Transition from Ferromagnetic Semiconductor to Ferromagnetic Metal with Enhanced Curie Temperature in Cr₂Ge₂Te₆ via Organic Ion Intercalation

Abstract

Magnetism in the two-dimensional limit has become an intriguing topic for exploring new physical phenomena and potential applications. Especially, the two-dimensional magnetism is often associated with novel intrinsic spin fluctuations and versatile electronic structures, which provides vast opportunities in 2D material research. However, it is still challenging to verify candidate materials hosting two-dimensional magnetism, since the prototype systems have to be realized by using mechanical exfoliation or atomic layer deposition. Here, an alternative manipulation of two-dimensional magnetic properties via electrochemical intercalation of organic molecules is reported. Using tetrabutyl ammonium (TBA+), we synthesized a (TBA)­Cr2Ge2Te6 hybrid superlattice with metallic behavior, and the Curie temperature is significantly increased from 67 K in pristine Cr2Ge2Te6 to 208 K in (TBA)­Cr2Ge2Te6. Moreover, the magnetic easy axis changes from the ⟨001⟩ direction in Cr2Ge2Te6 to the ab-plane in (TBA)­Cr2Ge2Te6. Theoretical calculations indicate that the drastic increase of the Curie temperature can be attributed to the change of magnetic coupling from a weak superexchange interaction in pristine Cr2Ge2Te6 to a strong double-exchange interaction in (TBA)­Cr2Ge2Te6. These findings are the first demonstration of manipulation of magnetism in magnetic van der Waals materials by means of intercalating organic ions, which can serve as a convenient and efficient approach to explore versatile magnetic and electronic properties in van der Waals crystals.

van der Waals Magnets: Material Family, Detection and Modulation of Magnetism, and Perspective in Spintronics

“…[ 164 ] Besides, the weak interlayer vdW interaction enables chemical intercalation to tune the magnetic properties of vdW magnets. As for bulk CrGeTe 3 , the intercalation of tetra‐butyl ammonium will induce heavy electron doping and modify the FM exchange mechanism, resulting in the increase of T C from 67 to 208 K (Figure 13d), [ 165 ] similar to the aforementioned ionic‐gating modulation. [ 143 ] What's more, the Na intercalation substantially increases the T C of bulk Fe 3 GeTe 2 , yet it is proved that this modulation arises from Fe 2− x Ge impurities.…”

“…Reproduced with permission. [ 165 ] Copyright 2019, American Chemical Society. e) The atomic force microscopic image (left panel) and optical microscopic image (right panel) of an exfoliated CrCl 3− x Br x flake.…”

Two‐Dimensional Metal Telluride Atomic Crystals: Preparation, Physical Properties, and Applications

“…There is an alternative manipulation of 2D ferromagnetic property through the electrochemical intercalation of organic molecules to construct (TBA)Cr 2 Ge 2 Te 6 hybrid superlattice, where TBA + is tetrabutyl ammonium, and the T C is distinctly increased from 67 K (Cr 2 Ge 2 Te 6 ) to 208 K ((TBA)Cr 2 Ge 2 Te 6 ). [ 201 ]…”

Surface/Interface Chemistry Engineering of Correlated‐Electron Materials: From Conducting Solids, Phase Transitions to External‐Field Response

“…[ 49 ] Recently, a facile and feasible approach had been developed to regulate the magnetism and electronic state of the van der Waals ferromagnetic Cr 2 Ge 2 Te 6 semiconductor. [ 50 ] Through an electrochemical intercalation method, the tetrabutyl ammonium (TBA + ) cations were intercalated into the interlayers of Cr 2 Ge 2 Te 6 to form the (TBA)Cr 2 Ge 2 Te 6 ( Figure a). The electronic and magnetization measurements showed that the (TBA)Cr 2 Ge 2 Te 6 exhibited metallic behavior and obviously increased Curie temperature from 67 K to 208 K compared with the pristine Cr 2 Ge 2 Te 6 semiconductor.…”

“…a,b) Reproduced with permission. [ 50 ] Copyright 2019, American Chemical Society. c) Diagrammatic representation of the phase incorporation strategy to achieve ferromagnetism of 2H‐MoS 2 nanosheets.…”