Ferromagnetic Monolayer Research Articles

Novel Magnetic Monolayers of Transition Metal Silicide

Two-dimensional (2D) magnetic materials are important for the potential applications in spintronics and magnetic storage. Twenty transition metal silicide (TMSi2) monolayers have been studied by first-principles calculations. Seven TMSi2 monolayers, i.e., TiSi2, VSi2, CrSi2, MnSi2, FeSi2,NbSi2, and MoSi2, are found to be magnetic. Among them, the formation energies of ferromagnetic TiSi2 and antiferromagnetic FeSi2 monolayers are lower than those of silicene. Meanwhile, a large magnetic moment of 2.512 μB per Mn atom is obtained in monolayer MnSi2 sheet, which has the same formation energy of silicene. Therefore, these three novel magnetic monolayers hold promise in future 2D magnetic devices.

Magnetic Vortices Induced by a Monopole Tip

A ferromagnetic monolayer with an easy-plane anisotropy scanned by a magnetic tip that is moved with constant velocity ν is studied using atomistic computer simulations. The spin dynamics are treated using the Landau-Lifshitz-Gilbert equation. To study the influence of the tip's field, it is modeled by a monopole field instead of a dipole field, which is a common near-field approximation of a scanning probe microscopy cantilever. The magnetic structures induced by the moving tip are analyzed with respect to the strength of the coupling as well as the scanning velocity, and the energy dissipation is calculated. The results agree with calculations in a continuum model using Thiele's equation, as well as with earlier results obtained from simulations using a dipolar tip. The quantitative influence of the field is illustrated using energetic arguments.

Preferential antiferromagnetic coupling of vacancies in graphene onSiO2: Electron spin resonance and scanning tunneling spectroscopy

Monolayer graphene grown by chemical vapor deposition and transferred to ${\mathrm{SiO}}_{2}$ is used to introduce vacancies by ${\mathrm{Ar}}^{+}$ ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy is considerably lower than the ion fluence, implying that most of the defects are single vacancies as expected from the low ion energy. The vacancies are characterized by scanning tunneling spectroscopy on graphene and highly oriented pyrolytic graphite (HOPG). A peak close to the Dirac point is found within the local density of states of the vacancies similar to the peak found previously for vacancies on HOPG. The peak persists after air exposure up to 180 min, such that electron spin resonance (ESR) at 9.6 GHz can probe the vacancies exhibiting such a peak. After an ion flux of $10/{\mathrm{nm}}^{2}$, we find an ESR signal corresponding to a $g$ factor of 2.001--2.003 and a spin density of $1--2\phantom{\rule{0.28em}{0ex}}\text{spins}/{\mathrm{nm}}^{2}$. The peak width is as small as 0.17 mT indicating exchange narrowing. Consistently, the temperature-dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of $\ensuremath{-}10$ K. Thus, the vacancies preferentially couple antiferromagnetically, ruling out a ferromagnetic graphene monolayer at ion induced spin densities of $1--2\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{2}$.

Relativistic and thermal effects on the magnon spectrum of a ferromagnetic monolayer

A spin model including magnetic anisotropy terms and Dzyaloshinsky–Moriya interactions is studied for the case of a ferromagnetic monolayer with C2v symmetry like Fe/W(110). Using the quasiclassical stochastic Landau–Lifshitz–Gilbert equations, the magnon spectrum of the system is derived using linear response theory. The Dzyaloshinsky–Moriya interaction leads to asymmetry in the spectrum, while the anisotropy terms induce a gap. It is shown that, in the presence of lattice defects, both the Dzyaloshinsky–Moriya interactions and the two-site anisotropy lead to a softening of the magnon energies. Two methods are developed to investigate the magnon spectrum at finite temperatures. The theoretical results are compared to atomistic spin dynamics simulations and good agreement is found between them.

