Physicists Describe Metamagnetism and Low-Carrier Kondo Effects of YbRh3Si7, an Eccentric Compound

In most families, there exists a misfit. However, Emilia Morosan, physicist at Rice University, has found out an entire class of eccentric compounds that could be helpful in elucidating the mysterious magnetic and electronic mechanisms of other quantum materials engineers are looking to use for next-generation electronics and computers.

The first member of the clan—a “semimetallic Kondo lattice” composed of ytterbium, rhodium, and silicon in the ratio of 1:3:7—has been described by Morosan and 30 co-authors in a paper published in the American Physical Society journal Physical Review X (PRX) this week. In the paper, two properties of YbRh₃Si₇ are described—“metamagnetism” and “low-carrier Kondo” effects—which have been rarely measured earlier in the same material.

Morosan, whose lab specializes in the design, discovery, and production of quantum materials, developed the new class of 1-3-7’s with assistance from the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative (EPiQS). According to her, not many 1-3-7’s had been reported in the scientific literature before her Moore-funded study. Of the number of compounds discovered by her team in the 1-3-7 class, three are ytterbium-based, four are magnetic, and “each is more surprising than the last,” she stated. Morosan was named a Moore Foundation EPiQS Materials Synthesis Investigator in 2014.

First, this gives us an opportunity to understand all of these, by themselves, and then to understand them in relationship to each other. For example, the structural and chemical differences between these are very small. The lattice parameters are almost identical. One would expect that the physical changes would therefore be minimal in these related compounds, but we’re finding dramatically different magnetic and transport properties. If we can understand why that happens in this family, it might allow us to search for compounds with the properties we want.

Elilia Morosan, Physicist, Rice University

The arrangement of atoms is ordered in YbRh₃Si₇ and all other crystals. Every crystal has its own unique structural pattern, or lattice. In crystals that include magnetic elements such as ytterbium or iron, the orderly arrangement of atoms in a lattice usually matches with the magnetic order.

For instance, each electron functions like a tiny spinning bar magnet, including a positive and negative magnetic pole at either end of its spin axis. The magnetic moment of the electron indicates the direction toward which the spin axis points, and in elements such as ytterbium and iron, which contain a number of electrons, the collective magnetic moment of atoms could be strong. In the case of ferromagnets—the materials used in a number of automobiles and refrigerators—all these magnetic moments point in one direction. In the case of antiferromagnets, such as YbRh₃Si₇, one half of the moments points in one direction and the half points in the opposite direction.

Tech firms are mostly interested in using spin in solid-state devices. Spintronics, which is an expanding field of technology, is committed to create spin-based technologies for data transfer, data storage, and computation, including fundamentally innovative types of chips for quantum computers.

For the researchers investigating novel magnetic materials, such as YbRh₃Si₇, one technique to explore the magnetic order is by coaxing the moments to point in another direction when an external magnetic field is applied. Physicists can gain better insights into the role played by the crystal lattice in ways the magnetic moments express themselves by measuring the amount of field energy required to change the direction toward which the magnetic moments point.

In a majorit of the materials, the magnetic moments of atoms slowly rotate toward the direction of the external field with an increase in intensity. In the case of metamagnets, the crystal field’s forces exert a pull to such an extent that the moments remain locked in place, even upon application of an external field. However, all the moments instantaneously snap into a new arrangement when the field energy reaches a critical level, where the new arrangement is more closely aligned to the field. With a sufficient increase in the field intensity, it would be possible to make the moments to align with the field, but “only through this progression of stepwise changes that are reminiscent of a devil’s staircase,” stated Morosan.

The unraveling of the metamagnetic transitions was the first hint that something unusual was happening in the crystallographic structure of YbRh₃Si₇.

“There are very few examples of metamagnetism in ytterbium-based compounds,” stated Macy Stavinoha, study co-author and a graduate student in Morosan’s group. “That transition clued us into looking at the underlying magnetic structure, which was quite complicated. We had to use a manifold of techniques to confirm what was involved.”

The eight-year experimental journey to decode the magnetic order of the material was headed by former PhD student and co-author Binod Rai and included travel to Tennessee’s Oak Ridge National Laboratory, Maryland’s National Institute of Standards and Technology, the United Kingdom’s Rutherford Appleton Laboratory, Florida’s National High Magnetic Field Laboratory, and New Mexico’s Los Alamos National Laboratory.

According to Morosan, the experiments helped her group decode the puzzling competition of forces—electronic, structural, and magnetic—that occurred in YbRh₃Si₇.

“There was nothing simple, in the sense that you could sit down, look at the data from an experiment and immediately tell what was going on,” she stated.

For instance, during the experiments, it was demonstrated that the metamagnetic transitions in YbRh₃Si₇ took place at lower fields upon applying the magnetic field perpendicular to the zero-field moment direction. This is contrary to the metamagnetic transitions in nearly all other ytterbium-based compounds, which take place when the applied field is parallel to the moment direction. Morosan noted that this indicates a delicate balance between the different energy scales in YbRh₃Si₇.

One more example of such competing energy scales in the material is the improved interaction between magnetic moments and conduction electrons. This interaction, called “Kondo screening,” emerges when carrier electrons—the flowing particles in electric current—interact with the magnetically aligned electrons in the ytterbium atoms. According to Stavinoha, it is confusing since the density of carrier electrons in YbRh₃Si₇ lower density compared to a majority of the familiar Kondo materials.

You rarely find multiple Kondo systems in one family of isostructural compounds. In the 1-3-7 family, we discovered three such Kondo systems with distinct magnetic and electronic properties. That combination of structural similarity and physical property dissimilarity present a great opportunity for comparative studies.

Macy Stavinoha, Graduate Student, Rice University

Morosan is a professor of physics and astronomy, chemistry and electrical and computer engineering at Rice and a member of the Rice Center for Quantum Materials, a university-wide attempt that makes the most out of global alliances and the strengths of more than 20 Rice research teams to address questions related to quantum materials.

Additional co-authors of the PRX paper include Alannah Hallas, Chien-Lung Huang, Vaideesh Loganathan, Tong Chen, Haoran Man, Scott Carr, Pengcheng Dai and Andriy Nevidomskyy, all of Rice; Shalinee Chikara, Xiaxin Ding, John Singleton, and Vivien Zapf, all of the National High Magnetic Field Laboratory at Los Alamos National Laboratory; Iain Oswald, Katherine Benavides, and Julia Chan, all of the University of Texas at Dallas; Rico Schönemann, Q.R. Zhang, Daniel Rhodes, Y.C. Chiu, and Luis Balicas, all of the National High Magnetic Field Laboratory at Florida State University; Huibo Cao and Adam Aczel, both of the Neutron Scattering Division at Oak Ridge National Laboratory; Qing Huang and Jeffrey Lynn, both of the Center for Neutron Research at the National Institute of Standards and Technology; Jonathan Gaudet of McMaster University; and Dimitri Sokolov of the Max Planck Institute for Chemical Physics of Solids.

The study was supported by the Gordon and Betty Moore Foundation, the National Science Foundation, the Department of Energy, the Welch Foundation, and the state of Florida.