Two-dimensional Ferromagnet Research Articles

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO3/SrTiO3 interfaces through elaborately controllable epitaxial growth of SrRuO3. After inserting a SrRuO3 buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru4+ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

FIRST-PRINCIPLES STUDY OF MAGNETIC ANISOTROPY ENERGY IN TWO-DIMENSIONAL FERROMAGNETIC CrI3

First-principles calculations are increasingly crucial in searching for new materials with customized properties or new functionality. First-principles calculations are superior in speed and cost, and comparable to experiments in terms of accuracy. Two-dimensional (2D) ferromagnetism materials were successfully fabricated experimentally in atomic thin films of CrI3. 2D intrinsic ferromagnetism with perpendicular magnetic anisotropy energy (PMAE) is an encouraging applicant compatible with high-density magnetic storage applications. In this study, spin density functional calculations are investigated on the magnetic anisotropy energy (MAE) in 2D monolayer ferromagnetic CrI3 using a first-principles electronic structure calculation. We include the MAE part calculation from the magnetocrystalline anisotropy energy (MCAE) that comes from the spin-orbit coupling (SOC). In the present calculation, the MCAE part, in addition to a usual scheme of total energy (TE), was evaluated using a grand-canonical force theorem (GCFT) scheme. The MCAE of 2D monolayer ferromagnetic CrI3 is 1.7 meV/unit cell and verified it has an easy-axis perpendicular to the crystal plane, which is a good agreement with experimental measurement. In the latter scheme, employing GCFT, we evaluated atom-resolved, k-resolved, and atomic k-resolved analyses for MCAE can be performed. From the GCFT result, the Cr atom indicates that it is the primary origin of PMAE in the atom-resolved MCAE. The negative MCAE contribution is in points, and the positive MCAE is mainly placed at the line in the 2D first Brillouin zone. Our systematic calculation in this work may also help design an effective structure of monolayer CrI3 in new 2D material magnetic sensor and spintronic device designs.

Exchange-mediated magnon-phonon scattering in monolayer CrI3

The interplay between magnons and phonons and its influences on magnon dissipation have attracted widespread attentions. Whereas the previous theoretical works were usually restricted to the interactions between low-frequency magnons and acoustic phonons in the long-wavelength region, in the present work we go beyond this limit and investigate the magnon relaxation in a two-dimensional ferromagnet, a monolayer ${\mathrm{CrI}}_{3}$, through magnon-phonon scattering channels mediated by the variation of the exchange strength resulting from all-wavelength acoustic phonons as well as the optical ones. With a precise description of the magnon and phonon Bloch states from first principles, we evaluate the magnon relaxation rate due to these exchange-mediated magnon-phonon scatterings and reveal rich features in its momentum dependence, which reflects nicely the role of the associated phonons with in-plane atomic vibrations. The magnon-number-conserving scattering is found to be orders of magnitude stronger than the magnon-number-nonconserving scattering due to the weak anisotropic exchange coupling or dipole-dipole interaction. Our first-principles-based approach provides a better solution for the quantitative evaluation of the magnon dissipation by phonons.

Nonperturbative approach to interfacial spin-orbit torques induced by the Rashba effect

Current-induced spin-orbit torque (SOT) in normal metal/ferromagnet (NM/FM) bilayers bears great promise for technological applications, but the microscopic origin of purely interfacial SOTs in ultra-thin systems is not yet fully understood. Here, we show that a linear response theory with a nonperturbative treatment of spin-dependent interactions and impurity scattering potential predicts damping-like SOTs that are strictly absent in perturbative approaches. The technique is applied to a two-dimensional Rashba-coupled ferromagnet (the paradigmatic model of a NM/FM interface), where higher-order scattering processes encoding skew scattering from nonmagnetic impurities allow for current-induced spin polarization with nonzero components along all spatial directions. This is in stark contrast to previous results of perturbative methods (neglecting skew scattering), which predict a coplanar spin-polarization locked perpendicular to the charge current as a result of conventional Rashba-Edelstein effect. Furthermore, the angular dependence of ensuing SOTs and their dependence upon the scattering potential strength is analysed numerically. Simple analytic expressions for the spin-density--charge-current response function, and related SOT efficiencies, are obtained in the weak scattering limit. We find that the extrinsic damping-like torques driven by impurity scattering reaches efficiencies of up to 7% of the field-like (Rashba-Edelstein) torque. Our microscopic theory shows that bulk phenomena, such as the spin Hall effect, are not a necessity in the generation of the damping-like SOTs of the type observed in experiments on ultra-thin systems.

