By Stephanie Laird

Summary: Roland Wiesendanger, a plenary lecturer at the recent joint NSS5/SP-STM2 international conference, discussed his research in understanding magnetic interactions at the single-atom layer using a subkelvin spin-polarized STM. During his presentation, Wiesendanger discussed the oscillatory behavior of the magnetic exchange coupling, which is critically impacted by distance, local environment and the substrate. Wiesendanger's research is paramount to the development of magnetic data storage devices, as he probes deeper into understanding the magnetic properties of single adatoms.

Roland Wiesendanger, a prominent nanoscientist conducting research on magnetic nanostructures at the single-atom level, delivered a plenary lecture at the recent Nanoscale Spectroscopy & Nanotechnology 5 and Spin-Polarized Scanning Tunneling Microscopy 2 (NSS5/SP-STM2) international conference entitled, “Oscillatory magnetic exchange coupling at the atomic level: a direct real-space study by a subkelvin spin-polarized STM.”

According to Wiesendanger, the current focus of his research is “to develop techniques to probe magnetic states of individual atoms and molecules; to understand the number of atoms or molecules needed to form a stable bit for storing information; to understand magnetics and spin fluctuations; and to determine the limitations for fast magnetic switching.”

In the past twenty years, spin-polarized (SP) STM, based on vacuum tunneling of spin-polarized electrons, has successfully been applied to study atomic-scale spin structures of ferrimagnetic, antiferromagnetic and ferromagnetic material systems. During his presentation, Wiesendanger presented recent studies of ground-state magnetic properties of individual magnetic adatoms on non-magnetic substrates as well as the interactions between them, which have become possible with the recent development of subkelvin SP-STM.

With the subkelvin SP-STM “we can now get single-spin sensitivity, resolve individual atoms, and probe the magnetic interactions at extremely small energy scales simultaneously,” said Wiesendanger. “This is needed because the energy scales involved in the various interactions are becoming very small. We can do very exciting measurements with the SP-STM system on individual atoms at surfaces and investigations on magnetic semiconductors,” he added.

“The principles underlying the indirect magnetic exchange interactions at the sub-milli-electronvolt energy scale are based on magnetic atoms or molecules on the surface that are interacting with the metallic substrate,” said Wiesendanger. “We can tune interactions by selecting particular types of magnetic substrates and atoms. This is needed to transfer information from one magnetic atom or molecule to another; this kind of transfer of information is mediated by the substrate.” We typically work with metal substrates in our research such as copper, gold or platinum, said Wiesendanger. “There are a great number of different substrates in order to tune the interaction of the magnetization of individual atoms and molecules.”

In his lecture, Wiesendanger presented SP-STM experiments performed at temperatures of 300 mK exhibiting indirect magnetic exchange interactions between individual paramagnetic adatoms as well as between adatoms and nearby magnetic nanostructures, which could directly be revealed in real space up to distances of several nanometers.

In both cases an oscillatory behavior of the magnetic exchange coupling, alternating between ferromagnetic and antiferromagnetic, were observed as a function of distance. Wiesendanger went on to demonstrate that “long-range oscillatory magnetic interaction between individual magnetic adatoms has significant consequences for their magnetic behavior which depends critically on the local environment.”

“We now can understand the interaction mechanism on a much smaller atomic scale, which gives hope that we can use the knowledge we have gained in a similar way as for magnetic multilayer systems,” said Wiesendanger. “Magnetic multi-layers are one dimensional – made of billions of atoms – but now we can study magnetic interactions at a much smaller scale. We are developing lots of ideas for how to use this information to create memory and logic devices. What we have studied here will form the basis for future concepts of magnetic data storage developments.”

The oscillatory behavior of the magnetic exchange coupling, alternating between ferromagnetic and antiferromagnetic, illustrates these interactions are strongly distance dependent, said Wiesendanger. “Depending on the distance between atoms and molecules, we can determine the magnetic moment of an individual atom or molecule. This gives us a lot of possibilities for designing the right geometry for the position of atoms or molecules in a magnetic device,” according to Wiesendanger.

By arranging atoms or molecules in a way we would like to have them we can gain full control over the type of magnetic interactions. Indirect exchange coupling is only one possibility for transferring data, he added, since we can also have a super-exchange mechanism. The dipole interactions that are present can also be used to transfer magnetic information. In all cases, the substrate plays an important role in mediating interactions, as does the distance, Wiesendanger stressed.

The role played by the local environment in determining the magnetic behavior of atoms and molecules is given by the substrate and by other atoms and molecules sitting close by, said Wiesendanger. “The spin state of a given atom is determined by many atoms in a given local environment. This can of course lead to a much more complex scheme of determining the spin state of an individual atom and transferring information. Many atoms in the neighborhood might determine the spin state of a given atom. If you design more complex arrangements, multiple interactions occur,” according to Wiesendanger, though it is important to start with simple arrangements in order to understand the interactions between two atoms or molecules. “In order to gain fundamental understanding, we need to start with the most simple arrangements; the next step is to study the interactions in a more complex case,” he said.

For measuring magnetization curves of single adatoms, the magnetic probe tip is positioned above a single atom sitting on a surface, then the spin-polarized current flow is measured as a function of an external magnetic field which is also applied, explained Wiesendanger. “In the case of a ferromagnetic atom we see a very nice hysteresis curve, which is a great surprise for a single magnetic atom on a non-magnetic substrate.”

According to Wiesendanger, these findings are significant because “we can combine atomic resolution and spin resolution, which has applications for magnetic semiconductors and molecular based systems as well as for magnetic metals. We can also study interactions between molecules, which is important because molecular spintronics is very promising. In addition to getting insight into the atomic structure of individual molecules we can now add the information about the spin dependent and magnetic properties of the molecule. The spin degree of freedom plays an important role in understanding interactions between molecules.” While Wiesendanger and his team have made significant leaps in understanding the oscillatory magnetic exchange coupling at the atomic level, there are still some challenges this research poses. “We are primarily working on a very detailed understanding of manipulating the spin state of single atoms or molecules,” said Wiesendanger. “Now we are going into a direction to manipulate the spin state purposely with the STM tip to develop a complete new concept for magnetic recording. All the information processing and magnetic storage devices today are mediated by magnetic stray fields. But by purposely increasing the spin polarized current, you can change the spin state of a single atom by making use of the spin transfer torque.”

Wiesendanger's research is concentrated on magnetic materials, but he is also doing a lot of measurements on molecular systems in general to provide a basis for understanding biological functions and interactions, he said. “One can probe the atomic structure of molecules and their electronic states, and progress has even been made in probing the vibrational states of molecules by the atomic force microscope. We can apply this technique to any kind of molecule, because the atomic force microscope does not rely on a conductive substrate.”

Wiesendanger's presentation on his current research illustrates promising advances in the area of magnetic interactions at the atomic level and its relevance in future magnetic data storage applications.