External In-plane Magnetic Field Research Articles

Effects of a random gauge field on the conductivity of graphene sheets with disordered ripples

We study the effect of disordered ripples on the conductivity of monolayer graphene flakes. We calculate the relaxation times and the Boltzmann conductivities associated with two mechanisms. First, we study the conductivity correction due to an external in-plane magnetic field $B_\parallel$. Due to the irregular local curvature found at graphene sheets deposited over a substrate, $B_\parallel$ can be mapped into an effective random magnetic field perpendicular to the graphene surface. Second, we study the electron momentum relaxation due to intrinsic pseudo magnetic fields originated from deformations and strain. We find that the competition between these mechanisms gives rise to a strong anisotropy in the conductivity tensor. This result provides a new strategy to quantitatively infer the strength of pseudo-magnetic fields in rippled graphene flakes.

Stopping power of two-dimensional spin quantum electron gases

Quantum effects can contribute significantly to the electronic stopping powers in the interactions between the fast moving beams and the degenerate electron gases. From the Pauli equation, the spin quantum hydrodynamic (SQHD) model is derived and used to calculate the stopping power and the induced electron density for protons moving above a two-dimensional (2D) electron gas with considering spin effect under an external in-plane magnetic field. In our calculation, the stopping power is not only modulated by the spin direction, but also varied with the strength of the spin effect. It is demonstrated that the spin effect can obviously enhance or reduce the stopping power of a 2D electron gas within a laboratory magnetic field condition (several tens of Tesla), thus a negative stopping power appears at some specific proton velocity, which implies the protons drain energy from the Pauli gas, showing another significant example of the low-dimensional physics.

Graphene nanoflakes in external electric and magnetic in-plane fields

The paper discusses the influence of the external in-plane electric and magnetic fields on the ground state spin phase diagram of selected monolayer graphene nanostructures. The calculations are performed for triangular graphene nanoflakes with armchair edges as well as for short pieces of armchair graphene nanoribbons with zigzag terminations. The mean field approximation (MFA) is employed to solve the Hubbard model. The total spin for both classes of nanostructures is discussed as a function of external fields for various structure sizes, for charge neutrality conditions as well as for weak charge doping. The variety of nonzero spin states is found and their stability ranges are determined. For some structures, the presence of antiferromagnetic orderings is predicted within the zero-spin phase. The process of magnetization of nanoflakes with magnetic field at constant electric field is also investigated, showing opposite effect of electric field at low and at high magnetic fields.

Tunneling electroresistance in multiferroic heterostructures

We demonstrate the room temperature polar switching and tunneling in PbZr0.52Ti0.48O3 (PZT) ultra-thin films of thickness 3–7 nm, sandwiched between platinum metal and ferromagnetic La0.67Sr0.33MnO3 (LSMO) layers, which also shows magnetic field dependent tunnel current switching in Pt/PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 heterostructures. The epitaxial nature, surface quality and ferroelectric switching of heterostructured films were examined with the help of x-ray diffraction patterns, atomic force microscopy, and piezo force microscopy, respectively. The capacitance versus voltage graphs show butterfly loops above the coercive field (> ±3 V) of PZT for small probe area (∼16 μm2). The effect of ferroelectric switching was observed in current density versus voltage curves with a large variation in high-resistance/low-resistance (HRS/LRS) ratio (2:1 to 100:1), however, these effects were more prominent in the presence of in-plane external magnetic field. The conductance is fitted with Brinkman’s model, and the parabolic conductance upon bias voltage implies electron tunneling governs the transport.

Magnetisation oscillations by vortex–antivortex dipoles

A vortex–antivortex dipole can be generated due to current with in-plane spin-polarisation, flowing into a magnetic element, which then behaves as a spin transfer oscillator. Its dynamics is analysed using the Landau–Lifshitz equation including a Slonczewski spin-torque term. We establish that the vortex dipole is set in steady state rotational motion due to the interaction between the vortices, while an external in-plane magnetic field can tune the frequency of rotation. The rotational motion is linked to the nonzero skyrmion number of the dipole. The spin-torque acts to stabilise the vortex dipole at a definite vortex–antivortex separation distance. In contrast to a free vortex dipole, the rotating pair under spin-polarised current is an attractor of the motion, therefore a stable state. The details of the rotating magnetisation configurations are analysed theoretically and numerically. The asymptotic behaviour of the rotating configurations provide results on their expected stability. Extensive numerical simulations reveal three types of vortex–antivortex pairs which are obtained as we vary the external field and spin-torque strength. We give a guide for the frequency of rotation based on analytical relations.

