“Switching” of the nanodomain state of the frustrated ferromagnet-antiferromagnet system by an external magnetic field

The phenomenon of ??switching?? of the domain walls generated by frustrations in a two-layer ferromagnet-antiferromagnet nanostructure has been studied using numerical simulation methods. This phenomenon manifests itself in the fact that, as the magnetic field increases, the ferromagnetic layer divided into nanodomains by domain walls perpendicular to the layer plane becomes single-domain, and the antiferromagnetic layer that is uniform in weak fields is divided into 180° domains by the domain walls perpendicular to the layer. The structures of these domain walls have been calculated in various magnetic fields.     

Kinetics of magnetization reversal in a thin La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite film

The kinetics of magnetization reversal of a thin LSMO film has been studied for the first time. It is shown that the magnetic domain structure critically depends on the conditions of structure formation. In the demagnetized state (after zero-field cooling from T _c ), a maze-like domain microstructure with perpendicular magnetization is formed in the film. However, after field cooling and/or saturating magnetization by a field of arbitrary orientation, the [110] direction of spontaneous magnetization in the film plane is stabilized; this pattern corresponds to macrodomains with in-plane magnetization. Further film magnetization reversal (both quasi-static and pulsed) from this state is implemented via nucleation and motion of 180° “head-to-head” domain walls. Upon pulse magnetization reversal, the walls “jump” at a distance proportional to the applied field strength and then undergo thermally activated drift. All dynamic characterisitcs critically depend on the temperature when the latter varies around the room temperature.     

Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.     

Shape instability in out of equilibrium magnetic domains observed in ultrathin magnetic films with perpendicular anisotropy

The magnetic configurations induced by the growth process in a thin film with perpendicular magnetisation have been observed by magnetic force microscopy (MFM). The FePd thin film has been grown by molecular beam epitaxy. A high uniaxial chemical ordering of the alloy into the tetragonal L1₀ structure induces the development of a large perpendicular anisotropy. As the growth process is performed below the Curie temperature of the FePd alloy, domain nucleation occurs during the growth process. The magnetic configuration has been imaged in the as grown state. As the equilibrium size of the magnetic domains decreases when the thickness of the layer increases, the domains obtained from spontaneous nucleation at the beginning of the growth of the thin film are submitted to very large strains as the layer thickness increases. At low thicknesses (low strains), the domain wall instability gives rise to an undulation of the domain walls. Thereafter, it leads to the formation of well-defined magnetic fingers, thus giving birth to the coexistence of two length scale in the domain structure. A quantitative estimation of the strain leading to the fingering instability is obtained. Last, the implications of these observations on the kinetic of domain wall distortion in ultrathin layers are discussed.     

The role of the wide-band-gap polymer layer for the existence of conductivity switching in the valve structure

The effect of electronic switching of the conductivity by a magnetic field lower than 500 mT has been implemented in the spin-valve structure in the “current perpendicular to plane” geometry with a transport layer of a wide-band-gap polymer to 1200 nm thick. The problem of spin transfer through the polymer layer has been discussed in the model of ballistic charge transport over nanoscale conducting channels.     

Individual domain wall resistance in submicron ferromagnetic structures

The resistance generated by individual domain walls is measured in a FePd nanostructure. Combining transport and magnetic imaging measurements, the intrinsic domain wall resistance is quantified. It is found positive and of a magnitude consistent with that predicted by models based on spin scattering effects within the walls. This magnetoresistance at a nanometer scale allows a direct counting of the number of walls inside the nanostructure. The effect is then used to measure changes in the magnetic configuration of submicron stripes under application of a magnetic field.     

Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy

While magnetoresistance (MR) has generally been found to be symmetric in applied field in nonmagnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. The antisymmetric MR and extraordinary Hall effect have been quantitatively accounted for by a theoretical model.     

Domain structure in magnetic films with a saturation magnetization higher than the anisotropy field

The domain structure (DS) of yttrium-iron garnet films with uniaxial anisotropy fields higher than ∼120 Oe was found to have a 3D character: there is a stripe domain structure of a certain type in the surface layer and a structure of another type in the film bulk. It was revealed that in the absence of an external magnetic field, the boundaries of both DSs are almost perpendicular, whereas with an increase in an external field applied in the film plane along the boundaries of the interior-volume DS, the boundaries of the surface DS are gradually reoriented along the external field. This phenomenon is theoretically explained on the basis of the micromagnetic model, which describes DS formation in ferrite films.     

