Session Q44: Topological Magnetic Materials in Space and Time

Quantum Imaging of Magnetic Phase Transitions in Intrinsic Magnetic Topological Nanoflakes

The family of MnBi₂Te₄ (Bi₂Te₃)_n has recently emerged as an appealing material platform to explore exotic quantum spin and charge transport behaviors in topological solid state systems. Notable examples include high-temperature quantum anomalous Hall effect, layer Hall effect, axion insulator phase, and many others. Establishing a detailed knowledge of the local magnetic properties of MnBi₂Te₄ (Bi₂Te₃)_n is highly desirable for a comprehensive understanding of these emergent material behaviors. Here, we introduce nitrogen-vacancy (NV) centers, optically active atomic defects in diamond, to directly image the local static and dynamic spin properties of MnBi₂Te₄ (Bi₂Te₃)_n nanoflakes, revealing the details of magnetic phase transition and domains at the nanoscale. We further explore the intriguing physics underlying the spin transport in MnBi₂Te₄ (Bi₂Te₃)_n. Our results highlight the significant advantages of NV centers in studying the local magnetic properties of emergent quantum materials, opening new opportunities for investigating the interplay between topology and magnetism.

    

Visualizing Temperature Driven Ferromagnetic to Antiferromagnetic Transition in MnSb2Te4

The intrinsic magnetic topological insulator MnBi₂Te₄ exhibits rich exotic quantum phenomena^1,2. However, the theoretically predicted Dirac mass gap is susceptible to the native magnetic defects, which often leads to conducting bulk state³. Therefore, understanding the role of magnetic defect essential in engineering topological materials. The same type of magnetic defects in the isostructural compound MnSb₂Te₄ shows higher concentration and tunability, making MnSb₂Te₄ an ideal platform to investigate the interplay among magnetic defect, topological band structure and magnetism. It has been reported that by tuning the Mn/Sb site mixing, ferromagnetic (FM) order can be introduced into MnSb₂Te₄ and compete with the antiferromagnetic order⁴. Magnetic imaging of ferromagnetic domains in some site mixing engineered MnSb₂Te₄ systems has been reported in our previous work⁵. In this work, we directly visualize the temperature driven FM-AFM phase transition with T_C = 24 K and T_N = 15 K in single crystals of MnSb₂Te₄ using cryogenic Magnetic Force Microscopy. Our work reveals the complex magnetic competition in MnSb2Te4 and paves the way for understanding the role of magnetic defect in the magnetism of MnSb2Te4.

Visualization of ferromagnetic domains in magnetic topological insulator/FeTe heterostructures in the superconducting state

The interfacial superconductivity of the topological insulator (TI)/antiferromagnetic FeTe heterostructures have attracted extensive attention in recent years. In this work, magnetic TI/FeTe heterostructures were synthesized and have a superconducting phase transition. In Hall resistance measurements, hysteresis loops can be observed below Curie temperature, but the resistance drops to zero when the sample enters the superconducting state. To confirm the ferromagnetism below superconducting temperature, we visualize ferromagnetic domains at 2.2 K using a cryogenic magnetic force microscope (MFM) with in-situ transport measurements. The evolution of domain nucleation and domain wall propagation under varying fields in zero resistance indicates a robust ferromagnetic order coexisting with superconductivity. Based on the domain population, we can construct ferromagnetic hysteresis loops in the superconducting state. The corresponding coercive fields are also in good agreement with reflective magnetic circular dichroism (RMCD) results.

