Domains and domain nucleation in magnetron-sputtered CoCr thinfilms

The magnetic domain configurations in magnetron-sputtered CoCr thin films have been examined by Lorentz transmission electron microscopy. The thinnest (10 nm) films display in-plane 180° domain walls, while thicker (50 nm) films exhibit out-of-plane dot-type domain structures. The dot domains were observed even in films that had not yet developed a columnar morphology. Intermediate thickness films show a featherlike contrast, indicating that both in-plane and out-of-plane magnetization components are present. Magnetization reversal is seen to occur by domain wall motion in films displaying in-plane anisotropy and by rotation for perpendicular anisotropy films. Intrinsic film stress was found to play a major role in determining the preferred magnetization direction, and thus the resulting magnetic domain configurations     

Current-induced spin-orbit torques

The ability to reverse the magnetization of nanomagnets by current injection has attracted increased attention ever since the spin-transfer torque mechanism was predicted in 1996. In this paper, we review the basic theoretical and experimental arguments supporting a novel current-induced spin torque mechanism taking place in ferromagnetic (FM) materials. This effect, hereafter named spin-orbit (SO) torque, is produced by the flow of an electric current in a crystalline structure lacking inversion symmetry, which transfers orbital angular momentum from the lattice to the spin system owing to the combined action of SO and exchange coupling. SO torques are found to be prominent in both FM metal and semiconducting systems, allowing for great flexibility in adjusting their orientation and magnitude by proper material engineering. Further directions of research in this field are briefly outlined.     

A noncollinear shear surface wave on a moving domain boundary in a ferromagnet

The possibility of the existence and the properties of a noncollinear shear surface wave on a moving domain boundary in a ferromagnet are considered.     

Non-volatile domain nucleation and growth in multiferroic BiFeO3 films

We have presented a systematical study of the domain nucleation and growth behaviors in multiferroic BiFeO(3) (BFO) films. Both the ferroelectric and the ferroelastic switching dynamics were investigated. Several environmental parameters, including the polarization orientations, the monodomain-like matrix, and the ordered domain walls as local boundaries, were well controlled by thin-film strain engineering through changing the vicinal angles of the substrates. The tip-based domain dynamics was studied by subsequent piezoresponse force microscope (PFM) imaging of the domain evolution under external voltage pulses. For the nanodomains written in the monodomain-like environment, the domain wall performed the thermal activated motion. The as-grown 71° domain walls can act as pinning centers for the ferroelectric domain growth driven by low fields; moreover, ferroelastic nucleation near a 71° domain wall will cause the deformation of the domain wall. The ferroelastic domain growth possessed relatively small activation fields, and therefore usually performed non-activated motion. This study revealed the effects of local environments on the dynamics forming nanoscale domains, and opened a pathway for applications in novel non-volatile functional devices.     

Current-induced torques in magnetic materials

The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.     

Current-induced domain-wall motion in U-shaped permalloy wire

The current-induced domain-wall motion has been observed by using a U-shaped permalloy wire. We observed two magnetic states in a U-shaped pattern. One is the vortex domain wall at the center of semicircular arc region of the U-shaped pattern, and the other is the continuous magnetic state without the domain wall in between. In general, the current density of the order of /spl sim/10/sup 7/ A/cm/sup 2/ is needed to drive a magnetic domain wall. In this paper, the critical current for domain-wall motion increases as the bias field increases. The bias field means the field deviated from the switching field of the wire.     

铁电薄膜纳米尺度畴分布及其生长机制

用溶胶-凝胶的方法制得(111)取向的PZT铁电薄膜,并喷涂于Pt(111)/Ti/SiO2/Si衬底上.用扫描力显微镜(SFM)的压电响应模式纳米级分辨率地观察到了几乎等间距或中间间距大于两侧的条状畴结构.这主要与晶粒和衬底间的应力应变有关.从这畴结构我们提出铁电畴形核和生长速率模型.其基本过程如下(1)由于薄膜和底电极间周期性的结构和成分起伏,致使畴结构在此相连处形核.(2)铁电畴横向生长至相临畴相遇,纵向生长至薄膜上表面.(3)在随后的过程中,由于单个畴间的能量差异致使横向生长再次启动.这些过程是连续的,难以正确的区分.     

