Main Text:Following the recent isolation of monolayer CrI3 1 , there has been a surge of new twodimensional van der Waals magnetic materials 2-12 , whose incorporation in van der Waals heterostructures offers a new platform for spintronics 5-8 , proximity magnetism 13 , and quantum spin liquids 14 . A primary question in this burgeoning field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a different magnetic ground state for ultrathin exfoliated films 1,5,6 but the origin is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order, finding a ten-fold enhancement in the interlayer exchange compared to bulk crystals. Moreover, temperatureand polarization-dependent Raman spectroscopy reveal that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets, such as RuCl3 14 .A key family of van der Waals magnets is the layered transition metal trihalides, which have been studied for decades as prototypical magnetic insulators 15-17 and as a platform for quasitwo-dimensional magnetism [18][19][20] . In the chromium trihalides, the Cr atoms are arranged in a honeycomb structure, with each Cr atom surrounded by six halide atoms in an octahedral geometry (Fig. 1a). The bulk crystals undergo a crystallographic phase transition from a monoclinic phase (space group C2/m) at room temperature to a rhombohedral phase (space group R3 " ) at low temperatures (below about 240 K for CrCl3 21 ). While the intralayer lattice spacings are largely
The magnetic order in CaK(Fe1−xNix)4As4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated to the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and non-collinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.PACS numbers:
Temperature dependent 57 Fe Mössbauer spectroscopy and specific heat measurements for CaK(Fe 1−x Ni x ) 4 As 4 with x = 0, 0.017, 0.033, and 0.049 are presented. No magnetic hyperfine field (e.g. no static magnetic order) down to 5.5 K was detected for x = 0 and 0.017 in agreement with the absence of any additional feature below superconducting transition temperature, T c , in the specific heat data. The evolution of magnetic hyperfine field with temperature was studied for x = 0.033 and 0.049. The long-range magnetic order in these two compounds coexists with superconductivity. The magnetic hyperfine field, B hf , (ordered magnetic moment) below T c in CaK(Fe 0.967 Ni 0.033 ) 4 As 4 is continuously suppressed with the developing superconducting order parameter. The B hf (T ) data for CaK(Fe 0.967 Ni 0.033 ) 4 As 4 , and CaK(Fe 0.951 Ni 0.049 ) 4 As 4 can be described reasonably well by Machida's model for coexistence of itinerant spin density wave magnetism and superconductivity [K. Machida, J. Phys. Soc. Jpn. 50, 2195(1981]. We demonstrate directly that superconductivity suppresses the spin density wave order parameter if the conditions are right, in agreement with the theoretical analysis. PACS numbers:1 arXiv:1810.07084v1 [cond-mat.supr-con]
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