8 research outputs found
Theory of moir\'e magnetism in twisted bilayer -RuCl
Twisted heterostructures of van der Waals materials have received much
attention for their many remarkable properties. Here, we present a
comprehensive theory of the long-range ordered magnetic phases of twisted
bilayer -RuCl via a combination of first-principles calculations
and atomistic simulations. While a monolayer exhibits zigzag antiferromagnetic
order with three possible ordering wave vectors, a rich phase diagram is
obtained for moir\'e superlattices as a function of interlayer exchange and
twist angle. For large twist angles, each layer spontaneously picks a single
zigzag ordering wave vector, whereas, for small twist angles, the ground state
involves a combination of all three wave vectors in a complex hexagonal domain
structure. This multi-domain order minimizes the interlayer energy while
enduring the energy cost due to the domain wall formation. Our results indicate
that magnetic frustration due to stacking-dependent interlayer exchange in
moir\'e superlattices can be used to tune the magnetic ground state and enhance
quantum fluctuations in -RuCl
Signatures of pressure-enhanced helimagnetic order in van der Waals multiferroic NiI
The van der Waals (vdW) type-II multiferroic NiI has emerged as a
candidate for exploring non-collinear magnetism and magnetoelectric effects in
the 2D limit. Frustrated intralayer exchange interactions on a triangular
lattice result in a helimagnetic ground state, with spin-induced improper
ferroelectricity stabilized by the interlayer interactions. Here we investigate
the magnetic and structural phase transitions in bulk NiI, using
high-pressure Raman spectroscopy, optical linear dichroism, and x-ray
diffraction. We obtain evidence for a significant pressure enhancement of the
antiferromagnetic and helimagnetic transition temperatures, at rates of
K/GPa, respectively. These enhancements are attributed to a
cooperative effect of pressure-enhanced interlayer and third-nearest-neighbor
intralayer exchange. These results reveal a general path for obtaining
high-temperature type-II multiferroicity via high pressures in vdW materials
Experimental realization of a single-layer multiferroic
Multiferroic materials have garnered wide interest for their exceptional
static and dynamical magnetoelectric properties. Intrinsic type-II
multiferroics exhibit an inversion-symmetry-breaking magnetic order which
directly induces a ferroelectric lattice distortion through mechanisms such as
the inverse Dzyaloshinskii-Moriya interaction. This direct coupling between the
magnetic and structural order parameters results in record-strength
magnetoelectric effects. Two-dimensional materials possessing such intrinsic
multiferroic properties have been long sought for harnessing magnetoelectric
coupling in nanoelectronic devices. Here, we report the discovery of type-II
multiferroic order in a single atomic layer of transition metal-based van der
Waals material NiI2. Using a combination of optical birefringence, second
harmonic generation, and Raman spectroscopy in bulk NiI2, we first identified
multiple independent and robust signatures of the multiferroic state.
Subsequently, we studied the evolution of the optical signatures as a function
of temperature and layer number, to find that the multiferroic state is robust
down to monolayer NiI2. These observations establish NiI2 as a new platform for
studying emergent multiferroic phenomena, chiral magnetic textures and
ferroelectricity in the two-dimensional limit
Effects of pressure on the electronic and magnetic properties of bulk NiI2
Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI2 is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the effects of hydrostatic pressure as a means to enhance this interlayer exchange and ultimately tune the electronic and magnetic response of NiI2. We study first the evolution of the structural parameters as a function of external pressure using first-principles calculations combined with x-ray diffraction measurements. We then examine the evolution of the electronic structure and magnetic exchange interactions via first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest-neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest-neighbor intralayer exchanges, which controls the magnetic frustration and determines the magnetic propagation vector q of the helimagnetic ground state, is also enhanced by pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature, indicating that pressure is an effective means to tune the magnetic response of NiI2
Theory of moiré magnetism in twisted bilayer α-RuCl3
Twisted heterostructures of van der Waals materials have received much attention for their many remarkable properties. Here, we present a comprehensive theory of the long-range ordered magnetic phases of twisted bilayer α-RuCl3 via a combination of first-principles calculations and atomistic simulations. While a monolayer exhibits zigzag antiferromagnetic order with three possible ordering wave vectors, a rich phase diagram is obtained for moiré superlattices as a function of interlayer exchange and twist angle. For large twist angles, each layer spontaneously picks a single zigzag ordering wave vector, whereas, for small twist angles, the ground state involves a combination of all three wave vectors in a complex hexagonal domain structure. This multi-domain order minimizes the interlayer energy while enduring the energy cost due to the domain wall formation. Our results indicate that magnetic frustration due to stacking-dependent interlayer exchange in moiré superlattices can be used to tune the magnetic ground state and enhance quantum fluctuations in α-RuCl3
Theory of Moiré Magnetism in Twisted Bilayer α‑RuCl<sub>3</sub>
Motivated
by the recent developments in moiré superlattices
of van der Waals magnets and the desire to control the magnetic interactions
of α-RuCl3, here we present a comprehensive theory
of the long-range ordered magnetic phases of twisted bilayer α-RuCl3. Using a combination of first-principles calculations and
atomistic simulations, we show that the stacking-dependent interlayer
exchange gives rise to an array of magnetic phases that can be realized
by controlling the twist angle. In particular, we discover a complex
hexagonal domain structure in which multiple zigzag orders coexist.
This multidomain order minimizes the interlayer energy while enduring
the energy cost due to domain wall formation. Further, we show that
quantum fluctuations can be enhanced across the phase transitions.
Our results indicate that magnetic frustration due to stacking-dependent
interlayer exchange in moiré superlattices can be exploited
to tune quantum fluctuations and the magnetic ground state of α-RuCl3
Effects of pressure on the electronic and magnetic properties of bulk NiI2
peer reviewedTransition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI2 is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the effects of hydrostatic pressure as a means to enhance this interlayer exchange and ultimately tune the electronic and magnetic response of NiI2. We study first the evolution of the structural parameters as a function of external pressure using first-principles calculations combined with x-ray diffraction measurements. We then examine the evolution of the electronic structure and magnetic exchange interactions via first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest-neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest-neighbor intralayer exchanges, which controls the magnetic frustration and determines the magnetic propagation vector q of the helimagnetic ground state, is also enhanced by pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature, indicating that pressure is an effective means to tune the magnetic response of NiI2