Session F44: Topological Kagome Magnets

Manipulation of Dirac band curvature and momentum-dependent g factor in a kagome magnet YMn6Sn6

The Zeeman effect describes the energy change of an atomic quantum state in a magnetic field. The magnitude and the direction of this change depend on the dimensionless Lande g-factor. In quantum solids, the response of the Bloch electron states to the magnetic field also exhibits the Zeeman effect with an effective g-factor that was theoretically predicted to depend on the momentum, and which may be particularly strong in topological and magnetic systems. However, the momentum dependence of the g-factor is difficult to extract and it has not been directly measured. In this talk, we will discuss the experimental discovery of a momentum-dependent g-factor in the kagome magnet YMn₆Sn₆. We use spectroscopic-imaging scanning tunneling microscopy to map the evolution of a massive Dirac band in the vicinity of the Fermi level as a function of the magnetic field. We find that electronic states at different lattice momenta exhibit different Zeeman energy shifts, giving rise to an anomalous g-factor that peaks around the Dirac point. Our work provides a momentum-resolved visualization of Dirac band curvature manipulated by a magnetic field, which will be relevant to other topological kagome magnets.

    

Intriguing Magnetism of the Topological Kagome Magnet TbMn6Sn6

Magnetic topological phases of quantum matter are an emerging frontier in physics and material science [1-3], of which kagome magnets appear as a highly promising platform. Here, we explore magnetic correlations in the recently identified topological kagome system TbMn₆Sn₆ using muon spin rotation, combined with local field analysis and neutron diffraction [1]. Our studies identify an out-of-plane ferrimagnetic structure with slow magnetic fluctuations which exhibit a critical slowing down below T^*_C1 ≅ 120 K and finally freeze into static patches with ideal out-of-plane order below T_C1 ≅ 20 K. The appearance of the static patches sets in at a similar temperature as the topological transport behaviors. We further show that a hydrostatic pressure of 2.1 GPa stabilizes the static out-of-plane topological ferrimagnetic ground state in the whole volume of the sample. Therefore the exciting perspective arises of a magnetically-induced topological system whose magnetism can be controlled through external control parameters. The present results [1] will stimulate theoretical investigations to obtain a microscopic understanding of the relation between the low-temperature volume-wise magnetic evolution of the static c-axis ferrimagnetic patches and the topological electronic properties in TbMn₆Sn₆.

 

[1] C. Mielke III et al., Comm. Phys. 5, 107 (2022).

[2] J.-X. Yin et al., Nature 583, 533-536 (2020).

[3] N.J. Ghimire and I.I. Mazin, Nature Materials 19, 137-138 (2020).

    

Influence of Ga doping on the magnetic anisotropy of the kagome-lattice magnet TmMn6Sn6.

The kagome-lattice compounds RMn6Sn6 (R is a rare earth element), where the Mn atoms form a kagome net in the basal plane, are currently attracting a great deal of attention as they’ve been shown to host complex magnetic textures and electronic topological states strongly sensitive to the choice of the R atom. Among the magnetic R atoms, TmMn6Sn6 orders with the easy-plane magnetization forming a complex magnetic spiral along the c-axis. Previous neutron studies carried on polycrystalline samples have shown that Ga doping changes the magnetic anisotropy from easy-plane to easy-axis.  Here we present magnetic and magnetotransport measurements on a single crystal and first principles calculations in the doping series TmMn₆Sn_6-xGa_x. We find that the magnetic properties are highly sensitive even to a small concentration of Ga. At small Ga concentrations, the in-plane anisotropy is maintained, which gradually changes to the out-of-plane anisotropy with increasing Ga.  We will discuss these observations with respect to the effect of modification of the Tm crystal field, introduced by Ga doping.

