Spin-injection mechanism of magnetization reversal and hysteresis of current in magnetic junctions

A novel mechanism is proposed for magnetization reversal by the current of magnetic junctions with two metallic ferromagnetic layers and thin separating nonmagnetic layer. The spin-polarized current flows perpendicularly to the interfaces between the ferromagnetic layers, in one of which the spins are pinned and in the other they are free. No domain structure is formed in the ferromagnetic layers. The current breaks spin equilibrium in the free layer, which manifests itself in the injection or extraction of spins. The nonequilibrium spins interact with the magnetization of the lattice due to the effective field of s-d exchange, which is current dependent. At currents exceeding a certain threshold value, this interaction leads to magnetization reversal. Two threshold currents for magnetization reversal have been obtained theoretically, which are reached as the current increases or decreases, respectively. Thus, the phenomenon of current hysteresis is found. The calculated results are in good agreement with experiments on magnetization reversal by current in three-layer junctions of composition Co(I)/Cu/Co(II) prepared in a pillar form.     

Current-induced domain formation in magnetic junctions

A junction between two ferromagnetic metal layers with fixed spins in one of them and free spins in the other (spin valve) is considered. The junction is placed in an external magnetic field that orients the free layer oppositely to the fixed layer. It is shown that the spin-polarized electron flow from the fixed layer to the free layer gives rise to stable motionless magnetic domains in the free layer, provided that the magnetic field and the thickness of the free layer are large enough.     

Possible mechanism for tunneling magnetoresistance in La0.9Ba0.1MnO3/Nb-doped SrTiO3 p-n junctions

The observed tunneling magnetoresistance (TMR) effect in La_0.9Ba_0.1MnO₃ (LBMO)/Nb-doped SrTiO₃ (Nb-STO) p⁺-n junctions is investigated and a possible mechanism responsible for the TMR generation is proposed by taking into account the dynamic spin accumulation and paramagnetic magnetization in the Nb-STO layer. Because of carrier diffusion across the dynamic domain boundaries in the Nb-STO layer and spin disordering in the LBMO layer, the tunneling resistance through the junction is high at zero magnetic field. The spin disordering is suppressed upon applying a non-zero magnetic field, which results in the spin-polarized tunneling in this ferromagnetic/depletion layer/dynamic ferromagnetic sandwiched structure and thus the observed TMR effect. The dependence of the TMR effect on the domain size in the LBMO layer, the tunneling current and temperature as well is explained, qualitatively consistent with the experimental observation.     

Switching of a spin valve with three magnetic layers

A spin valve with two pinned ferromagnetic layers sandwiching a free ferromagnetic layer with a thickness smaller than the spin diffusion length in the same layer and than the domain wall thickness is considered. The instability conditions are determined for various mutual orientations of the magnetization of the layers. The possibility of a considerable decrease in the instability threshold due to joint action of spin-polarized current and an external magnetic field is indicated. It is shown that in addition to collinear states, a nonequilibrium noncollinear state can exist, into which the system is switched after exceeding the instability threshold.     

Spin-polarized tunneling in a ferromagnetic graphene junction: Interplay between the exchange interaction and the orbital effect of the magnetic field

The influence of magnetic vector potential barrier (MVPB) on the spin-polarized transport of massless Dirac particles in ferromagnetic graphene is studied theoretically. The phenomenon of Klein tunneling of relativistic particles across a rectangular potential barrier prevents any of the massless fermions from being confined but they can be electrically confined by quantum dots with integrable dynamics (Bardarson et al., 2009) [36]. Utilization of only the in-plane exchange splitting in the ferromagnetic graphene cannot produce 100% spin polarization. This tunneling can be confined using the magnetic vector potential barrier, which leads to high degree of spin polarization. By combining the orbital effect and the Zeeman interaction in graphene junction, it is found that the junction mimics behavior of half-metallic tunneling junction, in which it acts as a metal to particles of one spin orientation but as an insulator or a semiconductor to those of the opposite orientation. The idea of the half-metallic tunneling junction can provide a source of ∼100% spin-polarized current, which is potentially very useful. Adjustment of the position of the Fermi level in ferromagnetic layer by placing a gate voltage on top of the ferromagnetic layer shows that reverse of the orientation of the completely spin-polarized current passing through the junction is controlled by adjusting the gate voltage. These interesting characteristics should lead to a practical gate voltage controlled spin filtering and spin-polarized switching devices as a perfect spin-polarized electron source for graphene-based spintronics.     

