Fri. Jan 21st, 2022

A strategy to control the spin polarization of electrons using helium

Figure showing a helium atom trapped between the STM tip and the sample. Credit: Coach et al.

Spintronics, also known as spin electronics, is a field of research that explores how the electrons’ inherent spin and its magnetic moment can be exploited by devices. Spintronic devices are promising for a wide range of applications, especially for efficient data storage and transfer.

The key requirement for spintronic devices is the ability to control and detect spin polarization of electrons. The spin polarization is essentially the degree to which the spin (i.e., the inherent angular momentum of electrons and other elementary particles) is adjusted in a particular direction.

Researchers at the University of St Andrews in the UK and other institutes worldwide have recently shown that helium can affect the spin polarization of the tunnel current and the magnetic contrast of a technique known as spin-polarized scanning tunneling microscopy (SP STM). Their results, published in Physical review letters, could have important implications for the development of new electronic devices.

In their previous research, the same group of researchers studied the magnetic sequence of the antiferromagnetic material iron telluride. Remarkably, they found that by collecting magnetic material from the surface of their sample using an STM tip, they were able to image the magnetic sequence of the sample.

“As part of my PhD project, I had to set up a new STM in a vector magnet, and one of the first measurements I set myself was to reproduce this imaging,” Christopher Trainer, one of the researchers who conducted the study told “I tried hard but could not make it work. This was a big puzzle for us because this measurement usually worked pretty straightforwardly until we found out that the new microscope had a leak in its vacuum seal so it was liquid. helium that we used to cool the experiment could enter the measuring chamber. ”

Based on their previous observations, Trainer and his colleagues set out to test the hypothesis that helium could affect their microscope’s ability to image the magnetic order. To do this, they fixed the helium leak and systematically added helium to their microscope measuring chamber. Their experiments revealed that helium trapped between the STM tip and their sample could completely suppress the microscope’s ability to detect the magnetic order.

A strategy to control the spin polarization of electrons using helium

Scanning tunneling microscope images of Iron Telluride surface before and after helium were added. In the vacuum image, the magnetic order appears as a stripe-like pattern, which disappears when helium is added. Credit: Coach et al.

“We would normally never have deliberately added helium to the vacuum can in our microscope because it risks destroying the STM head,” Peter Wahl, another researcher involved in the study, told “In fact, due to the high voltages required to control the tip position, arc discharges can be obtained in the wires, which effectively ‘burns’ the measuring head, the heart of our microscope. In retrospect, the key effect is (ie that we become sensitive to exchange interactions once there is a probe particle in the tunnel junction) was likely to be predictable, but no one had performed the measurement. ”

In their recent study, Trainer, Wahl, and their colleagues used an STM, a microscope that can be used to image surfaces at the atomic level, to measure a sample of iron telluride that exhibited an unusual antiferromagnetic order. In particular, STM microscopes work by exploiting the ability of electrons to ‘quantum tunnel’ through potential barriers that they would typically not be able to pass through.

“When you bring an atomically sharp tip extremely close to the surface of a sample (to well within a billionth of a meter), electrons can ‘jump’ between the tip and the sample,” Trainer explained. “By moving the tip across the sample surface, we can use this effect to build an atomic image of the sample surface. STM is also capable of imaging magnetic order if the microscope’s probe tip is magnetic.”

The main purpose of the experiments performed by Trainer, Wahl and their colleagues was to determine what effect helium atoms trapped between this tip and an iron telluride sample would have. By changing the voltage applied between the STM tip and their sample, the team was able to push the helium atoms out between the tip and the sample.

“We found that the voltage required to kick the helium out gives us access to its bonding energy and is dependent on the magnetic interaction between the tip and the sample and therefore by accurately measuring the voltage required to expel the helium. above the sample surface we could map the magnetic exchange interaction (or the magnetic force) between the tip and the sample, “Trainer explained.

Interestingly, the researchers also found that the presence or absence of helium in the tunnel compound dramatically affected the spin polarization of the tunnel electrons. This means that by applying different voltages to the sample and thus helium in the tunnel connection, the spin polarization of the tunnel current can be controlled.

A strategy to control the spin polarization of electrons using helium

An image showing the iron telluride surface recorded at a voltage when helium is pressed out between the tip and the sample. Bottom: A mapping of the energy needed to expel the helium atom from the tunnel connection. The energy required can be seen to vary with the underlying magnetic sequence, providing a way to map the magnetic exchange interaction. Credit: Coach et al.

“The two key results of our study are that we can control the spin polarization of the electrons tunneling between the tip and the sample using an applied voltage, as well as measure the exchange interaction between tip and sample without having to perform a force measurement as it had been done earlier, “said the coach.

In the future, the method of controlling spin polarization of electrons using an applied voltage presented by this team of scientists could enable the development of new spin electronic circuits and devices. Meanwhile, Trainer, Wahl, and their colleagues plan to conduct further research aimed at further testing the strategy introduced in their latest paper.

“There are many exotic quantum materials with complex magnetic phases that show interesting physics, but disappointingly, many of these materials are insulating, meaning they cannot be studied directly by a scanning tunnel microscope,” Trainer added. “One of our future research plans is to grow thin layers of these insulating magnetic materials on a metallic substrate that would allow the electrons from the microscope to tunnel through the insulating layer.”

Ultimately, Trainer and his colleagues hope that by applying a layer of helium to an insulating surface and collecting measurements with a magnetic tip, they will be able to measure the exchange interaction between the tip and the insulating layer. This in turn would allow them to characterize the magnetism of the insulating magnetic materials they are investigating, which would otherwise be undetectable with STM techniques.

“Our method provides a new way of imaging quantum magnetism, for example in frustrated magnetic systems,” Wahl said. “An interesting open question is how magnetic fluctuations would affect the exchange interaction and whether this method would be sensitive to fluctuating magnetic orders.”

Exotic magnetic states in miniature dimensions

More information:
C. Trainer et al, Probing magnetic exchange interactions with helium, Physical review letters (2021). DOI: 10.1103 / PhysRevLett.127.166803

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