Stability of interfaces in a random environment. A rigorous renormalization group analysis of a hierarchical model

We study a hierarchical model of domain walls in aD-dimensional bond disordered Ising model at low temperatures. Using a renormalization group method inspired by the work of Bricmont and Kupiainen for the random field Ising model, we prove the existence of rigid interfaces at low enough temperatures in dimensionsD>3.     

Striped phase in the cuprates as a semiclassical phenomenon

The striped phase, a novel type of electron solid, has been observed recently in a number of doped Mott-Hubbard insulators (including cuprates). This solid consists of a parallel array of charged-domain walls, bound states of carriers and Néel walls in the antiferromagnetic spin system. The existence of these states has been predicted well in advance of their experimental observation on the basis of semiclassical (‘Hartree-Fock’) theory. Nevertheless, it is not at all clear whether semiclassics yields a correct explanation. In this paper we will focus especially on the variety of striped phases realized in the cuprates, characterized by a domain wall filling of half a hole per domain wall unit cell. We will unfold the reasons why semiclassics, as applied to simple Hubbard models, favours strongly a filling of one hole per domain wall unit cell, as is for instance the case in the nickelates. Nevertheless, the occurrence of half-filled walls as semiclassical ground states cannot be excluded on general grounds. It might be that Hubbard models do not incorporate the microscopic situation correctly. Instead, we derive a qualitative criterion: in order to acquire a special stability on the semiclassical level, the half-filled domain walls should be characterized by a quadrupling of the period along the walls, involving a modulation in the longitudinal spin- and/or charge channel. 64.60. - i, 71.27. + a, 74.72. - h, 75.10. - b     

Domain wall creation in nanostructures driven by a spin-polarized current

We report on current-driven magnetization reversal in nanopillars with elements having perpendicular magnetic anisotropy. Whereas only the two uniform magnetization states are available under the action of a magnetic field, we observed current-induced Bloch domain walls in pillars as small as 50 x 100 nm(2). This domain wall state can be further controlled by current to restore the uniform states. The ability to nucleate and manipulate domain walls by a current gives insight into the reversal mechanisms of small nanoelements and provides new prospects for ultrahigh density spintronic devices.     

Breather states in magnetic domain wall racetrack memory samples

Magnetic domain wall racetrack memory samples allow for the controlled motion of isolated magnetic domain walls in a quasi one-dimensional geometry. Here we consider the possibility of the dynamical formation of bound states of domain walls that can be prepared via a suitably chosen external field. Upon switching off the external field, these domain walls oscillate around their common center of mass, with a frequency depending on the relative initial handedness of the domain wall. Such breather states may be observed by detecting the resulting magnetization oscillations.     

Localized and extended states in a disordered trap

We study Anderson localization in a disordered potential combined with an inhomogeneous trap. We show that the spectrum displays both localized and extended states, which coexist at intermediate energies. In the region of coexistence, we find that the extended states result from confinement by the trap and are weakly affected by the disorder. Conversely, the localized states correspond to eigenstates of the disordered potential, which are only affected by the trap via an inhomogeneous energy shift. These results are relevant to disordered quantum gases and we propose a realistic scheme to observe the coexistence of localized and extended states in these systems.     

Domain walls in gapped graphene

The electronic properties of a particular class of domain walls in gapped graphene are investigated. We show that they can support midgap states which are localized in the vicinity of the domain wall and propagate along its length. With a finite density of domain walls, these states can alter the electronic properties of gapped graphene significantly. If the midgap band is partially filled, the domain wall can behave like a one-dimensional metal embedded in a semiconductor and could potentially be used as a single-channel quantum wire.     

Field-driven creep motion of a composite domain wall in a Pt/Co/Pt/Co/Pt multilayer wire

We have studied the field-driven motion of a pair of coupled Bloch domain walls in a perpendicular magnetic anisotropy Pt/Co/Pt/Co/Pt multilayer Hall bar. The nucleation of an isolated but coincident pair of walls in the two Co layers, observed by Kerr microscopy, took place at an artificial nucleation site created by Ga⁺ ion irradiation. The average velocity v of the wall motion was calculated from time-resolved magnetotransport measurements at fixed driving field H, where the influence of the extraordinary Hall effect leads to the observation of voltages at the longitudinal resistance probes. We observed a good fit to the scaling relation lnv∝H^−1/4, consistent the motion of a single 1-dimensional wall moving in a 2-dimensional disordered medium in the creep regime: the two walls are coupled together into a 1-dimensional composite object.     

