Van der Waals epitaxy growth of 2D ferromagnetic Cr(1+δ)Te2 nanolayers with concentration-tunable magnetic anisotropy

Abstract

Cr(1+δ)Te2 are pseudo-layered compounds consisting of CrTe2 transition metal dichalcogenide (TMD) layers with additional (δ) self-intercalated Cr atoms. The recent search for ferromagnetic 2D materials revived the interest into chromium tellurides. Here, Cr(1+δ)Te2 nanolayers are epitaxially grown on MoS2 (0001), forming prototypical van der Waals heterostructures. Under optimized growth conditions, ultrathin films of only two TMD layers with a single intercalated Cr-layer are achieved, forming a 2D sheet with van der Waals surfaces. Detailed compositional and structural characterization by scanning tunneling microscopy, grazing incidence x-ray diffraction, and high-resolution Rutherford backscattering indicate the layer-by-layer growth and that the δ can be tuned by post-growth annealing in a range between ∼0.5 and 1. X-ray magnetic circular dichroism and magnetometry measurements demonstrate that all self-intercalated Cr(1+δ)Te2 nanolayers exhibit strong ferromagnetism with magnetic moments larger than 3μB per Cr-atom. The magnetic properties are maintained in the ultrathin limit of a material with a single intercalation layer. Interestingly, the magnetic anisotropy can be tuned from close to isotropic (δ = 1) to a desirable perpendicular anisotropy for low δ values. Thus, the bottom-up growth of these 2D Cr(1+δ)Te2 sheets is a promising approach for designing magnetic van der Waals heterostructures.

Layer-Number-Independent Two-Dimensional Ferromagnetism in Cr3Te4

“…All of those phases exhibit ferromagnetism with a relatively high Curie temperature ( T C ), and CrTe 2 , Cr 3 Te 4 , and CrTe in particular show room-temperature ferromagnetism with T C ≈ 310 K, 320 K, and 340 K, respectively. Such a rather high T C as well as its simple binary composition should be advantageous for 2D spintronics applications, and in fact a growing number of papers have been published in recent years reporting the magnetic properties of this material system down to the ultrathin regime, − including the realization of room-temperature ferromagnetism in 2D CrTe 2 , , 2D Cr 3 Te 4 , and 2D CrTe . On the other hand, recent studies on Cr–Te epitaxial thin films grown by molecular-beam epitaxy (MBE) demonstrated a large variation of their magnetic properties depending on the growth condition, ,,− implying their sensitive nature to the doping level of the system.…”

“…Such a rather high T C as well as its simple binary composition should be advantageous for 2D spintronics applications, and in fact a growing number of papers have been published in recent years reporting the magnetic properties of this material system down to the ultrathin regime, − including the realization of room-temperature ferromagnetism in 2D CrTe 2 , , 2D Cr 3 Te 4 , and 2D CrTe . On the other hand, recent studies on Cr–Te epitaxial thin films grown by molecular-beam epitaxy (MBE) demonstrated a large variation of their magnetic properties depending on the growth condition, ,,− implying their sensitive nature to the doping level of the system. Furthermore, a very recent study by angle-resolved photoemission spectroscopy (ARPES) measurements verified the existence of a characteristic semimetallic band region near the Fermi level, demonstrating a unique feature of this material system from a fundamental materials science viewpoint as well.…”

Stability of the In-Plane Room Temperature van der Waals Ferromagnet Chromium Ditelluride and Its Conversion to Chromium-Interleaved CrTe2 Compounds

“…The structure and composition control in the Cr–Te binary system is a known delicate problem. Indeed, a variety of compounds with different stoichiometry can be formed, including CrTe ( T Curie ∼ 200 K), Cr 2 Te 3 − ( T Curie ∼ 170 K), Cr 1+1/3 Te 2 ( T Curie ∼ 160–190 K), Cr 3 Te 4 , ( T Curie ∼ 240–320 K), Cr 3 Te 5 , Cr 4 Te 5 (both with a T Curie ∼ 320 K), and Cr 5 Te 8 − ( T Curie ∼ 100–220 K). None of these binary alloys are lamellar per se but are thought to consist of CrTe 2 layers where Cr atoms have an octahedral local environment with covalent Cr bridges between these layers.…”

Stability of the In-Plane Room Temperature van der Waals Ferromagnet Chromium Ditelluride and Its Conversion to Chromium-Interleaved CrTe$_2$ Compounds

“…Structure and composition control in the Cr-Te binary system is a known delicate problem. Indeed a variety of compounds with different stoichiometry can be formed, including CrTe 22 (T Curie ∼200 K), Cr 2 Te 3 [23][24][25] (T Curie ∼170 K), Cr 1+1/3 Te 2 26 (T Curie ∼160-190 K), Cr 3 Te 4 27,28 (T Curie ∼240-320 K), Cr 3 Te 5 , 29 Cr 4 Te 5 30 (both with a T Curie ∼320 K), and Cr 5 Te 8 [31][32][33] (T Curie ∼100-220 K). None of these binary alloys are lamellar per se, but are thought to consist of CrTe 2 layers where Cr atoms have octahedral local environment with covalent Cr bridges between these layers.…”