Magnetic Structure Determination: Advances and Applications

Conference: 2021: 71st ACA Annual Meeting
08/02/2021: 12:00 PM - 3:00 PM
2.1.4 
Oral Session 
Virtual  

Description

Magnetic symmetry is key to understanding and designing many quantum and topological properties in materials such as superconductors, quantum spin liquids, spin ices, topological insulators, chiral magnets, and skyrmions. The session will focus on various ideas on how to understand and design quantum and topological materials through magnetic and structural symmetries. We also welcome advanced magnetic and quantum characterization methods to enliven our discussions.

Presentations

Revealing exotic magnetic states through machine learning assisted modeling of neutron diffuse scattering data

Neutron scattering has a number of distinctive features. It is isotope sensitive, and readily detects light elements, as well as
having a magnetic scattering cross section. The scattering formulae are relatively simple but measurements involve large
amounts of noisy data. The data includes background and artifacts that are difficult to handle and in the case of diffuse and
inelastic scattering 3D and 4D data that are challenging to visualize. Further, there is no solution to the inverse scattering
problem. Experiments conventionally are limited by the lack of theory to interpret the scattering making it hard to optimally
perform experiments and analyze results. Data science approaches are needed to solve these challenges. Computational
simulations on wide ranges of cases provide the fundamental knowledge to interpret and guide the experiments. Integration
of machine learning into the neutron scattering pipeline provides a way to handle the large numbers of simulations and volume
of data and integrate these together. We have recently demonstrated integration of machine learning into neutron scattering
for diffuse and inelastic scattering. These involve 3D and 4D data sets and require sophisticated modeling. These are providing
new insights into exotic magnetic states and I will show two examples of frustrated magnetic materials where we have applied
these new approaches allowing the solution of difficult science problems. 

View Abstract 556

Author

David Tennant, Neutron Sciences Directorate, ORNL Oak Ridge, TN 

Reverse Monte Carlo refinement of single crystal diffuse neutron scattering and correlated magnetic disorder with program rmc-discord

A new software program for the analysis of single crystal diffuse scattering from disordered materials has recently been released. The rmc-discord Python package can extract short-range correlations from disordered crystals including the spin-spin correlations from magnetic diffuse neutron scattering data. An introduction to the program will be given including an overview of graphical user interface use for setting up a refinement. Using the program, it is shown with the mineral bixbyite, a geometrically frustrated magnetic, that reverse Monte Carlo can resolve subtle features of reciprocal space and differences between antiferromagnetic spin-pair correlation strength of nearest neighbor iron and manganese sites. Extensions to other systems generated by forward Monte Carlo method will also be discussed including future development for magnetic systems. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. 

View Abstract 585

Author

Zachary Morgan, Oak Ridge National Laboratory Oak Ridge, TN 

Magnetic vortices in a square lattice revealed by local magnetic susceptibilities and neutron diffuse scattering

Quantum materials (QMs) represent one of the most interesting and innovative areas in today's science due to their promising application potential in quantum computing and information technology. One of the challenges in quantum materials is to find materials with the subtle balance of interactions to display quantum phenomena. Neutron scattering is a key technique in exploring QMs. Recently, by employing polarized neutron scanttering and single crystal diffuse neutron scattering, a magnetic vortex liquid state is revealed in a rare-earth bilayer square lattice. It shows no long-range magnetic order upon cooling to 85 mK. Through the machine learning assisted optimization of the spin Hamiltonian, the polarized neutron diffraction and magnetic diffuse scattering together reveal the magnetic interactions of the tilted Ising spins, the role of impurities, and the origin of the short-range order vortex. 

View Abstract 614

Author

Erxi Feng, Oak Ridge National Laboratory Oak Ridge, TN 

Additional Author(s)

Anjana Samarakoon, Oak Ridge National Laboratory Oak Ridge, TN 
Xianghan Xu, Rutgers University Piscataway, NJ 
Chaowei Hu, University of California, Los Angeles Lso Angeles, CA 
Yaohua Liu, Oak Ridge National Laboratory Oak Ridge, TN 
Alan Tenant, Oak Ridge National Laboratory Oak Ridge, TN 
Ni Ni, university of california, los angeles Los Angeles, CA 
Sang-Wook Cheong, Rutgers University Piscataway, NJ 
Huibo Cao, Oak Ridge National Laboratory Knoxville, TN 

Coffee Break


Experimental Realization of Transverse Ising model on kagome and triangular lattice antiferromagnets

Transverse Ising model on a frustrated lattice is expected to host intriguing quantum disordered states at low temperatures due to the combination of an ice-like magnetic degeneracy and quantum-tunneling terms. We demonstrate that these models can be realized experimentally on rare-earth-based antiferromagnets, specifically, on the tripod kagome magnet Ho3Mg2Sb3O14, and the triangular lattice antiferromagnet TmMgGaO4. In both systems, Ising moments and intrinsic transverse fields originate from the crystal field of a non-Kramer's ion, whose magnitudes can be determined by an effective point charge analysis of the crystal field excitations [1]. Using neutron scattering, magnetic susceptibility, and thermodynamic measurements, (i) in Ho3Mg2Sb3O14, we observe a symmetry-breaking transition at 0.32  K to a partially ordered state which is characterized by a fragmentation of the magnetic moments and persistent inelastic magnetic excitations [2]; (ii) in TmMgGaO4, we found evidence for the existence of an intermediate Kosterlitz-Thouless phase between 0.9 K and 5 K which is characterized by short-range magnetic correlations and binding/unbinding of spin vortex-antivortex pair [3]. Our results point out a practical option to introduce quantum fluctuations in frustrated magnets and call for further experimental explorations of quantum magnets with short-range magnetic order.