Two-dimensional magnetic ordering in a multilayer structure

The effect of confinement from one, two or from all three directions on magnetic ordering has remained an active field of research for almost 100 years. The role of dipolar interactions and anistropy are important to obtain, the otherwise forbidden, ferromagnetic ordering at finite temperature for ions arranged in two-dimensional (2D) arrays (monlayers). We have demonstrated that conventional low-temperature magnetometry and polarized neutron scattering measurements can be performed to study short-range ferromagnetic ordering of in-plane spins in 2D systems using a multilayer stack of non-interacting monolayers of gadolinium ions formed by Langmuir-Blodgett (LB) technique. The spontaneous magnetization could not be detected in the heterogeneous magnetic phase observed here and the saturation value of the net magnetization was found to depend on the sample temperature and applied magnetic field. The net magnetization rises exponentially with lowering temperature and then reaches saturation following a T ln(βT) dependence. The T ln (βT) dependence of magnetization has been predicted from spinwave theory of 2D in-plane spin system with ferromagnetic interaction. The experimental findings reported here could be explained by extending this theory to a temperature domain of βT<1.     

Dynamics of magnetization in ferromagnet with spin-transfer torque

We review our recent works on dynamics of magnetization in ferromagnet with spin-transfer torque. Driven by constant spin-polarized current, the spin-transfer torque counteracts both the precession driven by the effective field and the Gilbert damping term different from the common understanding. When the spin current exceeds the critical value, the conjunctive action of Gilbert damping and spin-transfer torque leads naturally the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width. Driven by space- and time-dependent spin-polarized current and magnetic field, we expatiate the formation of domain wall velocity in ferromagnetic nanowire. We discuss the properties of dynamic magnetic soliton in uniaxial anisotropic ferromagnetic nanowire driven by spin-transfer torque, and analyze the modulation instability and dark soliton on the spin wave background, which shows the characteristic breather behavior of the soliton as it propagates along the ferromagnetic nanowire. With stronger breather character, we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. At last, we have theoretically investigated the current-excited and frequency-adjusted ferromagnetic resonance in magnetic trilayers. A particular case of the perpendicular analyzer reveals that the ferromagnetic resonance curves, including the resonant location and the resonant linewidth, can be adjusted by changing the pinned magnetization direction and the direct current. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. Th     

Pressure exerted on a domain wall by a current in a spin valve: Stoner ferromagnet model

The influence of an electric current flowing through a spin-valve perpendicular to its layers on a domain wall located in the free layer of the spin valve is studied. It is demonstrated that the nonequilibrium spin distribution generated by the current gives rise to a pressure exerted on the domain wall. This pressure is proportional to the current squared, and, for typical values of the magnetic parameters and a current density of 10⁷–10⁸ A/cm², its effect is similar to that of a magnetic field of several oersteds to several tens of oersteds. The magnitude and sign of the pressure are strongly dependent on the geometric and physical parameters of the device. The problem is solved using the model of itinerant-electron ferromagnetism. The relation of the discovered effect to experimental data on magnetization reversal induced by a spin-polarized current in such structures is discussed.     

