I consider the Hubbard model of graphene in an external magnetic field and in the Hartree-Fock approximation. In the continuum limit, the ground state energy at half filling becomes nearly symmetric under rotations of the three-component vector (N1,N2,m), with the first two components representing the Neel order parameter orthogonal to and the third component the magnetization parallel with the external magnetic field. When the symmetry breaking effects arising from the lattice, Zeeman coupling, and higher Landau levels are included the system develops a quantum critical point at which the antiferromagnetic order disappears and the magnetization has a kink. The observed incompressible state at filling factor one is argued to arise due to a finite third component of the Neel order parameter at these electron densities. Recent experiments appear consistent with vanishing N1 and N2, and finite N3, at the filling factors zero and one, respectively.
We propose that bilayer graphene can provide an experimental realization of deconfined criticality. Current experiments indicate the presence of Néel order in the presence of a moderate magnetic field. The Néel order can be destabilized by application of a transverse electric field. The resulting electric field induced state is likely to have valence bond solid order, and the transition can acquire the emergent fractionalized and gauge excitations of deconfined criticality.
We previously show [JETP Letters, {\bf 114}, 763 (2021)] that a graphene sample placed on a ferromagnetic substrate demonstrates a cooperative magnetoelectronic instability. The instability induces a gap in the electronic spectrum and a canting deformation of the magnetization near the graphene-substrate interface. In this paper we prove that the interaction between the electrons in graphene strongly enhances the instability. Our estimates suggest that in the presence of even a moderate interaction the instability can be sufficiently pronounced to be detected experimentally in a realistic sett…
We calculate the energy of the magnetostatic interaction between two ferromagnetic films with uniform magnetization and correlated interfaces (the "orange-peel" effect). The "orange-peel" coupling is anisotropic: the interaction is ferromagnetic when the films are magnetized in-plane; and it is antiferromagnetic when magnetization is out-of-plane. The interaction anisotropy can be used to distinguish the "orange-peel" effect from the interlayer exchange coupling.
We investigate the many-body effects of a magnetic adatom in ferromagnetic graphene by using the numerical renormalization group method. The nontrivial band dispersion of ferromagnetic graphene gives rise to interesting Kondo physics different from that in conventional ferromagnetic materials. For a half-filled impurity in undoped graphene, the presence of ferromagnetism can bring forth Kondo correlations, yielding two kink structures in the local spectral function near the Fermi energy. When the spin splitting of local occupations is compensated by an external magnetic field, the two Kondo ki…
We consider analytically current-driven dynamics of magnetic Néel walls in heavy metal/ferromagnetic metal/oxide trilayers where strong spin-orbit coupling and interfacial Dzyaloshinskii-Moriya interaction (i-DMI) coexist. We show that field-like spin-orbit torque (FL-SOT) with effective field along $\mathbf{n}\times\hat{\mathbf{J}}$ ($\mathbf{n}$ being the interface normal and $\hat{\mathbf{J}}$ being the charge current direction) and i-DMI induced torque can both lead to Walker breakdown suppression meanwhile leaving the wall mobility (velocity versus current density) unchanged. However, i-D…
The magnetic effects in ferromagnetic graphene basically depend on the principle of exchange interaction when ferromagntism is induced by depositing an insulator layer on graphene. Here we deal with the consequences of non-uniformity in the exchange coupling strength of the ferromagnetic graphene. We discuss how the in- homogeneity in the coordinate and momentum of the exchange vector field can provide interesting results in the conductivity analysis of the ferromagnetic graphene. Our analysis is based on the Kubo formalism of quantum transport.
