now studied seven materials with differing rare-earth atoms in total — from praseodymium to holmium — with their results published in the journal Nature Materials.

The problem in the development of spintronic materials is that perfectly designed crystals are required for such components as the smallest discrepancies immediately have a negative impact on the overall magnetic order in the material. As explained by Prof Krellner, “The rare earths melt at about 1000°C, but the rhodium that is also needed for the crystal does not melt until about 2000°C. This is why customary crystallisation methods do not function here.”

Instead the scientists used hot indium as a solvent. The rare earths, as well as the rhodium and silicon that are required, dissolve in this at about 1500°C. The graphite crucible was kept at this temperature for about a week and then gently cooled. As a result, the desired crystals grew in the form of thin disks with an edge length of 2–3 mm. These were then studied by the team with the aid of X-rays produced on the Berlin synchrotron BESSY II and on the Swiss Light Source at the Paul Scherrer Institute.

“The most important finding is that in the crystals which we have grown the rare-earth atoms react magnetically with one another very quickly and that the strength of these reactions can be specifically adjusted through the choice of atoms,” Prof Krellner said. This opens up the path for further optimisation.

There are still a great many problems to be solved on the path to market maturity, however. Thus, the crystals — which are produced in blazing heat — only deliver convincing magnetic properties at temperatures of less than -170°C.

“We suspect that the operating temperatures can be raised significantly by adding iron atoms or similar elements,” Prof Krellner said. “But it remains to be seen whether the magnetic properties are then just as positive.”

Image caption: The layered crystal structure, highlighting the layer-by-layer antiferromagnetic reversal along the [001] direction. Image courtesy of the study authors (cropped from the original) under CC BY 4.0

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