Optical manipulation of coupled spins in magnetic semiconductor systems: A path toward spin optoelectronics

Experimental results based on the optical excitations in the III–V-based ferromagnetic semiconductors are reviewed. On the bases of results obtained by both cw- and femto-second-pulse optical excitation, we point out the feasibility of magnetization rotation in the hole-mediated ferromagnetic semiconductor (Ga,Mn)As via the angular momentum and photon energy of light. Here, p–d exchange interaction is the effective channel that transmits a small change in spin axis of the valence band to the ferromagnetically coupled Mn spin sub-system. Within the limit of this picture, we also discuss a hole–Mn spin complex for which hole and Mn spins rotate and relax together upon optical excitation. Partial magnetization reversal observed in the experiments of the electrical current injection in (Ga,Mn)As-based magnetic-tunnel-junction devices is also reviewed in view of the effects caused by the spin-polarized holes. Here, we point out that a spin current of 10⁵ A/cm² may be reduced further if spin injection efficiency can be improved by the optimal designs of the device structure.     

Current controlled spin reversal of nanomagnets with giant uniaxial anisotropy

Since a giant magnetic anisotropy of 9 meV per atom has been realized on a Pt surface, we use the kinetic Monte Carlo method to study the spin dynamics of a nanomagnet that is made by putting a line of such adatoms on a thin metallic strip so that the fixed spins are coupled very weakly and a spin-polarized current can be injected into the strip. There is a magnetization hysteresis versus the current because of the giant anisotropy. The hysteresis loop is diminished exponentially with the temperature increasing. The magnetization can be controlled by injecting a spin-polarized current.     

Spin-injection radiation of terahertz waves in ferromagnetic structures

We propose and discuss the mechanisms of terahertz sd exchange radiation during transmission of the spin-polarized current in a structure from a thin steel rod pressured to an ultrathin ferromagnetic film. The two mechanisms considered are quantum transitions between the quasi-Fermi levels in the film near the rod and precession of injected spins around the film magnetization.     

Spin filter effect and large magnetoresistance in the zigzag graphene nanoribbons

The spin filter effect and magnetoresistance (MR) in the graphene nanoribbons with zigzag edges have been investigated by the non-equilibrium Green’s function method. Due to the spin-dependent current blocking effect, the ferromagnetic graphene/normal graphene junction can filter the spin in one direction, so a fully spin-polarized current is found. As the on-site energy μ_R in the right lead goes from negative to positive, the spin-down transmission would suddenly transforms from an `ON’ state to an `OFF’ state, however the spin-up transmission transforms from an `OFF’ state to an `ON’ state, so we can choose the current’s spin polarized direction by tuning μ_R. For the ferromagnetic graphene/ferromagnetic graphene junction the current for the antiparallel magnetization configuration is blocked, a very large MR is obtained. It is expected that these features may serve as a type of useful spintronic devices in future.     

Current-induced spin injection and surface torque in ferromagnetic metallic junctions

The joint influence of two current-induced effects, namely, longitudinal nonequilibrium spin injection and surface torque, on spin-valve-type ferromagnetic metallic junctions is considered theoretically. The current flows normally to layer boundaries. The analysis is based on solving a system of coupled equations of motion for mobile electron and lattice magnetizations. The boundary conditions for the equations of motion are derived from the continuity condition for the total magnetization flux in these subsystems. A dispersion relation is derived for spin wave fluctuations depending on the current through the junction. The fluctuations become unstable at currents exceeding some threshold value (usually, 10⁶?3 × 10⁷ A/cm²). The joint action of longitudinal spin injection and torque lowers the instability threshold. Current-induced spin injection decreases spin wave frequencies near the threshold and can strengthen magnetization pinning at the injecting contact.     

Effect of optical spin injection on ferromagnetically coupled Mn spins in the III-V magnetic alloy semiconductor (Ga,Mn)As

We report on the new type of photoinduced magnetization in ferromagnetic (Ga,Mn)As thin films. Optically generated spin-polarized holes change the orientation of ferromagnetically coupled Mn spins and cause a large change in magnetization, being 15% of the saturation magnetization, without the application of a magnetic field. The memorization effect has also been found as a trace after the photoinduced magnetization. The observed results suggest that a small amount of nonequilibrium carrier spins can cause collective rotation of Mn spins presumably through the p-d exchange interaction.     

