Effect of Ti seed layer on the magnetization reversal process of Co/NiFe/Al-oxide/NiFe junction films

科研費報告書収録論文(課題番号:13305001・基盤研究(A)(2) ・H13～H15/研究代表者:宮崎, 照宣/高品位微小トンネル接合へのスピン注入

Understanding the effect of curvature on the magnetization reversal of three-dimensional nanohelices

Comprehending the interaction between geometry and magnetism in three-dimensional (3D) nanostructures is of importance to understand the fundamental physics of domain wall (DW) formation and pinning. Here, we use focused electron beam-induced deposition to fabricate magnetic nanohelices with increasing helical curvature with height. Using electron tomography and Lorentz transmission electron microscopy, we reconstruct the 3D structure and magnetization of the nanohelices. The surface curvature, helical curvature and torsion of the nanohelices are then quantified from the tomographic reconstructions. Furthermore, by using the experimental 3D reconstructions as inputs for micromagnetic simulations we can reveal the influence of surface and helical curvature on the magnetic reversal mechanism. Hence, we can directly correlate the magnetic behavior of a 3D nanohelix to its experimental structure. These results demonstrate how control of geometry in nanohelices can be utilized in the stabilization of DWs and control of the response of the nanostructure to applied magnetic fields

Evidence for shallow implantation during the growth of bismuth nanocrystals by pulsed laser deposition

The shallow implantation of Bi species was analyzed for energy densities above 2 Jcm-2. The implantation range was shown to depend on the energy density used for ablation, which was related to the velocity of the Bi atoms and ions in the plasma. The kinetic energy of the Bi species in the plume generated at laser energy densities above 2 J cm-2 was estimated to be around 200 eV.This work has been partially supported by project TIC99-0866, CICYT (Spain). One of the authors (J.-P.B.) acknowledges support by the EPSRC and a Marie Curie Fellowship of the EC under Contract No. HPMT-CT-2000-00064.Peer Reviewe

In situ studies of the crystallization kinetics in Sb-Ge films

The crystallization process in SbxGe1-x alloy films has been observed during in situ annealing in a transmission electron microscope. Results are presented for two films with x=0.89 (89 at. % Sb) and x=0.71 (71 at. % Sb), which lie on either side of the eutectic composition (x=0.85). In the former films radial crystals are observed to grow rapidly from discrete nuclei, whereas in the latter films the crystallization process occurs through a near-planar front. In addition, quantitative data obtained from these experiments show that the Sb0.89Ge0.11 films have a higher activation energy for crystal growth and a lower temperature for the nucleation of crystals. Significant differences are observed between the crystallization processes for the two films studied, with the Sb0.89Ge0.11 film showing better potential for development as an ultrafast optical phase-change storage medium. © 1995 American Institute of Physics.This work has been partially supported by CICYT under the TIC93-0125 project. We are grateful to The British Council/Spanish Joint Research Program for traveling fi- nances, the Glasstone Benefaction (AKPL), Professor R. J. Brook for provision of laboratory facilities, and the Materials Modelling Laboratory at Oxford for computing facilities. Dr. Neil Long is thanked for operation of the FEG STEM and production of the composition map shown in Fig. 6.Peer Reviewe

Synthesis of uniformly distributed single- and double-sided zinc oxide (ZnO) nanocombs

Uniformly distributed single- and double-sided zinc oxide (ZnO) nanocomb structures have been prepared by a vapor-liquid-solid technique from a mixture of ZnO nanoparticles and graphene nanoplatelets. The ZnO seed nanoparticles were synthesized via a simple precipitation method. The structure of the ZnO nanocombs could easily be controlled by tuning the carrier-gas flow rate during growth. Higher flow rate resulted in the formation of uniformly-distributed single-sided comb structures with nanonail-shaped teeth, as a result of the self-catalysis effect of the catalytically active Zn-terminated polar (0001) surface. Lower gas flow rate was favorable for production of double-sided comb structures with the two sets of teeth at an angle of similar to 110 degrees to each other along the comb ribbon, which was attributed to the formation of a bicrystal nanocomb ribbon. The formation of such a double-sided structure with nanonail-shaped teeth has not previously been reported.Publisher's Versio

Active analog tuning of the phase of light in the visible regime by bismuth-based metamaterials