 

**The MFM studies at Rutgers are supported by DOE BES under Award No. DE-SC0018153.

    

Magnetic imaging of domain walls in antiferromagnetic topological insulator MnBi4Te7

MnBi₂Te₄ has been established as the first tangible candidate for an antiferromagnetic topological insulator (AFM-TI). Recent first principle calculations and ARPES measurements suggest that natural superlattice compounds (MnBi₂Te₄)(Bi₂Te₃)_n (n = 1, 2) are also AFM-TI. With n layers of Bi₂Te₃ inserted between MnBi₂Te₄ layers, the interlayer antiferromagnetic coupling reduces dramatically so that the metamagnetic transition becomes a spin-flip transition. Previously we discovered surface spin-flip transition preceding the bulk one using cryogenic magnetic force microscopy (MFM) [1].  In this talk, I will present our recent MFM results which uncover seemingly complex antiferromagnetic domain states via minor-loop field cycling into the bulk spin flip transition. By analyzing the surface spin-flip transition of the unconventional domains, we infer the formation of shallow antiphase domains induced by the minor-loop field annealing.  The combination of the stray fields from vertical and horizontal domain walls of shallow antiferromagnetic domains causes the complex appearance of antiferromagnetic domain pattern in magnetic imaging.

[1] W. Ge, J. Kim, Y-T Chan, D. Vanderbilt, J-Q Yan, W. Wu, PRL, 129, 107204 (2022).   

Milli-Kelvin Microwave Microscopy of Quantum Anomalous Hall States

Near-field microwave impedance microscopy (MIM) is a powerful technique that can visualize

sub-surface conductivity distribution with a spatial resolution on the order of 100 nm. In this

work, we report the implementation of a dilution-refrigerator-based MIM with a base

temperature of ~ 100 mK. The vibration noise of our apparatus with tuning-fork feedback control

is as low as 1 nm. Using this setup, we have successfully performed nanoscale imaging of

quantum anomalous Hall (QAH) states in magnetically (Cr and V) doped (Bi,Sb) 2 Te 3 thin films

grown on mica substrates. At high magnetic fields, the conductive chiral edge modes are clearly

observed in the MIM images, consistent with the quantization of Hall conductance in the

transport data. The two topological phase transitions near the coercive fields of Cr and V are also

vividly visualized in the field-dependent results. Our work establishes the experimental platform

for the investigation of nanoscale quantum phenomena under ultralow temperatures.   

Nanoscale quantum imaging of magnetic domains in noncollinear antiferromagnet Mn3Sn

Recently, non-collinear antiferromagnets have attracted immense interest owning to their topologically protected band structure, non-collinear magnetic order, and spontaneous time-reversal symmetry breaking. As a prominent material candidate of this family, Mn₃Sn has been the subject of intensive research, showing rich fundamental physics as well as enormous technological promise for practical applications. Here, we report quantum imaging of magnetic domains in noncollinear antiferromagnet Mn₃Sn and visualize their microscopic response to external magnetic field and spin-orbit torques at the nanoscale. Our results ascertain the advantages of NV quantum metrology in both spatial and field sensitivity for studying nanomagnetism hosted by emergent condensed matter systems. We highlight that the presented NV measurement platform could be extended naturally to a broad range of quantum materials, opening new opportunities for studying the interplay between topology and magnetism.   

Imaging Magnetic Domain Structure in Ferromagnetic Kagome Co3Sn2S2 by Magnetic Force Microscopy

Ferromagnetic Weyl semimetal Co₃Sn₂S₂, within which Co atoms form a quasi-2D Kagomé lattice, has attracted great attention because novel phenomena such as quantum anomalous Hall effect can emerge as the results of the interplay between topology and magnetism. While the existence of Weyl points and Fermi arcs have been realized in the ferromagnetic state at low temperature [1-2], an anomalous magnetic phase (A-phase) revealed by low-field magnetization, AC susceptibility [3] and transport [4] measurements slightly below its Curie temperature remain unsolved. Skyrmion phase [3], spin glass [5], co-existence of out-of-plane ferromagnetic and in-plane antiferromagnetic order [6], and a 2D phase transition within the domain walls [7] have been proposed. Here, we conducted comprehensive magnetic imaging study on Co₃Sn₂S₂ single crystals by using variable temperature magnetic force microscopy (MFM) across the A-phase. Our temperature dependent and magnetic field dependent MFM images, consistent with a recent Lorentz microscopy study [8], demonstrate the important role of magnetic bubble domain formation and the magnetic domain wall motion in the A-phase.

References:

[1] Liu et al., Science 365, 1282–1285 (2019).

[2] Morali et al., Science 365, 1286–1291 (2019).

