203 research outputs found

### Interplay between Kitaev interaction and single ion anisotropy in ferromagnetic CrI$_3$ and CrGeTe$_3$ monolayers

Magnetic anisotropy is crucially important for the stabilization of
two-dimensional (2D) magnetism, which is rare in nature but highly desirable in
spintronics and for advancing fundamental knowledge. Recent works on CrI$_3$
and CrGeTe$_3$ monolayers not only led to observations of the long-time-sought
2D ferromagnetism, but also revealed distinct magnetic anisotropy in the two
systems, namely Ising behavior for CrI$_3$ versus Heisenberg behavior for
CrGeTe$_3$. Such magnetic difference strongly contrasts with structural and
electronic similarities of these two materials, and understanding it at a
microscopic scale should be of large benefits. Here, first-principles
calculations are performed and analyzed to develop a simple Hamiltonian, to
investigate magnetic anisotropy of CrI$_3$ and CrGeTe$_3$ monolayers. The
anisotropic exchange coupling in both systems is surprisingly determined to be
of Kitaev-type. Moreover, the interplay between this Kitaev interaction and
single ion anisotropy (SIA) is found to naturally explain the different
magnetic behaviors of CrI$_3$ and CrGeTe$_3$. Finally, both the Kitaev
interaction and SIA are further found to be induced by spin-orbit coupling of
the heavy ligands (I of CrI$_3$ or Te of CrGeTe$_3$) rather than the commonly
believed 3d magnetic Cr ions

### Room Temperature Quantum Spin Hall Insulators with a Buckled Square Lattice

Two-dimensional (2D) topological insulators (TIs), also known as quantum spin
Hall (QSH) insulators, are excellent candidates for coherent spin transport
related applications because the edge states of 2D TIs are robust against
nonmagnetic impurities since the only available backscattering channel is
forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically,
honeycomb) lattice. Here, we propose that there exists the quantum spin Hall
effect (QSHE) in a buckled square lattice. Through performing global structure
optimization, we predict a new three-layer quasi-2D (Q2D) structure which has
the lowest energy among all structures with the thickness less than 6.0 {\AA}
for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69
eV). The electronic states of the Q2D BiF system near the Fermi level are
mainly contributed by the middle Bi square lattice, which are sandwiched by two
inert BiF2 layers. This is beneficial since the interaction between a substrate
and the Q2D material may not change the topological properties of the system,
as we demonstrate in the case of the NaF substrate. Finally, we come up with a
new tight-binding model for a two-orbital system with the buckled square
lattice to explain the low-energy physics of the Q2D BiF material. Our study
not only predicts a QSH insulator for realistic room temperature applications,
but also provides a new lattice system for engineering topological states such
as quantum anomalous Hall effect.Comment: 17pages, 4 figures Accepted by nano letter

### Evaluating Gilbert Damping in Magnetic Insulators from First Principles

Magnetic damping has a significant impact on the performance of various
magnetic and spintronic devices, making it a long-standing focus of research.
The strength of magnetic damping is usually quantified by the Gilbert damping
constant in the Landau-Lifshitz-Gilbert equation. Here we propose a
first-principles based approach to evaluate the Gilbert damping constant
contributed by spin-lattice coupling in magnetic insulators. The approach
involves effective Hamiltonian models and spin-lattice dynamics simulations. As
a case study, we applied our method to Y$_3$Fe$_5$O$_{12}$, MnFe$_2$O$_4$ and
Cr$_2$O$_3$. Their damping constants were calculated to be $0.8\times10^{-4}$,
$0.2\times10^{-4}$, $2.2\times 10^{-4}$, respectively at a low temperature. The
results for Y$_3$Fe$_5$O$_{12}$ and Cr$_2$O$_3$ are in good agreement with
experimental measurements, while the discrepancy in MnFe$_2$O$_4$ can be
attributed to the inhomogeneity and small band gap in real samples. The
stronger damping observed in Cr$_2$O$_3$, compared to Y$_3$Fe$_5$O$_{12}$,
essentially results from its stronger spin-lattice coupling. In addition, we
confirmed a proportional relationship between damping constants and the
temperature difference of subsystems, which had been reported in previous
studies. These successful applications suggest that our approach serves as a
promising candidate for estimating the Gilbert damping constant in magnetic
insulators.Comment: 14 pages, 11 figure

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