Electrical Probing of Sub-Néel Spin Dynamics in Two-Dimensional Antiferromagnets Using Graphene Heterostructures
Universidad Autónoma de Madrid（马德里自治大学）；Universidad de Sevilla（塞维利亚大学）；Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC—Universidad de Zaragoza（阿拉贡纳米科学与材料研究所，西班牙国家研究委员会—萨拉戈萨大学）；Condensed Matter Physics Center (IFIMAC) and Instituto Universitario de Ciencia de Materiales “Nicolás Cabrera” (INC), Universidad Autónoma de Madrid（凝聚态物理中心及“尼古拉斯·卡布雷拉”材料科学大学研究所，马德里自治大学）
We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot by using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction $U$ may lead to a edge ferromagnetic like behavior in the strained graphene quantum dot, and a possible room temperature transition is suggested. Around half filling, the ferromagnetic fluctuations at the zigzag edge is strengthened both markedly by the on-site Coulomb interaction and the strain, especially in low temperature region…
We propose and demonstrate that a EuO-induced and top-gated graphene ferromagnetic junction can be simultaneously operated as a spin filter as well as a spin valve. We attribute such a remarkable result to a coexistence of a half-metal band and a common energy gap for opposite spins in ferromagnetic graphene. We show that, both the spin filter and the spin valve can be effectively controlled by a back gate voltage, and they survive for practical metal contacts and finite temperature. Specifically, larger single spin currents and on-state currents can be reached with contacts with work function…
We investigate spin transport through ferromagnetic graphene vertical heterostructures where a sandwiched tunneling layer is either a normal or ferroelectric insulator. We show that the spin-polarization of the tunneling current is electronically controlled via gate voltages. We also demonstrate that the tunneling current of Dirac fermions can be prohibited when the spin configuration of ferromagnetic graphene sheets is opposite. The giant electroresistance can thus be developed by using the proposed heterostructure in this study. The effects of temperature on the spin transport and the giant …
We study the spin dynamics of a ferromagnetic insulator on which graphene is placed. We show that the Gilbert damping is enhanced by the proximity exchange coupling at the interface. The modulation of the Gilbert damping constant is proportional to the product of the spin-up and spin-down densities of states of graphene. Consequently, the Gilbert damping constant in a strong magnetic field oscillates as a function of the external magnetic field that originates from the Landau level structure of graphene. We find that a measurement of the oscillation period enables the strength of the exchange …
The spontaneous self-consistent generation of axial current and corresponding dipole-like magnetic field in a planar electron-positron system similar to graphene and related hetero-structures doped by charged impurity with charge $Z$ is explored. It is shown that this effect takes place for $Z \geq Z^{\ast}$ with $Z^{\ast}$ being a peculiar analogue of the Curie point in ferromagnetics. The properties of induced ferromagnetic state are studied in detail. It is shown also that the arising this way magnetic dipole leads to a significant decrease of the total Casimir (vacuum) energy of the system…
We propose a new mechanism to explain the quantum anomalous Hall (QAH) effect and the electric-field-induced topological phase transition in AB-stacked MoTe${}_2$/WSe${}_2$ moiré heterobilayers at $ν=1$ hole filling. We suggest that the Chern band of the QAH state is generated from an intrinsic band inversion composed of the highest two moiré hole bands with opposite valley numbers and a gap opening induced by two Coulomb-interaction-driven magnetic orders. These magnetic orders, including an in-plane $120^{\circ}$-Néel order and an in-plane ferromagnetic order, interact with moiré bands via c…
Room-temperature ferromagnetic semiconductor is vital in nonvolatile digital circuits and it can provide an idea system where we can make use of both charge and spin of electrons. However, seeking room-temperature ferromagnetic semiconductors is still just an appealing idea that has never been realized in practice up to now. Here we demonstrate that graphene monolayer, hybridized with underlying Ni(111) substrate, is the room-temperature ferromagnetic semiconductor that has been continuously searched for decades. Our spin-polarized scanning tunnelling microscopy (STM) experiments, complemented…
We calculated a spin-polarized conductance in the almost unexplored nanostructure "high temperature ferromagnetic insulator/ graphene/ ferroelectric film" with a special attention to the impact of electric polarization rotation in a strained multiaxial ferroelectric film. The rotation and value of polarization vector are controlled by a misfit strain. We proposed a phenomenological model, which takes into account the shift of the Dirac point due to the proximity of ferromagnetic insulator and uses the Landauer formula for the conductivity of the graphene channel. We derived analytical expressi…
Spin, anomalous, and valley Hall effects in graphene-based hybrid structures are studied theoretically within the Green function formalism and linear response theory. Two different types of hybrid systems are considered in detail: (i) graphene/boron nitride/cobalt(nickel), and (ii) graphene/YIG. The main interest is focused on the proximity-induced exchange interaction between graphene and magnetic substrate and on the proximity-enhanced spin-orbit coupling. The proximity effects are shown to have a significant influence on the electronic and spin transport properties of graphene. To find the …
