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Register a new userMagnetic and geometrical control of spin textures in the itinerant kagome magnet Fe$_3$Sn$_2$
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Magnetic materials with competing magnetocrystalline anisotropy and dipolar energies can develop a wide range of domain patterns, including classical stripe domains, domain branching, as well as topologically trivial and non-trivial (skyrmionic) bubbles. We image the magnetic domain pattern of Fe$_3$Sn$_2$ by magnetic force microscopy (MFM) and study its evolution due to geometric confinement, magnetic fields, and their combination. In Fe$_3$Sn$_2$ lamellae thinner than 3 $mu$m, we observe stripe domains whose size scales with the square root of the lamella thickness, exhibiting classical Kittel scaling. Magnetic fields turn these stripes into a highly disordered bubble lattice, where the bubble size also obeys Kittel scaling. Complementary micromagnetic simulations quantitatively capture the magnetic field and geometry dependence of the magnetic patterns, reveal strong reconstructions of the patterns between the surface and the core of the lamellae, and identify the observed bubbles as skyrmionic bubbles. Our results imply that geometrical confinement together with competing magnetic interactions can provide a path to fine-tune and stabilize different types of topologically trivial and non-trivial spin structures in centrosymmetric magnets.
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Magnetic materials with kagome crystal structure exhibit rich physics such as frustrated magnetism, skyrmion formation, topological flat bands, and Dirac/Weyl points. Until recently, most studies on kagome magnets have been performed on bulk crystals or polycrystalline films. Here we report the synthesis of high-quality epitaxial films of topological kagome magnet Fe$_3$Sn$_2$ by atomic layer molecular beam epitaxy. Structural and magnetic characterization of Fe$_3$Sn$_2$ on epitaxial Pt(111) identifies highly ordered films with c-plane orientation and an in-plane magnetic easy axis. Studies of the local magnetic structure by anomalous Nernst effect imaging reveals in-plane oriented micrometer size domains. The realization of high-quality films by atomic layer molecular beam epitaxy opens the door to explore the rich physics of this system and investigate novel spintronic phenomena by interfacing Fe$_3$Sn$_2$ with other materials.
We report on the observation of a large topological Hall effect (THE) over a wide temperature region in a geometrically frustrated Fe3Sn2 magnet with a kagome-bilayer structure. We found that the magnitude of the THE resistivity increases with temperature and reaches -0.875 {mu}{Omega}.cm at 380 K. Moreover, the critical magnetic fields with the change of THE are consistent with the magnetic structure transformation, which indicates that the real-space fictitious magnetic field is proportional to the formation of magnetic skyrmions in Fe3Sn2. The results strongly suggest that the large THE originates from the topological magnetic spin textures and may open up further research opportunities in exploring emergent phenomena in kagome materials.
A single ferromagnetic kagome layer is predicted to realize a Chern insulator with accompanying quantized Hall conductance, which upon stacking can become a Weyl-semimetal possessing large anomalous Hall effect (AHE) and magneto-optical activity. Indeed, in the kagome bilayer material Fe$_3$Sn$_2$, a large AHE was detected, however, it still awaits the direct probing of the responsible band structure features by bulk sensitive methods. By measuring the optical diagonal and Hall conductivity spectra, we identified their origin and determine the intrinsic contribution to the AHE. Facilitated by momentum and band decomposed first-principles calculations, we found that transitions around the K-point are responsible for a step edge at 0.25,eV in $sigma_{xx}$, while they strongly modify $sigma_{xy}$ even towards the DC limit. Together with a broad hump detected around 0.9,eV originating from multiple transitions, these excitations produce the static AHE.
Temperature- and frequency-dependent infrared spectroscopy identifies two contributions to the electronic properties of the magnetic kagome metal Fe$_3$Sn$_2$: two-dimensional Dirac fermions and strongly correlated flat bands. The interband transitions within the linearly dispersing Dirac bands appear as a two-step feature along with a very narrow Drude component due to intraband contribution. Low-lying absorption features indicate flat bands with multiple van Hove singularities. Localized charge carriers are seen as a Drude-peak shifted to finite frequencies. The spectral weight is redistributed when the spins are reoriented at low temperatures; a sharp mode appears suggesting the opening of a gap due to the spin reorientation as the sign of additional Weyl nodes in the system.
We investigate the low temperature magnetic properties of a $S=frac{5}{2}$ Heisenberg kagome antiferromagnet, the layered monodiphosphate Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$, using magnetization measurements and $^{31}$P nuclear magnetic resonance. An antiferromagnetic-type order sets in at $T_{rm N}=1.3$ K and a characteristic magnetization plateau is observed at 1/3 of the saturation magnetization below $T^* sim 5$ K. A moderate $^{31}$P NMR line broadening reveals the development of anisotropic short-range correlations within the plateau phase concomitantly with a gapless spin-lattice relaxation time $T_1 sim k_B T / hbar S$, which both point to the presence of a semiclassical nematic spin liquid state predicted for the Heisenberg kagome antiferromagnetic model.
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