Current-induced relaxation of charge imbalance in superconducting phase-slip centers

In superconductors with a relatively long inelastic scattering time, relaxation of charge imbalance generated in phase-slip centers will be dominated by elastic scattering in the presence of a supercurrent. The spatial dependence of the electrochemical potential is calculated. Important qualitative differences occur from the usual model of phase-slip centers, based on relaxation by inelastic scattering.Research supported by the Stichting voor Fundamenteel Onderzoek der Materie FOM.     

Current-induced formation of gradient crystals in block copolymer electrolytes

Conventional ordered phases such as crystals and liquid crystals have constant domain spacings. In this Letter, we report on the formation of coherently ordered morphologies wherein the domain spacing changes continuously along a specified direction. We have coined the term "gradient crystal" to refer to this structure, a signature of which is a small-angle X-ray scattering pattern that resembles a sundial. Gradient crystals composed of a gyroid morphology form spontaneously when ionic current is driven through a block copolymer electrolyte. We propose that this structure forms because it allows for a continuous change in domain spacing without requiring the introduction of defects. Previous studies have shown that applied electric fields ranging from 1000 to 40,000 V/mm can induce long-range structural order, alignment, and morphological transitions in block copolymers. Gradient crystals form under applied electric fields as low as 2.5 V/mm due to the presence of direct ionic currents that are absent in the aforementioned studies.     

Electric Current-induced Failure of 200-nm-thick Gold Interconnects

200-nm-thick Au interconnects on a quartz substrate were tested in-situ inside a dual-beam microscope by applying direct current, alternating current and alternating current with a small direct current component. The failure behavior of the Au interconnects under three kinds of electric currents were characterized in-situ by scanning electron microscopy. It is found that the formation of voids and subsequent growth perpendicular to the interconnect direction is the fatal failure mode for all the Au interconnects under three kinds of electric currents. The failure mechanism of the ultrathin metal lines induced by the electric currents was analyzed.     

Current-induced breakdown of superconductivity in constricted type I superconducting films

In simultaneous magneto-optical and electrical measurements on constricted type I superconducting films we have correlated step structure in the currentvoltage characteristic with the nucleation of individual flux-tube trains. A central feature of this experiment is the development of a sample geometry with which we can restrict the breakdown behavior to a single, localized flux-tube train running perpendicular to the transport current. Apparently, the step structure in the I–V characteristic results from a Gibbs free-energy barrier against the nucleation of flux-tube trains. The experiments are performed with films of lead and indium. From the I–V characteristics of the indium constrictions we conclude that the flux-tube nucleation rate can vary substantially with applied current beyond a voltage step at temperatures near T _c.Based on work performed under the auspices of the U.S. Energy Research and Development Administration.     

Current-induced butterfly shaped domains and magnetization switching in magnetic tunnel junctions

Patterned magnetic tunnel junctions (MTJs) with the layer structure of Ta (5 nm)/Ni₇₉Fe₂₁ (5 nm)/Cu (20 nm)/Ni₇₉Fe₂₁ (5 nm)/Ir₂₂Mn₇₈ (10 nm)/Co₇₅Fe₂₅ (4 nm)/Al (0.8 nm)-oxide/Co₇₅Fe₂₅ (4 nm)/Ni₇₉Fe₂₁ (20 nm)/Ta (5 nm) were fabricated using magnetron sputtering deposition and lithography. High tunnelling magnetoresistance ratios of 22 and 50% were obtained at room temperature before and after annealing, respectively. The evolution of leaf shaped images was observed via Lorentz transmission electron microscopy (LTEM) on the MTJs, which were deposited on a patterned and carbon-coated transmission electron microscopy grid. These leaf-like LTEM images correspond to a butterfly shaped domain structure that was confirmed by a micromagnetics simulation. When a large DC current or bias voltage was applied across the MTJ, the butterfly-like vortex domain structures could be induced to form in the free layer of the MTJ, resulting in a significant decrease of magnetization in the free layer. The existence of these butterfly shaped domains could be one of the major causes of the bias voltage dependence of the TMR ratio.     

