Reorientation Of Easy Axis Research Articles

Bifurcation structure and chaos in dynamics of nanomagnet coupled to Josephson junction.

Irregular easy axis reorientation features are observed in numerical simulations of the nanomagnet coupled to the Josephson junction. We study magnetization bifurcations and chaos that appear in this system due to the interplay of superconductivity and magnetism. The bifurcation structure of magnetization under the variation of Josephson to magnetic energy ratio as a control parameter demonstrates several precessional motions that are related to chaotic behavior and orbits with different periodicities in the ferromagnetic resonance region. The effect of an external periodic signal on the bifurcation structure is also investigated. The results demonstrate high-frequency modes of a periodic motion and a chaotic response near resonance. Far from the ferromagnetic resonance, we observe a quasiperiodic behavior. The obtained results explain the irregular reorientation of the easy axis and the transitions between different types of motion.

Reversible Switchability of Magnetic Anisotropy and Magnetodielectric Effect Induced by Intermolecular Motion

AbstractIntroducing magnetic switchability into artificial molecular machines is fascinating for precise control of magnetism via external stimuli. Herein, a field‐induced CoII single‐molecule magnet was found to exhibit the reversible switch of Jahn–Teller distortion near room temperature, along with thermal conformational motion of the 18‐crown‐6 rotor, which pulls the coordinated H2O to rotate through intermolecular hydrogen bonds and triggers a single‐crystal‐to‐single‐crystal phase transition with Twarm=282 K and Tcool=276 K. Interestingly, the molecular magnetic anisotropy probed by single‐crystal angular‐resolved magnetometry revealed the reorientation of easy axis by 14.6°. Moreover, ON/OFF negative magnetodielectric effects were respectively observed in the high‐/low‐temperature phase, which manifests the spin‐lattice interaction in the high‐temperature phase could be stronger, in accompanied by the hydrogen bonding between the rotating 18‐crown‐6 and the coordinated H2O.

Reversible Switchability of Magnetic Anisotropy and Magnetodielectric Effect Induced by Intermolecular Motion

Introducing magnetic switchability into artificial molecular machines is fascinating for precise control of magnetism via external stimuli. Herein, a field-induced CoII single-molecule magnet was found to exhibit the reversible switch of Jahn-Teller distortion near room temperature, along with thermal conformational motion of the 18-crown-6 rotor, which pulls the coordinated H2 O to rotate through intermolecular hydrogen bonds and triggers a single-crystal-to-single-crystal phase transition with Twarm =282 K and Tcool =276 K. Interestingly, the molecular magnetic anisotropy probed by single-crystal angular-resolved magnetometry revealed the reorientation of easy axis by 14.6°. Moreover, ON/OFF negative magnetodielectric effects were respectively observed in the high-/low-temperature phase, which manifests the spin-lattice interaction in the high-temperature phase could be stronger, in accompanied by the hydrogen bonding between the rotating 18-crown-6 and the coordinated H2 O.

Analytical magnetostatic model for 2D arrays of interacting magnetic nanowires and nanotubes.

A fully analytical model to describe the magnetostatic properties of these 2D nanocylinder arrays (tubes and wires) is presented. The model allows calculating the components of the effective demagnetizing field as a function of the cylinder height, inner and outer diameters, and the center-to-center distance. From these components, it is possible to calculate the shape anisotropy of the cylinder, the dipolar interaction between them, and the total magnetostatic energy. The model allows performing calculations very simply, using a simple spreadsheet or open-access software such as Geogebra. This allows analyzing the effect of each geometrical parameter in the different contributions to the magnetostatic energy. Amongst the most interesting findings is that the model describes naturally the magnetization easy-axis reorientation transition induced by the dipolar interaction, for which a general phase diagram has been calculated for both tubes and wires. For the case of nanowires, our results show a very good agreement with previously published results. While for nanotubes, the model predicts that the magnetization easy-axis reorientation transition is frustrated as the tube wall thickness decreases and reaches a critical value even when the distance between tubes is reduced to its lowest possible value.

