V. S. Gornakov

A field theory of piezoelectric media containing dislocations

A field theory is proposed to extend the standard piezoelectric framework for linear elastic solids by accounting for the presence and motion of dislocation fields and assessing their impact on the piezoelectric properties. The proposed theory describes the incompatible lattice distortion and residual piezoelectric polarization fields induced by dislocation ensembles, as well as the dynamic evolution of these fields through dislocation motion driven by coupled electro-mechanical loading. It is suggested that (i) dislocation mobility may be enhanced or inhibited by the electric field, depending on the polarity of the latter, (ii) plasticity mediated by dislocation motion allows capturing long-term time-dependent properties of piezoelectric polarization. Due to the continuity of the proposed electro-mechanical framework, the stress/strain and polarization fields are smooth even in the dislocation core regions. The theory is applied to gallium nitride layers for validation. The piezoelectric polarization fields associated with bulk screw/edge dislocations are retrieved and surface potential modulations are predicted. The results are extended to dislocation loops.

Nucleation and Evolution of Hybrid Spin Spiral in Soft/Hard Ferromagnetic Bilayer

The hybrid exchange spring behavior in a bilayer consisting of an epitaxial film of Feu200a(500u200aA)/Sm2Co7u200a(350u200aA) deposited onto a Cr (200 A) buffered MgO(110) substrate was investigated using the magneto-optical indicator film technique. Two critical magnitudes of the remagnetizing magnetic field were found that dramatically change the bilayer remanent magnetization. One of them is determined by exchange spring penetration in the interface; another is determined by hybrid spin spiral transformation to domain walls in the hard ferromagnet layer. Unexpected dependence of the direction of average remanent magnetization on the external magnetic field direction and value is revealed.

Stationary antiferromagnetic domains during magnetization reversal in an exchange-biased FeMn/Fe76Mn6C18 bilayer

Domain processes were observed at 300 K using the magneto-optic indicator film technique (MOIF) in an exchange-coupled ferromagnet (FM)/antiferromagnet (AF) bilayer Fe76Mn6C18 (150 A)/FeMn (100 A) deposited under the presence of a 0.4 mT magnetic field (H) applied in the plane of the sample. The hysteresis loop for this sample was comprised of two half-loops symmetrically shifted in opposite directions from the origin. At H=0, MOIF observations showed the presence of domains in the FM with magnetization (M) vectors along the axis of the preparation field separated by 180° walls. Upon field application along that axis, saturation of the FM was achieved by the nucleation and growth of domains. In this state, at not very high fields, it was possible to observe an unusual MOIF contrast at the location of the original FM domain walls in the as-prepared ground state, associated with the intersection of domain walls in the AF with the FM. Upon field reduction M reversed only in regions which had reversed during ...

Switching of domains and domain walls in Fe50Mn50/Ni81Fe19 bilayers with non-180° ferromagnetic domains

We have studied FeMn/Ni81Fe19 bilayers using vectorial vibrating-sample magnetometry (VSM) and the magnetooptic-indicator-film (MOIF) technique and observed stationary antiferromagnetic (AF) domain walls throughout the hysteresis process. The FeMn/Ni81Fe19 bilayer was demagnetized at 450 K and then cooled to 300 K in a zero field. The resultant hysteresis loop at 300 K clearly shows two loops, one shifted to the left of the origin and the other, to the right. Moreover, one loop is slanted and the other is square, indicating that the two sets of ferromagnetic (FM) domains have noncollinear magnetizations. At the zero applied field, the MOIF images clearly reveal the FM domains and domain walls. These domain walls, however, are hybrid domain walls connecting both FM and AF domain walls. With the application of a magnetic field, the FM domain walls can be swept. But, regardless of the applied field, including the case where the FM layer has been made into a single domain, domain walls at the original locations are still visible. Of the two sets of the FM domains with different magnetic-anisotropy axes, one set becomes reversed by rotation during magnetization reversal, while the other set becomes reversed by domain-wall nucleations and propagations.

Kinetics of magnetization reversal in a heterophase nanomagnet with spatially modulated anisotropy

Magnetization reversal modes in a thin-film NiFeCuMo ferromagnet (FM) with periodically varying in-plane anisotropy are studied by the magneto-optical indicator film (MOIF) technique. The uni-directional anisotropy in FM regions exchange-coupled to a FeMn antiferromagnet (AFM) film in the form of square mesh stripes is alternated by the uniaxial anisotropy in the FM regions inside this mesh. It is shown that the boundaries formed along the edges of these stripes, which separate FM regions with different anisotropy, crucially influence the kinetics of domain-structure transformation in both types of FM regions. It is established that the lateral exchange anisotropy in the ferromagnet, which is determined by the stabilization of the spin distribution in the FM layer along the FM-(FM/AFM) interface, leads to the asymmetry of the magnetization reversal in FM regions bordered with an FM/AFM structure. Anisotropy of the mobility of 180-degree “charged” and “uncharged” domain walls situated, respectively, perpendicular and parallel to the unidirectional anisotropy axis is revealed. The difference observed between the mobilities of charged and uncharged domain walls is attributed to the difference in the spin distribution in these walls with respect to the unidirectional anisotropy axis and is a key factor for the difference between the magnetization reversal kinetics in horizontal and vertical exchange-biased FM stripes. Drastic differences are revealed in the asymmetry of magnetization reversal processes in mutually perpendicular narrow stripes of FM/AFM structures. Possible mechanisms of magnetization reversal in low-dimensional FM-(FM/AFM) heterostructures are discussed with regard to the effect of domain walls localized on the edges of AFM layers.

