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Approaching the magnetic singularity



  Approaching the magnetic singularity
A domain wall (gray panel in the middle) separates regions with different spin orientations (green and blue arrows). MIT researchers discovered that a magnetic field used at a certain angle through a single crystal of a new magnetic quantum material makes it harder for electrons to cross this domain wall. Credit: Leon Balents

In many materials, electrical resistance and voltage change in the presence of a magnetic field which normally varies evenly as the magnetic field rotates. This simple magnetic response is based on many applications, including contactless current sensing, motion sensing and data storage. In a crystal, the way the charge and centrifugation of its electrons adjust and interact is subject to these effects. The use of the adaptation, called symmetry, is a key component in designing a functional material for electronics and the emerging field of spin-based electronics (spintronics).


Recently, a team of researchers from MIT, the French National Center for Scientific Research (CNRS) and the École Normale Supérieure (ESA) de Lyon, the University of California at Santa Barbara (UCSB), Hong Kong University of Science and Technology ( HKUST) and the NIST Center for Neutron Research, led by Joseph G. Checkelsky, assistant professor of physics at MIT, have discovered a new type of magnetically driven electrical response in a crystal composed of cerium, aluminum, germanium and silicon.

At temperatures below 5.6 Kelvin (equivalent to -449.6 degrees Fahrenheit), these crystals show a sharp improvement in electrical resistivity when the magnetic field is precisely adjusted within a 1

degree angle along the crystal's high symmetry direction. This effect, which the scientists have called "singular angle magnetoresistance", can be attributed to the symmetry – especially the arrangement of the magnetic moments of the cerium atoms. Their results are published today in the journal Science .

Novel response and symmetry

Like an old-fashioned watch designed for bell at. 12.00 and in no other way by the hands does the newly discovered magnetoresistance occur only when the direction or vector of the magnetic field points straight in line with the high symmetry axis in the crystal structure of the material. Turn the magnetic field more than one degree away from that axis and the resistor falls exactly.

"Instead of responding to the individual components of the magnetic field as a traditional material, the material here responds to the absolute vector direction," said Takehito Suzuki, a researcher in the Checkelsky group, who synthesized these materials and discovered the effect. "The observed sharp improvement that we call singular angle magnetoresistance implies a certain state that is only realized under these conditions."

Magnetoresistance is a change in electrical resistance of a material in response to an applied magnetic field. A related effect known as giant magnetoresistance is the basis of modern computer hard drives, and its discoverers were awarded the Nobel Prize in 2007.

"The observed improvement is so severely limited to the magnetic field along the crystalline axis of this material that it strongly suggests that symmetry plays a crucial role, "adds Lucile Savary, permanent CNRS researcher at the ESA de Lyon. Savary was Betty and Gordon Moore Postdoctoral Researcher at MIT from 2014-17, when the team began to collaborate.

To elucidate the role of symmetry, it is crucial to see the adaptation of the magnetic moments such as Suzuki and Jeffrey Lynn, NIST fellow, performed powder neutron diffraction studies on the BT-7 triple axis spectrometer at the NIST Center for Neutron Research (NCNR). The research team used the NCNR's neutron diffraction ability to determine the magnetic structure of the material, which plays a crucial role in understanding the topological properties and nature of the magnetic domains. A "topological condition" is one that is protected from general disorder. This was a key factor in lifting the mechanism in the singular response.

Based on the observed order system, Savary and Leon Balents, professor and permanent member of the Kavli Institute of Theoretical Physics at UCSB, constructed a theoretical model in which the spontaneous symmetry -breaking caused by magnetic torque control pairs to the magnetic field and the topological electronic structure . As a consequence of the coupling, switching between the uniformly ordered low and high resistivity states can be manipulated by the precise control of the magnetic field direction.

"According to experimental results, the model is unique, and was the key to understanding what was a mysterious experimental observation," says Checkelsky, the paper's senior author.

The universality of the phenomenon

"The interesting question here is whether the singular angular magnetoresistance can be widely observed in magnetic materials, or if this function can be considered ubiquitously, what is the key ingredient for constructing the materials with this effect, "Suzuki says.

The theoretical model indicates that the unique answer can actually be found in other materials and predicts material properties that are beneficial for realizing this function. One of the important ingredients is an electronic structure with a small number of free charges that occur in a point-like electronic structure called the nodal. The material in this study has so-called Weyl points that achieve this. In such materials, the allowed electron torque depends on the configuration of the magnetic sequence. Such control of the moment of these charges with the magnetic freedom degree allows the system to support switchable interface areas where the torque is mismatched between domains of different magnetic order. This mismatch also leads to the large increase in resistance observed in this study.

This analysis is further supported by the electronic structure calculation of the first principles carried out by Jianpeng Liu, research assistant professor at HKUST and Balents. Using more traditional magnetic elements such as iron or cobalt, rather than rare earth cerium, can offer a potential path to higher temperature observation of the singular angle magnetoresistance effect. The study also excludes a change in the arrangement of the atoms, called a structural phase transition, as a cause of the change in the resistivity of the cerium-based material.

Kenneth Burch, Bachelor of Science Program and Associate Professor of Physics at Boston College, whose laboratory examines Weyl materials, notes: "The discovery of remarkable sensitivity to magnetic angle is a completely unexpected phenomenon in this new class of materials. Not only on new applications of Weyl semimetals in magnetic sensing, but the unique coupling of electronic transport, chirality and magnetism. "Chirality is an aspect of electrons related to their spin that gives them either a left-handed or right-handed orientation.

The discovery of this sharp but narrowly limited resistance stop could ultimately be used by engineers as a new paradigm for magnetic sensors. Notes Checkelsky, "One of the exciting things about fundamental discoveries in magnetism is the potential for rapid adoption of new technologies. With the design principles, there is now a huge network to find these phenomena in more robust systems to unlock this potential . "


Improving magnetoresistance ratio in Heusler-based alloy


More Information:
T. Suzuki et al. Singular angle magnetoresistance in a magnetic nodal semimetal, Science (2019). DOI: 10.1126 / science.aat0348

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Massachusetts Technical Institute

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Approaching the magnetic singularity (2019, June 21)
June 21, 2019
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