Chirality switching and winding or unwinding of the antiferromagnetic NiO domain walls in Fe/NiO/Fe/CoO/Ag(001).

Fe/NiO/Fe/CoO/Ag(001) single crystalline films were grown epitaxially and investigated by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism. The bottom Fe layer magnetization is pinned through exchange coupling to the CoO layer and the top Fe layer magnetization can be rotated by an in-plane external magnetic field. We find that the NiO spins wind up to form a domain wall due to the perpendicular NiO/Fe interfacial coupling as the top layer Fe magnetization rotates from 0° to 90°, but switch wall chirality and unwind the wall as the Fe magnetization rotates from 90° to 180°. This observation shows that Mauri's 180° domain wall does not exist in perpendicularly coupled ferromagnetic-antiferromagnetic systems in the strong coupling regime.

Mueller matrix optical and magneto-optical characterization of Bi-substituted gadolinium iron garnet for application in magnetoplasmonic structures

A ferromagnetic garnet, used as a magneto-optical (MO) material in magneto-photonic and magneto-plasmonic structures, is characterized. We present a general procedure to determine optical and magneto-optical functions of the magneto-optic garnet by using Mueller matrix ellipsometry. In the first step, the optical functions (the refractive index spectra) of the (CaMgZr)-doped gallium-gadolinium garnet (sGGG) substrate and the Bi-substituted gadolinium iron garnet Gd1.24Pr0.48Bi1.01Lu0.27Fe4.38Al0.6O12 (Bi:GIG) are obtained in the spectral range from 0.73 eV to 6.42 eV (wavelength range 193 nm – 1.7 μm). Subsequently, the spectra of the magneto-optical tensor components are obtained by applying an external in-plane magnetic field in longitudinal and transverse geometry. The obtained functions are then used to fit the Mueller matrix spectra of a magneto-plasmonic structure with a gold grating on the magneto-optic garnet layer. This structure has recently been demonstrated to have strongly enhanced transverse magneto-optic Kerr response at visible and near-infrared frequencies. By taking possible fabrication imperfections (surface roughness, residual photo-resist layer, thickness deviation) into account, the measured strongly enhanced MO response fits very well to the numerical model predicting these exaltations.

Switching of perpendicular magnetization by spin-orbit torques in the absence of external magnetic fields.

Magnetization switching by current-induced spin-orbit torques is of great interest due to its potential applications in ultralow-power memory and logic devices. The switching of ferromagnets with perpendicular magnetization is of particular technological relevance. However, in such materials, the presence of an in-plane external magnetic field is typically required to assist spin-orbit torque-driven switching and this is an obstacle for practical applications. Here, we report the switching of out-of-plane magnetized Ta/Co(20)Fe(60)B(20)/TaO(x) structures by spin-orbit torques driven by in-plane currents, without the need for any external magnetic fields. This is achieved by introducing a lateral structural asymmetry into our devices, which gives rise to a new field-like spin-orbit torque when in-plane current flows in these structures. The direction of the current-induced effective field corresponding to this field-like spin-orbit torque is out-of-plane, facilitating the switching of perpendicular magnets.

Single-electron transport in a three-ion magnetic molecule modulated by a transverse field

We study single-electron transport in a three-ion molecule with strong uniaxial anisotropy and in the presence of a transverse magnetic field. Two magnetic ions are connected to each other through a third, nonmagnetic ion. The magnetic ions are coupled to ideal metallic leads and a back gate voltage is applied to the molecule, forming a field-effect transistor. The microscopic Hamiltonian describing this system includes inter-ion hopping, on-site repulsions and magnetic anisotropies. For a range of values of the parameters of the Hamiltonian, we obtain an energy spectrum similar to that of single-molecule magnets in the giant spin approximation where the two states with maximum spin projection along the uniaxial anisotropy axis are well separated from other states. In addition, upon applying an external in-plane magnetic field, the energy gap between the ground and first excited states of the molecule oscillates, going to zero at certain special values of the field, analogous to the diabolical points resulting from Berry phase interference in the giant spin model. Thus, our microscopic model provides the same phenomenological behavior expected from the giant spin model of a single-molecule magnet but with direct access to the internal structure of the molecule, thus making it more appropriate for realistic electronic transport studies. To illustrate this point, the nonlinear electronic transport in the sequential tunneling regime is evaluated for values of the field near these degeneracy points. We show that the existence of these points has a clear signature in the I–V characteristics of the molecule, most notably the modulation of excitation lines in the differential conductance.

