Session N72: Mn-Te Magnetic Topology IV

Strong Magneto-Elastic Coupling in Mn3X (X = Ge, Sn) revealed by Ultrasound

The chiral antiferromagnets Mn₃X (X = Sn, Ge) have recently attracted a large amount of attention due to their large anomalous Hall response at room temperature, linked to Weyl nodes close to the Fermi surface. Besides manipulation of the Hall angle with small magnetic and electric fields, the sign of the Hall voltage has been demonstrated to switch under hydrostatic and uniaxial pressure. Here, we perform Resonant Ultrasound Spectroscopy (RUS) and Pulse Echo Ultrasound measurements to further investigate strong magnetoelastic coupling in Mn₃X. We observe giant anomalies in the compressional elastic constants, related to a large change of Neel temperature with hydrostatic pressure. Additionally, we extract a sizeable piezomagnetic coefficient in Mn₃Ge, made possible through the unique combination of irreducible representations of in-plane shear strain and magnetic order parameter in Mn₃X. By examining the critical divergence of the bulk modulus close to the phase transition, we were able to place Mn₃Ge in the 3D-XY universality class. Further, access to the shear modulus c₆₆ offers the rare opportunity to extract the critical exponent for the nematic susceptibility.   

Ultrafast phonon-mediated melting of magnetism via itinerant spins in MnBi2Te4

A comprehensive understanding of spins away from equilibrium, including their interplay with lattice and electronic excitations is key to achieving fundamental breakthroughs in understanding how to manipulate topological phases with light. To this end, we study the ultrafast melting of magnetic order in the layered antiferromagnetic topological insulator MnBi₂Te₄. Here we use a multimodal experimental approach with ultrafast electron diffuse scattering to probe of nonequilibrium phonon dynamics, and magneto-optic Kerr rotation and resonant soft X-ray scattering are used to directly measure ultrafast dynamics of conduction and localized spins. Our experiments reveal that ultrafast demagnetization occurs through a previously unidentified p-like itinerant spin subsystem, with optical phonons acting as the primary channel for the dissipation of spin angular momentum. Localized Mn 3d⁵ spins disorder by a distinct, much slower thermalization mechanism, due to the quenched orbital angular momentum. The disparate demagnetization timescales in both spin subsystems provides evidence of a strong exchange coupling between the 3d­ localized spins and the p-like itinerant bands, which sheds light on the interatomic exchange pathways that enable magnetic topological phases.   

Magneto thermal conductivity and thermal Hall conductivity of single crystal MnBi through the spin-reorientation temperature range

The search for an experimental signature in transport properties of magnetic topological materials is a major thrust in materials research. MnBi, a hard hexagonal ferromagnet with a high Curie temperature and strong spin-orbit interactions, is a great candidate to investigate this due to two properties.  First, it has a unique spin reorientation (SR) behavior: under about 90 K the spins align in the ab-plane, above about 140 K the spins align along the c-axis, and in between 90 and 140 K the spins have some degree of freedom between these two extremes.¹ Second, it has an extremely large anomalous Nernst conductivity that is not simply explained by the band structure.² Here, we explore the magneto thermal conductivity and thermal Hall conductivity of MnBi and study the structure and topology of the domains using Lorentz Transmission Electron Microscopy to identify the correlation between thermal transport properties and the topological properties of the domains in the SR regime. We show that the domain growth in an applied magnetic field causes a large decrease in magneto thermal conductivity and interesting trends in thermal Hall conductivity; however, this process is complex in the SR temperature regime.

[1] McGuire et al.  PRB 90, 174425 (2014)

[2] He et al., Joule 5 1-11 (2021)   

Non-coplanar Magnetism in Mn3Sn

The magnetization-free anomalous Hall effect of Mn₃Sn yields topological non-trivial bands with Weyl nodes at the Fermi level. Yet the knowledge of its incommensurate phase remains insufficient. Considering incommensurate magnetism itself historically manifests strong electron correlations in unconventional superconductors and heavy fermion systems, studies on the incommensurate magnetism of Mn₃Sn provides further insights of the itinerate electrons in Mn₃Sn. With neutron scattering techniques, we were able to fully determine the incommensurate magnetic structure and analyze its dynamics. Our results show potential strong correlation and competing interactions in the magnetic Weyl semimetal Mn₃Sn.   

