Influence Of Shape Anisotropy Research Articles

Induced exchange bias in NiMn/CoFe multilayer thin films sputtered on a quartz substrate by field cooling

• CoFe/NiMn stack sputtered on a quartz substrate. • Influence of the annealing procedure on the exchange coupling. • Influence of the thickness of CoFe and NiMn on the exchange coupling. • Influence of shape anisotropy on the hysteresis loop.

Magnetisation configuration in arrays of permalloy rectangles and its impact on magnetisation reversal

The remanent domain structures of composite element magnetic barcodes have been imaged using photo-emission electron microscopy with contrast from x-ray magnetic circular dichroism (XMCD-PEEM) and analysed with reference to the results of micromagnetic simulations. The magnetisation configuration at the end of wide strips is found to be perpendicular to the majority magnetisation direction. This transitions to an incomplete rotation for nominal strip widths below 300 nm and is found to affect the mechanics of magnetisation reversal for nominal strip widths below 200 nm, owing to a difference in magnetisation orientation when an external magnetic field is applied that is just smaller than the magnetic coercivity of the structures and a corresponding change in reversal dynamics. This change in domain structure as strip width decreases is consistent with both the influence of shape anisotropy and with measurements of magnetic hysteresis. The magnetisation reversal characteristics of composite element structures are found to be dependent on the relative magnetisation configurations of neighbouring strips, which in turn are found to vary stochastically upon the application and removal of a magnetic field along the easy axis of the structure. It is found that the application of a canted field is necessary to ensure sharp, consistent magnetisation reversal of bits when writing a binary code. These results confirm that either improved lithography of narrower strips or non-rectangular elements would be necessary to further increase the number of individually programmable bits in a barcode.

Influence of Shape Anisotropy on the Emission of Low-Dimensional Semiconductors.

The emergence of precise and scalable synthetic methods for producing anisotropic semiconductor nanostructures provides opportunities to tune the photophysical properties of these particles beyond their band gaps, and to incorporate them into higher-order structures with macroscopic anisotropic responses to electric and optical fields. This perspective article discusses some of these opportunities in the context of colloidal semiconductor nanoplatelets, with a focus on the influence of confinement anisotropy on processes that dictate the emission.

Preparation and orientation mechanism analysis of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> magneto-optical single crystal film with out-of-plane orientation

Liquid-phase epitaxy (LPE) is one of the best techniques for the preparation of single crystal garnet films. However, the specific Faraday rotation angle of Yttrium iron garnet (YIG) is small, and its easy magnetization axis is parallel to the film surface. The YIG requires a large external saturation field, which cannot meet the development needs of magneto-optical devices. It is found that Bi-substituted YIG(Bi:YIG) film has a larger specific Faraday angle. By adjusting the easy magnetization axis of Bi: YIG perpendicular to the film surface, the saturation magnetization of Bi: YIG can be reduced, so that it can work under a small external magnetic field. This meets the development needs of miniaturization and energy saving of magneto-optical device. The saturation magnetization of garnet film can be effectively reduced by substituting Ga<sup>3+</sup> for YIG crystal, mainly for Fe<sup>3+</sup> at the 24d position of its tetrahedron. And the lattice constants of Gd<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub> (GGG) and YIG are 1.2383 nm and 1.2376 nm, respectively. However, the radius of Bi<sup>3+</sup> (10.8 nm) is larger than that of Y<sup>3+</sup> (9.0 nm), the lattice mismatch of garnet film increases with the incorporation of Bi<sup>3+</sup>. In order to neutralize the lattice expansion caused by Bi<sup>3+</sup>, Tm<sup>3+</sup> (8.69 nm) with a radius smaller than that of Y<sup>3+</sup> (9.0 nm) is selected. Based on the theoretical analysis of the magnetocrystalline anisotropy of garnet film, (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> mono-crystalline films with different growth temperatures and different thickness values are grown by LPE on GGG (111) substrates. The experimental results show that when the thickness of epitaxial film is greater than 1 μm, the influence of shape anisotropy on magnetocrystalline anisotropy can be ignored. With the increase of growth temperature, the substitution number of Bi<sup>3+</sup> ions decreases gradually, the lattice constant of epitaxial film decreases gradually, and the lattice mismatch first decreases and then increases. Then, the state of compressive stress gradually changes into that of tensile stress. Compared with growth-induced anisotropy, the stress-induced anisotropy is dominant in the change of magnetocrystalline anisotropy. The Verdet constant of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> film is 11.8 × 10<sup>4</sup> rad/Tm@1064 nm. The results show that the prepared (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> mono-crystalline films have great development potential in magneto-optical devices.