Long-distance coupling of spin qubits via topological magnons

We consider two distant spin qubits in quantum dots, both coupled to a two-dimensional topological ferromagnet hosting chiral magnon edge states at the boundary. The chiral magnon is used to mediate entanglement between the spin qubits, realizing a fundamental building block of scalable quantum computing architectures: a long-distance two-qubit gate. Previous proposals for long-distance coupling with magnons involved off-resonant coupling, where the detuning of the spin-qubit frequency from the magnonic band edge provides protection against spontaneous relaxation. The topological magnon mode, on the other hand, lies in between two magnonic bands far away from any bulk magnon resonances, facilitating strong and highly tuneable coupling between the two spin qubits. Even though the coupling between the qubit and the chiral magnon is resonant for a wide range of qubit splittings, we find that the magnon-induced qubit relaxation is vastly suppressed if the coupling between the qubit and the ferromagnet is antiferromagnetic. A fast and high-fidelity long-distance coupling protocol is presented capable of achieving spin-qubit entanglement over micrometer distances with $1\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$ gate speed and up to $99.9%$ fidelities. The resulting spin-qubit entanglement may be used as a probe for the long-sought detection of topological edge magnons.

Low-energy magnon excitations and emerging anisotropic nature of short-range order inCrI3

We report on the low-energy magnetic excitations and the evolution of local magnetic fields in ${\mathrm{CrI}}_{3}$ in a broad frequency regime of 30--330 GHz and in magnetic fields up to 15 T. Modeling the magnon branches by means of a domain-based ferromagnetic resonance model provides the microscopic parameters describing the magnetic excitations in the two-dimensional ferromagnet. Our data reveal the anisotropy gap of $\mathrm{\ensuremath{\Delta}}=80$ GHz at 2 K, which remarkably remains finite at ${T}_{\mathrm{C}}$ and vanishes only above ${T}_{\mathrm{\ensuremath{\Delta}}}\ensuremath{\simeq}80$ K, i.e., for $T\ensuremath{\gtrsim}1.3{T}_{\mathrm{C}}$. Concomitantly, anisotropic local magnetic fields are probed by a shift of the resonance lines in the same temperature regime. Well above ${T}_{\mathrm{C}}$, we detect short-range magnetic correlations up to 200 K. The observed changing nature of the short-range correlations at ${T}_{\mathrm{\ensuremath{\Delta}}}$ confirms the importance of spin-orbit coupling for the evolution of long-range ferromagnetism which develops from magnetically anisotropic short-range order. In addition, our analysis of the macroscopic magnetization enables to estimate the number of correlated spins well above ${T}_{\mathrm{C}}$ as well as the in-plane magnetic correlation length ${\ensuremath{\xi}}_{\mathrm{ab}}$.

Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer

In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature (TC) in thick samples. It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of TC, which is distinct from both the monolayer and the bulk. Our observation demonstrates unconventional 2D critical behavior on a honeycomb lattice.

Construction of Long-Range Magnetic Sequences on Different Surfaces of BaTiO3.

Using the first-principles spin-density-functional theory calculations, we studied the origin of ferromagnetism from non-magnetic ferroelectric barium titanate (BaTiO3 ) and found out vacancies in different surface can successfully contribute to the origin of ferromagnetism. Accurately, our findings demonstrate that both O and Ti vacancies induce ferromagnetism on the (001) and (010) surfaces of BaTiO3 , and the optimal Ti-O bond length can control the vacancy-induced spin density that is delocalized or concentrated in the real space outside the vacancy, and it helps to enhance our understanding on the long-range magnetic order induced by the vacancy. In addition, intrinsic magnetism is shown on the defect-free (110) surface, and the structure is found to be a near-ideal two-dimensional Ising ferromagnet with large magnetocrystalline anisotropy, and it supplies the platform for studying basic spin behavior of BaTiO3 and more according materials.

