Slimming diet for metal oxides

Properties of these material layers depend on the number of stacked atomic layers

Measuring the conductivity with light: The physicists working with Bernhard Keimer use infrared light from the synchrotron source ANKA at the Karlsruhe Institute of Technology to determine the electronic properties of layers of two and four layers of material. The sample is recognizable as a white-gray square mounted on the gold-colored cylinder. The laser beam falls from the right onto the sample and is reflected to the left on the detector. © Department Keimer / MPI for Solid State Research
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In the search for materials such as electronic components physicists can follow a new trail in the future: an international team of researchers has for the first time observed precisely how the physical properties of a substance - more specifically of the metal oxide lanthanum oxide - change if they are in two-dimensional rather than three-dimensional form is processed.

In fact, a layer of two layers of material shows quite different electronic and magnetic effects when cooled to very low temperatures than a layer of four layers. The fact that the physical characteristics can now also be controlled via the dimension opens up new possibilities for identifying substances from which the chips of the future could be made, according to the researchers in "Science".

Semiconductor industry reaches its limits

The semiconductor industry is gradually reaching its limits. As electronic components continue to shrink, interconnects and transistors are likely to shrink to atomic size. Such tiny structures can hardly be produced in a controlled manner using common methods. When they are in operation, they also generate so much heat because of their electrical resistance that they should quickly lose their shape. The era of semiconductor electronics could therefore come to an end in the foreseeable future.

Maybe metal oxides then offer as an alternative. Because among them, there are not only materials that are recommended as storage materials because of their magnetic properties - the metal oxides include superconductors that conduct electricity completely without resistance.

Customized properties of metal oxides

An international team led by Alexander Boris and Bernhard Keimer at the Max Planck Institute for Solid State Research in Stuttgart now shows a new way to tailor the properties of metal oxides. The researchers, who included scientists from the Max Planck Institute for Metals Research, the Paul Scherrer Institute in Villigen, Switzerland and the University of Friborg in Switzerland, have for the first time worked out exactly how the spatial dimension of a material reflects its physical behavior affected. display

We are thus turning specifically to a setpoint that physicists were previously unable to control in an imprecise way, "says Keimer. Neither have they been able to elucidate what effect the dimension has among all the other factors involved in electronic and magnetic behavior. And the effect is immense, as the researchers now noted.

Lanthanum nickel oxide examined

The scientists investigated the metal oxide lanthanum oxide LaNiO3, which also contains nickel in addition to the electronically inactive lanthanum and oxygen atoms. Not least of all, this composition was chosen because nickel brings with it a special type of electron whose magnetic moments are always good for physical surprises. In a massive piece, however, not much is noticeable, and this includes all samples that are thicker than four layers of material, so measure only a few nanometers: In this form lanthanum nickel belongs to the metallic ladders and the magnetic moments of the electrons whirl around each other like tumbling bar magnets. This also remained the case when the physicists cooled a sample of four layers of material almost to the absolute zero of the temperature at minus 273 degrees Celsius.

A 2D layer becomes insulator and antiferromagnetic

In a sample of two material layers, this changes completely, says Keimer: During cooling, the material lost its electrical conductivity at about minus 100 degrees Celsius. The thin layer puts the electrons in a predicament: they push each other off, but they can not stay out of the way. Therefore, they each stand more or less at an atom, and the flow of electricity stops.

A question of dimension: In a layer of two layers of lanthanum oxide, the electrons are localized on cooling at the very least on positively charged vortices of the nickel atoms. As the temperature decreases, the spins that give the electrons a magnetic moment align antiparallel. In a layer of four metal oxide layers, however, the electrons remain free to move even at low temperatures and their spins remain disordered. Department Keimer / MPI for Solid State Research

But that was not the only effect of the slimming cure for the metal oxide. When the physicists cooled the thin sample even further, to about 220 degrees Celsius, the material assumed a magnetic order, or more precisely, an antiferromagnetic one: the magnetic moments of the electrons align antiparallel, much like bar magnets alternating with their north and south poles.

Researchers work with laser beam evaporation

"So we can deliberately change the electronic and magnetic properties of the material by adding two layers of material, " says Keimer. Exactly controlling the thickness of the sample presented physicists with the first challenge in their investigation.

"The usual chemical processes do not really know what's coming out, " says Boris. Therefore, the researchers resorted to a physical method: laser beam evaporation, English Pulsed Laser Deposition (PLD). In a vacuum chamber they vaporize the lanthanum oxide in carefully metered quantities with laser pulses. The metal oxide deposits on a nearly perfectly flat and clean surface of the substrate and at the right temperature forms a fully ordered, even layer of the desired thickness.

Even more experimental challenges

However, the researchers had not yet mastered the experimental challenges. For in samples that are only a few atomic layers thick, the electronic and magnetic characteristics can only be determined with a few tricks. For example, to measure the conductivity of the sample, it hardly helps physicists to connect cables to two sides of the sample and measure the current flow.

"As accurate as the thin layers may have grown, somewhere, the support material always has an atomic level, which can then be found in the vapor-deposited layer, " explains Boris. An ordinary measurement of the conductivity would fail at such a stage because it interrupts the flow of current. Therefore, the researchers put to the test an intense, infrared light beam supplied by the ANKA synchrotron in Karlsruhe. The light waves from this source oscillate in one direction only. How this direction of vibration changes when the beam is reflected on the sample tells the researchers something about the mobility of the electrons in the material and thus about the conductivity.

Slow muons reveal the magnetic order

Determining an antiferromagnetic order in a layer of just two layers is at least as tricky. Because the magnetic moments cancel each other out exactly, they do not make themselves felt in an external magnetization. Therefore, the scientists relied on muons, unstable elementary particles that 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 Villigen, Switzerland, where there is a particle accelerator that supplies muons.

A probe for magnetic order: With the Swiss muon source at the Paul Scherrer Institute, an antiferromagnetic order can be observed in very thin layers. The sample is introduced through the main access of the measuring chamber. With the colorful contacts, the positron detectors are wired. © LMU / Paul Scherrer Institute

Space problem on microchips solved soon?

Only at the Paul Scherrer Institute can the researchers also regulate the speed at which the muons hit the sample. This is necessary in order to be able to look with them exactly into the layers of two or four material layers. Otherwise, the particles will rush through the lanthanum oxide and remain stuck somewhere in the substrate. Together with their colleagues from the University of Friborg, the scientists of the Paul Scherrer Institute scanned the magnetic order in the lanthanum nickel oxide layers. The muons with which they aimed at the samples disintegrate in the metal oxide layer though. However, the trajectory of their fragments reveals to the physicists the orientation of the magnetic moments in the material.

"In a similar way, we now want to investigate how the dimension of the sample affects the electronic properties of metal oxides that become superconducting below a certain temperature, " says Keimer. It may be possible for metal oxides to give properties in this way, which can also solve the growing space problem on microchips. (Science, 2011; doi: 10.1126 / science.1202647)

(MPG, 25.05.2011 - DLO)