Strong Above Room Temperature Intrinsic Magnetism of Freestanding Layered CrTe₂ Crystal

<b>Abstract:</b> Magnetism in van der Waals (vdW) layered materials has attracted worldwide attention in fundamental condensed matter material research. Magnetism in low dimension materials has many exciting emerging properties and great potential for applications in low power spintronics, quantum computing, data storage, etc. Most monolayer (ML) vdW materials have been demonstrated to have Curie temperatures (Tc) below 300 K [1] or near room temperature [2]. This is because thermal fluctuations destroy the long-range magnetic order in a ML material according to Mermin-Wagner theorem. Thus, demands for the growth of bulk vdW layered materials to increase its Tc to above room temperature remains challenging.<br/>Only recently, bulk 1T-CrTe₂ crystal of mm size was synthesized from K deintercalation of KCrTe₂ and its room temperature Tc (310 K) was reported [3]. In this work, we present synthesized CrTe₂ crystals made from a similar method. We present their structure, chemical stoichiometry, vibrational, and magnetic properties of free standing CrTe₂crystal measured by transmission electron microscopy (TEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), energy dispersive X-ray spectroscopy (EDS), Raman scattering, superconducting quantum interference device<b> </b><b>(</b>SQUID), and surface magneto-optical Kerr effect (SMOKE) techniques. TEM and XRD show CrTe₂has crystalline layered structure with (0001) out-of-plane orientation with bulk lattice constants. EBSD shows a six-fold hexagonal crystal symmetry. EDS and Raman spectra show correct stoichiometry of 1 to 2 for Cr to Te ratio. All data support the synthesized crystals are of high quality. The temperatures dependent magnetization shows a phase transition at a Tc (317 K) above room temperature. The critical exponents extracted from the critical region are consistent with the classic 3D Ising model’s prediction. SMOKE measurement of crystal at room temperature shows hysteresis loops with low coercivity (~20 Oe). Since the 632 nm laser wavelength’s penetration depth depends on the laser incident angle, the probing depth is within sub-tens to tens of nm below surface, therefore the magnetic hysteresis loop measured by SMOKE is effectively from ultrathin layers of the CrTe₂ crystal.<br/><b>Acknowledgements: </b>This work is supported by the Center for Functional Nanomaterials, Brookhaven National Laboratory under DE-SC0012704, Penn State 2D Crystal Consortium −Materials Innovation Platform under NSF DMR-2039351, and the NYSTAR Focus Center at RPI, C180117.<br/><b>References:</b><br/>1. Huang, B., et al., Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature, 2017. 546: p. 270.<br/>2. Bonilla, M., et al., Strong room-temperature ferromagnetism in VSe₂ monolayers on van der Waals substrates. Nature Nanotechnology, 2018. 13(4): p. 289-293.<br/>3. Freitas, D.C., et al., Ferromagnetism in layered metastable 1T-CrTe₂. Journal of Physics: Condensed Matter, 2015. 27(17): p. 176002.