Peak in Angular Dependence of Coercivity in a Hexagonal Array of Permalloy Spherical Nanocaps

Micromagnetic simulations of coercivity as a function of external magnetic field direction were performed for a hexagonal array of hemispherical Permalloy nanocaps. The analysis was based on hysteresis loops for arrangements of nanocaps of variable thickness (5 nm and 10 nm). The angular dependence of coercivity had a maximum at about 80° with respect to the arrangement plane. An increase in coercivity with nanocap thickness is related to the magnetization reversal mechanism, where the dipole energy of individual caps generates an effective intermediate axis, locking the magnetic moments. The coercivity has maximum values of 109 Oe for 5 nm and 156 Oe for 10 nm thickness. The remanence decreases monotonically with angle. This is associated with the influence of shape anisotropy, where the demagnetizing field in the plane of the array is much smaller than the demagnetizing field perpendicular to the plane.

Influence of Shape Anisotropy and Temperature on Magnetostrictive Behavior in Single-Crystal Galfenol Alloys

Fe-Ga (Galfenol) is a rare-earth-free alloy with useful magnetostrictive properties of nominal cost and robust mechanical properties advantageous for sensing, actuating, and energy harvesting. Understanding magnetostriction and magnetization behaviors under stress while at temperatures up to +250 °C is needed to explore the potential of harsh environment applications for magnetostrictive non-contact sensors, for use in applications such as monitoring torque changes in engines and rotorcraft shafts. Galfenol thin patches have the potential to operate in these applications, but alloy performance at these temperatures while under stress has not been studied. Thus, in this paper, we investigate simultaneous effects of temperature and compressive stresses on magnetic and magnetostrictive behaviors of single crystal Fe-Ga samples at temperatures from +25 °C to +250 °C while under compressive stresses of 0, 15, and 23 MPa. The samples are also subjected to temperatures up to +250 °C and no stress to acquire simultaneous magnetostriction and magnetization values. Building upon this experimental data, this paper attempts to relate temperature dependence and shape anisotropy to determine the effect on magnetic susceptibility at elevated temperatures. Understanding the roles of temperature and compressive stress as well as shape anisotropy on magnetostrictive, Fe-Ga alloys can provide an advanced understanding of the material properties and thus facilitate their use in a wider range of applications.

Size and shape-tailored hydrothermal synthesis and characterization of nanocrystalline LaPO4:Eu3+ phosphor

A facile hydrothermal route has been demonstrated for the synthesis of Eu3+ doped LaPO4 with distinct and well-controlled morphologies (spherical and rod-like). Morphological control has been established by fine-tuning the pH of the precursor solution, and formation of different morphologies at different pH values has been explained by taking into consideration the chemical potentials of the various species present in the precursor solution. At constant pH of the mother liquor, an increase in the autoclaving temperature from 120°C to 140°C is seen to change the aspect ratio of the nanorods. A red shift in the charge transfer band is observed in the excitation spectra of the nanocrystalline samples (both nanorod and nanosphere) with respect to the bulk. A change in the (red/orange) peak intensity ratio in the photoluminescence emission spectra of bulk, spherical and rod-like nanocrystalline samples reveals that the host morphology plays a vital role in determining the local symmetry of Eu3+ in LaPO4 matrix. Decay-time measurement of the as-prepared morphologies at 620nm (for 5D0–7F2 transition) leads to a maximum value for nanorods (3.7ms), while the corresponding values for nanospheres and bulk samples are somewhat smaller (2.58ms and 2.99ms respectively). This clearly indicates the influence of shape anisotropy on the photoluminescence properties of the as-synthesized nanoarchitectures.

The adjustable anisotropy field in FeCoTiO/SiO2/FeCoTiO trilayer films by oblique sputtering and stripe patterning

A series of FeCoTiO thin films were deposited on Si (100) substrates using oblique sputtering and stripe patterning at the same time and the static and high frequency magnetic properties were studied in details. For the single-layered films, if the anisotropy fields induced by the two methods are in the same direction, the effective anisotropy field will be greatly enhanced, closed to 300 Oe. But if the two anisotropy fields are perpendicular to each other, there will be an opposite result. In the FM/NM/FM sandwich structures, the influence of shape anisotropy will be suppressed by the exchange coupling effect between the two FM layers. The resonance frequency and permeability are still above 3.5 GHz and 75 even the width of stripes change from 40 µm to 10 µm.