The magnetoelectric effect of the CFO thin film by coupling a P(VDF-co-TrFE) piezoelectric layer

The CoFe2O4 (CFO) thin films are prepared by the sol-gel method combined with the spin-casting process, and the related SEM and XRD results show that a 9-layer spinning of the original sol together with an annealing temperature of 700 °C is the appropriate condition for obtaining this CFO film with an excellent magnetic performance. The CFO films are subsequently compounded with a ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-co-TrFE)]. After polarizing the obtained CFO/P(VDF-co-TrFE) double-layer composite film under a high electrical field of 275 MV/m, the P(VDF-co-TrFE) layer shows the character of normal ferroelectricity that has a large dielectric permittivity (ɛ33/ɛ0 = 9.5) and a high piezoelectric value (d31 = 20 pC/N). Interestingly, we also find that the magnetization of this electrically poled CFO/P(VDF-co-TrFE) film increases from 85.58 to 99.15 emu/cm3 at a magnetic field of 1.8 T since the electrostrictive effect may lead to the re-orientation of magnetic domains and thereby resulting in a magnetization easy-axis reorientation effect in the out-of-plane direction of the CFO thin film. Consequently, a large magnetoelectric coupling coefficient (αE31) of 508.5 MV/cm Oe is obtained through calculation provided the interfaces between CFO and P(VDF-co-TrFE) have an appropriate compatible coefficient of about 0.5. The results indicate that the CFO/P(VDF-co-TrFE) double-layer film is a potential multiferroic composite as an attractive candidate for memory storage and spintronic devices.

Orientational instability of nematic liquid crystal in a homeotropic cell with boundary conditions controlled by an electric field

ABSTRACTWe study the orientational instability of the director in a homeotropic nematic liquid crystal (NLC) cell in a DC electric field. The electric field is applied along or perpendicular to a cell surface depending on whether anisotropy of dielectric permittivity of NLC is positive or negative. The easy axis on one of the cell polymer substrates is allowed to deviate in perpendicular to the substrate plane due to the influence of the NLC and the electric field. It was established that the orientational instability of the director can have a threshold as well as be thresholdless which depends on the character of the coupling of the easy axis with the electric field. The temporal behaviour of a director and the easy axis during transition to the stationary state after turning on electric field and returning to the initial homogeneous state after turning off the field was investigated. In the case of the negative anisotropy of the static permittivity, the comparison of experimental and calculated time dependences of the easy axis reorientation angle let us to estimate values of the mobile easy axis viscosity coefficient and coupling parameter describing the coupling between the easy axis and the electric field.

Light- and electro- induced Fredericks transition in liquid crystal cells with photosensitive chalcogenide glasses surfaces As2S3

In this paper we represent light- and electro- induced Fredericks transition in liquid crystal cells with 5 CB on thin photosensitive chalcogenide glassy films As2S3 as command surfaces. Easy axis reorientation in a cell with nematic liquid crystals under the low intensity illumination with non-polarized beam (0.7…2mWt; 473nm) and decreasing of the Fredericks transition in dc field were observed. We suggest that easy axis reorientation with the optical illumination in the presence of a dc electric field is due to a light-induced desorption process on the photosensitive surface As2S3. We believe that the decreasing of the Fredericks transition voltage is caused by light-induced free charges in chalcogenide layer that discharge electrical double layer on the LC - chalcogenide interface. As a consequence, the energy of interaction between the liquid crystal molecules and the constant electric field near the surface overcome the anchoring energy, and, as a result, the reorientation of the director of LC in polar plane takes place in this case. From this point of view the utilization of the effects of light-induced anisotropy in inorganic materials, in particular, in chalcogenide glasses for photoalignment of LCs looks very promising.

Tunable High-Field Magnetization in Strongly Exchange-Coupled Freestanding Co/CoO Core/Shell Coaxial Nanowires.

The exchange bias properties of Co/CoO coaxial core/shell nanowires were investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. First, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting predefined magnetization states to the antiferromagnetic (AFM) shell, as corroborated by micromagnetic simulations. Second, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops attributed to the AFM easy axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, was thus experimentally and theoretically demonstrated.

Atomic-scale control of magnetic anisotropy via novel spin–orbit coupling effect in La2/3Sr1/3MnO3/SrIrO3 superlattices

Magnetic anisotropy (MA) is one of the most important material properties for modern spintronic devices. Conventional manipulation of the intrinsic MA, i.e., magnetocrystalline anisotropy (MCA), typically depends upon crystal symmetry. Extrinsic control over the MA is usually achieved by introducing shape anisotropy or exchange bias from another magnetically ordered material. Here we demonstrate a pathway to manipulate MA of 3d transition-metal oxides (TMOs) by digitally inserting nonmagnetic 5d TMOs with pronounced spin-orbit coupling (SOC). High-quality superlattices comprising ferromagnetic La2/3Sr1/3MnO3 (LSMO) and paramagnetic SrIrO3 (SIO) are synthesized with the precise control of thickness at the atomic scale. Magnetic easy-axis reorientation is observed by controlling the dimensionality of SIO, mediated through the emergence of a novel spin-orbit state within the nominally paramagnetic SIO.

