Spintronics, also known as spin electronics, is a research field that
explores how the intrinsic spin of electrons and its magnetic moment can be
exploited by devices. Spintronic devices are promising for a wide range of
applications, particularly for efficiently storing and transferring data.
The key requirement for spintronic devices is the ability to control and
detect the spin polarization of electrons. The spin polarization is
essentially the degree to which the spin (i.e., the intrinsic angular
momentum of electrons and other elementary particles) is aligned with a
specific direction.
Researchers at University of St Andrews in the U.K. and other institutes
worldwide have recently shown that helium can influence the spin
polarization of the tunneling current and magnetic contrast of a technique
known as spin-polarized scanning tunneling microscopy (SP STM). Their
findings, published in Physical Review Letters, could have important
implications for the development of new electronic devices.
In their previous research, the same research group investigated the
magnetic order in the antiferromagnetic material iron telluride. Remarkably,
they found that by collecting magnetic material from their sample's surface
using an STM tip, they could image the sample's magnetic order.
"As part of my Ph.D. project, I was to set up a new STM in a vector magnet
and one of the first measurements I set out to do was reproduce this
imaging," Christopher Trainer, one of the researchers who carried out the
study, told Phys.org. "I tried hard, but couldn't get it to work. This was a
huge puzzle to us because usually, this measurement worked fairly
straightforwardly, until we found that the new microscope had a leak in its
vacuum seal so that the liquid helium that we used to cool the experiment
could enter the measurement 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's measurement 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.
"We would usually never have deliberately added helium in the vacuum can of
our microscope, because it risks destroying the STM head," Peter Wahl,
another researcher involved in the study, told Phys.org. "In fact, due to
the high voltages required to control the tip position, one can get arc
discharges in the wiring, effectively 'burning' the measurement head, the
heart of our microscope. In hindsight, the key effect, (i.e., that we become
sensitive to exchange interactions once there is a probe particle in the
tunneling junction) was probably predictable, but nobody had carried out 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. Notably, STM microscopes work by leveraging the
ability of electrons to 'quantum tunnel' through potential barriers that
they would not typically be able to pass through.
"When bringing an atomically sharp tip extremely close to the surface of a
sample (to well within one 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 up an atomic
picture of the sample's surface. The STM is also able
to image magnetic order if the probe tip of the microscope is magnetic."
The key objective of the experiments conducted 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 could eject the helium atoms
from between the tip and the sample.
"We found that the voltage that is required to kick out the helium gives us
access to its binding energy and is dependent on the
magnetic interaction between the tip and the sample and so by precisely
measuring the voltage required to eject the Helium across the sample
surface we could map out 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 tunneling junction dramatically impacted the spin-polarization
of the tunneling electrons. This means that by applying different voltages
to the sample and consequently the helium in the tunneling junction one can
control the spin-polarization of the tunneling current.
"The two key results of our study are that we can control the spin
polarization of the electrons that tunnel between the tip and the
sample using an applied voltage, as well as measure the exchange
interaction between tip and sample without having to undertake a force
measurement, as had been done previously," Trainer said.
In the future, the method for controlling the spin polarization of electrons
using an applied voltage presented by this team of researchers could enable
the development of new spintronic circuits and devices. Meanwhile, Trainer,
Wahl and their colleagues plan to conduct further studies aimed at testing
the strategy introduced in their recent paper further.
"There are many exotic quantum materials with complex magnetic phases that
show interesting physics however disappointingly many of these materials are
insulating which means that they cannot be directly studied by a scanning
tunneling microscope," Trainer added. "One of our future research plans is
to grow thin layers of these insulating magnetic materials on a metallic
substrate which 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 would in turn allow them to characterize the
magnetism of the insulating magnetic materials they examine, which would
otherwise be undetectable by STM techniques.
"Our method provides a new way to image 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."
Reference:
C. Trainer et al, Probing Magnetic Exchange Interactions with Helium,
Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.166803
Tags:
Physics