Half-metallicity of a kagome spin lattice: the case of a manganese bis-dithiolene monolayer

The spin ordering in kagome lattices has long been studied in the search for real materials with a spin-liquid ground state. The synthesis of a nickel bis-dichiolene complex (Ni3C12S12) nanosheet (T. Kambe et al., J. Am. Chem. Soc., 2013, 135, 2462) paved a way for realizing real two-dimensional kagome lattices. Using first-principles calculations, we predicted that a ferromagnetic kagome spin lattice with S = 3/2 on lattice vertices can be achieved in an Mn3C12S12 monolayer formed by substituting Ni with Mn atoms in nonmagnetic Ni3C12S12. Monte Carlo simulations on the basis of the Ising model suggest that it has a Curie temperature of about 212 K. A ferromagnetic Mn3C12S12 monolayer is half metallic with high carrier mobility in one spin channel and a band gap of 1.54 eV in another spin channel, which is quite promising for spintronic device applications. Additionally, a small band gap opens up at the Dirac point of the kagome bands due to the spin-orbital coupling effects, which may be implementable for achieving a quantum anomalous Hall effect.

Emergent coherent states and flow rectification in active magnetic colloidal monolayers

We report on an emergent coherent behavior of a ferromagnetic colloidal monolayer suspended at a liquid–air interface and powered by a single-axis alternating in-plane magnetic field. Large-scale coherent states are manifested by formation of dynamic vortex arrays within the colloidal monolayer. Emergence of the dynamic coherence and self-organized patterns in this non-equilibrium colloidal system is attributed to a spontaneous symmetry breaking due to subtle interplay between magnetic, steric, and hydrodynamic forces acting on individual colloidal particles. Collective response of the magnetic colloidal monolayer induces in-plane rectified hydrodynamic surface flows which in turn entrains the particles, resulting in a variety of coherent states.

First-principles study of nanometer-sharp domain walls in ferromagnetic Fe monolayers under in-plane strain

We investigated a nanometer-sharp magnetic domain wall (DW) structure in a free-standing Fe(110) monolayer and studied the crucial role of in-plane strain using fully unconstrained noncollinear ab initio spin-density-functional theory calculations within the generalized gradient approximation. The DW width is calculated to be 0.86 nm. A precise vector-field description of the magnetization density revealed that a noncollinear character in the DW was spatially confined between atoms, whereas a collinear and high magnetization density was localized around each atom. In the rapid rotation of magnetic moments in the DW, we found an electron rearrangement from the dzx and dx2−y2 states to the dxy, dyz and dz2 states due to a shift of band structures. Applied tensile and compressive in-plane strains both bring about narrower DWs in the monolayer except when the strain is small. The strain dependence of the DW width is discussed in terms of both exchange interaction and magnetocrystalline anisotropy.

Tunneling time and Hartman effect in a ferromagnetic graphene superlattice

Using transfer-matrix and stationary phase methods, we study the tunneling time (group delay time) in a ferromagnetic monolayer graphene superlattice. The system we peruse consists of a sequence of rectangular barriers and wells, which can be realized by putting a series of electronic gates on the top of ferromagnetic graphene. The magnetization in the two ferromagnetic layers is aligned parallel. We find out that the tunneling time for normal incident is independent of spin state of electron as well as the barrier height and electron Fermi energy while for the oblique incident angles the tunneling time depends on the spin state of electron and has an oscillatory behavior. Also the effect of barrier width on tunneling time is also investigated and shown that, for normal incident, the Hartman effect disappears in a ferromagnetic graphene superlattice but it appears for oblique incident angles when the x component of the electron wave vector in the barrier is imaginary.

Dynamic behaviors of domain wall in FM/AFM bilayers

The magnetic domain evolution behaviors of ferromagnetic (FM) monolayer and ferromagnetic (FM)/antiferromagnetic (AFM) bilayer are compared and analyzed. The results indicate that the equivalent width, mass and velocity of magnetic domain wall are changed, then the coercivity is enhanced and the exchange bias is present due to the exchange coupling between anti- ferromagnetic and ferromagnetic layer. The results also show that the equivalent width, mass and velocity of magnetic domain wall for FM /AFM bilayers system could be varied with the change of the net magnetization of antiferromagnetic layer, the magnetic anisotropy constants of FM and AFM layer, the exchange coupling constant of antiferromagnetic layer, interface exchange coupling constant and the temperature, and the relevant influences on the coercivity and exchange bias are discussed. So, the physical mechanisms of the emergence of exchange bias and enhancement of coercivity are discovered by the formation and evolution of the domain wall.