[3] Kassem et al., PRB 96, 014429 (2017).

[4] Lachman et al., Nat. Commun. 11, 560 (2020).

[5] Lachman et al., Nat. Commun. 11, 560 (2020).

[6] Guguchia et al., Nat. Commun. 11, 559 (2020).

[7] Lee et al., Nat Commun 13, 3000 (2022).

[8] Sugawara et al., Phys. Rev. Materials 3, 104421 (2019).   

Defect-driven Competition between the Interlayer Magnetic Interactions in MnSb2Te4

Isostructural to the first realized antiferromagnetic (AF) topological insulator (TI) MnBi₂Te₄ (MBT), MnSb₂Te₄ (MST) demonstrates a complex magnetic behavior due to the heavy anti-site mixing between the Mn and Sb atoms. Targeted sample preparations reveal that small variations in the anti-site defect concentration result in samples having either the ferromagnetic (FM) or the AF ground state. Here we compare the inelastic neutron scattering results of the spin dynamics in the FM and AF MST single-crystal samples to MBT. In MST, the anti-site mixing introduced disorder that disrupts the spin waves, and also promotes new energetic AF couplings, both of which are absent in MBT. Based on the quantitative analysis using linear spin wave theory and classical atomistic spin dynamics simulations, we propose a scheme of anti-site defect-driven competition between the FM and AF interlayer interactions in MST, and discuss the implication of anti-site mixing in Mn(Bi,Sb)₂Te₄ and Mn-doped Bi₂Te₃ magnetic TIs.   

Interlayer coupling and magnonic features of van der Waals magnetic topological insulator MnBi2Te4

The interplay between topology and magnetism offers new opportunity for realizing exotic quantum phenomena. MnBi₂Te₄ (MBT) exhibits novel intrinsic magnetic topological properties, including the quantum anomalous Hall effect and the axion insulator state. As a two-dimensional van der Waals antiferromagnetic material, MBT can be exfoliated into atomically thin flakes, which enables studies of layer-number-dependent magnetism. Here, we present results of Raman spectroscopy experiments on 1- to 5-septuple-layer-thick MBT flakes, which provide significant new insights into interlayer coupling, spin collective motion, and ordering. In the low-frequency regime, we observed strong interlayer phonon peaks coming from substantial interfacial coupling. In addition, we observed another rising peak with magnonic feature below the Néel temperature in the presence of an external magnetic field. These results provide new systematic information on the layer-number-dependent spin and phonon properties of this topological magnetic material

Photo-induced Critical Magnetization Dynamics in Magnetic Topological Insulator MnBi2Te4

We investigate the ultrafast magnetization dynamics of the antiferromagnetic topological insulator MnBi₂Te₄ (MBT) using time-resolved magneto-optical Kerr effect (trMOKE) measurements as a function of temperature, magnetic field, fluence, and polarization. MBT has generated substantial interest as the first intrinsic magnetic topological insulator, hosting topologically nontrivial states that may have applications to spintronics and quantum computing. Furthermore, theory predicts that the magnetic ground state of MBT lies in proximity to several competing orders due to frustration from large next-nearest neighbor AFM exchange [1] along with strong coupling between electronic [2], magnetic, and lattice degrees of freedom [3]. This opens the door for the optical control and engineering of nonequilibrium states of matter. Here, we characterize the photo-induced nonequilibrium dynamics across the  phase diagram, building upon previous work [4]. We show that the dynamics are well defined across most of the phase diagram and exhibit critical behavior with metastable dynamics near the AFM-spin flop transition.

