Mutsuhiro Shima

Ordered Ni nanowire tip arrays sticking out of the anodic aluminum oxide template

We present a method for making highly ordered arrays of Ni nanowire (NW) tips fully exposed over the surface of anodic aluminum oxide (AAO) templates with uniform exposed lengths. Ni NWs are electrochemically deposited in the nanochannels of the AAO templates, and the templates surface is selectively etched to expose Ni NW tips of uniform lengths, which can be tuned by adjusting the etching time. The magnetic domain structure and magnetic hysteresis of the Ni NW tips were studied at room temperature, and the results indicate strong magnetic anisotropy for the NW arrays and magnetic coercivities significantly larger than that of bulk Ni. The ordered NW tips fully exposed over the surface of the AAO templates with uniform lengths could have various practical applications.

Reduced magnetization in magnetic oxide nanoparticles

Magnetic oxide nanoparticles have been studied to elucidate the effects of nanoscale finite size on the magnetic behavior of the particles. Magnetite nanoparticles synthesized by coprecipitation show superparamagnetism at room temperature with reduced saturation magnetization MS. The MS value decreases and approaches zero with decreasing particle size. Yttrium iron garnet (YIG) nanoparticles also show a similar trend. The magnetization of nanoparticles estimated using the Langevin function with the particle size distribution indicates that the reduced magnetization can be consistently explained by the existence of a spin-disordered surface layer with the thickness of 1–2nm. The results found in magnetite and YIG nanoparticles suggest that the reduced magnetization can be commonly observed among magnetic oxide nanoparticles due to the existence of spin disordered surface layer.

Superparamagnetic behavior in ultrathin CoNi layers of electrodeposited CoNi/Cu multilayer nanowires

We present evidence that in a very thin regime the magnetic layers become discrete islands and superparamagnetic in multilayered CoNi(1–17nm)∕Cu(4.2nm) nanowires grown by pulsed electrodeposition using a hole pattern of anodized alumina templates. Magnetic hysteresis loops measured at room temperature using a vibrating sample magnetometer show that superparamagnetism appears at t(CoNi)<1.7nm, due to a volumetric reduction of the CoNi layers that may result in discontinuity of the layer or formation of islands. The magnetic hysteresis loops for the superparamagnetic nanowires can be represented by the Langevin function. The temperature dependence of coercivity data obtained for the superparamagnetic nanowires using a superconducting quantum interference device indicates that the magnetization reversal can be consistently explained by the Stoner-Walfarth model for coherent rotation. The volumetric reduction accounted for the observed superparamagnetism is probably due to an electrochemical exchange reaction...

Perpendicular giant magnetoresistance of electrodeposited Co∕Cu-multilayered nanowires in porous alumina templates

The giant magnetoresistance (GMR) of multilayered Co∕Cu nanowires potentiostatically electrodeposited inside the pores of an anodized alumina template was studied in the current perpendicular to the plane (CPP) geometry. The maximum magnetoresistance change of 13.5% was observed for Co(8nm)∕Cu(10nm) nanowires at room temperature. The interfacial roughness and/or an intermixing between Co and Cu layers were varied by changing the Cu deposition potential. However, the CPP-GMR value only slightly decreases as the Cu deposition potential increases to the positive value. This indicates that the interfacial roughness and/or intermixing are not a crucial factor in determining the CPP-GMR value of the Co∕Cu nanowires, which is argued to be due to the uncoupled magnetic layers. The x-ray diffraction shows that Co∕Cu nanowires with 300nm diameter have a face-centered-cubic structure with a strong (111) texture along the wire axes; their magnetic easy axes are perpendicular to the wire axes as determined from the CP...

Magnetic properties of Sb-doped FePt nanoparticles

Sb-doped FePt nanoparticles with an average diameter of 8.5nm were prepared by thermal decomposition of platinum acetylacetonate, antimony acetate, and iron pentacarbonyl. Upon annealing to ∼300°C for 30min, nanoparticles with XSb=0.14 and 0.23 show Hc>500mT, and L10 ordering parameter S values of ∼0.83–0.87. Transmission electron microscopy of the annealed assemblies shows no observable nanoparticle coalescence at 300°C. Low-temperature coercivity measurements with a superconducting quantum interference device indicate the presence of particles that exhibit superparamagnetism, probably due to the large particle size distribution or inhomogeneity in Sb incorporation. Our results underscore the necessity to synthesize monodisperse FePt nanoparticles with controlled composition to maximize ferromagnetic behavior.