Multiferroic CoFe2O4–BiFeO3 core–shell nanofibers and their nanoscale magnetoelectric coupling

Abstract

Ground-state magnetic phase diagram of bow-tie graphene nanoflakes in external magnetic field

The magnetic phase diagram of a ground state is studied theoretically for graphene nanoflakes of bow-tie shape and various sizes in external in-plane magnetic field. The tight-binding Hamiltonian supplemented with Hubbard term is used to model the electronic structure of the systems in question. The existence of the antiferromagnetic phase with magnetic moments localized at the sides of the bow-tie is found for low field and a field-induced spin-flip transition to ferromagnetic state is predicted to occur in charge-undoped structures. For small nanoflake doped with a single charge carrier, the low-field phase is ferrimagnetic and a metamagnetic transition to ferromagnetic ordering can be forced by the field. The critical field is found to decrease with increasing size of the nanoflake. The influence of diagonal and off-diagonal disorder on the mentioned magnetic properties is studied. The effect of off-diagonal disorder is found to be more important than that of diagonal disorder, leading to significantly widened distribution of critical fields for disordered population of nanoflakes.

Effect of contact geometry on spin-transport signals in nonlocal (Ga,Mn)As/GaAs devices

We report on spin-valve experiments in lateral spin-injection devices with different geometries of (Ga,Mn)As/GaAs spin Esaki diode contacts. We study the influence of the geometry of the contacts, i.e., their widths and the crystallographic orientation, on the magnetization reversal process and the resulting pattern observed in the spin-valve signal. We find that tuning of the magnetic anisotropy of the narrow (Ga,Mn)As stripes by means of lithographically induced anisotropic strain relaxation allows one to realize parallel, antiparallel, and even orthogonal configurations of magnetizations in injector and detector contacts. Understanding of the switching between these configurations during sweeping of the external in-plane magnetic field is crucial for a proper interpretation of the measured nonlocal spin signals.

Magnetization dynamics in permalloy films with stripe domains

Through a systematic investigation of the field-dependent dynamic magnetization of a series of NiFe films with and without stripe domains in conjunction with the static magnetization process, we demonstrate that the experimental rotatable anisotropy field is not a fixed value but strongly varied with the external in-plane magnetic field, being qualitatively associated with the emergence of stripe domains. Moreover, the frequency linewidth spectra of the films with stripe domains show an abnormal behavior with three distinct regimes which are strongly correlated with both the static magnetization process and the competition between external magnetic field and dynamic anisotropy field. The results are discussed in terms of the effect of inhomogeneous magnetization associated with the formation of stripe domains and the field-dependent dynamic anisotropy that cause the broadening of frequency linewidth.

Current-induced instability of a composite free layer with antiferromagnetic interlayer coupling

Stability conditions for a spin valve with a composite free layer consisting of two antiferromagnetically coupled films---known as synthetic antiferromagnet or ferrimagnet---is studied theoretically by means of the linearized coupled Landau-Lifshitz-Gilbert equations. The Lyapunov and Routh-Hurwitz methods have been used to examine stability of the free layer subject to external in-plane magnetic field and electric current flowing perpendicularly to the layers' plane. A simple formula for the critical current density, valid for a variety of composite free layer structures, also has been derived. The analytical results are compared with those obtained from numerical simulations in which we took into account the spin transfer torque in the diffusive spin-dependent transport model. An excellent agreement between the analytical and numerical results has been achieved.

Transverse spin penetration length in metallic spin valves

A semiclassical description of diffusive spin transport in spin valves, which takes into account the transverse components of spin accumulation, is used to calculate the second harmonic voltage response to a low-frequency current. The description is applied to single as well as dual spin valves, with the magnetic moment of the sensing layer slightly tilted out of the equilibrium position by an in-plane external magnetic field. In the case of double spin valves, only the antiparallel configuration is considered since the spin torque in this configuration is enhanced, while in the parallel configuration it is significantly reduced. In both cases considered, the second harmonic voltage response and the relevant magnetoresistance are shown to be significantly dependent on the transverse spin penetration length.

Theory of ground-state switching in an array of magnetic nanodots by application of a short external magnetic field pulse

A theory of a ground-state switching in an array of axially magnetized cylindrical magnetic dots arranged in a square lattice is developed. An array can be switched into a quasiregular chessboard-antiferromagnetic state by the application of a short pulse of external in-plane magnetic field having a sufficiently long trailing front. The statistical properties of an array magnetization in its final (after switching) state are determined at the linear stage of growth of unstable collective spin-wave modes of the array under the action of a time-dependent magnetic field, and depend critically on the rate of the field decrease: the slower this decrease, the more regular is the final magnetization state. An analytical procedure is presented that allows one to relate the statistical properties of the final demagnetized state of the array and the linewidth of the array's microwave absorption to the parameters of the external switching pulse. The comparison of the developed analytic theory with the results of numerical simulations is presented and demonstrates good agreement between analytical and numerical results.