Magnetic and magnetoelectric transport properties of TbMn6Sn6 thin films

The kagome lattice is an excellent model system for studying frustrated magnetism, electronic correlation and topological electronic structure. Recently, quantum oscillations and the anomalous Hall effect in single-crystals of the kagome compound TbMn₆Sn₆ hint at the realization of a quantum-limit two-dimensional Chern phase. We have synthesized epitaxial TbMn₆Sn₆ thin films using molecular beam epitaxy. We have characterized the magnetization, magnetoresistance, and anomalous Hall effects of these thin films, and compare our results to those reported for bulk single-crystals in the literature. The realization of high-quality TbMn₆Sn₆ thin films may present new possibilities in the search for topological quantum phenomena in kagome metals.   

Ab initio studies of electronic structures and magnetic properties in RMn6Sn6 (R =Gd, Tb, Dy, Ho, and Er)

Using ab initio methods, we systematically investigate the electronic structures and intrinsic magnetic properties in RMn₆Sn₆ with R = Gd, Tb, Dy, Ho, and Er. The calculations show that TbMn₆Sn₆ has an easy-axis anisotropy, DyMn₆Sn₆ and HoMn₆Sn₆ have easy-cone anisotropy, and ErMn₆Sn₆ has an easy-plane anisotropy, all agreeing well with experiments and explained by the Mn coordination of the rare-earth atoms. The Mn sublattice is found to have an easy-plane anisotropy of similar amplitude in all RMn₆Sn₆ compounds. Band structures of various RMn₆Sn₆ compounds share great similarities near the Fermi level as they mostly consist of non-4f bands. Multiple Dirac crossings occur at the Brillouin zone corners and are opened by spin-orbit coupling; most of them are strongly K_z-dependent. The most prominent 2D-like (barely-K_z-dependent) Dirac crossing at K lies 0.6–0.7 eV above the Fermi level. However, additional on-site correlations within Mn-d electrons can have profound effects on crossing energies and gap sizes. Finally, we discuss the effects of spin reorientation and the effects caused by the surface on the topological band structures.   

Magnetization compensation temperature and frustration-induced topological defects in ferrimagnetic anti-perovskites

Anti-perovskites can display promising properties such as superconductivity [1] and topological band gaps [2–4]. Ferrimagnets can have many promising features, such as a so-called magnetization compensation temperature (at which the total magnetization vanishes, as a result of cancellation between magnetic moments of different sublattices) and the perpendicular magnetic anisotropy which allows robust and ultrasmall skyrmions at room temperatures [5–7]. Such features make anti-perovskite ferrimagnets very promising candidates for realizing high-density, low-cost, and energy-efficient skyrmionic device technology. 

In this work, first-principles calculations and ab-initio based Monte Carlo simulations are done to investigate magnetic properties and topological spin textures in the anti-perovskite ferrimagnet -- Mn₄N. We reveal the existence of a sizeable magnetization compensation temperature that is driven by the different temperature behaviors of three types of magnetic Mn ions when Mn₄N is in thermal equilibrium. We also report the discovery of metastable topological states, including nanometric hedgehog-anti-hedgehog pairs, that are induced by frustrated exchange interactions.

Furthermore, we look at other candidates from the Materials Project that could host similar magnetic and topological properties. Thus, we hope that this work will motivate experimental confirmations of magnetization compensation temperature and topological phases such as hedgehog-anti-hedgehog pairs in anti-perovskite ferrimagnets and will be put to use to design novel spintronic devices.   

Anomalous Exchange Resonance Driven by Spin-Orbit Torques in Intrinsic Antiferromagnetic Topological Insulator

In antiferromagnetic topological insulator MnBi2Te4, the interplay between topological electrons and magnetization dynamics in the form of spin torques and charge pumping remains an open question. Using the Berry phase theory, we develop a universal formalism to quantify the spin-orbit torques (SOTs) and their reciprocal effects in layered MnBi2Te4, where the symmetry of SOTs exhibits an even-odd layer-number pattern. The dynamical consequences of our finding are demonstrated by the SOT-driven magnetic resonances. In particular, we identify an anomalous exchange (AE) mode in the tri-septuple layer (SL) case, where the magnetic moments from the top and bottom SL rotate with a pi phase difference while those in the middle SL stay static. Unlike the ordinary exchange mode and the acoustic mode, the AE mode does not react to a microwave electromagnetic field. Therefore, the AE resonance can only be excited by the peculiar SOTs we find, providing a unique way to verify the SOTs in MnBi2Te4. Our study lays the theoretical foundation of high-efficiency electric-field-induced spin dynamics in intrinsic antiferromagnetic topological insulators.   