Field Angle and Thickness Dependence of Coercivity in Electrodeposited CoNi&#8211;Cu Multilayer Nanowires

Variation in the coercivity of CoNi-Cu multilayer nanowires grown by electrodeposition using a hole pattern of alumina templates has been systematically studied as a function of the CoNi layer thickness and the angle between an applied magnetic field and the nanowire axis. We have found that the magnetization reversal for the nanowires with thin disk-shaped CoNi layers is in the coherent rotation mode, while that for the nanowires with long rod-shaped CoNi layers can be interpreted by a combination of the coherent rotation and curling modes. With decreasing CoNi layer thickness t(CoNi) from 6.8plusmn0.8 to 1.7plusmn0.2 nm, the coercivity rapidly decreases from 305 to 10 Oe, indicating that for t(CoNi)<1.7plusmn0.2 nm, the CoNi layers of the nanowires become superparamagnetic at room temperature. The room temperature superparamagnetism is further confirmed by magnetization measurements at low temperatures using a superconducting quantum interference device. The estimated blocking temperatures for t(CoNi)=1.7plusmn0.2 and 1.0plusmn0.1 nm are 181 and 136 K, respectively

Towards batch fabrication and assembly of 3D microstructures: A sequential assembly planner with new hard magnet configuration

We propose a new method for assembly of 3D microstructures. Hard magnets are arranged in a novel 120deg offset configuration, allowing for in-situ magnetization. The new configuration makes the use of hard magnets feasible in batch fabrication, and hard magnets allow for the use of a uniform external field for application of magnetic torque. A sequential planner was developed to calculate layout and magnet orientations for given assemblies, based on this new magnet configuration. We describe the new magnet configuration, experimentally test the configuration, and present a planner that generates a planar layout and magnet orientations.

Biological Crystallization of Self-Aligned Iron Oxide Nanoparticles

Crystal growth and magnetic behavior of iron oxide nanoparticles assembled with biomolecules have been investigated. The nanoparticles assembled with trypsin molecules exhibit superparamagnetism at room temperature with blocking temperature (~80 K) significantly lower than those without trypsin (~140 K). This is attributed to reduced magnetostatic couplings between particles due to increased distance between particles separated by trypsin molecules. Moreover, the synthesized nanoparticle-biomolecule assemblies consist of a unique one-dimensional self-assembled arrays of nanoparticles found by structural analysis using transmission electron microscopy. The moire fringes observed from the particle arrays indicate that the particles are aligned with slight misorientation of their crystallographic axes. Such an unusual formation of nanoparticle arrays may be relevant to specific ligand sites in trypsin molecules and the magnetostatic interparticle couplings

Compositional dependence of magnetic anisotropy in chemically synthesized Co3−xFexO4 (0 ≤ x ≤ 2)

Magnetic anisotropy of Co3− x Fe x O4 (CFO, 0 ≤ x ≤ 2) thin-film and powder samples prepared by a sol–gel method has been investigated as a function of Fe composition x. Structural analyses by X-ray diffraction show that CFO powder samples exhibit diffraction peaks associated with the spinel structure when x < 2, while CFO thin-film samples with thickness of 130–510 nm yield the peaks when 0 ≤ x ≤ 2. CFO thin-film samples are highly (111)-oriented with the Lotgering factor greater than 0.9 when 0.6 ≤ x ≤ 1.3. The magnetic anisotropy constant K 1 of CFO powder samples estimated from their room-temperature hysteresis loops yields a minimum when x = 0.9. Relatively large in-plane magnetic anisotropy (K eff = 5.7 × 105 erg/cm3) is observed for the CFO thin-film sample when x = 1.3. With increasing x, the magnetic easy axis of the spinel CFO changes from 〈111〉 to 〈100〉 when x = 0.9.

Magnetization reversal in electrodeposited CoNi/Cu multilayer nanowires

Magnetization reversal and superparamagnetic behavior in electrodeposited CoNi/Cu multilayer nanowires with the CoNi layer thicknesses t(CoNi) ranging from nanometer to micrometer scales were investigated using vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID). The angular dependence of the magnetization measurements with respect to the wire axis shows that CoNi/Cu nanowires exhibit different magnetization reversal processes depending on t(CoNi). The reversal for nanowires with thin disk-shaped CoNi layers (nanometer thickness) is of coherent rotation type, while that for long rod shaped CoNi layers (micrometer thickness) is consistent with a combination of coherent rotation and curling modes. The coercivity of the nanowires measured at room temperature decreases when t(CoNi) decreases (< 7 nm) and becomes nearly zero when t(CoNi) further decreases (< 2 nm), indicating a superparamagnetic behavior in the thin CoNi layers.