Session F72: Magnetic Topological Materials II

Stimulated nucleation of skyrmions in a geometrically asymmetric structure

Understanding the dynamics of skyrmion nucleation and manipulation is important for applications in spintronic devices. Here, we have investigated the magnetic texture transformation using Lorentz transmission electron microscopy (LTEM) in a centrosymmetric magnet Fe₃Sn₂ with an engineered geometrical asymmetry in the form of a thickness gradient. Using sample tilting and tunable magnetic field strength in the LTEM we developed a new approach for skyrmion nucleation at magnetic domain boundaries. This approach enables nucleation of isolated skyrmions in Fe₃Sn₂ using in-plane fields (< 5 mT) that are two orders of magnitude lower than the previously reported critical magnetic field (~800 mT). Micromagnetic simulations combined with LTEM experiments reveal that the rotatable anisotropy and thickness dependence of the response to the external in-plane field are the critical factors for the skyrmion formation. These results suggest that magnetic materials with rotatable anisotropy are potential skyrmionic systems and provide a novel approach for the manipulation of skyrmions in spintronic devices.   

Neutron scattering investigation of the field-induced magnetic orders in EuIn2As2.

We recently demonstrated that hexagonal EuIn₂As₂ exhibits a zero-field low-symmetry helical antiferromagnetic order that makes the compound a stoichiometric magnetic topological-crystalline axion insulator[1]. It is predicted that the topological properties of the surface states can be tuned according to the direction and strength of an applied magnetic field. We continue our investigation of EuIn₂As₂ by performing single-crystal neutron diffraction experiments while using an external magnetic field in order to determine field-induced changes to the magnetic order. With increasing field applied in the basal plane, our results are consistent with the spins first flopping out of the plane while keeping a complex in-plane spiral periodicity. The out of plane component is then progressively suppressed and a coplanar fan-type structure appears before the fully field polarized state prevails. The intermediate-field magnetic ordering may give rise to exotic topology associated with a non-coplanar spin texture.   

Ferromagnetic EuCd2As2 as a candidate Weyl ‘Hydrogen Atom’

We are in an intriguing era of condensed matter physics where much of our efforts are focused on the search for solutions to relativistic quantum mechanical wave-equations in solid state materials.  However, whereas originally these exotic particles were born of 'beautiful' simplifications of the Dirac equation, in materials they require a careful tuning of numerous factors to achieve - and even then, rarely is the ideal situation found leaving some ambiguity to the quasiparticles’ observation. For the case of the massless chiral Weyl particle, most host systems have numerous Weyl points and other trivial bands near the Fermi energy adding higher order interactions and obscuring the desired physics in all but ARPES measurements.  Here we present the ferromagnetic EuCd₂As₂ as a Weyl semi-metal with a more ideal set of conditions – a single pair of Weyl points generated by broken time reversal symmetry which live close to the Fermi Energy. Using a combination of neutron scattering, density functional theory and careful transport measurements we elucidate the relevant symmetries, their implications for the band structure and show the resultant transport properties suggesting the clear presence of topological physics.   

Engineering magnetic topological insulators in Eu-based Zintls

In Zintl materials, the rocksalt-like charge separation between the cationic and anionic frameworks offers a vital platform for the exploration of magnetic topological insulators. When the cations are magnetic and the anions are non-magnetic, it often causes the materials to behave as two independent subunits, with separate magnetic and electronic behaviors. In addition, the clear separation between the electronic and magnetic energy scales allows us to tune the electronic structure of the polyanionic network and then consider the effect of magnetism. This band engineering approach is achieved by employing doping, element substitution, pressure, and epitaxial strain. Specifically, starting from the trivial antiferromagnetic insulator Eu₅In₂Sb₆, we utilize density functional theory to engineer band inversions by applying chemical pressure via isoelectronic substitution, and use external pressure and epitaxial strain to control the bulk energy gaps. Our approach results in the prediction of three new antiferromagnetic topological insulator candidates: Eu₅Ga₂Sb₆, Eu₅Tl₂Sb₆, and Eu₅In₂Bi₆.   

Discovery of magnetic topological insulators using chemistry

In designing magnetic topological insulating states one often runs into mutually exclusive co-existence of topology and electron correlations. In order to achieve an intrinsic magnetic topological insulator state, there needs to be a separation between the density of states close to the Fermi level and the states from magnetism. Using this requirement, we identify Zintl compounds to be promising to find a sweet spot between topology and electron correlations. Zintl compounds have rocksalt-like charge separation between the cationic and anionic frameworks that are charge separated from one another. If the cationic framework is magnetic and the anionic framework is non-magnetic, consists of heavy atoms, and has covalent bonding, then these serve as essential structural motifs for magnetic topological insulators. Using this concept, we identify the Eu5M2X6 (M = Ga,In,Tl, X = Sb,Bi) family of materials as a rich source of topologically trivial and non-trivial insulators. In this work we will present the synthesis, magnetism, optical and transport behavior of members of this family with an aim of identifying an intrinsic axion insulator.   

Magnetic Structure of the Zintl Material Eu5In2Sb6

Eu₅In₂Sb₆ is a Zintl material that displays perhaps the greatest (negative) colossal magnetoresistance of observed stoichiometric antiferromagnetic materials. It has also been proposed as an axion insulator candidate. To our knowledge, the magnetic structure of Eu₅In₂Sb₆ has not yet been reported. Using neutron diffraction, we find the 14 K transition previously observed in heat capacity and magnetization gives k=0 order while the 7 K transition is to a multi-k structure with a second k=(0,0,1/2) propagation vector. We report refined magnetic structures for both phases. This work contributes to our understanding of both magnetic and exotic transport properties in Eu₅In₂Sb₆. It also suggests the magnetic structure of substituted variants, which may realize the axion state.   

