Current driven magnetization dynamics in ferromagnetic nanowires with a Dzyaloshinskii-Moriya interaction

We study current-induced magnetization dynamics in a long thin ferromagnetic wire with a Dzyaloshinskii-Moriya interaction (DMI). We find a spiral domain wall configuration of the magnetization and obtain an analytical expression for the width of the domain wall as a function of the interaction strengths. Our findings show that above a certain value of DMI a domain wall configuration cannot exist in the wire. Below this value we determine the domain wall dynamics for small currents, and calculate the drift velocity of the domain wall along the wire. We show that the DMI suppresses the minimum value of current required to move the domain wall. Depending on its sign, the DMI increases or decreases the domain wall drift velocity.     

Superconductivity and spin density waves

In the electron-electron interaction the r-space structure caused by magnetic fluctuations at the phase transition from a nonmagnetic metal to an antiferromagnetic metal gives rise to a d-wave attractive interaction for Cooper pairing. This is a contribution to some total electron-electron interaction which in total may or may not give rise to Cooper pairing and superconductivity.     

Low frequency dynamics of disordered XY spin chains and pinned density waves: from localized spin waves to soliton tunneling

A long-standing problem of the low-energy dynamics of a disordered XY spin chain is reexamined. The case of a rigid chain is studied, where the quantum effects can be treated quasiclassically. It is shown that, as the frequency decreases, the relevant excitations change from localized spin waves to two-level systems to soliton-antisoliton pairs. The linear-response correlation functions are calculated. The results apply to other periodic glassy systems such as pinned density waves, planar vortex lattices, stripes, and disordered Luttinger liquids.     

Self-consistent calculation of spin transport and magnetization dynamics

A spin-polarized current transfers its spin-angular momentum to a local magnetization, exciting various types of current-induced magnetization dynamics. So far, most studies in this field have focused on the direct effect of spin transport on magnetization dynamics, but ignored the feedback from the magnetization dynamics to the spin transport and back to the magnetization dynamics. Although the feedback is usually weak, there are situations when it can play an important role in the dynamics. In such situations, simultaneous, self-consistent calculations of the magnetization dynamics and the spin transport can accurately describe the feedback. This review describes in detail the feedback mechanisms, and presents recent progress in self-consistent calculations of the coupled dynamics. We pay special attention to three representative examples, where the feedback generates non-local effective interactions for the magnetization after the spin accumulation has been integrated out. Possibly the most dramatic feedback example is the dynamic instability in magnetic nanopillars with a single magnetic layer. This instability does not occur without non-local feedback. We demonstrate that full self-consistent calculations generate simulation results in much better agreement with experiments than previous calculations that addressed the feedback effect approximately. The next example is for more typical spin valve nanopillars. Although the effect of feedback is less dramatic because even without feedback the current can make stationary states unstable and induce magnetization oscillation, the feedback can still have important consequences. For instance, we show that the feedback can reduce the linewidth of oscillations, in agreement with experimental observations. A key aspect of this reduction is the suppression of the excitation of short wavelength spin waves by the non-local feedback. Finally, we consider nonadiabatic electron transport in narrow domain walls. The non-local feedback in these systems leads to a significant renormalization of the effective nonadiabatic spin transfer torque. These examples show that the self-consistent treatment of spin transport and magnetization dynamics is important for understanding the physics of the coupled dynamics and for providing a bridge between the ongoing research fields of current-induced magnetization dynamics and the newly emerging fields of magnetization-dynamics-induced generation of charge and spin currents.     

Incommensurate spin density waves in iron aluminides

Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al), and Fe(43Al) show n varying from 11 to 6 for q-->=2pi(h+/-1/n,k+/-1/n,l+/-1/n)/a(0), where (h,k,l) and a(0) characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel.     

Electrically driven spin dynamics of paramagnetic impurities

The spin dynamics of dilute paramagnetic impurities embedded in a semiconductor GaAs channel of a conventional lateral spin valve has been investigated. It is observed that the electron spin of paramagnetic Mn atoms can be polarized electrically when driven by a spin valve in the antiparallel configuration. The transient current through the MnAs/GaAs/MnAs spin valve bears the signature of the underlying spin dynamics driven by the exchange interaction between the conduction band electrons in GaAs and the localized Mn electron spins. The time constant for this interaction is observed to be dependent on temperature and is estimated to be 80 ns at 15 K.     