Empirical relation between the Hall voltage and domain wall position in a Hall bar geometry

The anomalous Hall effect has been used as a versatile tool for the measurement of various transport phenomena in magnetic systems, particularly those with perpendicular magnetic anisotropy. The anomalous Hall voltage responds not only to the magnetization state but also to the position of magnetic domain walls when the magnetic domain passes through the Hall bar structure. In this study, an empirical relation was developed between the Hall voltage and domain wall position in the Hall bar geometry. This relation was first developed by numerical simulations and then, confirmed by analytical formulae. The validity of the empirical relation was finally verified by experimental results. The present empirical relation provides an experimental method for the electric detection of the position of magnetic domain walls.     

Effect of piezomagnetism on coupling of electric and magnetic domain walls in hexagonal manganites

The profiles of antiferromagnetic domain walls in hexagonal manganites RMnO₃ are obtained numerically depending on anisotropy and internal strain due to the lattice distortion at the ferroelectric domain walls. It is found that the piezomagnetism can lower the free energy of the system thus it favors the coupling between electric and magnetic domain walls. Due to the piezomagnetic effect, the clamped antiferromagnetic domain walls with spin orientation angle ψ changing from 0 to π have different profiles comparing with those of ψ changing from 0 to -π, and the former is energetically more favorable than the latter when the internal strain is tensile at the FEL domain walls while it is the contrary for compressive strain. Moreover, the strongest coupling between the FEL domain walls and the favorable AFM domain walls can be achieved at an optimized internal strain.     

Perpendicular surface magnetic anisotropy in an amorphous ferromagnet induced by adsorption occurring through the mechanism of the formation of hydrogen bonds

It has been found that the domain structures of amorphous magnetically soft iron-based ferromagnetic ribbons in an atmosphere of water vapor and methyl alcohol are significantly different. In the former case, several domains separated by domain walls oriented at an angle of 30°–40° to the long side of the sample are observed. In the latter case, two domains separated by one wall located in the middle of the sample in parallel to its long side are observed. In the former case, the normal component of the magnetization on the sample surface has been detected using the polar magnetooptical Kerr effect. In the latter case, the normal component of the magnetization is almost absent. The observed effects are reversible. The normal component of the magnetization is induced by the desorption of water and methyl alcohol molecules, which are absorbed through the mechanism of the formation of hydrogen bonds, from the sample surface. According to the performed estimate, the effective field of the perpendicular magnetic anisotropy reaches a value of 1.6 kA/m.     

Electrical resistivity due to electron scattering with magnetic domain walls and magnetic properties of Ni-Fe alloy thin films

The physical properties of magnetic domain walls and electrical conductivity of permalloy thin films under external magnetic fields were studied. Using a magnetic force microscope (MFM), we observed the variation of domain configurations with the change of applied magnetic field for different film thicknesses of 245, 320, and 415 nm. A superconducting quantum interference device (SQUID) was exploited to measure the magnetization loop for the applied magnetic field either parallel or perpendicular to the normal direction of the surface. We also found that the resistivity increases significantly as the electrical current conduction changed from parallel to perpendicular to the domain walls.     

Atomic force microscopy study of Gd(Co,Cu)<Subscript>5</Subscript> single-crystal surface relief

A comprehensive study of the magnetic properties, magnetic viscosity, and microstructure of Gd(Co,Cu)₅ intermetallic compounds has been carried out using vibrating-coil magnetometry and atomic force microscopy. High-quality images of liquation inhomogeneities at the basal planes of Gd(Co_{1 − x} Cu _x )₅ single crystals with x = 0.1−0.6 have been obtained. Their size distribution curves have been plotted, and a correlation between the size of the structure inhomogeneities and the effective width of domain walls has been established.     

Spontaneous edge <Emphasis Type="Italic">E</Emphasis> × <Emphasis Type="Italic">B</Emphasis> sheared flow development studies in the TJ-II stellarator

The influence of plasma density and edge gradients in the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. It has been experimentally observed that the generation of spontaneous perpendicular sheared flow (i.e. the naturally occurring shear layer) requires a minimum plasma density or gradient. It has been found that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increase. The resulting shearing rate is close to the one required to trigger a transition to improved confinement regimes with reduction of edge turbulence, showing that spontaneous sheared flows and fluctuations keep themselves near marginal stability. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Tarragona, Spain, July 3–4, 2005.     

