Covalent mixing in the two-dimensional ferromagnet CrSiTe₃ evidenced by magnetic x-ray circular dichroism

The low-temperature electronic structure of the van der Waals ferromagnet CrSiTe3 has been 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&nbsp;L2,3&nbsp;and Te&nbsp;Mb&nbsp;edges on in vacuo cleaved single crystals, give strong evidence for hybridization-mediated super-exchange between the Cr atoms. The observed chemical shift in the XAS, as well as the comparison of the XMCD with the calculated Cr L2,3&nbsp;multiplet spectra, confirm a strongly covalent bond between the Cr&nbsp;3d(eg) and Te 5p&nbsp;states. Application of the XMCD sum rules gives a non-vanishing orbital moment, supporting a partial occupation of the&nbsp;eg&nbsp;states, apart from the&nbsp;t2g. Also, the presence of a non-zero XMCD signal at the Te Mb edge confirms a Te 5p&nbsp;spin polarization due to mixing with the Cr&nbsp;eg&nbsp;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 CrSiTe3&nbsp;and CrGeTe3. This demonstrates the interplay between electron correlation and ferromagnetism in insulating two-dimensional materials.</p

Tailoring the magnetic exchange interaction in MnBi2Te4 superlattices via the intercalation of ferromagnetic layers

The intrinsic magnetic topological insulator MnBi2Te4&nbsp;(MBT) provides a platform for the creation of exotic quantum phenomena. Novel properties can be created by modification of the MnBi2Te4&nbsp;framework, but the design of stable magnetic structures remains challenging. Here we report ferromagnet-intercalated MnBi2Te4&nbsp;superlattices with tunable magnetic exchange interactions. Using molecular beam epitaxy, we intercalate ferromagnetic MnTe layers into MnBi2Te4&nbsp;to create [(MBT)(MnTe)m]N&nbsp;superlattices and examine their magnetic interaction properties using polarized neutron reflectometry and magnetoresistance measurements. Incorporation of the ferromagnetic spacer tunes the antiferromagnetic interlayer coupling of the MnBi2Te4&nbsp;layers through the exchange-spring effect at MnBi2Te4/MnTe hetero-interfaces. The MnTe thickness can be used to modulate the relative strengths of the ferromagnetic and antiferromagnetic order, and the superlattice periodicity can tailor the spin configurations of the synthesized multilayers.</p

Spin-orbit coupled spin-polarised hole gas at the CrSe2-terminated surface of AgCrSe2

In half-metallic systems, electronic conduction is mediated by a single spin species, offering enormous potential for spintronic devices. Here, using microscopic-area angle-resolved photoemission, we show that a spin-polarised two-dimensional hole gas is naturally realised in the polar magnetic semiconductor AgCrSe2 by an intrinsic self-doping at its CrSe2-terminated surface. Through comparison with first-principles calculations, we unveil a striking role of spin-orbit coupling for the surface hole gas, unlocked by both bulk and surface inversion symmetry breaking, suggesting routes for stabilising complex magnetic textures in the surface layer of AgCrSe2

Covalency, correlations, and inter-layer interactions governing the magnetic and electronic structure of Mn3Si2Te6

Mn3Si2Te6 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 Mn3Si2Te6. 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 Mn3Si2Te6, which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbours and anti-symmetric 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 Mn3Si2Te6