In-plane Domain Structure Research Articles

Spin–wave dynamics in perpendicularly magnetized antidot multilayers

Using all-optical time-resolved magneto-optical Kerr effect measurements we demonstrate an efficient modulation of the spin–wave (SW) dynamics via the bias magnetic field orientation around nanoscale diamond shaped antidots that are arranged on a square lattice within a [Co(0.75 nm)/Pd(0.9 nm)]8 multilayer with perpendicular magnetic anisotropy (PMA). Micromagnetic modeling of the experimental results reveals that the SW modes in the lower frequency regime are related to narrow shell regions around the antidots, where in-plane (IP) domain structures are formed due to the reduced PMA, caused by Ga+ ion irradiation during the focused ion beam milling process of antidot fabrication. The IP direction of the shell magnetization undergoes a striking change with magnetic field orientation, leading to the sharp variation of the edge localized (shell) SW modes. Nevertheless, the coupling between such edge localized and bulk SWs for different orientations of bias field in PMA systems gives rise to interesting Physics and attests to new prospects for developing energy efficient and hybrid-system-based next-generation nanoscale magnonic devices.

Out-of-plane polarization reversal and changes in in-plane ferroelectric and ferromagnetic domains of multiferroic BiFe0.9Co0.1O3 thin films by water printing

BiFe0.9Co0.1O3 is a promising material for an ultra-low-power-consumption nonvolatile magnetic memory device because local magnetization reversal is possible through application of an electric field. Here, changes in ferroelectric and ferromagnetic domain structures in a multiferroic BiFe0.9Co0.1O3 thin film induced by “water printing”, which is a polarization reversal method involving chemical bonding and charge accumulation at the interface between the liquid and the film, was investigated. Water printing using pure water with pH = 6.2 resulted in an out-of-plane polarization reversal from upward to downward. The in-plane domain structure remained unchanged after the water printing process, indicating that 71° switching was achieved in 88.4% of the observation area. However, magnetization reversal was observed in only 50.1% of the area, indicating a loss of correlation between the ferroelectric and magnetic domains because of the slow polarization reversal due to nucleation growth.

High-frequency spin torque oscillation in orthogonal magnetization disks with strong biquadratic magnetic coupling

In this study, we numerically investigate the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a strong biquadratic magnetic coupling. The orthogonal configuration consists of top and bottom layers with in-plane and perpendicular magnetic anisotropy sandwiching a nonmagnetic spacer. The advantage of an orthogonal configuration is the high efficiency of spin transfer torque leading a high STO frequency; however, maintaining the STO in a wide range of electric current is challenging. By introducing biquadratic magnetic coupling into the orthogonal structure of FePt/spacer/Co90Fe10, Ni80Fe20 or Ni, we were able to expand the electric current region in which the stable STO is realized, resulting in a relatively high STO frequency. For example, approximately 50 GHz can be achieved in an Ni layer at a current density of 5.5 × 107 A/cm2. In addition, we investigated two types of initial magnetic state: out-of-plane and in-plane magnetic saturation; this leads to a vortex and an in-plane magnetic domain structure after relaxation, respectively. The transient time before the stable STO was reduced to between 0.5 and 1.8 ns by changing the initial state from out-of-plane to in-plane.

In-plane magnetic anisotropy and domain structure of FeNi films deposited in the presence of a magnetic field of various configurations

FeNi films in circular shape with 3 mm in diameter obtained by magnetron sputtering in the presence of a constant magnetic field of various configurations are studied. The presence of a field created by a permanent magnet with a diameter of 1.2 mm, the magnetization of which is oriented perpendicular to the plane of the substrate, leads to the formation of a vortex-like magnetic structure in the film, characterized by diffuse domain walls. Keywords: thin magnetic films, induced magnetic anisotropy, magnetic domain structure, magnetron sputtering.