Hybrid states of a cavity-photon–vortex coupled system in a superconductive cavity

As the Abrikosov vortex lattice has recently been found in van der Waals heterostructures constructed by a two-dimensional (2D) ferromagnet and a superconductor, we propose the realization of cavity-photon–vortex coupling in a superconductive cavity to construct a new hybrid quantum system in this paper. We study the corresponding hybrid states therein, including the exceptional lines (ELs) in the parameter space. Considering that the parameters of our system are adjustable by external magnetic field and temperature, our system and the ELs are much easier to be realized in experiments. Furthermore, the numerical results show that the corresponding hybrid states can be switched by tuning the source of AC, which makes this hybrid system more advantageous to realize hybrid quantum computing in the future. Moreover, for practical use in detecting hybrid states and the vortex dynamics, the transmission amplitude of an external transverse electric wave through the cavity is also studied.

Robust Second-Order Topological Insulators with Giant Valley Polarization in Two-Dimensional Honeycomb Ferromagnets.

Magnetic topological states have attracted great attention that provide exciting platforms for exploring prominent physical phenomena and applications of topological spintronics. Here, using a tight-binding model and first-principles calculations, we put forward that, in contrast to previously reported magnetic second-order topological insulators (SOTIs), robust SOTIs can emerge in two-dimensional ferromagnets regardless of magnetization directions. Remarkably, we identify intrinsic ferromagnetic 2H-RuCl2 and Janus VSSe monolayers as experimentally feasible candidates of predicted robust SOTIs with the emergence of nontrivial corner states along different magnetization directions. Moreover, under out-of-plane magnetization, we unexpectedly point out that the valley polarization of SOTIs can be huge and much larger than that of the known ferrovalley materials, opening up a technological avenue to bridge the valleytronics and higher-order topology with high possibility of innovative applications in topological spintronics and valleytronics.

First-principles study of two-dimensional half-metallic ferromagnetism in NiXCl (X = S and Se) monolayer

Half-metallic ferromagnets with the character of making maximum use of spin polarization become the center of scientific research. Specifically, two-dimensional (2D) ferromagnetic materials, due to their 100% spin polarization at the Fermi level, are highly desirable for spintronic devices. Using first-principles calculations, we demonstrated that the NiSCl and NiSeCl monolayers are intrinsic 2D ferromagnetic half-metals. The predicted Curie temperatures of the NiSCl and NiSeCl monolayers are 311 and 180 K by Monte Carlo simulations, respectively. The calculated electronic structures using the Heyd–Scuseria–Ernzerhof hybrid functional showed that their half-metallic bandgaps are 1.06 and 0.53 eV, and 3.40 and 2.68 eV for NiSCl and NiSeCl monolayers, respectively. These results indicated that 2D NiSCl and NiSeCl are promising half-metallic materials.

Importance of magnetic shape anisotropy in determining magnetic and electronic properties of monolayer VSi2P4

Two-dimensional (2D) ferromagnets have been a fascinating subject of research, and magnetic anisotropy (MA) is indispensable for stabilizing the 2D magnetic order. Here, we investigate magnetic anisotropy energy (MAE), magnetic and electronic properties of by using the generalized gradient approximation plus U approach. For large U, the magnetic shape anisotropy (MSA) energy has a more pronounced contribution to the MAE, which can overcome the magnetocrystalline anisotropy (MCA) energy to evince an easy-plane. For fixed out-of-plane MA, monolayer undergoes ferrovalley (FV), half-valley-metal (HVM), valley-polarized quantum anomalous Hall insulator (VQAHI), HVM and FV states with increasing U. However, for assumptive in-plane MA, there is no special quantum anomalous Hall (QAH) state and spontaneous valley polarization within considered U range. According to the MAE and electronic structure with fixed out-of-plane or in-plane MA, the intrinsic phase diagram shows common magnetic semiconductor, FV and VQAHI in monolayer . At representative U = 3 eV widely used in references, can be regarded as a 2D-XY magnet, not Ising-like 2D long-range order magnets predicted in previous works with only considering MCA energy. Our findings shed light on importance of MSA in determining magnetic and electronic properties of monolayer .