Analytical study of magnetization dynamics driven by spin-polarized currents

An analytical approach is presented for the study of magnetization dynamics driven by spin-polarized currents. Two cases are considered: (i) magnetic layers with in-plane uniaxial anisotropy; (ii) magnetic layers with uniaxial anisotropy and applied field perpendicular to the layer plane. Theoretical predictions are obtained for the existence of stationary modes and self-oscillations of magnetization by solving the deterministic Landau-Lifshitz-Gilbert equation with Slonczewski spin-torque term. Thermal fluctuations are studied by deriving the corresponding Fokker-Planck equation for the magnetization probability distribution. Analytical procedures to estimate the effective potential barrier separating self-oscillatory regimes and/or stationary modes are proposed.     

Conductivity from charge or spin density waves

Below a Peierls transition the coupled electron phonon collective mode plays an important role in the conductivity of one-dimensional metal models such as have been recently postulated for various organic compounds. Within the jellium model, or in an incommensurate situation, the mode frequency goes to zero for q → 0 and is responsible for the infinite conductivity first proposed by Fröhlich. Impurities, lattice commensurability and three dimensional ordering introduce a gap into the mode spectrum. The low frequency conductivity and a large dielectric constant are predicted. Similar effects are predicted for a spin density wave.     

Current-driven magnetization dynamics in magnetic trilayers with a tilted spin polarizer

The magnetization dynamics is studied theoretically in the tilted-polarizer magnetic trilayers. Utilizing stability analysis, we obtain the phase diagrams in the plane defined by the current density and the direction of pinned-layer magnetization. With the pinned-layer magnetization oriented in a certain range, one can realize different magnetic states, such as quasi-parallel and quasi-antiparallel stable states, in-plane and out-of-plane precessions, and out-of-plane stable states by varying the current. We find that the free-layer magnetization prefers reversal for small deviation of the fixed-layer magnetization from the film plane, while precession for big deviation.     

Effect of surface spin waves on the magnetization of thin films

The theoretical calculation of the magnetization across the thickness of the film shows that it may be either lower or higher near the surface than at the centre of the film depending on the surface anisotropy. For intermediate surface anisotropy values the plots of magnetization vs distance from the surface show several extrema. The temperature dependences of magnetization show only small departures from the usual theoretical behaviour of thin films for the cases when surface spin waves are present. The given examples are calculated for uniaxial h.c.p. Heisenberg ferromagnet.     

Time-correlated soliton tunneling in charge and spin density waves

We consider a model in which an electric field induces quantum nucleation of kink-antikink pairs in a pinned charge or spin density wave. Pair nucleation events, prevented by Coulomb blockade below a pair creation threshold, become correlated in time above threshold. The model provides a natural explanation for the observed (i) small density wave polarization below threshold in NbSe (3), (ii) narrow band noise, (iii) coherent oscillations, and (iv) mode-locking at high drift frequencies.     

Energy relaxation in disordered charge and spin density waves

We investigate collective effects in the strong pinning model of disordered charge and spin density waves (CDWs and SDWs) in connection with heat relaxation experiments. We discuss the classical and quantum limits that contribute to two distinct contribution to the specific heat (a Cv T-2 contribution and a Cv T contribution respectively), with two different types of disorder (strong pinning versus substitutional impurities). From the calculation of the two level system energy splitting distribution in the classical limit we find no slow relaxation in the commensurate case and a broad spectrum of relaxation times in the incommensurate case. In the commensurate case quantum effects restore a non vanishing energy relaxation, and generate stronger disorder effects in incommensurate systems. For substitutional disorder we obtain Friedel oscillations of bound states close to the Fermi energy. With negligible interchain couplings this explains the power-law specific heat Cv T observed in experiments on CDWs and SDWs combined to the power-law susceptibility (T)T-1+ observed in the CDW o-TaS3.     