Film thickness dependence of in-plane ferroelastic domain structure in constrained tetragonal PbTiO3 films induced by isotropic tensile strain

In this study, the ferroelectric phase of PbTiO3 (PTO) thin films was grown on cubic single-crystal KTaO3 (KTO) substrates using metal–organic chemical vapor deposition. X-ray diffraction (XRD) was used to reveal an a1/a2 domain structure, which remained unchanged down to a film thickness of 2 nm. The a1 and a2 polydomains do not have a simple tetragonal symmetry because aa∥ and aa⊥ do not have the same values. The crystallographic tilt angle, α, is defined based on the rotation angle of the PTO lattice with respect to the cubic phase of KTO substrates. The in-plane tetragonal distortion (ca∥/aa∥) and α decreased with the decrease in the film thickness, following the in-plane tetragonal geometric equation: α=tan−1(ca∥aa∥)−45°. The isotropic tensile strains induced in-plane polarization directions along the [100] and [010] axes of the substrates. These axes are formed via the a1/a2 polydomain of the tetragonal-like phase. Moreover, synchrotron in-plane grazing incidence XRD and piezoelectric force microscopy were used to reveal the thickness dependency of the periodic domain width of the ferroelastic a1/a2 domain. The periodic domain width in the PTO films decreased, following Kittel's law, with the reduction in the film thickness.

Magnetoelectric properties of three-layered composite thin film fabricated by pulsed laser deposition

Magnetostriction-piezoelectric composite films present promising application prospects due to the presence of the magnetoelectric effect. In this paper, a sandwich structure Bi4Ti3O12/CoFe2O4/Bi4Ti3O12 composite film was prepared in a pulsed laser deposition system. The morphology, EDX, XPS, ferroelectric, piezoelectric, ferromagnetic and magnetoelectric properties were investigated in detail. In the sample, the atomic percentages of Bi, Ti and O elements measured by EDX are 35.373%, 12.824% and 46.578%, respectively. The out-of-plane electric domain structure and the in-plane magnetic domain structure show that BTO and CFO phases have excellent ferroelectricity (Pr = 1.57 μC/cm−2) and ferromagnetism (Mr = 104.83 emu/cc), respectively. The piezoelectric properties are proportional to the ferroelectric properties, so the composite thin films also have good piezoelectric properties (d33max = 192 p.m./v). Moreover, a great magnetoelectric coupling coefficient (∼11.95 mV/cm·Oe) was obtained by mechanical strain transfer and interface coupling. By adding a ferroelectric layer to the composite thin film, the clamping effect can be reduced and the coupling between the ferroelectric and the ferromagnetic layers can be increased, providing the experimental basis for the research of magnetoelectric coupling effect.

Growth of Metastable α-Ga<sub>2</sub>O<sub>3</sub> Epitaxial Thin Film on Flexible Synthetic Mica by Insertion α-Fe<sub>2</sub>O<sub>3</sub> Buffer Layer

Metastable α-Ga2O3 thin films with a corundum structure were grown epitaxially on flexible synthetic mica. The κ-Ga2O3 thin films were grown in a temperature range of 450‒600 °C, without a buffer layer. In contrast, the α-Ga2O3 thin films were grown in a wider temperature range of 350‒600 °C, by inserting corundum-structured α-Fe2O3 buffer layers. X-ray diffraction (XRD) rocking curve measurements revealed that the α-Ga2O3 thin film grown at 425 °C had the highest degree of crystallinity. Cross-sectional transmission electron microscopy (TEM) observations and an XRD φ scan revealed that the epitaxial relationship between the thin film and the substrate via the buffer layer was (0001) α-Ga2O3 [11‒20] || (001) synthetic mica [100]. Furthermore, when TEM observation was performed close to the surface of the α-Ga2O3 thin film at a high magnification, a lattice image derived from the out-of-plane (0001)-plane orientation was observed. However, it was also revealed that the α-Ga2O3 thin films did not have a single crystal structure but rather an in-plane domain structure. By conducting selected area electron diffraction (SAED) at the interface area, it was determined that the α-Fe2O3 buffer layer was polycrystalline. This implies that α-Ga2O3 thin films were epitaxially grown while forming the in-plane domains on the polycrystalline buffer layers. The results of this study indicate the flexible applications of α-Ga2O3 thin films, which have significant potential for use as power sources and deep-ultraviolet devices.