Self-field effects in window-type Josephson tunnel junctions

The properties of Josephson devices are strongly affected by geometrical effects such as those associated with the magnetic field induced by the bias current. The generally adopted analysis of Owen and Scalapino (1967 Phys. Rev. 164, 538) for the critical current, Ic, of an in-line Josephson tunnel junction in the presence of an in-plane external magnetic field, He, is revisited and extended to junctions whose electrodes can be thin and of different materials, i.e., of arbitrary penetration depth. We demonstrate that the asymmetry of the magnetic diffraction pattern, Ic(He), is ascribed to the different electrode inductances, for which we provide empirical expressions. We also generalize the modeling to the window-type junctions used nowadays and discuss how to take advantage of the asymmetric behavior in the realization of some superconducting devices. Further we report a systematic investigation of the diffraction patterns of in-line window-type junctions having a number of diverse geometrical configurations and made of dissimilar materials. The experimental results are found to be in agreement with the predictions and clearly demonstrate that the pattern asymmetry increases with the difference in the electrode inductances.

Standing spin waves in magnonic crystals

The features of standing spin waves (SWs) excited during ferromagnetic resonance in three different one-dimensional magnonic crystals (MC) are intensively studied. The investigated magnonic crystals were: an array of air-spaced cobalt stripes, an array of air-spaced permalloy (Py) stripes, and a bi-component MC composed of alternating Co and Py stripes. All MC structures were made by etching technique from Co and Py thin films deposited onto Si substrates. Two configurations are considered with the in-plane external magnetic field applied parallel or perpendicular to the stripes. The supporting calculations are performed by the finite element method in the frequency domain. A number of intensive SW modes occurred in periodic structures under ferromagnetic resonance conditions as a consequence of standing spin waves excitation. These modes were analyzed theoretically in order to explain the origins of SW excitations. With the support of numerical calculations, we analyze also the possible scenarios for the occurrence of standing SWs in the investigated structures. It is demonstrated that the SW propagation length is an important factor conditioning the standing SW formation in MCs.

Tunable diffusion of magnetic particles in a quasi-one-dimensional channel

The diffusion of a system of ferromagnetic dipoles confined in a quasi-one-dimensional parabolic trap is studied using Brownian dynamics simulations. We show that the dynamics of the system is tunable by an in-plane external homogeneous magnetic field. For a strong applied magnetic field, we find that the mobility of the system, the exponent of diffusion, and the crossover time among different diffusion regimes can be tuned by the orientation of the magnetic field. For weak magnetic fields, the exponent of diffusion in the subdiffusive regime is independent of the orientation of the external field.

Technical Note: Building a combined cyclotron and MRI facility: Implications for interference

With the introduction of hybrid PET∕MRI systems, it has become more likely that the cyclotron and MRI systems will be located close to each other. This study considered the interference between a cyclotron and a superconducting MRI system. Interactions between cyclotrons and MRIs are theoretically considered. The main interference is expected to be the perturbation of the magnetic field in the MRI due to switching on or off the magnetic field of the cyclotron. MR imaging is distorted by a dynamic spatial gradient of an external inplane magnetic field larger than 0.5-0.04 μT∕m, depending on the specific MR application. From the design of a cyclotron, it is expected that the magnetic fringe field at large distances behaves as a magnetic dipolar field. This allows estimation of the full dipolar field and its spatial gradients from a single measurement. Around an 18 MeV cyclotron (Cyclone, IBA), magnetic field measurements were performed on 5 locations and compared with calculations based upon a dipolar field model. At the measurement locations the estimated and measured values of the magnetic field component and its spatial gradients of the inplane component were compared, and found to agree within a factor 1.1 for the magnetic field and within a factor of 1.5 for the spatial gradients of the field. In the specific case of the 18 MeV cyclotron with a vertical magnetic field and a 3T superconducting whole body MR system, a minimum distance of 20 m has to be considered to prevent interference. This study showed that a dipole model is sufficiently accurate to predict the interference of a cyclotron on a MRI scanner, for site planning purposes. The cyclotron and a whole body MRI system considered in this study need to be placed more than 20 m apart, or magnetic shielding should be utilized.