Active and analog tuning of the phase of light is needed to boost the switching performance of photonic devices. However, demonstrations of this type of tuning in the pivotal visible spectral region are still scarce. Herein we report active analog tuning of the phase of visible light reflected by a bismuth-based metamaterial, enabled by a reversible solid-liquid transition. This metamaterial, fabricated by a lithography-free approach, consists of two-dimensional assemblies of polydisperse plasmonic bismuth nanostructures embedded in a refractory and transparent aluminum oxide matrix. Analog tuning of the phase is achieved by controlled heating of the metamaterial to melt a fraction of the nanostructures. A maximum tuning of 320 deg (1.8pi) is observed upon complete melting of the nanostructures at 230 degrees Celsius. This tuning is reversible by cooling to 25 degrees Celsius. In addition, it presents a wide hysteretic character due to liquid bismuth undercooling. This enables the phase achieved by this analog approach to remain stable over a broad temperature range upon cooling and until re-solidification occurs around 100 degrees Celsius. Therefore, bismuth-based metamaterials are appealing for applications including optical data storage with enhanced information density or bistable photonic switching with a tunable "on" state

Lorentz transmission electron microscopy and magnetic force microscopy characterization of NiFe/Al-oxide/Co films

科研費報告書収録論文(課題番号:13305001・基盤研究(A)(2) ・H13～H15/研究代表者:宮崎, 照宣/高品位微小トンネル接合へのスピン注入

Optical and magneto-optical properties of Fe nanoparticles

The optical and magneto-optical properties of Fe nanoparticles with sizes ranging from 2 to 8 nm, embedded in amorphous Al2O3, are studied as a function of their size and shape. The optical properties were measured using spectroscopic ellipsometry, whereas the magneto-optical properties were determined in two different Kerr configurations: polar and transverse. A generalization of different effective medium approximations is used to describe and analyze experimental data in nanocomposite media. In this generalization, the shapes of the nanoparticles are considered as an input parameter. The optical and magneto-optical parameters show clearly different values as a function of the nanoparticle size. A reasonable agreement between the theoretical calculations and experimental data is found when the average size of the nanoparticles is larger than 4 nm. On the other hand, the experimental and theoretical curves differ for smaller sizes, implying that below 4 nm the optical and magneto-optical constants of the particles deviate from the bulk behavior.J. L. M. wants to acknowledge Comunidad de Madrid’s Consejería de Educación y Cultura for financial support. This work was partially supported by TIC99-0866, MAT98-0974, MAT99-1063, CICYT (Spain).Peer reviewe

Lorentz transmission electron microscopy and magnetic force microscopy characterization of NiFe/Al-oxide/Co films

Magnetization reversal process of NiFe/Al-oxide/Co junction films was observed directly using Lorentztransmission electron microscopy (LTEM) and magnetic force microscopy(MFM).In situmagnetizing experiments performed in both LTEM and MFM were facilitated by a pair of electromagnets, which were mounted on the sample stages. A two-stage magnetization reversal process for the junction film was clearly observed in LTEM with NiFe magnetization reversed first via domain wall motion followed by Co magnetization reversal via moment rotation and domain wall motion. Reversal mechanism and domain characteristics of the NiFe and Co layers showed very distinctive features. The magnetization curve of the junction filmmeasured using alternating gradient force magnetometry showed a nonzero slope at the antiparallel magnetization configuration region, which implies that magnetization directions of the NiFe and Co layers were not exactly antiparallel due to Co moment rotation existed in that region. After the magnetization reversal of the Co was complete, MFM images revealed some magnetic contrast, which suggests that an out-of-plane magnetization component remained in the Co layer. Such magnetic contrast disappeared at higher magnetic fields when the Co moments further rotated and aligned parallel to the applied field direction

Coexistence of Merons with Skyrmions in the Centrosymmetric van der Waals Ferromagnet Fe5GeTe2

Fe$_{5-x}$GeTe$_2$ is a centrosymmetric, layered van der Waals (vdW) ferromagnet that displays Curie temperatures $T_c$ (270-330 K) that are within the useful range for spintronic applications. However, little is known about the interplay between its topological spin textures (e.g., merons, skyrmions) with technologically relevant transport properties such as the topological Hall effect (THE), or topological thermal transport. Here, we show via high-resolution Lorentz transmission electron microscopy that merons and anti-meron pairs coexist with N\'{e}el skyrmions in Fe$_{5-x}$GeTe$_2$ over a wide range of temperatures and probe their effects on thermal and electrical transport. We detect a THE, even at room $T$, that senses merons at higher $T$s as well as their coexistence with skyrmions as $T$ is lowered indicating an on-demand thermally driven formation of either type of spin texture. Remarkably, we also observe an unconventional THE in absence of Lorentz force and attribute it to the interaction between charge carriers and magnetic field-induced chiral spin textures. Our results expose Fe$_{5-x}$GeTe$_2$ as a promising candidate for the development of applications in skyrmionics/meronics due to the interplay between distinct but coexisting topological magnetic textures and unconventional transport of charge/heat carriers.Comment: In press. Four figures in the main text. Includes SI file with 19 additional figure