    

Tuning TbMn6Sn6 via Cr doping

Kagome materials have gained much attention due to the coexistence of flat bands and Dirac points in their band structures which may become topologically non-trivial with the addition of spin-orbit coupling. The RT₆X₆ (R = rare earth, T = transition metal, X = Si, Ge, Sn) kagome family is of particular interest due to the kagome layers being comprised solely of T. The ferrimagnet TbMn₆Sn₆ was recently claimed to be a quantum-limit Chern topological magnet [1], but the physics of this system is still being debated [2]. We present data on single crystals of Tb(Cr_xMn_1-x)₆Sn₆ showing that doping with the largely non-magnetic Cr effectively tunes this system. The ferrimagnetic transition temperature and subsequent spin-reorientation temperature seen in TbMn₆Sn₆ both decrease with doping, the anomalous Hall effect is significantly altered with doping, and for a large range of x an exchange bias is observed that exists up to 100K. These responses provide evidence that doping can play a large role in engineering the magnetic and topological phases of the RT₆X₆ family.

 

[1] J.-X. Yin et al., Nature 583, 533-536 (2020)

[2] D. C. Jones et al., arXiv: 2203.17246   

Incommensurate magnetic order in Topological Antiferromagnet ErMn6Sn6

RMn₆Sn₆ (R166) compounds (R = Gd-Lu, Y) with kagome Mn bilayers separated by triangular R layers have the possibility to tune topological states within the kagome layer via the magnetism of the R layers. In particular, competition between uniaxial R and easy-plane Mn anisotropies leads to a temperature-dependent spin-reorientation transition which can switch topological electronic phases. Here we present detailed single-crystal neutron diffraction data for Er166 as a function of temperature, showing complex incommensurate antiferromagnetic (AFM) order between temperatures of T_ER ≈ 88 and T_N ≈ 350 K. The easy-plane ferrimagnetic order involving both Mn and Er sublattices below T_ER is in agreement with previous reports.  Above T_ER, the Er and Mn lattices potentially decouple, and we find the Bragg peaks corresponding to incommensurate spiral-like magnetic order.  Unlike Y166, we report even and odd higher-order harmonics of the AFM propagation vector. The second-order peaks, if magnetic, could indicate a zero-field fan-type order. On the other hand, second-order peaks could indicate a structural transition induced by magnetoelastic coupling.   

Large Room Temperature Anomalous Transverse Thermoelectric Effect in Kagome Antiferromagnet YMn6Sn6

Kagome magnets possess several novel nontrivial topological features owing to the strong correlation between topology and magnetism that extends to their applications in the field of thermoelectricity. Conventional thermoelectric (TE) devices use the Seebeck effect to convert heat into electrical energy. In contrast, transverse thermoelectric devices based on the Nernst effect are attracting recent attention due to their unique transverse geometry, which uses a single material to eliminate the need for a multitude of electrical connections compared to conventional TE devices. Here, a large anomalous transverse thermoelectric effect of ≈2 µV K⁻¹ at room temperature in a kagome antiferromagnet YMn₆Sn₆ single crystal is obtained. The obtained value is larger than that of state-of-the-art canted antiferromagnetic (AFM) materials and comparable with ferromagnetic systems. The large anomalous Nernst effect (ANE) can be attributed to the net Berry curvature near the Fermi level. Furthermore, the ANE of the AFM YMn₆Sn₆ exceeds the magnetization scaling relationship of conventional ferromagnets. The results clearly illustrate that AFM material YMn₆Sn₆ is an ideal topological material for room-temperature transverse thermoelectric applications.   

Topological Nernst Effect in ternary kagome compound ScMn6Sn6

Electromagnetic phenomenon topological Hall effect (THE) and its thermoelectric equivalent, topological Nernst effect (TNE), are known to be the two signature properties of a skyrmion lattice (SkL) phase. Recent reports on YMn₆Sn₆ show that THE can also be observed in spiral magnets with no static spin chirality, which arises the necessity of looking for TNE in similar materials. With the aim of investigating the origin of these hallmark properties (THE and TNE) of SkL in other spiral magnets we perform the electrical and thermal transport study on ScMn₆Sn₆. Our results show that along with the THE, ScMn₆Sn₆ gives rise to the topological Nernst effect as well. So far only three other materials have been reported to exhibit TNE, which puts ScMn₆Sn₆ in a small group of materials hosting multiple topological effects. The similarity between the critical fields and the critical temperatures of THE and TNE suggest that both these phenomena have the same origin. In this talk we will discuss our attempt to understand the origin of these effects.

