A new dimension in materials research

The electronic and magnetic properties of thin films depend on the number of atomic layers which are stacked on top of each other

May 25, 2011

In the future, physicists will be able to follow a new lead in their search for new materials for electronic components, for example. An international team of researchers headed by scientists at the Max Planck Institute for Solid State Research in Stuttgart is the first to accurately observe how the physical properties of a substance – or to be more precise of the metal oxide lanthanum nickel oxide – change when it is used in two-dimensional, instead of three-dimensional form. In fact, a film consisting of two layers of material exhibits completely different electronic and magnetic effects when cooled to very low temperatures than does a film comprising four layers. The ability to control the physical characteristics via the dimension as well opens up new possibilities to identify materials from which the chips of the future could be made.

A 2D layer becomes an insulator and anti-ferromagnetic

A question of dimension: As they cool, the electrons in a film made of two layers of lanthanum nickel oxide are first localized to positively charged cores of the nickel atoms. If the temperature decreases further, the spins, which give the electrons a magnetic momentum, align so as to be anti-parallel. In a film of four layers of metal oxide, however, the electrons remain freely mobile even at low temperatures, and their spins remain unordered.

“In a sample made of two layers of material, this changes completely,” says Bernhard Keimer. When cooled to around minus 100 degrees, the material lost its electrical conductivity. The thin layer puts the electrons in a predicament: they repel each other, but can no longer distance themselves from each other to any great extent. They therefore remain more or less stuck at an atom, and the current stops.

This was not the only effect which the slimming treatment had on the metal oxide, however. When the physicists cooled the thin sample even further, to around minus 220 degrees Celsius, the material assumed a magnetic order, or to be more precise, an anti-ferromagnetic one: the magnetic moments of the electrons align themselves so as to be antiparallel, just like bar magnets which are lying alternately with their north and south poles next to each other.

“We can alter the electronic and magnetic properties of the material in a specific way by adding two layers of the material,” says Bernhard Keimer. The first challenge for the physicists in their investigation was to discover how to control the thickness of the sample in such an exact way. “With the usual chemical methods one does not really have an accurate idea of what the final result will be,” says Alexander Boris, who made an essential contribution to the work. The researchers therefore made use of a method found in physics: pulsed laser deposition (PLD). Working in a vacuum chamber, they used laser pulses to vaporize the lanthanum nickel oxide in carefully dosed quantities. The metal oxide deposits on an almost perfectly plane and clean surface of the carrier material and at the right temperature forms a completely ordered, plane layer with the required thickness.

However, this did not mean that the researchers had now mastered the experimental challenges, because in samples which are only a few atomic layers thick the electronic and magnetic characteristics can only be determined with a few tricks. The physicists can hardly attach cables to two sides of the sample and measure the current in order to determine the conductivity of the sample, for instance. “No matter how accurately the thin layers have grown, the carrier material will always have an atomic step somewhere. This atomic step can then also be found in the deposited layer,” explains Alexander Boris. A conventional measurement of conductivity would fail at such a step because it interrupts the current flow. The researchers therefore directed an intense, infrared laser beam produced by the ANKA synchrotron in Karlsruhe onto the sample. The light waves from this source oscillate in one direction only. How this direction of oscillation changes when the beam is reflected at the sample provides the researches with information on the mobility of the electrons in the material and thus on its conductivity.

Slow muons shed light on the magnetic order

It is at least as tricky to determine an anti-ferromagnetic order in a film made up of just two layers. Since the magnetic moments cancel each other out exactly in this case, the order does not produce any external magnetisation. The scientists therefore put their faith in muons, instable elementary particles which are produced in particle accelerators. They resemble electrons, but have a much weaker magnetic moment. “Muons are therefore suitable as sensitive probes for the magnetic order,” says Thomas Prokscha, researcher at the Paul Scherrer Institute in the Swiss town of Villingen, home to a particle accelerator which supplies muons.

A probe for the magnetic order: The Swiss muon source at the Paul Scherrer Institute enables the researchers to observe the anti-ferromagnetic order in very thin films. The sample is introduced via the main access to the measuring chamber. The colourful contacts are used to connect the positron detectors.

Only the Paul Scherrer Institute has the facility which enables the researchers to also regulate the speed with which the muons impact on the sample. This is necessary in order for them to be able to take a precise look into the film of two or four layers of material. If the speed is not regulated, the particles race through the lanthanum nickel oxide and only get stuck when they are somewhere inside the carrier material. The scientists at the Paul Scherrer Institute teamed up with their colleagues from the University of Fribourg to probe the magnetic order in the lanthanum nickel oxide layers. Although the muons, which they targeted at the samples, decay in the metal oxide layer, the trajectories of their fragments tell the physicists what the orientation of the magnetic moments in the material is.

“We now want to adopt a similar approach to investigate how the dimension of the sample influences the electronic properties of metal oxides, which become superconducting below a certain temperature”, says Bernhard Keimer. It is possible that they can give metal oxides properties which can also be used to solve the increasing problem of space on microchips.


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