We have presented here the consequences of the non-uniform exchange field on the spin transport issues in spin chiral configuration of ferromagnetic graphene. Taking resort to the spin orbit coupling (SOC) term and non-uniform exchange coupling term we are successful to express the expression of Hall conductivity in terms of the exchange field and SOC parameters through the Kubo formula approach. However, for a specific configuration of the exchange parameter we have evaluated the Berry curvature of the system. We also have paid attention to the study of SU(2) gauge theory of ferromagnetic gra…
Graphene has been predicted to develop a magnetic moment by proximity effect when placed on a ferromagnetic film, a promise that could open exciting possibilities in the fields of spintronics and magnetic data recording. In this work, we study in detail the interplay between the magnetoresistance of graphene and the magnetization of an underlying ferromagnetic insulating film. A clear correlation between both magnitudes is observed but we find, through a careful modelling of the magnetization and the weak localization measurements, that such correspondence can be explained by the effects of th…
Carbon (C) doped hexagonal boron nitride (hBN) has been experimentally reported to be ferromagnetic at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work we derive a mechanistic understanding of ferromagnetism with graphene islands in hBN from first principles and mean-field Hubbard model. We find a general property, that in bipartite lattices where the sublattices differ in on-site energies, as in hBN, the ordering between local magnetic moments can be substantial and predominantly anti-ferromagnetic (AFM) if they are embedded in the sa…
Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfac…
We build a tight-binding Hamiltonian describing Co/Ni over graphene, contemplating ATOP (a Co/Ni atom on top of each Carbon atom of one graphene sublattice) and HCP (one Co/Ni atom per Graphene plaquette) configurations. For the ATOP configuration the orbitals involved, for the Co/Ni, are the $d_{z^2-r^2}$ which most strongly couple to one graphene sublattice and the $d_{xz}$, $d_{yz}$ orbitals that couple directly to the second sublattice site. Such configuration is diagonal in pseudo-spin and spin space, yielding electron doping of the graphene and antiferro-magnetic ordering in the primitiv…
We study ferromagnetism and its stability in twisted bilayer graphene. We work with a Hubbard-like interaction that corresponds to the screened Coulomb interaction in a well-defined limit where the Thomas-Fermi screening length $l_\text{TF}$ is much larger than monolayer graphene's lattice spacing $l_g \ll l_\text{TF}$ and much smaller than the Moiré super lattice's spacing $ l_\text{TF} \ll l_{\text{Moiré}}$. We show that in the perfectly flat band "chiral" limit and at filling fractions $\pm 3/4$, the saturated ferromagnetic (spin and valley polarized) states are ideal ground states candidat…
A number of quantum Hall isospin ferromagnetic (QHIFM) states have been predicted in the relativistic zero Landau level (LL) of graphene monolayer. These states, especially the states at LL filling factor v = 0 of charge-neutral graphene, have been extensively explored in experiment. To date, identification of these high-field broken-symmetry states has mostly relied on macroscopic transport techniques. Here, we study splitting of the zero LL of graphene at partial filling and demonstrate a direct approach by imaging the QHIFM states at atomic scale with a scanning tunneling microscope. At hal…
We study spin-wave excitations in a circular ferromagnetic nanodot in different inhomogeneous, topologically non-trivial magnetization states, specifically, vortex and skyrmion states. Gradual change in the strength of the out-of-plane magnetic anisotropy and the Dzyaloshinskii-Moriya exchange interaction leads to continuous phase transitions between different stable magnetic configurations and allows for mapping of dynamic spin modes in and between the vortex, Bloch-type skyrmion and Néel-type skyrmion states. Our study elucidates the connections between gyrotropic modes, azimuthal spin waves…
In the quantum Hall regime of graphene, antiferromagnetic and spin-polarized ferromagnetic states at the zeroth Landau level compete, leading to a canted antiferromagnetic state depending on the direction and magnitude of an applied magnetic field. Here, we investigate this transition at 2.7 K in graphene Hall bars that are proximity coupled to the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. From nonlocal transport measurements, we demonstrate an induced magnetic exchange field in graphene, which lowers the magnetic field required to modulate the magnetic state in graphene. These results …
We report the fabrication of one-dimensional (1D) ferromagnetic edge contacts to two-dimensional (2D) graphene/h-BN heterostructures. While aiming to study spin injection/detection with 1D edge contacts, a spurious magnetoresistance signal was observed, which is found to originate from the local Hall effect in graphene due to fringe fields from ferromagnetic edge contacts and in the presence of charge current spreading in the nonlocal measurement configuration. Such behavior has been confirmed by the absence of a Hanle signal and gate-dependent magnetoresistance measurements that reveal a chan…
We study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localised at different spots depending on their spin. We found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarised electric current can be genera…
Graphene on an insulating ferromagnetic substrate---ferromagnetic insulator or ferromagnetic metal with a tunnel barrier---is expected to exhibit giant proximity exchange and spin-orbit couplings. We use a realistic transport model of charge-disorder scattering and solve the linearized Boltzmann equation numerically exactly for the anisotropic Fermi contours of modified Dirac electrons to find magnetotransport signatures of these proximity effects: proximity anisotropic magnetoresistance, inverse spin-galvanic effect, and the planar Hall resistivity. We establish the corresponding anisotropies…