*The work on Ho3Mg2Sb3O14 was sponsored by DE-SC0018660 and the work on TmMgGaO4 by NSF-DMR-1750186.
[1] Z. Dun et al., Physical Review Research 3 (2), 023012 (2021).
[2] Z. Dun et al., Physical Review B 103 (6), 064424 (2021).
[3] Z. Dun et al., Physical Review X 10 (3), 031069 (2020). 

View Abstract 520

Author

Zhiling Dun SANTA CLARA, CA 

Determine anisotropic g-tensor of rare-earth magnet using polarized neutron powder diffraction

Rare-earth-based quantum materials provide a rich playground for realizing exotic spin-orbital-entangled physics, such as Kitaev spin liquids. A model built from pseudo spin-1/2 operators is typically used to describe the low-energy collective physics in the rare-earth magnets. The critical link between such theoretical models and magnetic correlations observed in experiments is the effective g-tensor. Despite being a single-ion property of magnetic ion, an accurate determination of the g-tensor is not a trivial task, which often requires a combined effort of fitting bulk susceptibility, magnetization, and crystal electric field (CEF) excitations. For systems with low site-symmetry, it is particularly challenging, because the number of free parameters in the CEF model is generally greater than the number of modes observed in experiments. Polarized neutron powder diffraction is capable of probing site-dependent local susceptibility tensor, therefore provides a model-free approach for accurate determination of the ground state g-tensor. It can be applied in a wide range of rare-earth magnets and constitutes a key step toward understanding the collective physics. In this talk, I will demonstrate the this approach in the study of a Kagome-lattice spin-liquid candidate Er3Mg2Sb3O14, benefit from recent development of polarized neutron capability at the DEMAND instrument at High Flux Isotope Reactor, Oak Ridge National Laboratory. 

View Abstract 665

Author

Xiaojian Bai Oak Ridge, TN 

Additional Author(s)

Huibo Cao, Oak Ridge National Laboratory Knoxville, TN 
Erxi Feng, Oak Ridge National Laboratory Oak Ridge, TN 
Zhiling Dun SANTA CLARA, CA 
Chenyang Jiang, Oak Ridge National Laboratory Oak Ridge, TN 

Structural, magnetic ordering process and the magnetic excitations in spinel FeMn2O4

Spinel FeMn2O4 has been reported to exhibit a rough cation distribution of (Mn2+)A(Mn3+Fe3+)BO4, with Mn2+ mainly occupying the tetrahedral (A) sites forming a diamond lattice, and Fe3+ and Mn3+ sharing the octahedral (B) sites randomly to form a pyrochlore lattice. Powder neutron diffraction reveals a cubic-tetragonal structural transition at ~595 K upon cooling due to the Jahn-Teller distortion of MnO6 octahedra. A collinear ferrimagnetic order with antiparallel moments along c axis between A and B sites is found below ~373 K, and non-collinear ferrimagnetic order induced by the spin canting at B site appears below ~50 K. At 8 K, we observe a spin gap of approximately 5 meV arising from single-ion anisotropy at the orbitally active Mn3+ site. Spin wave dispersions in both the collinear and non-collinear ferrimagnetic ordered regions have been mapped out via inelastic neutron scattering on large single crystals. We demonstrate that the orbital degree of freedom of B-site Mn3+ ion and its coupling to spin and lattice degrees of freedom play a key role in the structural and magnetic properties in FeMn2O4. 

View Abstract 652

Author

Qiang ZHANG

Additional Author(s)

Wei Tian, ORNL knoxville, TN 
Masaaki Matsuda, ORNL knoxville, TN 
Andrew Christianson, ORNL knoxville, TN 
Tao Hong, ORNL knoxville, TN 
Roshan Nepal, ORNL knoxville, TN 
John DiTusa, ORNL knoxville, TN 
Stephen Nagler, ORNL knoxville, TN 
Rongying Jin1, ORNL Baton Rouge, LA 

Centers of Spatial and Time Inversion Symmetry in Magnetoelectric Crystalline Materials

Just as atomic structure dictates the properties of crystalline materials, the behavior of magnetic materials are dictated by their magnetic structure. Over the past several years we have researched antiferromagnetic materials in which the magnetic point group symmetry permits the formation of higher order toroidal moments. Since the magnetic moments in these candidate materials are ordered, this implies the toroidal moments would be ordered in a periodic structure as well. For example, if the arrangement of magnetic moments produces toroidal moments aligned along one direction in the crystal, the material would possess ferrotoroidal order.