Equations for domain walls in a coordinate system of the ferroelectric phase

Two methods are suggested for writing equations for domain walls in a coordinate system of the ferroelectric phase in ferroelastics and multiaxial ferroelectrics. The equations for domain walls in ferroelectric barium titanate and ferroelastic lead orthophosphate are derived. It is shown that suborientation states are possible in these crystals. The suggested methods make it possible to find the matrices of the transformation from the coordinate system of the paraelectric phase to a coordinate system of the ferroelectric phase for each orientation state.     

Conduction through 71° domain walls in BiFeO3 thin films

Local conduction at domains and domain walls is investigated in BiFeO(3) thin films containing mostly 71° domain walls. Measurements at room temperature reveal conduction through 71° domain walls. Conduction through domains could also be observed at high enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime, electrons trapped at defect states are temperature activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in BiFeO(3).     

Tunable conductance of magnetic nanowires with structured domain walls

We show that in a magnetic nanowire with double magnetic domain walls, quantum interference results in spin-split quasistationary states localized mainly between the domain walls. Spin-flip-assisted transmission through the domain structure increases strongly when these size-quantized states are tuned on resonance with the Fermi energy, e.g., upon varying the distance between the domain walls which results in resonance-type peaks of the wire conductance. This novel phenomenon is shown to be utilizable to manipulate the spin density in the domain vicinity. The domain wall parameters are readily controllable, and the predicted effect is hence exploitable in spintronic devices.     

Stabilization of domain walls between traveling waves by nonlinear mode coupling in Taylor-Couette flow

We present a new mechanism that allows the stable existence of domain walls between oppositely traveling waves in pattern-forming systems far from onset. It involves a nonlinear mode coupling that results directly from the nonlinearities in the underlying momentum balance. Our work provides the first observation and explanation of such strongly nonlinearly driven domain walls that separate structured states by a phase generating or annihilating defect. Furthermore, the influence of a symmetry breaking externally imposed flow on the wave domains and the domain walls is studied. The results are obtained for vortex waves in the Taylor-Couette system by combining numerical simulations of the full Navier-Stokes equations and experimental measurements.     

Metastable domain wall dynamics in magnetic nanowires

We study the formation and control of metastable states of pairs of domain walls in cylindrical nanowires of small diameter where the transverse walls are the lower energy state. We show that these pairs form bound states under certain conditions, with a lifetime as long as 200 ns, and are stabilized by the influence of a spin polarized current. Their stability is analyzed with a model based on the magnetostatic interaction and by 3D micromagnetic simulations. The apparition of bound states could hinder the operation of devices.     

Competition between domain walls and the reverse magnetization in the magnetic relaxation of a Pt/Co/Ir/Co/Pt spin switcher

A multilayer Pt/Co/Ir/Co/Pt/GaAs heterostructures demonstrates a long term (to several hours) magnetic relaxation between two stable states of the magnetization of the system. The magnetization reversal of the heterostructure layers occurs both due to the formation of nuclei of the reverse magnetization domains and as a result of their further growth by means of motion of domain walls. The competition between two these processes provides a nonexponential character of the magnetic relaxation. At 300 K, the contributions of these processes to the relaxation are commensurable, while, at temperatures lower than 200 K, the contribution of the nucleation is suppressed and the magnetic relaxation occurs as a result of motion of the domain walls.     

Suppression of spatial chaos via noise-induced growth of arrays of spatial solitons

Domain walls with oscillatory tails are commonplace in models of spatially extended nonlinear optical devices. Their interaction and locking at discrete distances lead to asymptotically stable spatial disorder. We show that noise in the presence of domain walls with oscillatory tails can suppress spatial disorder by privileging highly correlated dynamical states consisting of arrays of spatial solitons.     