Topological magnons in the honeycomb-kagome lattice

Materials with magnon Hall effect have potential applications in the field of spintronics and magnonics. The experimental observations of the magnon Hall effect in three-dimensional pyrochlore ferromagnets and two-dimensional kagome ferromagnets inspired the search for topological magnons in various lattice structures. The honeycomb-kagome (HK) lattice (also known as the edge-centered honeycomb lattice) can be seen as the combination of the honeycomb and kagome lattices. Hence, the Dzyaloshinskii–Moriya (DM) interaction is allowed and topological magnons are expected in the HK lattice, as the cases in the honeycomb and the kagome lattices alone. Here, we study the topological magnons in the HK lattice by calculating its band structure, Chern number, edge states and thermal Hall conductivity. It is shown that there are rich topological phases and phase transitions with the tuning of the model parameters. The finite thermal Hall conductivity induced by the DM interaction also has interesting behaviors, which are related to the topological phase transitions.

Topological magnons in one-dimensional ferromagnetic Su–Schrieffer–Heeger model with anisotropic interaction

Topological magnons in a one-dimensional (1D) ferromagnetic Su–Schrieffer–Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the intercellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii–Moriya interaction and pseudo-dipolar interaction, also can induce the emergence of the non-trivial phase with two degenerate in-gap edge states separately localized at the two ends of the 1D chain, while the intracellular interactions instead unfavors the topological phase. The interplay among them has synergistic effects on the topological phase transition, very different from that in the two-dimensional (2D) ferromagnet. These results demonstrate that the 1D magnons possess rich topological phase diagrams distinctly different from the electronic version of the SSH model and even the 2D magnons. Due to the low dimensional structural characteristics of this 1D topological magnonic system, the magnonic crystals can be constructed from bottom to top, which has important potential applications in the design of novel magnonic devices.

Piezoelectric ferromagnetism in two-dimensional materials via materials screening

We predict that monolayer ${\mathrm{FeCl}}_{2}$ is a two-dimensional piezoelectric ferromagnet (PFM) with easy-axis magnetism and a Curie temperature of 260 K. Our ab initio calculations combined with data mining reveal 2H-${\mathrm{FeCl}}_{2}$ as the only easy-axis 2D monolayer PFM, and that its magnetic anisotropy increases many-fold with moderate hole doping. We develop an analysis based on magnetic anisotropies densities that explain the magnetic and doping-dependent behavior of ${\mathrm{FeCl}}_{2}$, as well as ${\mathrm{VSe}}_{2}$ and ${\mathrm{CrI}}_{3}$, and can enable the design of future 2D magnetically ordered materials.

Tunable magnetic anisotropy in two-dimensional CrX3/AlN (X=I,Br,Cl) heterostructures

Controlling the magnetic anisotropy of two-dimensional (2D) ferromagnets (FMs) is of great importance for the development of next generation spintronics. Here, by combining the 2D FM chromium trihalides $\mathrm{Cr}{X}_{3} (X=\mathrm{I},\mathrm{Br},\mathrm{Cl})$ monolayer (ML) with the fully hydrogenated bilayer AlN (AlN-2L) together, the electronic and magnetic properties, especially the magnetic anisotropies, of $\mathrm{Cr}{X}_{3}\text{/}\mathrm{AlN}$ heterostructures ($\mathrm{Cr}{X}_{3}$ HSs) are systematically investigated based on first-principles calculations. When the polarization direction of AlN-2L reverses, the magnetic easy axis of $\mathrm{Cr}{X}_{3}$ MLs can be switched between out-of-plane and in-plane directions. Particularly the magnetic anisotropic energy is tuned by a maximum of 200% variation in $\mathrm{Cr}{\mathrm{I}}_{3}$/AlN. It is shown that the different effects of AlN-2L on the modulation of the magnetic easy axis of $\mathrm{Cr}{X}_{3}$ MLs are related to their different interfacial charge transfer/redistribution across their interfaces. In addition, a transition from FM semiconductor to half-metal is found in all $\mathrm{Cr}{X}_{3}$ MLs when the polarization direction points toward $\mathrm{Cr}{X}_{3}$ MLs. Our results suggest a feasible avenue for the design of van der Waals HSs to realize the control of magnetic anisotropy in 2D FMs.