Multiple order spin-wave resonance in composition gradient sputtering FeCoB thin films

FeCoB films with different B doping contents and different thicknesses were deposited by composition gradient sputtering. The results show that in-plane anisotropy fields and exchange constants change with the increasing B contents and increasing thickness, respectively. Both results of the composition-gradient films and the thickness-dependent films and the micromagnetic simulation indicate that multiple order spin-wave resonances are easy to obtain in the films with the large in-plane anisotropy field. We observed four resonance peaks including three perpendicular standing spin waves. The hysteresis loop and magnetic domain results indicate that such films also have good magnetic softness and an in-plane homogeneous domain structure.

Preparation of epitaxial yttria-stabilized zirconia with non-quilibrium t' phase and ferroelastic domain by pulsed laser deposition

The non-equilibrium tetragonal t' phase of yttria-stabilizedzirconia (YSZ) plays a key role in improving the toughness of YSZ ceramics owing to its own ferroelastic domain toughening mechanism. Due to the disadvantages of traditional processes for preparing the YSZ t' phase, which requires high heating temperature, difficult to control defects and inability to regulate microstructure, it is not conducive to the preparation of YSZ by these methods. We report the use of substrate clamping and non-equilibrium growth process of PLD to render stable t' epitaxial YSZ films with multiple domain structure on (0001) Al2O3 substrates. Thin film structure and epitaxial relationship with substrate were subsequently analyzed by XRD, RSM, and HRTEM. It was found that the crystal orientation of the YSZ film changes as a function of oxygen pressure during deposition. When oxygenpressure is larger than 100 mtorr, YSZ films with highly oriented ferroelastic domains are obtained. Based on the symmetry relationship, we know that the film has three in-plane domain structures. The epitaxial relationship between the film and the substrate is (002) [110] YSZ // (0001) [112¯0] Al2O3.

Ferroelastically protected polarization switching pathways to control electrical conductivity in strain-graded ferroelectric nanoplates

Charged domain walls have rarely been observed in ferroelectric materials because of instability by large electrostatic energy. In this work, the authors have stabilized charged domain structures by ferroelastic clamping and manipulated resultant electric conductance in ferroelectric nanomaterials. Ferroelectric nanoplates subjected to compressive misfit strain at the bottom but less external stress on the sidewalls exhibit a radial-quadrant in-plane ferroelectric domain structure that is electrostatically unstable but emerges due to the flexoelectric effect associated with the misfit strain relaxation. They report that significant electronic conduction occurs near the edge of a ferroelectric nanoplate and the enhanced electronic conduction can be suppressed by 180${}^{\ensuremath{\circ}}$ polarization switching in a reversible way.

Ferromagnetic Fe2O3 nanopatterns prepared using dip-pen lithography

Ferromagnetic Fe2O3 nanorings and nanorods were deposited on an SiO2/Si substrate by dip-pen lithography using an iron nitrate Fe(NO3)3·9H2O solution. Magnetic force microscopy (MFM) studies have shown that ferromagnetic Fe2O3 nanorings and nanorods have in-plane, oriented, and magnetic domain structures. In particular, nanorings have the vortex magnetization state, not the onion state. Their small-size fabrication and stable magnetic properties may suggest future applicability for next-generation magnetic media memory devices.

Stabilization of metastable ɛ-Fe2O3 thin films using a GaFeO3 buffer

This paper reports a simple and robust route to stabilize metastable ɛ-Fe2O3 as an epitaxial thin film using a GaFeO3 (GFO) buffer in pulsed laser deposition. The use of an isostructural GFO buffer widens the range of film growth, allowing for stabilization on a range of substrates even with different surface symmetries, such as Al2O3 (0001), SrTiO3 (111), and Y:ZrO2 (100). All films grown on these substrates were c-axis oriented with the characteristic in-plane domain structures. Magnetization of these buffered thin films showed considerable magnetic anisotropy at 350 K. In particular, in-plane magnetization was found to be relatively larger in the films grown on the Y:ZrO2 (100) compared to the other cases. Heterostructuring ɛ-Fe2O3 on GFO is promising in the applications of ε-Fe2O3 for magnetic and multiferroic devices in that it provides a larger growth window and an effective means to optimize the film properties beyond the simple substitution of Fe with Ga.