Simulation of spin-polarized scanning tunneling microscopy images of nanoscale non-collinear magnetic structures

We apply an efficient method to calculate spin-polarized scanning tunneling microscopy (SP-STM) images of nanostructures with complex non-collinear magnetic order. The model is based on the spin-polarized version of the Tersoff–Hamann model of STM and the independent orbital approximation for the surface electronic structure. For its application, only the knowledge of the arrangement of the magnetic moments of the surface atoms is required. In spite of its simplifications, calculated SP-STM images of periodic collinear and non-collinear magnetic spin structures are in many cases in excellent agreement with those obtained from computationally much more demanding ab initio calculations. Especially for surfaces of chemically equivalent atoms, the atomic scale SP-STM images are dominated by the magnetic structure and depend much less on the accurate electronic structure. This suggests the application of the method to more complex non-collinear magnetic structures such as domain walls in antiferromagnets, spin-spiral states, spin glasses, or disordered states. Based on the model, we predict SP-STM images of helical spin-spiral states in ultra-thin films. PACS 68.37; 75.70; 75.30     

Frequency dependence of aging, rejuvenation and memory in a disordered ferroelectric

We characterize in details the aging properties of the ferroelectric phase of KTa_1-xNb_x O₃ (KTN), where both rejuvenation and (partial) memory are observed. In particular, we carefully examine the frequency dependence of several quantities that characterize aging, rejuvenation and memory. We find a marked subaging behaviour, with an a.c. dielectric susceptiblity scaling as ω, where t _w is the waiting time. We suggest an interpretation in terms of pinned domain walls, much along the lines proposed for aging in a disordered ferromagnet, where both domain wall reconformations and overall (cumulative) domain growth are needed to rationalize the experimental findings. Received 10 November 2000 and Received in final form 20 February 2001     

Spin-dependent scattering in multilayered magnetic rings

Narrow mesoscopic NiFe/Cu/Co elliptical rings exhibit room-temperature giant magnetoresistance with distinct resistance levels corresponding to three different micromagnetic states. The highest and lowest resistance states of the multilayer rings correspond to the Co layer being in a bidomain state, antiparallel or parallel, respectively, to the NiFe, while the intermediate resistance corresponds to the Co layer being in a vortex state. Micromagnetic simulations suggest that the behavior of these rings is dominated by magnetostatic interactions between the domain walls in the Co and NiFe layers. Additional magnetization states in the NiFe at low applied fields can account for the minor loop behavior.     

Strain relief and disorder in commensurate water layers formed on Pd(111)

Water adsorbs and desorbs intact on Pd(111), forming a hydrogen-bonded wetting layer whose structure we examine by low energy electron diffraction (LEED) and He atom scattering (HAS). LEED shows that water forms commensurate (√3 × √3)R30° clusters that aggregate into a partially ordered, approximately (7 × 7) superstructure as the layer completes. HAS indicates that the water layer remains disordered on a local (approximately 10 ?) scale. Based on workfunction measurements and density functional theory simulations we propose that water forms small, flat domains of a commensurate (√3 × √3)R30° water network, separated by disordered domain boundaries containing largely H-down water. This arrangement allows the water layer to adapt its density and relieve the lateral strain associated with adsorbing water in the optimum flat atop adsorption site. We discuss different possibilities for the structure of these domain walls and compare this strain relief mechanism to the highly ordered, large unit cell structures formed on surfaces such as Pt(111).     

Reorientation,multidomain states and domain walls in diluted magnetic semiconductors

The competition between surface/interface and intrinsic anisotropies yields a number of specific reorientation effects and strongly influences magnetization processes in diluted magnetic semiconductors as (Ga,Mn)As and (In,Mn)As. We develop a phenomenological theory to describe reorientation transitions and the accompanying multidomain states applicable to layers of these magnetic semiconductors. It is shown that the magnetic phase diagrams of such systems include a region of four-phase domain structure with four adjoining areas of two-phase domains as well as several regions with coexisting metastable states. We demonstrate that the parameters of isolated domain walls in (Ga,Mn)As nanolayers are extremely sensitive to applied magnetic field and can vary in a broad range. This can be used in microdevices of magnetic semiconductors with pinned domain walls. For (Ga,Mn)As epilayers with perpendicular anisotropy the geometrical parameters of domains have been calculated.