Bell-state generation for spin qubits via dissipative coupling

We theoretically investigate the dynamics of two spin qubits interacting with a magnetic medium. A systematic theoretical framework for this qubit-magnet hybrid system is developed in terms of the equilibrium properties of the magnetic medium. Our particular focus is on the induced dissipative coupling between the spin qubits. In contrast to the conventional wisdom that dissipation is detrimental to quantum effects, here we show that a sizable long-lifetime entanglement can be established via a dissipative environment, in the absence of any coherent coupling. Moreover, we demonstrate that maximally-entangled two-qubit states (Bell states) can be achieved in this scheme when complemented by proper postselection. In this situation, there is a dynamical phase transition separated by an exceptional point. The resultant Bell state is robust against weak random perturbations and does not require the preparation of a particular initial state. Our study may find applications in quantum information science, quantum spintronics, and for sensing of nonlocal quantum correlations.   

Evolution of band topology with antisite defects in the Sb-doped MnBi4Te7

  Magnetic topological materials attract worldwide attention because the interplay between band topology and magnetism may lead to novel topological quantum phenomena, such as quantum anomalous Hall effect and quantized magnetoelectric effect. Here we propose a tunable magnetic and topological state in Sb doping a newly discovered intrinsic antiferromagnetic (AFM) topological insulator (TI) MnBi₄Te₇. Our first-principles calculation shows that the topological ground state transfer from AFM TI to ferromagnetic axion insulator in the low doping region, and then exhibit a type-I ferrimagnetic Weyl semimetal that resulted from strong antisite disorder in the high doping region. Our calculation demonstrates that the doping series provides a fruitful platform with continuously tunable magnetism and topology for investigating emergent phenomena.   

Kagome Magnet RMn6Sn6: First-principles study

Kagome lattice is a two-dimensional honeycomb structure network with triangular corner sharing. The band structure of kagome lattice has been theoretically predicted to possess both flat bands and Dirac states in the Brillouin zone. In addition, the band degeneracy will be lifted and present a topological nontrival gap as considering time-invariant intersite spin-orbit coupling. Recently, a new kagome family RMn₆Sn₆ has been found to provide a tunable magnetic configuration by replacing different rare earth elements R. For example, TbMn₆Sn₆ (R=Tb) shows a ferrimagnetic state with our-of-plane spin orientation. Based on first-principles calculations, we identify a highly orbital-selective massive Dirac state forming by Mn d_x2-y2/d_xy with 35 meV large energy gap at K point, which is well consistent with our tunneling measurement [1]. Remarkably, our calculation exhibits the strong Berry curvature in this gapped Dirac state, supporting TbMn₆Sn₆ may be an intrinsic Chern insulator. Besides TbMn₆Sn₆, we find an in-plane spin configuration in GdMn₆Sn₆. Our first-principles calculations demonstrate a magnetic nodal-line state resulted from the nearly gapless Dirac cone around K point in this easy-plane ferrimagnet spin structure. Our calculation reveals an orbital-selective and a tunable topological gap in kagome magnet RMn₆Sn₆ and provide a natural platform to investigate the interplay between topology and magnetism.   