[1] B. Li, J.-Q. Yan, D.M. Pajerowski, E. Gordon, A.-M. Nedic, Y. Sizyuk, L. Ke, P.P. Orth, D. Vaknin, and R.J. McQueeney, Phys. Rev. Lett. 124, 167204 (2020)

[2] M. Köpf, J. Ebad-Allah, S.H. Lee, Z.Q. Mao, and C.A. Kuntscher Phys. Rev. B 102, 165139 (2020)

[3] H. Padmanabhan, M. Poore, P.K. Kim, et al., Nature communications 13 (1), 1-10 (2022)

[4] H. Padmanabhan, V.A. Stoica, P.K. Kim, M. Poore, ... Advanced Materials, 2202841 (2022)   

Time-resolved measurements of magnetization dynamics in few-layer MnBi2Te4

MnBi₂Te₄ is a layered magnetic material which has been under intense study in recent years. It combines strong in-plane ferromagnetism with weaker out-of-plane antiferromagnetism, effectively becoming a one-dimensional antiferromagnet. Mechanical exfoliation allows study of ultrathin (<10 layers) samples, allowing a wide variety of experimental tests. We present measurements of the ultrafast magnetization dynamics using pump-probe techniques across varying thicknesses, showing both slow thermally driven behaviours and magnon oscillations characteristic of such a material. Our results also highlight the apparent limitations of the current theoretical models, suggesting some need for improvements our understanding of the magnetic interactions.   

Separation of electron and magnetic order dynamics in magnetic topological insulator MnBi2Te4

MnBi₂Te₄ is one of the few known intrinsic magnetic topological insulators. Various exotic properties have been demonstrated, including the axion insulating state and the quantum anomalous Hall effect. Despite a plethora of equilibrium spectroscopic studies, a number of mysteries remain unresolved. In particular, the interaction between the topological electronic band and magnetism in this material, manifested by a broken time-reversal symmetry gap, is under intense debate. Meanwhile, time-resolved spectroscopies provide a possible route to disentangle electronic, magnetic, and lattice degrees of freedom in the time domain, which may shed light on the fundamental mysteries in MnBi₂Te₄. To understand the electron-phonon and electron-magnon coupling in MnBi₂Te₄, we have performed time- and angle-resolved photoemission spectroscopy (trARPES) experiments using 1.5 eV infrared pump and 6 eV ultraviolet probe. At very low pumping fluences, the electronic ensemble reaches transient temperatures that are orders of magnitude higher than the Néel temperature, yet signatures of magnetism are barely changed. This spectroscopic evidence indicates the separation of the dynamics of the electrons and magnetic order in this material.

    

Ultrafast coherent interlayer phonon dynamics in atomically thin layers of MnBi2Te4

The atomically thin MnBi₂Te₄ crystal is a novel magnetic topological insulator, exhibiting exotic quantum physics. Here we report a systematic investigation of ultrafast carrier dynamics and coherent interlayer phonons in few-layer MnBi₂Te₄ as a function of layer number using time-resolved pump-probe reflectivity spectroscopy. Pronounced coherent phonon oscillations from the interlayer breathing mode are directly observed in the time domain. We find that the coherent oscillation frequency, the photocarrier and coherent phonon decay rates all depend sensitively on the sample thickness. The time-resolved measurements are complemented by ultralow-frequency Raman spectroscopy measurements, which both confirm the interlayer breathing mode and additionally enable observation of the interlayer shear mode. The layer dependence of these modes allows us to extract both the out-of-plane and in-plane interlayer force constants. Our studies not only reveal the interlayer van der Waals coupling strengths, but also shed light on the ultrafast optical properties of this novel two-dimensional material.   

Interfaces of B20 compounds

Materials in modern devices are nominally restricted by various impurities and defects, reducing desired transport phenomena and increasing waste heat. This restriction leads to a solid investment to create high-quality single crystals. However, 2D planar defects can potentially increase select transport phenomena, such as the spin and anomalous Hall effect in systems that break inversion symmetry. We use the first principal calculations to investigate 2D planar twinning interfaces in B20 compounds. Intuitive tight-binding Hamiltonians are used to understand the role of symmetry and to reduce the complexity of the interface band structure. We employ supercell calculations where the area of the interface is restricted to the primitive cell interface. The calculations are carried out for four materials, magnetic, metallic, topological, and insulating. The analysis focuses on the most energetically favorable relaxed interface compared to the pristine bulk, where strain can further increase stability. Our results show that the spin and anomalous Hall effect can be significantly increased due to the increase in spin-orbit coupling with the change in atomic potential at the interface.