Observation of ferromagnetic order and inverted hysteresis in EuCd2P2

EuCd₂P₂ has been reported to show a giant, magnetic-field-sensitive resistance peak at 18 K, which is well above the antiferromagnetic (AFM) transition at 11 K in this material [1]. The physics origin of the resistance peak remains an open question, which leaves the space to explore magnetic structures above the Neel temperature. We observe a clear hysteresis in EuCd₂P₂ via magneto-optical Kerr effect microscopy, indicating an unexpected ferromagnetic (FM) order. The hysteresis is observed both below and above the AFM transition. Remarkably, the hysteresis becomes inverted above the Neel temperature, which coincides with the emergence of the resistance peak. The out-of-plane moment also correlates with the off-diagonal linear magneto-optical effect [2] observed in this material.

[1] Z. Wang, J. D. Rogers, X. Yao, R. Nichols, K. Atay, B. Xu, J. Franklin, I. Sochnikov, P. J. Ryan, D. Haskel, and F. Tafti, Colossal Magnetoresistance without Mixed Valence in a Layered Phosphide Crystal, Adv. Mater. 33, 2005755 (2021).

[2] V. V. Eremenko, Yu. G. Litvinenko, N. K. Kharchenko, and V. M. Naumenko, Magneto-Optics and Spectroscopy of Antiferromagnets (Springer New York, New York, NY, 1992).   

Thermal Transport Properties of Ho2RhIn8

The arrangement of the rare-earth atoms in the tetragonal structure of R₂TX₈ (R = rare earth, T = transition metal, X = In, Ga) takes a quasi two dimensional (2D) character. All of these compounds order antiferromagnetically with Neel temperatures (T_N) ranging between 4 – 45 K.  Ho₂RhIn₈ is one of the most interesting compounds among this very large family of closely related materials.

It shows two successive metamagnetic transitions at 2 K, when an external magnetic field is applied along the c axis. Its magnetic properties are driven mainly by crystalline electric effects and the RKKY interactions. We have grown single crystals of  Ho₂RhIn₈ and have studied these metamagnetic transitions through thermal and electrical transport properties. These transitions are absent in the Hall resistivity but they appear around the same fields in the thermal conductivity, thermal Hall conductivity, and the longitudinal electrical resistivity. We discuss these metamagnetic transitions and their relations to the possibility of the Ho₂RhIn₈ compound being a magnetic topological candidate.   

Investigating the phase diagram of thin film MnSi

We have charted the magnetic phase diagram of thin film MnSi grown on a Si substrate via magnetization, magnetic susceptibility, first-order reversal, planar Hall, polarised neutron reflectometry and small-angle neutron scattering data, tracking carefully the field and temperature history. Our experimental results are supported by micromagnetic simulations, which jointly reveal a magnetic phase diagram dominated by a field-induced unwinding of an out-of-plane propagating helical wavevector. Below 2 K a discrete phase regime can be discerned unambiguously. These observations provide insights into the integral role of magnetic anisotropy and dimensionality on the low-temperature phase diagram of thin film MnSi.   

Investigating the temperature dependent electronic and magnetic properties of ferromagnetic topological insulators (CrxSb1-x)2Te3

We have investigated the bulk Cr 3d electronic and magnetic properties of ferromagnetic topological insulators (Cr_xSb_1-x)₂Te₃ by using soft x-ray absorption spectroscopy (XAS) and x-ray circular magnetic dichroism (XMCD). (Cr_xSb_1-x)₂Te₃ exhibits a relatively high Curie temperature with Tc ~100 K and ~200 K for Cr concentration x ~0.15 and x ~0.35, respectively. We have carried out temperature (T) dependent (25 K - 300 K) Cr L-edge XAS and XMCD measurements which show systematic changes and an unconventional T-dependent leading edge shift across Tc, associated with a magnetic gap opening in the unoccupied Cr 3d bulk states. The XMCD signal and magnetic gap as a function of T follow a power-law behaviour, consistent with bulk magnetization experiments and mean-field theory results. Full-multiplet charge transfer cluster model simulations of the Cr L-edge XAS and XMCD spectra are in good agreement with the experiment and indicate (Cr_xSb_1-x)₂Te₃ is a negative charge transfer system.   

Magnetic skyrmions Hall effect

Recently, tremendous attention has been focused on the continuous scaling of Moore's law as well as the advanced non-von Neumann computing architecture. Spintronics may go beyond classical electronics and offer low power logic/computing components to further increase the performance of modern information and communication technologies meanwhile reducing the energy consumption. In particular, magnetic skyrmions – topologically non-trivial spin textures, have been endowed with great expectations as promising candidates for next-generation spintronic device applications. However, there is a major roadblock for skyrmionics device applications – the skyrmion Hall effects, which may lead to skyrmions annihilation at the sample edge. In this talk, Prof. Yan Zhou will discuss his recent work of eliminating/suppressing the skyrmion Hall effects, which may overcome the main bottleneck of practical applications of skyrmionic racetrack memory and logic devices. In the last part of his talk, he will briefly discuss some novel device concepts based on these intriguing topological spin textures (such as neuromorphic or in-memory computing etc).