Magnon excitations in ultrathin Fe layers: The influence of the Dzyaloshinskii-Moriya interaction

High wave-vector magnon excitations in ferromagnetic Fe monolayer and double-layer grown on W(110) are investigated using spin-polarized electron energy loss spectroscopy. The magnon dispersion relation is obtained up to the Brillouin zone boundary. A direct comparison among different systems shows that the magnons in the Fe monolayer are extremely soft and are even softer than the acoustic surface mode of Fe(110).By measuring the spectra in both energy loss and gain regions on a double-layer Fe film at room temperature and by reversing the sample magnetization, it is demonstrated that the magnon dispersion is asymmetric with respect to the sign of the wave-vector. The asymmetric dispersion relation is attributed to the degeneracy breaking of the magnons due to the presence of the Dzyaloshinskii-Moriya interaction.

Magnetoelectric response of the time-reversal invariant helical metal

We derive compact analytical expressions for the coupled spin-charge susceptibility of a clean helical metal at the surface of a three dimensional topological insulator (TI). These expressions lead to unconventional non-collinear RKKY interactions between two impurity magnetic moments placed on the surface of a TI, and predict the generation of electric currents by time-dependent magnetic moments. We determine the influence of gate and bias voltages on the interlayer exchange coupling between two single-domain ferromagnetic monolayers deposited on top of a TI.

Magnon softening in a ferromagnetic monolayer: A first-principles spin dynamics study

We study the Fe/W(110) monolayer system through a combination of first principles calculations and atomistic spin dynamics simulations. We focus on the dispersion of the spin waves parallel to the [001] direction. Our results compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett. 102, 177206], and correctly capture a drastic softening of the magnon spectrum, with respect to bulk bcc Fe. The suggested shortcoming of the itinerant electron model, in particular that given by density functional theory, is refuted. We also demonstrate that finite temperature effects are significant, and that atomistic spin dynamics simulations represent a powerful tool with which to include these.

Electric field effects on magnetocrystalline anisotropy in ferromagnetic Fe monolayers

The magnetocrystalline anisotropy (MCA) for an Fe monolayer on MgO substrate (Fe/MgO) and that sandwiched by MgO (MgO/Fe/MgO) is investigated by means of the first principles full-potential linearized augmented plane-wave method, and the effects of an external electric field on the MCA is discussed. In both systems, the MCA is found to be modified through a change in the d band structure around Fermi level by the external electric field.

Determination of the Mosaic Angle Distribution of Grafoil Platelets Using Continuous-Wave NMR Spectra

We described details of a method to estimate with good accuracy the mosaic angle distributions of microcrystallites (platelets) in exfoliated graphite like Grafoil which is commonly used as an adsorption substrate for helium thin films. The method is based on analysis of resonance field shifts in continuous-wave (CW) NMR spectra of 3He ferromagnetic monolayers making use of the large nuclear polarization of the adsorbate itself. The mosaic angle distribution of a Grafoil substrate analyzed in this way can be well fitted to a gaussian form with a 27.5±2.5 deg spread. This distribution is quite different from the previous estimation based on neutron scattering data which showed an unrealistically large isotropic powder-like component.