Breakdown of antiferromagnet order in polycrystalline NiFe/NiO bilayers probed with acoustic emission

Magnetization reversal of polycrystalline NiFe/NiO bilayers was investigated using magneto-optical indicator film imaging and acoustic emission techniques. Sporadic acoustic signals were detected in a constant magnetic field after the magnetization reversal. It is suggested that they are related to elastic waves excited by sharp shocks in the NiO layer with strong magnetostriction. Their probability depends on the history and number of repetitions of the field cycling, thus testifying the thermal-activation nature of the long-time relaxation of an antiferromagnetic order. These results provide evidence of spontaneous thermally activated switching of the antiferromagnetic order in NiO grains during magnetization reversal in ferromagnet/antiferromagnet (FM/AFM) heterostructures. The respective deformation modes are discussed in terms of the thermal fluctuation aftereffect in the Fulcomer and Charap model which predicts that irreversible breakdown of the original spin orientation can take place in some antif...

Statistical and Multifractal Properties of Barkhausen Jumps in Exchange-Coupled Antiferromagnetic/Ferromagnetic Bilayers

Statistical and multifractal properties of Barkhausen jumps in exchange-coupled ferromagnet/antiferromagnet bilayers are studied on Co/IrMn and NiFe/NiO heterostructures using a magneto-optical indicator film technique. The statistical analysis proves a nonstochastic character of magnetization jumps. In particular, power-law behavior is observed for Co/IrMn samples. Furthermore, the statistics depends on the ferromagnet layer thickness and antiferromagnet layer material structure. The average jump size displays an asymmetry between the forward and backward branches of the hysteresis loop, particularly pronounced for the structure with a thin Co layer. In spite of the observation of such an asymmetry in the activity of the domain nucleation and pinning centers, the statistical distributions of jumps do not show any significant differences for two branches of the hysteresis loop. The conclusion on a nonrandom character of the magnetization process is supported by the multifractal analysis which reveals the presence of correlations in the time arrangement of the Barkhausen jumps.

Multiscale Analysis of Acoustic Emission during Plastic Flow of Al and Mg Alloys: From Microseconds to Minutes

Recent studies of plastic deformation using high-resolution experimental techniques bear witness that deformation processes are often characterized by collective effects emerging on an intermediate scale between the scales describing the dynamics of individual crystal defects or the macroscopic plastic flow. In particular, the acoustic emission (AE) reveals intermittency of plastic deformation in various experimental conditions, which is manifested by the property of scale invariance, a characteristic feature of self-organized phenomena. Some materials, e.g., Al or Mg alloys, display a macroscopic discontinuity of plastic flow due to the Portevin-Le Chatelier effect or twinning. These materials are therefore of special interest for the study of collective effects in plasticity. The present work reviews the results of a multiscale investigation of AE accompanying plastic deformation of such model alloys. The AE is analyzed by methods borrowed from the theory of nonlinear dynamical systems, including statistical and multifractal analyses.

Direct experimental study of the effect of dislocations on the magnetization reversal in a quasi-two-dimensional ferromagnet with unidirectional anisotropy

The effect of dislocations on the elementary acts of the magnetization reversal in the epitaxial heterostructure NiFe/NiO/MgO(001) has been studied using the magneto-optical indicator film technique. It has been found that the edge dislocations grouped along the 〈110〉 slip planes lead to the formation of quasi-one-dimensional domains in the permalloy film with the induced uniaxial anisotropy oriented along these planes the direction of which differs by 90° from that of the uniaxial anisotropy in the dislocation-free part of the heterostructure. The micromechanism of the observed effect has been discussed taking into account the effect of dislocations on the orientation of spins in the antiferromagnet and their exchange interaction with the spins of the ferromagnet on the interphase plane.

Chapter 8 Rotational hysteresis and chirality of the spin spiral structure in exchange-coupled heterostructures

Evolution of the net magnetization M of a nanocomposite magnetic bilayer consisting of exchange-coupled soft ferromagnetic/antiferromagnetic films under a rotating magnetic field H has been investigated using an advanced magnetooptic-indicator-film technique. The magnetization reversal of the ferromagnetic film was found to proceed via an exchange-spring formation due to twisting of the layers of the spin system. Two magnetization-reversal modes were found. At high fields, M rotates synchronously with the field. At low fields, there are stages of M and H rotation in either the same or opposite directions. The revealed features of the magnetization-reversal processes are compared with those for soft/hard ferromagnetic bilayers. The experimental data are discussed in terms of the hybrid spin-spring nucleation and evolution with allowance for the unidirectional anisotropy affected by interface imperfections and crystal-lattice defects.