Verification of a new quantum simulation approach through its application to two-dimensional Ising lattices

A new quantum simulation approach has been applied in the present work to the two-dimensional (2D) ferromagnetic and antiferromagnetic Ising lattices to calculate their magnetic structures, magnetizations, free energies and specific heats in the absence of an external magnetic field. Surprisingly, no size effects could be observed in our simulations performed for the Ising lattices of different sizes. Most importantly, our calculated spontaneous thermally averaged spins for the two kinds of systems are exactly same as those evaluated with quantum mean field theory, and the magnetic structures simulated at all chosen temperatures are perfectly ferromagnetic or antiferromagnetic, verifying the correctness and applicability of our quantum model and computational algorithm. On the other hand, if the classical Monte Carlo (CMC) method is applied to the ferromagnetic 2D Ising lattice with S=1, it is able to generate correct magnetization well consistent with Onsager's theory; but in the case of S=1/2, the computational results of CMC are incomparable to those predicted with the quantum mean field theory, giving rise to very much reduced magnetization and considerably underestimated Curie temperature. The difficulty met by the CMC method is mainly caused by its improperly calculated exchange energy of the randomly selected spin in every simulation step, especially immediately below the transition temperature, where the thermal averages of spins are much less than 1/2, however they are assigned to ±1/2 by CMC to evaluate the exchange energies of the spins, such improper manipulation is obviously impossible to lead the code to converge to the right equilibrium states of the spin systems.     

Field sweep rate dynamics in magnetic tunnel junctions

The dynamical behavior of magnetic tunnel junctions (MTJs) was investigated by varying the magnetic field sweep rate from 0.01 mT/s to 10 T/s in a magneto optical Kerr effect set-up. The bias fields of the pinned and free ferromagnetic electrodes were found to drastically decrease above a field sweep rate of 1 T/s. This decrease in the bias fields coincides with a change in the magnetization reversal process from domain wall motion at low-field sweep rates to domain nucleation at high-field sweep rates. The nucleation of inverse domains in the ferromagnetic layer changes the interfacial spin structure of the antiferromagnetic layer and therefore the magnitude of the exchange bias effect. Furthermore, the nucleation of domains induces a discontinuous magnetic charge density at the tunnel barrier interfaces and this reduces the interlayer coupling between the two ferromagnetic electrodes of the MTJ.     

Anisotropic spin relaxation in graphene

Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the nonlocal geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field B applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B=0 and B=2 T shows a 20% decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin-orbit effective fields in the in-plane and out-of-plane directions and discuss the role of the Elliott-Yafet and Dyakonov-Perel mechanisms for spin relaxation.     

Size and anisotropy effects on magnetic properties of antiferromagnetic nanoparticles

Based on the Heisenberg model taking into account single-ion anisotropy and using a Green's function technique we have studied the influence of size and anisotropy effects on magnetization M, Neel temperature T_N, coercive field H_c and spin excitation energy of antiferromagnetic nanoparticles. The properties are compared with those of ferromagnetic nanoparticles. We have shown that the enhanced magnetization M and coercive field H_c of antiferromagnetic nanoparticles is a surface effect, which is due to uncompensated surface spins. Moreover, the shape of the coercive field curve can be significantly influenced by surface magnetic anisotropy.     

The influence of fixed spins on the thermodynamic behavior of an Ising ferromagnet

We study the thermodynamic behavior of a ferromagnetic Ising system on a Bethe lattice in the presence of given boundary conditions. More specifically, we study the interface of the system when the spins on half of the surface are fixed opposite to the spins on the other half. We find an interface width that remains finite in the whole range (0,T _c ), a feature due to the special topology of the Bethe lattice. We also study the case where the spin on a certain lattice site belonging to a domain is fixed in a direction opposite to the domain magnetization at all temperaturesT _c . We obtain the influence of that spin on the local magnetization, and we find that the fixed spin nucleates a local domain that extends over a distance of only a few lattice sites from it at all temperaturesT _c .