Coexistence of triplet superconductivity and spin density waves

We discuss the possibility of the coexistence of spin density waves (antiferromagnetism) and triplet superconductivity as a particular example of a broad class of systems where the interplay of magnetism and superconductivity is important. We focus on the case of quasi-one-dimensional metals, where it is known that antiferromagnetism is in close proximity to triplet superconductivity in the pressure versus temperature phase diagram. Over a range of pressures, we propose an intermediate nonuniform phase consisting of antiferromagnetic and triplet superconducting orders. In the coexistence region, we propose a flop transition in the spin density wave order parameter vector, which affects the nature of the superconducting state and leads to the appearance of several new phases.     

Nonautonomous helical motion of magnetization in ferromagnetic nanowire driven by spin-polarized current and magnetic field

Under the generalized gradient approximation (GGA), the electronic and magnetic properties are studied for H-terminated zigzag edge Si nanoribbon (ZSiNR) decorated with a single C chain by using the first-principles projector augmented wave (PAW) potential within the density function theory (DFT) framework. The results show that either a perfect ZSiNR or a single C chain decorated ZSiNR, the ferromagnetic state is preferred over the antiferromagnetic state. But a single C chain decorated ZSiNR is more stable than the perfect one. Furthermore, the electronic and magnetic properties of a ZSiNR can be modulated in detail by a single C chain at different positions.     

Micromagnetic modeling of magnetization dynamics driven by spin-transfer torque in magnetic nanostructures

Magnetization orientation of a nanoscale ferromagnet can be manipulated by an electric current via spin-transfer torque(STT) effect,which holds great promise in the applications of non-volatile magnetic random access memory(MRAM) and spintorque oscillators.We review the fundamental mechanism and experimental progress of the STT effect.Then,different formula of STT torque has been classified,which can be added to the conventional Landau-Lifshitz-Gilbert equation.After that,we show some simulation results that mainly concern the STT-driven vortex dynamics,magnetization oscillations excited by a perpendicular polarizer,and the detail dynamics by in-plane and out-of-plane dual spin polarizers.     

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically T _g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T _g/T). The glass cross-over temperature T_g related to correlations among solitons is equal to the average energy barrier and scales like T _g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap. Received 12 December 2001     

Anatomy of field effects on magnetization dynamics and spin transfer noise

Spin transfer-related phenomena in nanomagnets have attracted extensive studies. In this paper we shall focus on analysis of individual and combined effects of the external, anisotropy, and demagnetization fields on magnetization dynamics and spin transfer noise. It is found that individual roles of the external, anisotropy, and demagnetization fields, as well as the combined roles of external plus anisotropy fields and anisotropy plus demagnetization fields, do not change the behavior of current induced magnetization switching. Such magnetization reversal procedures are of low noise. Our dynamics and power spectral density calculations show that it is the demagnetization field that plays a major role in inducing spin transfer noise: the demagnetization field itself or in combination with the anisotropy field will result in wave-like switching; moreover, the demagnetization field, together with the external field (not too small), will lead to precession and hence the system would be in noisy states. Our modeling work for an elliptical Py alloy is qualitatively consistent with Cornell's experiment and simulation [Science 307 (2005) 228].     

Charge current driven by spin dynamics in disordered Rashba spin-orbit system

Pumping of charge current by spin dynamics in the presence of the Rashba spin-orbit interaction is theoretically studied. Considering a disordered electron, the exchange coupling and spin-orbit interactions are treated perturbatively. It is found that the dominant current induced by spin dynamics is interpreted as a consequence of the conversion from spin current via the inverse spin Hall effect. We also find that the current has an additional component from a fictitious conservative field. The results are applied to the case of a moving domain wall.     

Spin waves in paramagnetic bcc iron: spin dynamics simulations

Large scale computer simulations are used to elucidate a long-standing controversy regarding the existence, or otherwise, of spin waves in paramagnetic bcc iron. Spin dynamics simulations of the dynamic structure factor of a Heisenberg model of Fe with first principles interactions reveal that well defined peaks persist far above Curie temperature Tc. At large wave vectors these peaks can be ascribed to propagating spin waves; at small wave vectors the peaks correspond to overdamped spin waves. Paradoxically, spin wave excitations exist despite only limited magnetic short-range order at and above Tc.