After reviewing the candidates for ferrotoroidal order [1], we have undertaken several detailed neutron diffraction studies on members in the lithium transition metal phosphate series, including LiMPO4 (M = Co, Mn) [2] and the solid solution series LiMnxCo1-xPO4 and LiMnxFe1-xPO4. Previously at this conference, we discussed the evolution of the structure and magnetic moment in these series. All except LiMnPO4 were found to have the magnetic space group symmetry Pnma'. Based upon their tunability and magnetic symmetry, we concluded the entire series were candidate ferrotoroidic materials. In a subsequent year, we discussed in more detail the models for the toroidal moments in these and other candidate materials.

We have continued to study the structure and magnetism in this series with additional magnetization and diffraction experiments. We have also focused on understanding how the intersecting spatial and time inversion symmetry elements in the magnetic structure permit or forbid the ordering of toroidal moments. In this talk, I will present how having sites of -1' symmetry are crucial to modeling ordered toroidal moments, since both the magnetic and non-magnetic atoms contribute to the formation of toroidal moments. Using the series of lithium transition metal phosphates studied, I will explain how different arrangements of toroidal moments can be classified as not only ferrotoroidal, but also antiferrotoroidal or non-toroidal.

[1] Gnewuch, S.; Rodriguez, E. J. Solid State Chem. 2019, 271, 175 – 190.
[2] Gnewuch, S.; Rodriguez, E. Inorg. Chem. 2020, 59, 5883-5895. 

View Abstract 713

Author

Stephanie Gnewuch, University of Maryland, College Park Maryland, MD 

Additional Author

Efrain Rodriguez, University of Maryland College Park, MD 

MnBi2Te4.nBi2Te3: a happy marriage of magnetism and topology

Magnetic topological material provides a great platform for discovering new topological states, such as the axion insulators, the Chern insulators, and the 3D quantum anomalous Hall (QAH) insulators. Recently, MnBi2Te4 was discovered to be the first material realization of an intrinsic antiferromagnetic topological insulator (TI) where the QAH effect was observed at a record high temperature in its two-dimensional limit. Since the interplay of the magnetism and band topology determines their topological natures, understanding and manipulating the magnetism inside magnetic TIs will be crucial. In this talk, I will present our discovery of two new magnetic topological materials MnBi2Te4.nBi2Te3 (n=1 and 3) which consist of alternating [MnBi2Te4] and n[Bi2Te3] layers [1, 2]. I will show that by reducing the interlayer magnetic coupling with the increasing number of spacer [Bi2Te3] layers, MnBi2Te4.nBi2Te3 can be tuned from Z2 antiferromagnetic TIs (n=0,1,2) to ferromagnetic axion insulators. Furthermore, I will show what we have learned on magnetism and sample defects in this family of materials from the neutron diffraction experiments [3].

[1] C. W. Hu, et.al, Nature Communications, 11, 97 (2020)
[2] C. W. Hu, et.al, Science Advances, 6, eaba4275 (2020)
[3] L. Ding, et.al, J. Phys. D: Appl. Phys. 54 174003 (2021) 

View Abstract 671

Author

Ni Ni, university of california, los angeles Los Angeles, CA 

La2O3-type structure magnetic topological candidates

Within the condensed matter science community, interest in topological materials has grown rapidly. An exciting direction is the combination of topological electronic states with additional quantum phases including magnetic topological materials, which have a range of applications including spintronics, thermoelectric and photovoltaics. Previously, our group discovered the trigonal La2O3-type Mg3Bi2 material to be a type-II nodal-line semimetal. The strong spin orbit coupling (SOC) Bi layers in this structure create an ideal environment for the topological electronic states. To drive this material even further magnetic elements can be introduced, potentially leading to a coupling between the magnetism and topological quasiparticles. In this work we explore various La2O3-type structure magnetic topological candidates consisting of either 3d or 4f magnetic elements. Various techniques have been utilized to investigate the intricate tuning of the magnetic and electronic properties in these materials including neutron diffraction, revealing their magnetic structure and dynamics. As a result, this work has provided a further understanding of the ability to control the magnetic and topological properties. 

View Abstract 624

Author

Madalynn Marshall, Rutgers University New Brunswick, NJ 

Additional Author(s)

Weiwei Xie
Huibo Cao, Oak Ridge National Laboratory Knoxville, TN 
Antonio M. dos Santos, Oak Ridge National Laboratory Oak Ridge, TN 

Magnetic properties in new half-Heusler-type compound

Heusler and half-Heusler compounds were found to have intriguing properties including magnetic ordering, topological non-trivial state, superconductivity etc. Here we report the synthesis and crystal structure of a novel compound crystallizing in half-Heusler-type structure which was never disovered before. The new comopund paves a way to realize half-Heusler compounds and provides a new platform to investigate and tune physical properties in half-Heusler compounds. 

View Abstract 698

Author

Xin Gui, Princeton University Princeton, NJ 

Additional Author(s)

Huibo Cao, Oak Ridge National Laboratory Knoxville, TN 
Robert Cava, Princeton University Princeton, NJ