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Approaching the magnetic singularity



 Approaching the magnetic singularity
A field wall separates regions with different spin orientations (green and blue arrows). MIT researchers found that a magnetic field applied to a particular 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, usually varying smoothly as the magnetic field rotates. This simple magnetic response underlies many applications including contactless current sensing, motion sensing, and data storage. In a crystal, the way that the charge and spin of its electrons align and interact underlies these effects. Using the nature of the alignment, called symmetry, is a key ingredient in designing a functional material for electronics and the emerging field of spin-based electronics (spintronics).
                                               


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

At temperatures below 5.6 kelvins (corresponding to -449.6 degrees Fahrenheit), these crystals show a sharp increase in electrical resistivity when the magnetic field is precisely aligned within a 1

degree angle along the high symmetry direction of the crystal. This effect, which researchers have called "singular angular magnetoresistance," can be attributed to the symmetry-in particular, the ordering of the cerium atoms' magnetic moments. Their results are published today in the journal Science .

Novel response and symetry

Like an old-fashioned clock designed to chime at 12:00 and at no other position of the hands, the newly discovered magnetoresistance occurs only when the direction of the magnetic field is pointed in line with the high-symmetry axis in the material's crystal structure. Turning the magnetic field more than a degree away from that axis and the resistance drops precipitously

"Rather than responding to the individual components of the magnetic field like a traditional material, here the material responds to the absolute vector direction," says Takehito Suzuki, a research scientist in the Checkelsky group who synthesized these materials and discovered the effect. "The observed sharp enhancement, which we call singular angular magnetoresistance, implies a distinct state realized only under those conditions."

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

"The observed enhancement is so highly bound to the magnetic field along the crystalline axis in this material that it strongly suggests symmetry plays and critical role, "Lucile Savary, permanent CNRS researcher at ENS de Lyon, adds. Savary was a Betty and Gordon Moore Postdoctoral Fellow at MIT from 2014-17, when the team began collaborating.

To elucidate the role of symmetry, it is crucial to see the alignment of the magnetic moments, for which Suzuki and Jeffrey Lynn, a NIST fellow, conducted powder neutron diffraction studies on the BT-7 triple axis spectrometer at the NIST Center for Neutron Research (NCNR). The research team used NCNR's neutron diffraction capabilities to determine the material's magnetic structure, which plays an essential role in understanding its topological properties and nature of the magnetic domains. A "topological state" is one that is protected from common disorder.

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

"The agreement of the model with the experimental results is outstanding and was the key to understanding what was a mysterious experimental observation, "says Sterling, the senior author's paper.

The universality of the phenomenon

"The interesting question here is whether or not singular angular magnetoresistance can be widely observed in magnetic materials and, if this feature can be ubiquitously observed, what is the key "Suzuki says.

The theoretical model indicates that the singular response may indeed be found in other materials and predicts material properties beneficial for realizing this feature. One of the important ingredients is an electronic structure with a small number of free charges, which occurs in a point-like electronic structure referred to as a nodal. The material in this study has so-called Weyl points that achieve this. In such materials, the allowed electron momentum depends on the configuration of the magnetic order. Such control of the magnitude of these charges by the magnetic degree of freedom allows the system to support switchable interface regions where moments are mismatched between domains of different magnetic order.

This analysis is further supported by the first-principles electronic structure calculation performed by Jianpeng Liu, research assistant professor at HKUST, and Balents. Using more traditional magnetic elements such as iron and cobalt rather than rare earth cerium may offer a potential path to higher temperature observation of the singular angular magnetoresistance effect.

Kenneth Burch, graduate program director and associate professor of physics at Kenneth Burch, Ph.D. Boston College, whose laboratory investigates Weyl materials, notes: "The discovery of remarkable sensitivity to magnetic angle is a completely unexpected phenomenon in this new class of materials." This result suggests not only new applications of Weyl semimetals in magnetic sensing but unique coupling of electronic transport, chirality and magnetism. " Chirality is an aspect of electrons related to their spin, which gives them either a left-handed or right-handed orientation.

The discovery of this sharp but narrowly confined resistance peak could eventually 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 adoptions for new technologies." With the design principles now in hand, we are casting a broad net to find this phenomenon in more robust systems to unlock this potential . "
                                                                                                                        


Magnetoresistance ratio enhancement in Heusler-based alloy


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

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Massachusetts Institute of Technology

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                                                 Approaching the magnetic singularity (2019, June 21)
                                                 retrieved 21 June 2019
                                                 from https://phys.org/news/2019-06-approaching-magnetic-singularity.html
                                            

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