The low‐temperature electronic structure of the van der Waals ferromagnet is investigated. This ferromagnetic semiconductor has a magnetic bulk transition temperature of 33 K, which can reach up to 80 K in single‐ and few‐layer flakes. X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr and Te edges in vacuo‐cleaved single crystals, give strong evidence for hybridization‐mediated superexchange between the Cr atoms. The observed chemical shift in the XAS, as well as the comparison of XMCD with the calculated Cr multiplet spectra, confirms a strong covalent bond between the Cr () and Te states. Application of the XMCD sum rules gives a nonvanishing orbital moment, supporting a partial occupation of the states, apart from . Also, the presence of a nonzero XMCD signal at the Te edge confirms a Te spin polarization due to mixing with the Cr bonding states. The results strongly suggest that superexchange, instead of the previously suggested single‐ion anisotropy, is responsible for the low‐temperature ferromagnetic ordering of 2D materials such as and . This demonstrates the interplay between electron correlation and ferromagnetism in insulating 2D materials.

The intrinsic magnetic topological insulator MnBi2Te4 (MBT) has provided a platform for the successful realization of exotic quantum phenomena. To broaden the horizons of MBT-based material systems, we intercalate ferromagnetic MnTe layers to construct the [(MBT)(MnTe)m]N superlattices by molecular beam epitaxy. The effective incorporation of ferromagnetic spacers mediates the anti-ferromagnetic interlayer coupling among the MBT layers through the exchange spring effect at the MBT/MnTe hetero-interfaces.2 Moreover, the precise control of the MnTe thickness enables the modulation of relative strengths among the constituent magnetic orders, leading to tunable magnetoelectric responses, while the superlattice periodicity serves as an additional tuning parameter to tailor the spin configurations of the synthesized multi-layers. Our results demonstrate the advantages of superlattice engineering for optimizing the magnetic interactions in MBT-family systems, and the ferromagnet-intercalated strategy opens up new avenues in magnetic topological insulator structural design and spintronic applications.

Mn 3 Si 2 Te 6 is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in-to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations to probe the elemental-selective electronic structure and magnetic order in Mn 3 Si 2 Te 6 . Through these, we identify a marked Mn-Te hybridization, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in Mn 3 Si 2 Te 6 , which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbors and antisymmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilizing the ferrimagnetic order in Mn 3 Si 2 Te 6 .

The size of the orbital moment in Fe3O4 has provided a long standing and contentious debate. In this paper we make use of ferromagnetic resonance (FMR) spectroscopy and x-ray magnetic circular dichroism (XMCD) to provide complementary determinations of the size of the orbital moment in "bulk-like" epitaxial Fe3O4 films grown on Yttria-stabilized zirconia (111) substrates. Annealing the 100 nm as-grown films to 1100 C in a reducing atmosphere improves the stoichiometry and microstructure of the films allowing for bulk like properties to be recovered as evidenced by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). In addition, in-plane angular FMR spectra exhibit a cross over from a 4-fold symmetry to the expected 6-fold symmetry of the (111) surface, together with an anomalous peak in the FMR linewidth at ~10 GHz; indicative of low Gilbert damping in combination with two-magnon scattering. For the bulk-like annealed sample, a spectroscopic splitting factor g ≈ 2.18 is obtained using both FMR and XMCD techniques, providing evidence for the presence of a finite orbital moment in Fe3O4.

The topological surface states (TSSs) in topological insulators (TIs) offer exciting prospects for dissipationless spin transport. Common spin-based devices, such as spin valves, rely on trilayer structures in which a non-magnetic (NM) layer is sandwiched between two ferromagnetic (FM) layers. The major disadvantage of using high-quality single-crystalline TI films in this context is that a single pair of spin-momentum locked channels spans across the entire film, meaning that only a very small spin current can be pumped from one FM to the other, along the side walls of the film. On the other hand, using nanocrystalline TI films, in which the grains are large enough to avoid hybridization of the TSSs, will effectively increase the number of spin channels available for spin pumping. Here, we used an element-selective, x-ray based ferromagnetic resonance technique to demonstrate spin pumping from a FM layer at resonance through the TI layer and into the FM spin sink.