Tunable room-temperature ferromagnetism in Co-doped two-dimensional van der Waals ZnO

Abstract

The recent discovery of ferromagnetism in two-dimensional van der Waals crystals has provoked a surge of interest in the exploration of fundamental spin interaction in reduced dimensions. However, existing material candidates have several limitations, notably lacking intrinsic room-temperature ferromagnetic order and air stability. Here, motivated by the anomalously high Curie temperature observed in bulk diluted magnetic oxides, we demonstrate room-temperature ferromagnetism in Co-doped graphene-like Zinc Oxide, a chemically stable layered material in air, down to single atom thickness. Through the magneto-optic Kerr effect, superconducting quantum interference device and X-ray magnetic circular dichroism measurements, we observe clear evidences of spontaneous magnetization in such exotic material systems at room temperature and above. Transmission electron microscopy and atomic force microscopy results explicitly exclude the existence of metallic Co or cobalt oxides clusters. X-ray characterizations reveal that the substitutional Co atoms form Co2+ states in the graphitic lattice of ZnO. By varying the Co doping level, we observe transitions between paramagnetic, ferromagnetic and less ordered phases due to the interplay between impurity-band-exchange and super-exchange interactions. Our discovery opens another path to 2D ferromagnetism at room temperature with the advantage of exceptional tunability and robustness.

A room temperature polar magnetic metal

“…Non-centrosymmetric magnets offer an extraordinary platform for exploring fascinating magnetic, quantum topological phases, owing to broken crystal symmetries. [1][2][3][4][5][6][7] As an example, magnetic skyrmions [8][9][10] have been observed in polar magnets and are usually stabilized by an antisymmetric Dzyaloshinskii-Moriya interaction (DMI). In polar magnets such as GaV 4 S 8 (C 3v ) [11], VOSe 2 O 5 (C 4v ) [12], GaV 4 Se 8 (C 3v ) [13] , and PtMnGa (C 3v ) [14] with C nv crystal symmetry, the DMI confines the magnetic modulation direction vector to be perpendicular to the polar axis.…”

Magnetic Anisotropy Control with Curie Temperature above 400 K in a van der Waals Ferromagnet for Spintronic Device

“…To confirm the maximum T c value at even higher temperatures, we fit the $$R_{AHE}^{\rm{s}}--T$$ data (Figure 3e) based on the critical power‐law form α(1 − ( T / T c )) β (where α is a constant, β is the critical exponent), [ 35–37 ] and obtain a T c value of around 400 K. Such a T c value is consistent with the estimated value by the method of Arrott plots (Figure 3f). Compared with those methods of T c engineering with intercalation, [ 5 ] chemical doping, [ 38–40 ] and defect engineering, [ 41,42 ] pressure engineering can serve as a clean method to enhance T c (up to 400 K) and to realize new ferromagnetic metastable phases.…”

2D Ultrawide Bandgap Semiconductors: Odyssey and Challenges

“…Until now, various methods have been exploited to improve the property of ultraviolet photodetectors, like van der Waals heterojunction integration, [201][202][203][204][205][206][207][208][209][210] ferroelectric modulation, [211] surface functionalization, [212] and doping. [213][214][215]…”