Influence of Shape Anisotropy on Magnetization Dynamics Driven by Spin Hall Effect

As the lateral dimension of spin Hall effect based magnetic random-access memory (SHE-RAM) devices is scaled down, shape anisotropy has varied influence on both the magnetic field and the current-driven switching characteristics. In this paper, we study such influences on elliptic film nanomagnets and theoretically investigate the switching characteristics for SHE-RAM element with in-plane magnetization. The analytical expressions for critical current density are presented and the results are compared with those obtained from macrospin and micromagnetic simulation. It is found that the key performance indicators for in-plane SHE-RAM, including thermal stability and spin torque efficiency, are highly geometry dependent and can be effectively improved by geometric design.

Focused particle beam-induced processing.

Focused particle beam-induced processing.

Influence of shape anisotropy on reversal process in coupled ellipsoidal magnetic dot array

The magnetization reversal of two configurations of linear magnetic ellipsoidal arrays has been studied by using the micromagnetic simulations. The effect of magnetostatic interaction on the switching field is investigated by altering aspect ratio of ellipsoids while boundary edge spacing is kept fixed. The simulation results showed that, when the field is parallel to the ellipsoid easy axis, the shifted switching field is more than the case when the field is parallel to the ellipsoid hard axis. However, in both configurations the shifted switching field accompany with the more relaxed magnetic pattern.

The influence of shape anisotropy on vortex nucleation in Pacman-like nanomagnets

In this paper we explore magnetic properties of Permalloy Pacman-like (PL) nanomagnets in external in-plain magnetic field. PL nanomagnets represent unique magnetic systems with broken symmetry, which are perspective as non-volatile memory elements. In these nanomagnets both bits, chirality and polarity of a single vortex state, can be easily read and written by in-plane magnetic field only. In the experimental part of this work we show that namely chirality of the∼1-μm large PL nanomagnet can be red easily by magnetic force microscopy method. The easy bit reading is enabled due to coupling of the polarity magnetization vector to the magnetic charges located at the surface of the PL missing sector. Using micromagnetic simulations we show the influence of spatial anisotropy on vortex nucleation and annihilation fields in the PL nanomagnets. Angular dependence of the vortex nucleation field is analysed in detail for PL nanomagnets of different diameter, thickness, and missing-sector dimensions. Best control of the ground state can be achieved for diameters not exceeding 100nm, thicknesses from 40 to 45nm, and for the missing sector angles from 30 to 60°.

The influence of shape anisotropy on the microstructure of magnetic dipolar particles

In the first part of the present contribution we study theoretically the ground states of magnetic rods and ellipsoids with point dipoles. We consider two different orientations of dipoles: the dipole moment oriented along the short axis of the anisotropic particle and the dipole moment aligned along the long axis. We show that depending on the particle asphericity the ground states of ellipsoids and rods might be a chain and a ring (daisy) in two dimensions, or a carpet and a bracelet in three dimensions. If the ellipsoids or rods with the dipole moment coaligned with the long axis are elongated enough, the head-to-tail configuration of moments becomes less favourable than the antiparallel side-by-side pair of dipoles. This structural crossover in the ground state drives crucial changes in the microstructure of systems of anisotropic particles when the thermal energy starts being comparable to that of the interparticle interaction. We investigate the latter regime in the second part of our manuscript and provide a detailed analysis of the interparticle correlations using molecular dynamics simulations. We extensively study the influence of aspect ratios and different dipolar strengths on the structural properties, internal pressure and initial susceptibility of anisotropic particles' systems. We conclude that the particle shape anisotropy could be effectively used as a control parameter for the system microstructure.

Non-destructive method for evaluation of deterioration of austenitic stainless steel using initial magnetic phase

We report a novel non-destructive evaluation method (NDE) that relies on the magnetic properties of the inherent ferromagnetic phase in austenitic stainless steel (SUS) parts to assess the state of damage. The effects of the volume fraction, internal stress, and shape anisotropy of the ferromagnetic phase on the electromagnetic impedance signal used in the NDE are discussed on the basis of experimental results for cold-rolled SUS 304 steel plates of different thicknesses with and without stress relief annealing. In particular, the influence of shape anisotropy is clarified on the basis of measurements conducted on 0.5-mm-thick samples both parallel and perpendicular to the rolling direction. The results of the measurements show that the parameters obtained by the proposed method are indicative of damage.

