Waltzing with Spintronics

IBM and ETH Zurich scientists in Zurich recently published a paper in Nature Physics which revealed the first-ever direct mapping of the formation of a persistent spin helix in a semiconductor. This new understanding in spintronics not only gives scientists unprecedented control over the magnetic movements inside devices but also opens new possibilities for creating more energy efficient electronics.

IBM scientists Matthias Walser
Matthias Walser is one of the scientists that made the discovery and he answered a few questions about himself and his research.

1. How did you get interested in your field of study?

Matthias Walser: During my time at university, IBM brought MRAMs from research to the real world and I heard more about this during a lecture by IBM scientist Rolf Allenspach, who for many years has been teaching at ETH Zurich about ferromagnetism and spintronics. Although I already had some interest in semiconductors and magnetism before, this lecture finally motivated me to apply for a PhD position in this field.

Side note: This paper was influential on Matthias, IBM Journal of Research and Development, Volume: 50, DOI 10.1147/rd.501.0003, (2006)

2. Your paper in Nature Physics goes back to a theoretical proposal in 2003. What made you believe that this proposal could be brought to reality in a lab?

MW: I think it is important to start a PhD with a fascinating idea: For my research it was the spin field-effect-transistor (FET). The first concept for a spin-FET was proposed in 1990, and it became famous by the name of its initiators - the Datta and Das transistor. The proposal by J. Schliemann and D. Loss follows the initial device concept in many ways, but overcomes one essential hurdle: The device does not require ballistic transport or in other words, it has the potential to work at much higher temperatures – surely a crucial benefit for applications.

But back to the original question, some proposals are more likely to work in practice than others, but there is no guarantee and possibly some subtleties hidden in the small print. Exemplary, the problem of efficient electrical spin injection into semiconductors is still unsolved. Certainly it is helpful if some knowledge and understanding is already available, but it also requires the time and means to develop and experiment with new materials and techniques. 

3. If you had to narrow it down to one thing, what was the major improvement in the paper?

Figure shows the measured spatial
and temporal spread of a spin helix.
MW: To make it clear, we can not present a spin-FET, but we can nicely illustrate how the physics behind non-ballistic spin-FET - the so-called persistent spin helix - looks in a real piece of material. 

When I started my work in 2009, J. D. Koralek and American co-workers just published the first experimental evidence for the strong lifetime enhancement that was predicted by theory. Loosely speaking, our major improvement on that is, that we can also make “photographic” recordings of the spin helix in a “stroboscope-like” way, and so we can directly map the formation of this fascinating spin state.

4. How did you know what material and technique to use and what was the ah-ha moment?

MW: Despite that silicon continues to be the number one material for the semiconductor industry, new materials such as III/V semiconductors (e.g. Gallium arsenide) have made their way to the market in the form of integrated circuits, infrared light-emitting diodes, and highly efficient solar cells. 

Material growth experts, such as our collaborators at ETH Zurich, can grow very clean and perfectly crystalline layers of such III/V semiconductor materials. By the choice of materials and the growth of multi-layer systems, one can design the optical, electrical, and magnetic properties in a certain regime -- which makes them a perfect test platform for a variety of physical phenomena.

The imaging is another story. We needed a technique that could monitor the temporal and spatial evolution of the tiny magnetic moment of an ensemble of electron spins. Two polarized laser pulses in a pump and probe configuration offer this capability if they are focused on a small spot. We first had to work on such a setup. 

The moment I saw the first periods of the spin helix appearing on the measurement monitor, I knew for sure that our setup works, and that the piece of material we use met the required properties.

5. What is next for this research?

MW: Primarily we are excited to have the technique and material ready to study such a spin helix. As we have a good understanding of the material parameters, we can now optimize the material structure. For example, there is still room for extending the lifetime of the spin helix beyond 1.1 nanoseconds. It will also be interesting to study how small constrictions and higher temperatures affect the spin helix.

In nanometer-sized devices such as CMOS devices today, we are approaching the boundary where quantum mechanical phenomena become important and interesting to study. The physics of spin is quite different than the physics of electric charge, and there is much to learn and improve until spintronics logic devices become competitive.

Thanks for your time Matthias.

Gern geschehen. (My pleasure.)

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