Unraveling peculiar magnetism and band topology in Mn3Sb

Magnetic, pseudogap, topological, magnetostructural, and elastic behaviors of Mn 3 Sb have been unraveled. The ferrimagnetism (FIM) is described by localized and delocalized electron magnetism resulting in dif­ferent magnetic moments on Mn atoms, confirming the neutron diffraction data. The identified magne­tostructural properties are due to the non-equivalent Mn atoms in its lowest symmetry structure. The electronic structure is also unique due to variable valance states of Mn atoms. The magnetic moment (4.10 μ_B ) of non-equivalent Mn1 atom is antiparallely aligned with the magnetic moments (2.34 μ _B ) of Mn2 and Mn3 atoms. The estimated Curie temperature, T C , is higher than the room temperature, which may have above the room temperature applications in spintronic devices. The band structure and density of states (DOS) show the characteristics of band topology (opening of a gap in Dirac-like band features) and pseudogap, respectively, around the Fermi level. While expanding the unit cell, the tetragonal FIM ground state

transforms to the cubic primitive FIM phase, however, the contraction transforms it to the L1 ₂ ferromag­ netic (FM) phase. The estimated elastic constants, bulk to shear modulus ratio, and elastic anisotropy factor indicate that Mn 3 Sb exhibits mechanically stable, ductile, and anisotropic behaviors, respectively.   

Morphology and Magneto-Transport Properties of Submicron Grains of the Topological Antiferromagnet Mn3Sn

The topological antiferromagnet Mn₃Sn [0] exhibits ferromagnetic-like responses such as the Anomalous Hall and Nernst Effects (AHE/ANE) due to its time-reversal symmetry breaking magnetic structure [1,2]. Recently, Mn₃Sn thin films have been of interest due to their role in spintronics [3,4], and studies of submicron Mn₃Sn particles could expand the material’s use in nanotechnology. In this work, Mn₃Sn films with varying thickness t were fabricated by DC magnetron sputtering on Si/SiO₂ substrates at crystallization temperature. Through atomic force microscopy (AFM) measurements, our films were seen to be amalgamations of submicron formations. When t ≦ 20 nm, individual isolated grains made up the films. Depending on t, the sizes of these grains vary from tens to hundreds of nanometers. AHE was observed in the Mn₃Sn grains covered by a conducting layer, suggesting that our grains possess finite Berry curvature in momentum space, which could further cement the material’s role in spintronics.   

Colossal angular magnetoresistance in a ferrimagnetic nodal-line semiconductor Mn3Si2Te6

Topological magnets, where both magnetism and nontrivial band topology coexist, have emerged as promising candidates to realize novel electronic and spintronic functionalities, because their topological band degeneracy can be readily tuned by spin configurations, thus dramatically modulating electronic conduction. Here we propose a new class of topological magnets, namely, magnetic nodal-line semiconductors, in which spin-polarized conduction or valence bands possess topological nodal-line degeneracy. Taking a layered ferrimagnet Mn₃Si₂Te₆ as a model system, we show that the topological band degeneracy, driven by chiral molecular orbital states, is lifted depending on the spin orientation, which leads to a metal-insulator transition in the same ferrimagnetic phase. As a result, we have observed extremely large angular magnetoresistance exceeding a trillion percent per radian, which we call colossal angular magnetoresistance. Our findings highlight that magnetic nodal-line semiconductors are a promising platform for realizing extremely sensitive spin- or orbital-dependent functionalities.   

Transport and magnetic properties of the topological (Weyl) semimetal: Hexagonal - (Mn1-αFeα)3Ge (α = 0 – 0.3)

In the case of Mn₃Ge, the anomalous Hall effect (AHE) has its origins in the topological Weyl nodes. The AHE can be controlled by tuning of the Weyl points relative to the Fermi surface, by suitable dopants of the parent phase. Therefore, we have explored the electrical transport and magnetic properties of the single crystal (Mn_1-αFe_α)₃Ge to study the change in AHE and chiral anomaly with Fe doping. Clear signatures of the AHE and chiral anomaly were observed for samples up to α = 0.22, in the temperature regime where magnetization behaves the same as the parent sample. However, the strength of AHE and chiral anomaly decreases with an increase in Fe doping and vanishes beyond α = 0.22. To predict the origin of AHE in doped samples, the ground state magnetic structure of α = 0.22 was determined using single-crystal (polarized and unpolarized) neutron diffraction techniques. We observed that the magnetic structure of the doped sample remains the same as that of the parent compound in the temperature regime where AHE was observed. These observations led us to two main conclusions: (i) the Weyl points are very likely to be present in the doped samples, and (ii) the characteristics of the Weyl points can be tuned significantly by suitable doping of the Weyl semimetals.