Domain walls in antiferromagnetically coupled multilayer films

We report experimentally observed magnetic domain-wall structures in antiferromagnetically coupled multilayer films with perpendicular anisotropy. Our studies reveal a first-order phase transition from domain walls with no net moment to domain walls with ferromagnetic cores. The transition originates from the competition between dipolar and exchange energies, which we tune by means of layer thickness. Although observed in a synthetic antiferromagnetic system, such domain-wall structures may be expected to occur in A-type antiferromagnets with anisotropic exchange coupling.     

Magnetic phase diagram of the spin-valve structure with an antiferromagnetic oxide layer

The magnetic phase diagram of a three-layer ferromagnet-antiferromagnet-ferromagnet structure with an antiferromagnetic oxide layer of variable thickness has been investigated. It has been predicted that there are three ranges of thicknesses in which domain walls created by the frustration of the exchange interaction between the layers have different structures. The structure of these domain walls induced by edges of atomic steps at the interfaces between the layers has been described. The predictions of the theory have been compared with the available experimental results, in particular, with the data indicating the existence of the nanodomain state.     

Two-dimensional micromagnetic simulation of domain structures in films with combined anisotropy

The transformation of the domain structure of micrometer-thick films with variations in the induced uniaxial anisotropy constant with the easy magnetization axis perpendicular to the film surface has been investigated using numerical micromagnetic simulation in the framework of a two-dimensional model of the magnetization distribution. The case where the tetra-axial crystallographic anisotropy exists in the film with uniaxial magnetic anisotropy has been considered. The transformation of the open domain structure into the structure with a magnetic flux closed inside the sample has been investigated in detail, and new types of 109-degree and 90-degree vortex-like domain walls and periodic domain structures have been obtained.     

Recovery of soft magnetic properties of FeNiSm films by Ta interlayer

The magnetic properties of FeNiSm thin films with different thicknesses, different Ta interlayer thicknesses and different numbers of Ta interlayers were investigated. The single layer FeNiSm shows in-plane uniaxial anisotropy at a thickness below critical value, but shows weak perpendicular anisotropy with a stripe domain structure at thickness above the critical value. Experiments indicate that one or more Ta interlayers inserted into thick FeNiSm films with weak perpendicular anisotropy were effective not only in canceling the perpendicular anisotropy, but also in recovering the in-plane uniaxial anisotropy. Blocking of the columnar growth of FeNi grains by the Ta interlayer is considered to be responsible for this spin reorientation phenomenon. Moreover, the magnetization reversal mechanism in FeNiSm films with uniaxial anisotropy can be ascribed to coherent rotation when the applied field is close to the hard axis and to domain-wall unpinning when the applied field is close to the easy axis. The dynamic magnetic properties of FeNiSm films with uniaxial anisotropy were investigated in the frequency range 0.1-5 GHz. The degradation of the soft magnetic properties of magnetic thin films due to the growth of columnar grains can be avoided by insertion of a Ta interlayer.     

Field-induced phase transitions in magnetooptical films with a small positive anisotropy constant

We study phase transitions induced by a static magnetic field in magnetically uniaxial films with a small positive anisotropy constant. The phase diagram of these objects is determined in the H _∥-H _⊥ plane, where H _∥ and H _⊥ are, respectively, the components of the magnetizing field along and perpendicular to the surface normal. The stability boundary is located for all of the main types of domain configurations observed: a simple stripe domain structure, a stripe domain structure with periodic bending by surface distortions in the profile of the domain walls, and hexagonal lattices of cylindrical magnetic bubbles. Zh. éksp. Teor. Fiz. 111, 283–297 (January 1997)     

Features of magnetization reversal in LCMO/YBCO heterostructures

The change in the magnetic domain structure due to the proximity of a superconductor has been experimentally investigated for the first time. The complex character of magnetization reversal at temperatures below critical, caused by the mutual long-range effect of a superconductor and a magnet, has been shown. In particular, it is found that even magnetization reversal of the heterostructure by an in-plane field leads to the formation of Abrikosov vortices in the superconductor, carrying a flux perpendicularly to the film plane. It is shown that this is a consequence of the transformation of narrow domain walls into wide stripes due to the interaction with scattering fields from the superconductor. In turn, after penetration of the magnetic flux into the superconductor at some depth, the scattering fields cause backward magnetization reversal of the external film edge, as a result of which vortices with oppositely directed fluxes enter the crystal and propagate in the superconductor bulk in the form of chains along twins, as in the case of magnetization by a perpendicular magnetic field. Thus, at longitudinal magnetization, the flux enters the superconducting film in the form of wide stripes with alternating perpendicular induction, which is explained by the long-range interaction of the scattering fields of the superconductor with the manganite magnetization.