Electric and magnetic fields in photoalignment of liquid crystals

AbstractThe results of investigation of slow surface dynamics (easy axis gliding), induced by secondary illumination of photoaligned substrates with linearly polarized UV or blue light in the presence of electric (magnetic) fields, are presented for the first time. The initial surface orientation of nematic liquid crystal (NLC) was provided, in accordance with the standard photoalignment technique, via preliminary UV treatment of the glass substrates coated by sulfonic azo‐dye SD1 film. The experiments fulfilled for the two geometries, A and B, corresponding to the normal (A) and parallel (B) orientation of fields relatively to liquid crystal (LC) layer, revealed different effects induced by electric (magnetic) fields. For geometry A, strong electric field applied, in combination with a linearly polarized blue light, to the planar layer in a sandwich‐like LC cell with two photoaligned substrates results in simultaneous reorientation of easy axis on both substrates. It demonstrates the possibility of an azimuthal rotation of monodomain planar samples of LCs. For geometry B, usage of “in‐plane” electric (magnetic) fields in the cell, composed of photoaligned and rubbed substrates, speeds up reorientation process of NLC easy axis respectively to pure light‐induced reorientation. It provides electric control of operating times in previously proposed optical rewritable technology.

The Temperature-sensitivity of a Critical Electric Field Induced Magnetic Easy-axis Reorientation Ferromagnetic/ferroelectric Layered Heterostructures

We have investigated the effects of temperature on the magnetic easy axis reorientation (EARs) in polycrystalline ferromagnetic (FM) nano-scale thin films, i.e. (001)-oriented cubic films such as Fe, Ni, CFO and (0001)-oriented hexagonal films such as Co, grown on ferroelectric (FE) substrates, for example PZT, BTO substrates. The model of FM/FE bilayered heterostructures has been applied to study the total free energy of the FM films. By minimizing this energy, we have shown that temperature has a significant effect on a critical electric field induced magnetic reorientation.

Dynamic polarization grating based on a dye-doped liquid crystal controllable by a single beam in a homeotropic-planar geometry

We present a dynamic polarization grating based on a dye-doped liquid crystal cell that is controllable by a single pump beam in a binary homeotropic-planar configuration produced through selective rubbing. Upon single pump beam irradiation, the azo dyes in the liquid-crystal (LC) layer diffuse and adsorb onto the planar LC-anchoring surface due to trans-cis photo-isomerization. It is found that the dynamic polarization grating effect results mainly from the photo-induced easy axis reorientation by the amount of dye molecules adsorbed on the planar LC-alignment surface in a single-beam control scheme. The initial LC-anchoring conditions and the dynamic behavior of the dye adsorption strongly influence the repetitive writing-erasing processes by the single pump beam.

Spin torque switching and scaling in synthetic antiferromagnet free layers with in-plane magnetization

We study the field-induced and the current-induced switching of synthetic antiferromagnet free layers with in-plane magnetization. In practical cases, the energy barrier relevant for the thermal stability depends mainly on the magneto-crystalline anisotropies of the layers. We derive the spin-flop and the direct overwrite fields and their dependence on the asymmetry of the thickness of the two layers of the SyF. This is used to explain why the SyF magnetizations are much more robust to external fields than the anisotropy would tell. We then calculate the spin-torque instability threshold current densities for the acoustical and the optical excitations of the SyF, taking into account that there are spin torques acting on the two layers of the SyF. The acoustical excitation has the lowest instability current. Based on these findings, we finally discuss the scaling of SyF free layers by combining thermal stability requirements and dielectric breakdown avoidance constraints. Scaling can be conducted with either the aim of minimizing the write current or with the aim of minimizing the free layer surface. An absolute minimum of write current can be reached for relatively large cells (166×166 nm2) provided a zero effective magnetization is induced by a proper choice of the free layer thicknesses. Depending on how close the technology can reliably approach to the thickness yielding the easy axis reorientation transition while keeping bulk damping, this current minimum could in principle be as low as 14 μA in the macrospin approximation. If in contrast one aims at small free layer surfaces, then the present CoFeB alloy-based technology cannot reach dimensions below 90×90 nm2.

Effects of polarization azimuth in dynamics of electrically assisted light-induced gliding of nematic liquid-crystal easy axis

We experimentally study the reorientation dynamics of the nematic liquid crystal easy axis at photoaligned azo-dye films under the combined action of in-plane electric field and linearly polarized reorienting UV light at varying polarization azimuth, ϕp. At non-zero values of the azimuth, ϕp≠0, as opposed to the case where the polarization vector of the light is parallel to the initial easy axis (ϕp=0), the easy axis reorientation was observed to be most pronounced outside the interelectrode gaps. In the regions between electrodes with non-vanishing electric field, it is found that the dynamics of reorientation slows down with ϕp and the sense of easy axis rotation is independent of the sign of ϕp. A generalized version of the phenomenological model that was previously developed to describe the electrically assisted light-induced gliding is applied to interpret the experimental data.

