Robust intrinsic ferromagnetism and half semiconductivity in stable two-dimensional single-layer chromium trihalides

Abstract

Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial to develop lowdimensional spintronic devices. Using density functional theory, we show that single-layer chromium trihalides (SLCTs) (CrX3,X=F, Cl, Br and I) constitute a series of stable 2D intrinsic FM semiconductors. A free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low cleavage energy and a high in-plane stiffness. Electronic structure calculations using the HSE06 functional indicate that both bulk and single-layer CrX3 are half semiconductors with indirect gaps and their valence bands and conduction bands are fully spin-polarized in the same spin direction. The energy gaps and absorption edges of CrBr3 and CrI3 are found to be in the visible frequency range, which implies possible opt-electronic applications. Furthermore, SLCTs are found to possess a large magnetic moment of 3µB per formula unit and a sizable magnetic anisotropy energy. The magnetic exchange constants of SLCTs are then extracted using the Heisenberg spin Hamiltonian and the microscopic origins of the various exchange interactions are analyzed. A competition between a near 90 • FM superexchange and a direct antiferromagnetic (AFM) exchange results in a FM nearestneighbour exchange interaction. The next and third nearest-neighbour exchange interactions are found to be FM and AFM respectively and this can be understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction. Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo simulations and the values can further increase by applying a biaxial tensile strain. The unique combination of robust intrinsic ferromagnetism, half semiconductivity and large magnetic anisotropy energies renders the SLCTs as promising candidates for next-generation semiconductor spintronic applications.

Magnetic behavior and spin-lattice coupling in cleavable van der Waals layered CrCl3 crystals

“…The inset shows a separate specimen determined to be one monolayer thick. It is expected that the single-layer CrCl 3 may display ferromagnetism at low temperature, as observed in CrI 3 [30] and as predicted by first principles calculations for all of the chromium trihalides [19,20].…”

“…This gave a very low cleavage energy of 0.10−0.13 J/m 2 . Zhang et al calculate a cleavage energy of 0.3 J/m 2 using a different vdW-DF [19]. Monte Carlo simulations using their first principles calculations results for monolayer CrCl 3 suggest a Curie temperature of 49 K, which is predicted to increase with applied strain.…”

“…Other recently discovered materials include Fe 3 GeTe 2 [14], a metal with itinerant ferromagnetism below T C =220-230 K [14][15][16]. Chromium triiodide, and transition metal halides in general [17], have also been put forth as candidates for cleavable magnetic materials [18][19][20][21]. Experimentally, CrI 3 is a ferromagnet with T C = 61 K [18,22,23], CrBr 3 is a ferromagnet with T C = 37 K [24], and CrCl 3 is an antiferromagnet with an ordering temperature near 17 K [25,26].…”

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

“…Recent additions include spin-valley coupled semiconductors 3 , Ising superconductors 4-6 that can be tuned into a quantum metal 7 , possible Mott insulators with tunable charge-density waves 8 , and topological semi-metals with edge transport 9,10 . Despite this progress, there is still no 2D crystal with intrinsic magnetism [11][12][13][14][15][16] , which would be useful for many technologies such as sensing, information, and data storage 17 . Theoretically, magnetic order is prohibited in the 2D isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem 18 .…”

“…Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect 22 , while in trilayer the interlayer ferromagnetism observed in the bulk crystal is restored. Our work creates opportunities for studying magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for realizing magnetoelectronics 13,23 , and van der Waals engineering for novel interface phenomena 17 . …”