Physicists from Masaryk University observed the fluctuating behavior of magnetic moments, which was predicted by a theory from the sixties of the last century. They contributed to a hot debate in condensed matter physics. Their experiments were performed with samples fabricated in the scientific facility CEITEC Nano. They investigated the properties of these samples in a unique spectrometer at the Paul Scherrer Institute in Switzerland. They published the results of their research in the scientific journal Nature Communications.
Physicists have been investigating antiferromagnets for a long time. Antiferromagnets look at first sight as nonmagnetic since the magnetic moments are compensated and the total magnetic moment is zero. “We have focused on properties of a specific type of antiferromagnets – on two-dimensional antiferromagnets. We can imagine them as an ultrathin layer with a thickness of only one monolayer. One nonmagnetic example of such a material is graphene. Its discovery was awarded a Nobel Prize in 2010. We have worked with a layer of lanthanum iron oxide,” explained Adam Dubroka from the Institute of Condensed Matter Physics at the Faculty of Science of Masaryk University.
The team of scientists from Masaryk University observed a change in the behavior of magnetic moments in antiferromagnets in custom-designed samples of lanthanum iron oxide monolayers. The fluctuating or inconstant magnetic moments in these materials can be a basis for new magnetic states or high-temperature superconductivity. The principles of magnetism are typically used in electronics, particularly for data storages. The unique magnetic properties of the investigated antiferromagnets can be a basis for new materials, such as high-temperature superconductors, that can have important future usage in high-tech medical diagnostic devices.
Following the so-called Mermin-Wagner theorem published in the sixties of the last century, a specific class of two-dimensional magnetic materials cannot exhibit magnetic order because thermal fluctuations destroy it. However, this prediction deals only with infinitely large samples. Since then, the behavior of samples with laboratory size has been a topic of scientific debate. “We have performed the experiments on a set of our samples. We have contributed to the lively discussion of whether the theorem describes even samples with laboratory sizes well. We have shown that yes,” Dubroka summarized their research. “With the samples of ultrathin layers, so-called superlattices, of few (one, two or three) atomic monolayers of antiferromagnet lanthanum iron oxide (LaFeO3), we have shown that specifically the sample with one monolayer exhibits fluctuations predicted by more than fifty years ago by D. Mermin and H. Wagner, despite that the recent theories predict the opposite,” he has added.
The first essential part of the research involved the fabrication of the samples. Michal Kiaba, a doctoral student at the Insititute of Condensed Matter Physics at Masaryk University, performed this task. He fabricated the samples in a vacuum using a pulsed laser, with the precision of individual atoms. The used deposition chamber is equipped with an electron gun that can monitor individual atomic layers during the growth of the samples and thus allow the control of the growth. This part of the research was performed in the research infrastructure CEITEC Nano, the joint laboratory of Masaryk University and Brno Technical University.
Consecutively, the samples were measured in a unique spectrometer using the decay of muons with low energy located at the Paul Scherrer Institute in Switzerland. Muons are unstable particles about 200 times heavier than electrons, with a half-life of 2 microseconds. The decay of the muons is used in the measurements for studies of the magnetic moments. An important parameter of the measurements is the temperature of the sample, which reached close to absolute zero. The researchers performed three sequences of measurements during two years.
The importance of the research of physicists of Masaryk University was recognized by Professor Christian Bernhard from the University of Fribourg, who, in the report of the dissertation thesis of Michal Kiaba, wrote: “Personally, I am of the impression that the presented research is likely to become a milestone in modern experimental condensed matter physics. The obtained result is not only of great fundamental importance, it is also a masterpiece of combining growth of high quality heterostructures with control at the monolayer scale, with the use of a worldwide unique experimental technique, like low-energy muon spin rotation to address a long-standing theoretical theorem that is central to the physics of low-dimensional magnetic and superconducting systems.“
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