Electrically assisted light-induced azimuthal gliding of the nematic liquid-crystal easy axis on photoaligned substrates

We study azimuthal gliding of the easy axis that occurs in nematic liquid crystals brought in contact with the photoaligned substrate (initially irradiated azo-dye film) under the action of reorienting UV light combined with in-plane electric field. For irradiation with the linearly polarized light, dynamics of easy axis reorientation is found to be faster as compared to the case of nonpolarized light. Another effect is that it slows down with the initial irradiation dose used to prepare the azo-dye film. This effect is interpreted by using the previously suggested phenomenological model. We present the theoretical results computed by solving the torque balance equations of the model that agree very well with the experimental data.

Easy axis reorientation and magneto-crystalline anisotropic resistance of tensile strained (Ga,Mn)As films

We present a study of magnetic anisotropy by using magneto-transport and direct magnetization measurements on tensile strained (Ga,Mn)As films. The magnetic easy axis of the films is in-plane at low temperatures, while the easy axis flips to out-of-plane when temperature is raised or hole concentration is increased. This easy axis reorientation is explained qualitatively in a simple physical picture by Zener’s p–d model. In addition, the magneto-crystalline anisotropic resistance was also investigated experimentally and theoretically based on the single magnetic domain model. The dependence of sheet resistance on the angle between the magnetic field and [1 0 0] direction was measured. It is found that the magnetization vector M in the single-domain state deviates from the external magnetic field H direction at low magnetic field, while for high magnetic field, M continuously moves following the field direction, which leads to different resistivity function behaviors.

GAIN AND NOISE IN ERBIUM-DOPED FIBER AMPLIFIER (EDFA) - A RATE EQUATION APPROACH (REA)

We have investigated the effects of temperature on the magnetic easy axis reorientation (EARs) in polycrystalline ferromagnetic (FM) nano-scale thin films, i.e. (001)-oriented cubic films such as Fe, Ni, CFO and (0001)-oriented hexagonal films such as Co, grown on ferroelectric (FE) substrates, for example PZT, BTO substrates. The model of FM/FE bilayered heterostructures has been applied to study the total free energy of the FM films. By minimizing this energy, we have shown that temperature has a significant effect on a critical electric field induced magnetic reorientation.

Direct local observation of magnetic anisotropy in microscopic Ga1−xMnxAs dots

The effect of the patterning of a Ga1−xMnxAs (x=0.08) film into an array of microscopic dots on its magnetic properties is locally studied by means of scanning Hall probe microscopy. The measured stray field patterns indicate the presence of a single domain state with a uniform magnetization within the Ga1−xMnxAs dots. The magnetic anisotropy of the dot array is directly imaged, revealing a [100] easy axis in the as-grown sample and a [11¯0] easy axis orientation after annealing. In contrast to a temperature dependent anisotropy in the plain Ga1−xMnxAs film, no magnetic easy axis reorientation is observed within the experimentally accessible temperature range.

Induced easy-axis reorientation in Ni/Pd multilayers upon Ar sputtering pressure

We have studied the easy-axis reorientation of Ni/Pd multilayer with varying Ar sputtering pressure. All the Ni/Pd multilayers prepared by dc-magnetron sputtering at an Ar sputtering pressure of 2 mTorr show in-plane magnetic anisotropy. However, room-temperature perpendicular magnetic anisotropy was observed in Ni/Pd multilayers prepared at an Ar sputtering pressure of 7 mTorr. To understand the origin of the easy-axis reorientation from in-plane to out-of-plane with varying the sputtering pressure, the magnetoelastic anisotropy was quantitatively determined from delicate in situ stress and ex situ magnetostriction coefficient measurements. We have found that the observed easy-axis reorientation was ascribed to the enhancements of the surface anisotropy as well as the magnetoelastic anisotropy with increasing Ar sputtering pressure.

Shape induced in-plane magnetic anisotropy reorientation in epitaxial hexagonal close packed cobalt dots

The in-plane magnetic easy-axis reorientation was investigated for the arrays of (10⋅0) hcp Co rectangular dots 0.2 μm in width with a variable length from 3 to 5 μm perpendicular to the c axis. In the system, the magnitude of the shape anisotropy contribution is determined by a lateral aspect ratio of the dot, while the magnetocrystalline anisotropy contribution is temperature dependent. The torque curves for the dot arrays exhibited a characteristic fourfold symmetry due to the easy-axis reorientation. The reorientation angle from the initial easy axis was found to be dependent on the value of in-plane demagnetization factors as well as the temperature variable first and second magnetocrystalline anisotropy constants. The experimental results are in good agreement with the calculated effective magnetic anisotropy.