 

    

Interplay between local symmetry breaking, magnetic order and Weyl states in Co3Sn2S2

Co₃Sn₂S₂, a magnetic Weyl semimetal with a kagome lattice of cobalt ions, has triggered intense interest recently. Co₃Sn₂S₂ was proposed to exhibit a coexistence of ferromagnetic (FM) order and antiferromagnetic (AFM) order below T_C≈ 175 K, followed by a pure ferromagnetic (FM) order below T_A≈ 135 K.  The ferromagnetic order along the c axis was confirmed from our half-polarized neutron technology below T_C. Using neutron total scattering, we found a striking local symmetry breaking from rhombohedral R-3m to monoclinic Cm co-emerges with the onset of ferromagnetic order below T_C. The mismatch of local and average crystallographic structures indicates that Co₃Sn₂S₂ becomes an intrinsically lattice disordered system below T_C. This provides new insight to the previously puzzling magnetic phase separation and spin glass like state in Co₃Sn₂S₂. Our DFT calculations reveal that both the symmetry-allowed 120° antiferromagnetic orders support new Weyl states in T_A<T<T_C, with distinct numbers and locations of Weyl points. Upon cooling below T_A, inelastic neutron scattering revealed highly anisotropic spin-wave dispersions and linewidths. Modeling the spin-wave spectra shows that the ground-state FM order is dominated by the FM third-neighbor “across-hexagon” J_d coupling with a weak frustrated next-nearest-neighbor bond. Furthermore, our DFT calculation indicates that the local symmetry breaking plays a detrimental role in the formation of the Weyl points associated with the FM order by breaking mirror symmetries and is expected to induce a broad topological surface band like feature. Our study unveils an intimate interplay between local symmetry breaking, magnetic order and Weyl states in Co₃Sn₂S₂.

    

Large Magnetocaloric Effect in RMn6Sn6 Systems

Magnetic refrigeration is a more efficient and environmentally friendly technology than the conventional gas-cycle cooling. The main challenge, however, is finding materials with large enough magnetocaloric effect (MCE). A giant MCE has been found in materials with large rare-earth content but the search for earth-abundant systems has proven to be extremely challenging. Here we present our results on the Kagome magnets RMn₆Sn₆ (R=Rare Earth) that order near and above room temperature with a considerable MCE and with less than 8% rare-earth content. The materials exhibit a range of different magnetic structures depending on the specific rare earth element. We measured the MCE in these systems in order to determine how it evolves along the later Lanthanides and what magnetic transitions, structures, and anisotropies lead to the largest change of entropy. Our investigations into the RMn₆Sn₆ materials with (R=Tb, Ho, Er, and Lu) show that transitions from the paramagnetic state to a ferro- or ferrimagnetic state result in a larger entropy change than transitions between different magnetically ordered states or between para- and antiferromagnetic states. Our investigation also shows a larger MCE in the earlier lanthanides which are ferrimagnetic compared to the later Lanthanide materials which are antiferromagnetic.   

Evolution of spin structure in Kagome metal Sc0.8Hf0.2Mn6Sn6 under magnetic field

Much interest has been drawn to the magnetic Kagome materials because of the presence of rich phases including quantum spin liquid, flat electronic bands and topological electronic behavior. In particular, a topological Hall effect in the absence of crystallographic inversion symmetry breaking is observed in the Kagome-net magnet YMn₆Sn₆ near room temperature (240 K) with in-plane magnetic field applied (µ₀H > 2 Tesla). Such effect is believed to associated with noncollinear spin structures like the double-fan spin structure [1] or transverse conical spiral configuration [2,3]. Similar behavior has been observed in the Hf-doped Sc_0.8Hf_0.2Mn₆Sn₆ except with a much-reduced critical field. We have used single crystal neutron diffraction to investigate the evolution of the complex spin configuration with magnetic field applied both in in-plane and along the c-axis. The detailed modification in spin structure is discussed in the context of the competitions between exchange interactions and magnetic anisotropies.

[1]          Q. Wang et al., Phys. Rev. B 103, 014416 (2021).

[2]          N. J. Ghimire et al., Sci. Adv. 6, eabe2680 (2020).

[3]          R. L. Dally et al., Phys. Rev. B 103, 094413 (2021).