Modulating Magnetic Properties by Tailoring In-Plane Domain Structures in Hexagonal YMnO3 Films.

Periodic structures and the coupling of multiorder parameters in complex oxides heterojunctions can generate exotic properties, of interest both for fundamental researches and for device applications. Here, we report a self-assembling in-plane periodic domain structure, and the resulting rich magnetic states, in a h-YMnO3 thin film fabricated on c-face sapphire substrate. Detailed structural investigations at atomic-level reveal the fashion of alternating domains under tensile or compressive strains separated by a boundary region. Tuned by this in-plane domain structure, the abnormal magnetic properties, such as the ferromagnetic enhancement and the unexpected spin glass state (below ∼38 K), are realized. Moreover, the existence of ferroelectric polarization is confirmed by scanning transmission electron microscopy, which brings in the chances of magnetoelectric coupling effect. These results manifest the close connections between the magnetic properties and such in-plane microstructures, suggesting the possibility of tuning the coupling effects via strain engineering in the hexagonal manganite film.

Effect of lanthanum doping on tetragonal-likeBiFeO3with mixed-phase domain structures

The recent discoveries of both chemical-driven and strain-driven morphotropic phase boundaries (MPBs) in $\mathrm{BiFe}{\mathrm{O}}_{3}$ (BFO) thin films have opened up new horizons in developing high-performance lead-free piezoelectrics. An attempt to bridge these two MPBs is made by doping La into highly strained BFO thin films with the coexistence of tetragonal-like and rhombohedral-like phases. The structural, morphological, and ferroelectric properties of such films are investigated. It is observed that La doping changes the energy landscape between the tetragonal-like and the rhombohedral-like polymorphs due to the chemical pressure imposed by the La substitution. Polar instability is found upon increasing La doping for the in-plane polarization component, which correlates with the vanishing of in-plane ferroelectric domain structures. The transition sequence of the in-plane ferroelectric polarization resembles that previously reported for the bulklike rhombohedral phase of BFO under continuous La doping, indicating the universality of the chemical-alloying effect on the ferroelectric order.

ハードディスクドライブ用磁気記録ヘッドにおける磁化挙動解析手法の実際と将来展望

With the increase in the recording density of hard disk drives, the narrower magnetic pole and read-sensor widths in writing and reading heads are required, and techniques used for the magnetic behavior analysis in a nanoscale area is essential. The observation of the in-plane domain structure of the writer pole for a perpendicular recording (PMR) head was performed by electron holography in order to elucidate the mechanism of the pole erasure originating from the instability of the magnetic domain state. The results revealed that the stability of the domain structure is strongly related to the domain wall trapping, and the pole erasure can be suppressed by realizing the stable domain structures. With respect to a current perpendicular to plane (CPP)-giant magneto-resistive (GMR) head, which has been promising candidates of a next-generation reading head element, the insertion of non-magnetic materials, such as Cu, between the ferromagnetic pinned and free layers is known to increase the MR ratio. For realizing the practical use of the CPP-GMR head, effects of the insertion of materials on the increase in the MR ratio were investigated by X-ray magnetic circular dichroism (XMCD). The results revealed that XMCD is a powerful technique to obtain information on electronic states and magnetic moments with the help of the theoretical electronic band calculation. The potential application of secondary cantilever resonance magnetic force microscopy and spin-polarized scanning electron microscope to the in-situ magnetic imaging of PMR heads will be also discussed.

Effects of electron irradiation on the ferroelectric 180° in-plane nanostripe domain structure in a thin film prepared from a bulk single crystal of BaTiO3 by focused ion beam

Effects of electron irradiation on the ferroelectric 180° in-plane nanostripe domain structure in a thin film prepared from a bulk single crystal of BaTiO3 by focused ion beam were studied. The domain structure transformed into a characteristic 90° in-plane nanostripe domain structure under intense electron irradiation. In particular, an unconventional triangular 90° in-plane nanostripe domain structure was observed. Polarization analysis suggests the existence of an incomplete or half tetradomain vortex at the boundary of the triangular domain structure. Together with the help of phase-field simulations using time-dependent Ginzburg–Landau equations, it is suggested that such a domain structure is created by an anisotropic in-plane electric field, which is plausibly induced by an anisotropic interaction of the incident electron beam with the ferroelectric material.