Vortex matter in superconductor/ferromagnet hybrids

Vortex pinning in type-II superconducting films can be effectively controlled by combining these films with different ferromagnetic nanostructures. In this article an overview is presented of different types of ferromagnetic pinning centers. The investigated hybrid structures consist of Pb films that are deposited on top of arrays of ferromagnetic dots with in-plane magnetization (IMP) or out-of-plane magnetization (OPM), ferromagnetic films with IPM or OPM that contain arrays of submicron holes (antidots), or continuous films with OPM and a magnetic domain structure. Interesting effects such as field-polarity dependent vortex pinning and the dependence of the pinning strength on the domain structure of the ferromagnet are observed. Our experiments demonstrate that vortex pinning in superconductors is strongly influenced by the magnetic properties of the different ferromagnetic pinning centers.     

Coping with Brown's paradox: The pinning and nucleation of magnetic domain walls at antiphase boundaries

The critical field for unpinning or nucleation of a domain wall in a ferromagnetic crystal at an antiphase boundary (APB) is calculated as a function of the magnetic coupling constant η across the APB. It turns out that for positive values of η pinning occurs and that for negative values there is very strong pinning and also nucleation.     

Current-induced instabilities of hydromagnetic flow in a small gap between rotating conducting cylinders

Summary.  In considering the stability of an electrically conducting fluid between rotating perfectly conducting cylinders with a current-induced pressure gradient acting in the azimuthal direction and with an applied axial magnetic field, the assumption of small-gap approximation is made and the governing equations with respect to both axisymmetric and non-axisymmetric disturbances are solved by a direct numerical procedure. A parametric study covering wide ranges of Q, the Hartmann number which represents the strength of the axial magnetic field, and β, a parameter characterizing the ratio of current-induced and rotation velocities, is conducted for the situation where the outer cylinder is stationary and the inner cylinder is rotating. It is found that the stability characteristics are thoroughly different from those of the case of weakly conducting walls. The variation of the onset mode is shown in the (β, Q)-plane, and the transition of the corresponding neutral curves is discussed in detail. Results for the critical Taylor number and wavenumber pertaining to the critical disturbances are presented. The critical values of radial current density required for the onset of instability are also determined. Received May 8, 2002; revised November 11, 2002 Published online: May 8, 2003 The financial support for this work from National Science Council of Taiwan, ROC, through Grant No. NSC 89-2212-E-132-006 is gratefully acknowledged.     

Josephson effect of a superconducting ferromagnet

A point contact tunneling technique has been used to investigate the existence of superconductivity in a long-range, magnetically ordered phase in the reentrant pseudoternary system (Er_{1 – x}Ho_x)Rh₄B₄ in the vicinity of the lower critical temperature T _{c 2}. In this experiment, Josephson-like current-voltage characteristics could be observed in a Nb-Nb oxide-(Er_0.58Ho_0.42)Rh₄B₄ junction even in a temperature region below T _{c 2} (1.8–1.96 K), that is, in the magnetically ordered phase.This work was carried out while on leave of absence.Sponsored by Suisse National Science Foundation.Work in La Jolla sponsored by the National Science Foundation under contract No. NSF/DMR77-08469.     

Induction time in nucleation kinetics

Abstract

The full expression for the induction time in homogeneous nucleation given by Wu has been evaluated for small critical nucleus sizes. It is shown, using parameters relevant to the Cu-Co alloy system, that Wu's analytical approximation for large critical nucleus sizes holds to quite small values (~10 atoms). A more elaborate expression from Shneidman and Weinberg was also examined; it yields slightly different predictions.