The magnetoresistive effect induced by stress in spin-valve structures

Using a method of free energy minimization, this paper investigates the magnetization properties of a ferromagnetic (FM) monolayer and an FM/antiferromagnetic (AFM) bilayer under a stress field, respectively. It then investigates the magnetoresistance (MR) of the spin-valve structure, which is built by an FM monolayer and an FM/AFM bilayer, and its dependence upon the applied stress field. The results show that under the stress field, the magnetization properties of the FM monolayer is obviously different from that of the FM/AFM bilayer, since the coupled AFM layer can obviously block the magnetization of the FM layer. This phenomenon makes the MR of the spin-valve structure become obvious. In detail, there are two behaviors for the MR of the spin-valve structure dependence upon the stress field distinguished by the coupling (FM coupling or AFM coupling) between the FM layer and the FM/AFM bilayer. Either behavior of the MR of the spin-valve structure depends on the stress field including its value and orientation. Based on these investigations, a perfect mechanical sensor at the nano-scale is suggested to be devised experimentally.

Giant Modification of the Magnetocrystalline Anisotropy in Transition-Metal Monolayers by an External Electric Field

Controlling and designing quantum magnetic properties by an external electric field is a key challenge in modern magnetic physics. Here, from first principles, the effects of an external electric field on the magnetocrystalline anisotropy (MCA) in ferromagnetic transition-metal monolayers are demonstrated which show that the MCA in an Fe(001) monolayer [but not in Co(001) and Ni(001) monolayers] can be controlled by the electric field through a change in band structure, in which small components of the p orbitals near the Fermi level, which are coupled to the d states by the electric field, play a key role. This prediction obtained opens a way to control the MCA by the electric field and invites experiments.

Magnons in a Ferromagnetic Monolayer

We report the first observation of high wave vector magnon excitations in a ferromagnetic monolayer. Using spin-polarized electron energy loss spectroscopy, we observed the magnon dispersion in one atomic layer (ML) of Fe on W(110) at 120 K. The magnon energies are small in comparison to the bulk and surface Fe(110) excitations. We find an exchange parameter and magnetic anisotropy similar to that from static measurements. Our results are in sharp contrast to theoretical calculations, indicating that the present understanding of magnetism of the ML Fe requires considerable revision.

Heisenberg Model of a Ferromagnetic Monolayer Deposited on a Non-Magnetic Bulk Substrate

A model describing a Heisenberg ferromagnetic monolayer (in a magnetic field) interacting with its nonmagnetic bulk substrate is formulated. We use reduced-density operators to show that physical properties of the monolayer are affected by its interaction with the environment. Particularly the influence of the substrate lattice vibrations on the monolayer exchange parameter is examined. The Gaussian-type orbitals were used to calculate the distance dependence of the exchange parameter and the many-body Green functions to calculate the temperature dependence of the magnetization. Finally, the influence of the Debye temperature of the substrate on the magnetization of the monolayer is depicted. Although the resultant effect is not prominent, we state that interaction of ultrathin magnetic films with their environment has to be taken into account in the construction of the reduced-density operator.

Cantilever measurements of surface stress, surface reconstruction, film stress and magnetoelastic stress of monolayers.

We review the application of cantilever-based stress measurements in surface science and magnetism. The application of thin (thickness appr. 0.1 mm) single crystalline substrates as cantilevers has been used successfully to measure adsorbate-induced surface stress changes, lattice misfit induced film stress, and magneto-elastic stress of ferromagnetic monolayers. Surface stress changes as small as 0.01 N/m can be readily measured, and this translates into a sensitivity for adsorbate-coverage well below 0.01 of one layer. Stress as large as several GPa, beyond the elasticity limits of high strength materials, is measured, and it is ascribed to the lattice misfit between film and substrate. Our results point at the intimate relation between surface stress and surface reconstruction, stress-induced structural changes in epitaxially strained films, and strain-induced modifications of the magneto-elastic coupling in ferromagnetic monolayers.

Coexistence of Ferromagnetism and Paramagnetism in a Ferromagnetic Monolayer

The ferromagnetic-paramagnetic phase transition in an Fe monolayer grown on a 2 ML Au/W(110) substrate and capped with a 1 ML Au layer was studied with spin polarized low electron energy microscopy. When the data are analyzed as in previous studies, not only the Curie temperature T(C) but also the effective critical exponent beta depends upon the terrace width. Taking finite size effects into account gives the terrace width-independent two-dimensional Ising model value beta=1/8.