Epitaxial Structure of (001)- and (111)-Oriented Perovskite Ferrate Films Grown by Pulsed-Laser Deposition.

We report epitaxial growth and structures of SrFeO2.5 (SFO) films on SrTiO3 (STO) (001) and (111) substrates by pulsed-laser deposition. Reflection high-energy electron diffraction intensity oscillations were observed during the initial growth on both substrates, reflecting a layer-by-layer growth mode of the formula unit cell. It was found that the films were stabilized with a monoclinic structure that was derived from the original orthorhombic structure of bulk Brownmillerite. Using an X-ray reciprocal space mapping technique, in-plane domain structures and the orientation relationship were investigated. In addition, the impact of laser spot area on the epitaxial structures was studied. For the films grown on the (001) STO, the orientation relationship was robust against the change of the laser spot area: SFO(001)//STO(001) and SFO(100)//STO(100) for the out-of-plane and the in-plane, respectively, with the [001] axis tilted toward the 4-fold a- and b-axes by ∼1.4°, whereas nearly (111)-oriented films were obtained on the (111) STO, exhibiting a complicated manner of tilting that depended on laser spot area. The observed variation in tilting configurations can be understood in terms of possible atomic arrangements at the SFO/STO interface. These results present a guide to control the heteroepitaxial growth and structure of (111)-oriented noncubic perovskites.

Electron holography observation of in-plane domain structure in writer pole for perpendicular recording heads

A direct observation of the in-plane domain structure of the writer pole of a perpendicular recording head was performed by electron holography in order to investigate the cause of pole erasure due to the instability of the domain wall behavior. The instability of domain structure of the writer pole generated a stray field on the air bearing surface of the writer pole. The domain wall trapping at the pole tip was found quite effective to form a stable domain structure in the remanent state, and it reflected domain wall energy change and initial domain structure of the writer pole.

Spin Reorientation Transitions in Perpendicularly Exchange-Coupled Thin Films Studied Using Element Specific Imaging

Spatial variations in the spin-reorientation transition of an exchange-coupled Co-wedge/YIG (Y3Fe5O12) bilayer, from perpendicular to in-plane domain structure, was studied using magnetic force microscopy (MFM) and photo-emission electron microscopy (PEEM). Even though MFM measurements of the YIG film showed perpendicular stripe domains, it was not possible to unambiguously resolve the domain structure of only the top ferromagnetic metal layer because of complications arising from the stray fields of the much thicker YIG underlayer. Hence, using element-specific, X-ray magnetic circular dichroism (XMCD) of the transition metal L3,2 edges (Co and Fe, respectively) for magnetic contrast, PEEM measurements were carried out to resolve the domain structure of the individual Co and YIG layers. The two were identical up to a Co thickness of 4.5 nm, confirming that the Co layer was exchange coupled with the YIG underlayer; however, a transition of the Co domains, from perpendicular to in-plane, was observed at thickness of 4.5-6 nm. For thicker regions of the Co film (>6 nm), a sizeable portion of the Co layer showed in-plane domains; their spins were perpendicular to the domain walls of the stripe domains with Co XMCD values in the range of -5% to +5%-a value much smaller than that of the perpendicular domains

Multiple magnetic resonances in permeability spectra of thick CoTaZr films

Multiple resonance peaks were observed below the ferromagnetic resonance in the permeability spectra of CoTaZr films with thickness up to 3μm. Analysis shows the power-law-like dependence of peak frequencies on the film thickness and the peak order, and the observed behavior was identical for both blanket and patterned CoTaZr samples. Experimental findings suggest that the observed modes corresponding to the multiple resonance peaks may not be related to the in-plane domain structure but rather are likely associated with a mechanism in the out-of-plane direction. Multiple resonance peaks disappeared when the lamination structures were used.