Figure
1. The atomic structure and transport properties of PtSe2 flakes. (a)
Atomic-resolution high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) image of a few-layer PtSe2 flake showing its 1T phase
feature and A–A stacking configuration. (b) Longitudinal resistance of s3 as a
function of temperature in log plot from 200 K to 2 K. (c) Isotropic NMR when
the magnetic field is applied in ab plane of PtSe2 flake s3. Inset shows the
schematic diagram of magnetotransport measurements. θ labels the angle between
magnetic field and current. (d) MR at various temperatures when the magnetic
field is perpendicular to the current in s3. Credit: Peking University.
The
emergence of two-dimensional (2-D) materials provides an excellent platform for
exploring and modulating exotic physical properties in the 2-D limit, and has
driven the development of modern condensed matter physics and nanoelectronic
devices. Among various exotic physical properties, 2-D magnetism is one of the
most important topics, which shows potential application in spintronics. In
recent years, researchers have discovered a series of intrinsic 2-D magnetic
materials, such as CrI3, Fe3GeTe2, etc. However, most of the yet discovered 2-D
magnetic materials are instable in atmosphere, which limits further
investigation and the application of 2-D magnetism. Therefore, the key issue is
how to induce magnetism in air-stable 2-D materials.
Recently,
Professor Wang Jian at Peking University, in collaboration with Professor Duan
Wenhui at Tsinghua University, and Professor Zhang Yanfeng at Peking
University, detected localized magnetic moments induced by Pt vacancies in
transition metal dichalcogenide PtSe2 flakes, and revealed the origin and
flake-thickness dependence of the localized magnetic moments. The paper
entitled "Magnetic Moments Induced by Atomic Vacancies in Transition Metal
Dichalcogenide Flakes" was published online in Advanced Materials.
Professor Wang at Peking University, Professor Duan at Tsinghua University and
Professor Zhang at Peking University are the corresponding authors of this
paper. Ge Jun, Luo Tianchuang at Peking University, Lin Zuzhang at Tsinghua
University, and Shi Jianping at Wuhan University contributed equally to this
work (joint first authors).
PtSe2 flakes with thicknesses of 8-70 nm were grown by chemical vapor deposition (CVD), and their high crystalline quality was confirmed by transmission electron microscopy and selected area electron diffraction. The researchers further fabricated PtSe2 devices of different thicknesses and studied their electrical transport properties. The longitudinal resistance decreases with the decrease of temperature in high temperature regime, which is typical metallic behavior. Interestingly, on further decreasing the temperature, the longitudinal resistance increases logarithmically and then tends to saturate at ultralow temperatures.
Figure
2. Theoretical interpretation for the local magnetic moment in PtSe2 flakes.
(a) An illustration of the local magnetic moments (denoted by red arrows) and a
Pt-vacancy defect (the blue circle) placed in the topmost layer. (b) Electronic
density of states of the p orbitals of the three neighboring selenium atoms of
the Pt-vacancy. (c) The energy of different magnetic configurations (labeled by
the angle β between the magnetic-moment direction and the z axis), where the
energy zero corresponds to the magnetic configuration with out-of-plane
magnetic moment (i.e., β = 0). Credit: Peking University
At low
temperatures, isotropic negative magnetoresistance (NMR) is detected when an
in-plane magnetic field is applied. Further analysis shows that the logarithmic
increase of the longitudinal resistance with the decrease of temperature and
the isotropic NMR originate from Kondo effect. The well-known Kondo effect
usually arises in a non-magnetic metal doped with magnetic impurities,
resulting from the exchange interaction between the spins of conduction
electrons of non-magnetic host and magnetic impurities. However, the
characterization results have demonstrated that there are no magnetic elements
in PtSe2 flakes.
The origin
of the localized magnetic moments in PtSe2 flakes is revealed by theoretical
calculations. The Pt vacancy defects are inevitable to arise during the growth
of the PtSe2 flakes. The Pt vacancies result in an asymmetric distribution of
the occupied spin majority and minority states of the p orbitals of the three
neighboring selenium atoms, finally giving rise to the localized magnetic
moments. Surprisingly, the observed magnetic moments seem to be
thickness-dependent. When reducing the thickness of flakes, the localized
magnetic moment becomes larger. Theoretically, the local magnetic moment in the
sample is mainly contributed by the Pt vacancies on the sample surface. With
decreasing thickness of the PtSe2 flake, the surface-to-bulk ratio increases,
leading to an increase of relative proportion of surface vacancies. As a
result, the averaged magnetic moment induced per defect increases with the
decreasing thickness, which is consistent with the experimental observations.
This work provides a new route for the modulation of magnetism at the atomic scale
in non-magnetic 2-D materials, especially in air-stable 2-D materials, and has
potential significance in the development of spintronics and quantum
information.