Effects of vortex chirality and shape anisotropy on magnetization reversal of Co nanorings (invited)

The effects of vortex chirality and shape anisotropy on the magnetization reversal of thin Co nanorings with and without slots have been investigated using off-axis electron holography and micromagnetic simulations. Stepped hysteresis loops were determined and typical well-defined states, including onion states, vortex states, flux-closure (FC) states, and omega (Ω) states were identified during reversal for different element shapes. The chirality of the vortex (vortices) formed after the onion states determined the switching mechanism. The Co nanorings and slotted rings with applied field parallel to the slots showed multistep switching via onion-FC-onion mode, also involving the formation and annihilation of vortex (vortices), whereas slotted rings with applied field perpendicular to the slots exhibited simple one-step switching by abrupt chirality reversal of the FC states. The influence of shape anisotropy on switching fields is also discussed in terms of demagnetization energy.

Influence of shape anisotropy on microwave complex permeability in carbonyl iron flakes/epoxy resin composites

To explore the mechanism of carbonyl iron flake composites for microwave complex permeability, this paper investigates the feature of the flakes. The shape anisotropy was certified by the results of the magnetization hysteresis loops and the Mossbauer spectra. Furthermore, the shape anisotropy was used to explain the origin of composite microwave performance, and the calculated results agree with the experiment. It is believed that the shape anisotropy dominates microwave complex permeability, and the natural resonance plays main role in flake.

Magnetization reversal in patterned ferromagnetic and exchange-biased nanostructures studied by neutron reflectivity (invited)

We have measured the off-specular polarized neutron reflectivity of periodic arrays of micron-sized rectangular polycrystalline ferromagnetic Co bars and exchange-biased Co∕CoO bars, which were prepared by a combination of electron-beam lithography and evaporation techniques. The intensity of the first-order off-specular neutron satellite reflection was monitored as function of the magnetic field parallel to the long edge of the bars, allowing analysis of the magnetization reversal process using the four spin-polarized scattering cross sections. The neutron data are compared with calculations based on a micromagnetic simulation. The influence of shape anisotropy on the reversal mechanism is demonstrated.

Influence of shape anisotropy on photoluminescence characteristics in LaPO 4:Eu nanowires

One dimensional LaPO 4:Eu nanowires with different ratio of length to width (10–30) were synthesized by hydrothermal method and studied. The results indicated that there existed two crystalline sites and their relative proportion changed with the variation of the ratio of length to width. It is significant to observe that luminescent quantum efficiency of Eu 3+ increased linearly with the increase of the ratio of length to width.

Irregular domain patterns in structured magnetic thick films

Domain structures in patterned magnetostrictive NixFe1−x films are observed by Kerr microscopy. The micrometer-thick films display different types of domain patterns varying with saturation magnetostriction. In negative magnetostrictive and tensile-stressed films, regular closure domains are replaced by edge curling walls seen for the first time in single layer films. The influence of shape anisotropy is strongly reduced. Small changes in material composition drastically alter the domain structures. For positive magnetostriction films, an “anisotropy structured” pattern is observed. Deviations from a regular Landau state are due to local effective magnetic anisotropy changes induced by stress relaxation at the sample edges.

Temperature dependence of magnetization reversal in magnetostrictive glass-coated amorphous microwires

Abstract The temperature dependence of the magnetic behaviour of Fe-based glass-coated amorphous microwires with positive and large magnetostriction constant (around 30 ppm at room temperature) has been studied in the temperature range from 2 to 390 K. For long enough microwires (about 10 mm), hysteresis loops are squared shaped denoting the existence of a single domain structure and the magnetization reversal by a single Barkhausen jump. In the case of short microwires (around 2 mm) drastic changes are observed at low temperature where such magnetic bistability is lost. This is interpreted considering the influence of shape anisotropy: for short wires the demagnetizing field becomes large enough to destroy the single domain structure. On the other hand, the changing magnetoelastic anisotropy induced by coating when reducing measuring temperature is also considered. Particular emphasis is additionally done to study the dependence of the switching field for magnetization reversal and that of its fluctuations on temperature and frequency of exciting field.