Quartic asymmetric exchange for two-dimensional ferromagnets with trigonal prismatic symmetry

We suggest a possible origin of noncollinear magnetic textures in ferromagnets (FMs) with the $D_{3h}$ point group symmetry. The suggested mechanism is different from the Dzyaloshinskii-Moriya interaction (DMI) and its straightforward generalizations. The considered symmetry class is important because a large fraction of all single-layer intrinsic FMs should belong to it. In particular, so does a monolayer Fe$_3$GeTe$_2$. At the same time, DMI vanishes identically in materials described by this point group, in the continuous limit. We use symmetry analysis to identify the only possible contribution to the free energy density in two dimensions that is of the fourth order with respect to the local magnetization direction and linear with respect to its spatial derivatives. This contribution predicts long-range conical magnetic spirals with both the average magnetization and the average chirality dependent on the spiral propagation direction. We relate the predicted spirals to a recent experiment on Fe$_3$GeTe$_2$. Finally, we demonstrate that, for easy-plane materials, the same mechanism may stabilize bimerons.

Weighted averages in population annealing: Analysis and general framework.

Population annealing is a powerful sequential Monte Carlo algorithm designed to study the equilibrium behavior of general systems in statistical physics through massive parallelism. In addition to the remarkable scaling capabilities of the method, it allows for measurements to be enhanced by weighted averaging [J. Machta, Phys. Rev. E 82, 026704 (2010)1539-375510.1103/PhysRevE.82.026704], admitting to reduce both systematic and statistical errors based on independently repeated simulations. We give a self-contained introduction to population annealing with weighted averaging, generalize the method to a wide range of observables such as the specific heat and magnetic susceptibility and rigorously prove that the resulting estimators for finite systems are asymptotically unbiased for essentially arbitrary target distributions. Numerical results based on more than 10^{7} independent population annealing runs of the two-dimensional Ising ferromagnet and the Edwards-Anderson Ising spin glass are presented in depth. In the latter case, we also discuss efficient ways of measuring spin overlaps in population annealing simulations.

Temperature-dependent magnetic order of two-dimensional ferromagnetic Cr2Ge2Te6 single crystal

The vigorous development of two-dimensional materials has attracted much attention in fields including physics, chemistry, materials, and energy in recent years. As an important part among the two-dimensional (2D) material family, 2D ferromagnets provide unique physical properties and potential applications. However, the similarities and differences between two-dimensional ferromagnetic materials and traditional ferromagnetic materials in terms of magnetic properties have not been studied thoroughly. To this end, combining the ferromagnetic resonance (FMR) and superconducting quantum interference vibrating sample magnometer (SQUID VSM), we systematically study the changes in the internal magnetic order of the 2D van der Waals crystal Cr2Ge2Te6 (CGT). As the temperature rising to Curie Temperature (TC), the in-plane and out-of-plane resonance fields of CGT gradually merge into a same field, indicating the changes of anisotropic. The amplitude of ferromagnetic resonance shows a peak when the temperature is below TC, which may be due to the competition between damping progress and magnetization degradation during the heating process. This work paves the way for a better understanding of the mechanism of two-dimensional ferromagnetism.

Prediction of a first-order phase transition in two-dimensional ferromagnets in the presence of random fields

Random magnetic fields suppress the long-range magnetic ordering through the formation of the magnetic domains. The size of the domains is determined by the competition among the exchange interaction, the magnetic anisotropy, and the strength of the random field. Here we theoretically investigate the temperature dependence of the magnetization of the two-dimensional domains for the anisotropic Heisenberg model with a random magnetic field. We find that magnetization of the domains displays a first-order phase transition in which the magnetization is discontinuous at a critical temperature. Moreover, the first-order transition persists even in the presence of an external magnetic field whose magnitude is smaller than the strength of the disorder. The above unusual first-order phase transition can be experimentally tested with doped two-dimensional magnets.