No abstract

We report direct observation of current-driven magnetic domain wall (DW) displacement by using a well-defined single DW in a microfabricated magnetic wire with submicron width. Magnetic force microscopy visualizes that a single DW introduced in a wire is displaced back and forth by positive and negative pulsed current, respectively. The direct observation gives quantitative information on the DW displacement as a function of the intensity and the duration of the pulsed current. The result is discussed in terms of the spin-transfer mechanism.

The motion of a magnetic domain wall in a submicrometer magnetic wire was detected by use of the giant magnetoresistance effect. Magnetization reversal in a submicrometer magnetic wire takes place by the propagation of a magnetic domain wall, which can be treated as a "particle." The propagation velocity of the magnetic domain wall was determined as a function of the applied magnetic field.

It was found that high current density needed for the current-driven domain wall motion results in the Joule heating of the sample. The sample temperature, when the current-driven domain wall motion occurred, was estimated by measuring the sample resistance during the application of a pulsed-current. The sample temperature was 750 K for the threshold current density of 6.7 x 10^11 A/m2 in a 10 nm-thick Ni81Fe19 wire with a width of 240 nm. The temperature was raised to 830 K for the current density of 7.5 x 10^11 A/m2, which is very close to the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5 x 10^11 A/m2, an appearance of a multi-domain structure in the wire was observed by magnetic force microscopy, suggesting that the sample temperature exceeded the Curie temperature.Comment: 13 pages, 4 figure

Our recent work has demonstrated that well-defined hollow interiors can be created inside Prussian Blue (PB) nanoparticles through controlled chemical etching in the presence of poly(vinylpyrrolidone) (Angew. Chem., Int. Ed. 2012, 51, 984). By calcination of these PB nanoparticles as starting precursors, we can successfully synthesize nanoporous iron oxides with hollow interiors. From scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the original hollow cavities of PB nanocubes are shown to be retained after crystal transformation to iron oxides. Also, the obtained hollow iron oxides show a very high surface area because of their nanoporous shells, as illustrated by N2 gas adsorption–desorption analysis. By tuning the applied calcination temperatures and selecting the PB nanoparticles with different hollow cavities, crystalline α-Fe2O3, and γ-Fe2O3 can be selectively formed in the products without formation of any impurity phases. Field-dependent magnetization measurements indicate that the nanoporous hollow iron oxides exhibit a very interesting superparamagnetic property at room temperature. Especially, nanoporous hollow γ-Fe2O3 particles with well-developed crystallinity possess sufficient saturation magnetization (M s) value. Such reasonable M s value, together with the superparamagnetic property, high specific surface area, and internal hollow cavity, makes our nanoporous iron oxides a very promising platform for future biomedical applications.

Epitaxial thin films of ordered double-perovskite La2NiMnO6 were deposited on SrTiO3, (LaAlO3)0.3–(Sr2AlTaO6)0.7, and LaAlO3 substrates by a pulsed-laser deposition method. A rock-salt-type ordering for Ni2+ and Mn4+ ions was confirmed through structural and magnetic measurements. Despite the difference in heteroepitaxial constraints on the crystal structure, the magnetic properties of the films were quite similar to each other and also to those of bulk La2NiMnO6.

We study ferromagnetic properties of Heusler-alloy Co2FeSi epilayers grown on silicon (Si). The magnetic moment and in-plane magnetic anisotropy of the Co2FeSi/Si(111) epilayers vary significantly with the growth temperature (TG) even in the low-temperature region (TG≤200 °C). These features are induced by reaction phases formed at the interface between Co2FeSi and Si. At TG=100 °C, however, we can obtain both highly ordered L21 structures on Si and high-quality Co2FeSi/Si heterointerfaces at the same time. This fact will open a road to realize a Co-based half-metallic spin injector and detector for Si-based spintronic devices.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approaches based on group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature (T C). Here, we establish a predictive and quantitative relationship between T C and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A 3 B 2 O 7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr 1−x Ca x) 3 Sn 2 O 7 (x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic-size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d 0 and 6s 2) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with T C = 410 K for Sr 3 Sn 2 O 7. We also find that the T C increases linearly up to 800 K with increasing the Ca 2+ content, i.e., with decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A 3 B 2 O 7 ferroelectrics, indicating that T C correlates with the simple crystal-chemistry descriptor, t, based on the ionic-size mismatch. This study provides a predictive guideline for estimating T C of a given material, which would complement the grouptheoretical and first-principles design approach. Additional ND and SXRD analyses, first-principles calculation results, and Mössbauer spectroscopy (PDF).