Mikiko Saito

A soft magnetic CoNiFe film with high saturation magnetic flux density and low coercivity

Magnetic materials are classed as ‘soft’ if they have a low coercivity (the critical field strength Hc required to flip the direction of magnetization). Soft magnetic materials are a central component of electromagnetic devices such as step motors, magnetic sensors, transformers and magnetic recording heads. Miniaturization of these devices requires materials that can develop higher saturation flux density, Bs, so that the necessary flux densities can be preserved on reducing device dimensions, while simultaneously achieving a low coercivity. Common high-Bs soft magnetic films currently in use are electroplated CoFe-based alloys electroplated CoNiFe alloys and sputtered Fe-based nanocrystalline and FeN films. Sputtering is not suitable, however, for fabricating the thick films needed in some applications, for which electrochemical methods are preferred. Here we report the electrochemical preparation of a CoNiFe film with a very high value of Bs (2.0–2.1 T) and a low coercivity. The favourable properties are achieved by avoiding the need for organic additives in the deposition process, which are typically used to reduce internal stresses. Our films also undergo very small magnetostriction, which is essential to ensure that they are not stressed when an external magnetic field is applied (or conversely, that external stresses do not disrupt the magnetic properties). Our material should find applications in miniaturization of electromechanical devices and in high-density magnetic data storage.

New soft magnetic CoNiFe plated films with high B/sub s/=2.0-2.1 T

A CoNiFe film with saturation magnetic flux density (B/sub s/) greater than 2.0 tesla (T) has been prepared for the first time as a soft magnetic film; the coercivity (H/sub c/) of the film is less than 160 A/m (2.0 Oe). This success was achieved by formulating a new plating bath and operating conditions to form fine grains. The film has a low H/sub c/ of less than 160 A/m, a low saturation magnetostriction (/spl lambda//sub s/) of approximately 10/sup -6/, and a high B/sub s/ of 2.0-2.1 T. The present invention is expected to contribute to accelerating the development of not only the technology of high-density magnetic recording but also the field of magnetic materials in general.

Influence of Crystalline Structure and Sulfur Inclusion on Corrosion Properties of Electrodeposited CoNiFe Soft Magnetic Films

The corrosion resistance of soft magnetic films is an important property to be considered in the manufacture of magnetic devices. We investigated the corrosion behavior of the electrodeposited CoNiFe film with desired soft magnetic properties by varying the crystalline structure and the amount of included sulfur. The corrosion property in 2.5% NaCI solution depends largely on the sulfur content and also on the structure of the film. Although the CoNiFe film contains more than 13 atom % Fe, the film of face‐centered to body‐centered cubic mixed crystals exhibits a high anticorrosion property because of very small grain size with essentially no sulfur inclusion (<0.1 atom %).

Newly developed inductive write head with electroplated CoNiFe film

A newly developed CoNiFe film is investigated from the viewpoint of its application to MR write heads. Film composition is Co/sub 65/Ni/sub 12/Fe/sub 23/, prepared by electroplating to obtain a b.c.c. plus f.c.c. mixed phase at that composition. Films with mixed phases show improved soft magnetic properties (B/sub s/ is 2.0-2.1 T and the magnetostriction coefficient is +1.8/spl times/10/sup -6/ to +7.4/spl times/10/sup -6/) and also show improved corrosion resistance. Performance of a head with a double layer structure of a 0.3 /spl mu/m CoNiFe film and a 4.1 /spl mu/m NiFe film is investigated, and a short throat height is found to give better overwrite performance.

Corrosion Properties of Electroplated CoNiFe Films

Electroplated CoNiFe films with a saturation flux density as high as 2.1 T are potentially useful in high-density magnetic recording heads. The authors found that films electroplated at a high current density (15 mA/cm{sup 2}) from a bath without saccharin have a sufficient anodic pitting-corrosion potential ({minus}65 mV). The authors also found that the pitting-corrosion potential of films electroplated under a low current density (5 mA/cm{sup 2}) from saccharin-free baths have anodic pitting-corrosion potentials of less than {minus}300 mV. However, the corrosion resistance improved after annealing at temperatures above 100 C. The crystal-grain boundaries in the as-plated film that electroplated under a low current density from saccharin-free baths are not clear (i.e., that the phase is amorphous). But the crystal grain boundaries in the annealed film are clear. Films electroplated from baths containing saccharin also have anodic pitting-corrosion potentials of less than {minus}300 mV. Their corrosion resistance did not improve when they were annealed at 250 C. The deterioration of the corrosion resistance is attributed to the defects that increase the face-centered cubic (111) lattice constant. One of the most important characteristics of a highly corrosion-resistant CoNiFe film is fine crystal structure with very few defects.

Trap-state passivation of titania nanotubes by electrochemical doping for enhanced photoelectrochemical performance

TiO2 nanotubes are widely investigated materials for photoelectrochemical water splitting, but the presence of trap states limits their performance by facilitating the recombination of electron/hole pairs. In this investigation we unequivocally demonstrate that the photocurrent improvement observed in TiO2 nanotubes after performing electrochemical doping with hydrogen or lithium ions is due to trap state passivation. Specifically, electrochemical impedance spectroscopy evidences that trap state defects disappear upon electrochemical doping, concurrent with an increase in the electron lifetime and faster photocurrent transients. This results in a two-fold enhancement in the photocurrent under simulated sunlight at 1.0 V vs. SCE. Li intercalation was confirmed and the structure as well as the composition of the modified nanotubes was elucidated by GDOES, XPS, and TEM.

Miniaturized thin-film magnetic field probe with high spatial resolution for LSI chip measurement

It is important to obtain the absolute value of current flowing through each power line on a large-scale integrated (LSI) circuit by measurement because this current on an LSI chip is regarded as conductive noise. We have developed a thin-film magnetic field probe that has spatial resolution high enough to obtain the absolute value of high-frequency power current on an LSI chip. Spatial resolution was enhanced by miniaturizing the shielded loop coil, the detection part of the probe. The outer size of the new coil is 50/spl times/22 /spl mu/m. In taking measurements with the new probe over a 60 /spl mu/m wide microstrip line used as a device under test (DUT), we obtained a 6 dB decrease point of 40 /spl mu/m, which indicates the spatial resolution of the probe. This value is comparable to the typical width of power lines on an LSI chip, around 50 /spl mu/m and is less than half that of our conventional probes, around 90 /spl mu/m. In measurements with the new probe over an LSI chip, we obtained such a fine magnetic near-field distribution that the magnetic fields generated from the lines on the chip were separated. On-chip decoupling was also confirmed by using the new probe. The new probe enables direct verification of a circuit design for suppressing electromagnetic interference (EMI), while conventional coarse mapping of the magnetic near-field cannot be used to evaluate such conductive noise.

Co-Ni-Fe write heads with a 10-/spl mu/m yoke length for high-speed recording

We have developed a Co-Ni-Fe write head with a short yoke length for high-speed recording. By reducing the yoke length to 9.5 /spl mu/m, the eddy currents induced in a yoke with a relatively low resistivity (0.2 /spl mu//spl Omega/m) were reduced. The head of this short yoke had good write performance for a medium with a coercivity of 400 kA/m (5000 Oe) at frequencies up to 250 MHz (the overwrite less than -30 dB, and nonlinear transition shift less than 7%).

Evaluation of the Crystal Structure, Film Properties, and B s of Electroplated CoNiFe Films

The factors which affect the saturation magnetic flux density (B s ) of electroplated CoNiFe films were onvestigated and the results are reported. The Fe composition, the ratio of body-centered cubic (110) to face-centered cubic (111) peak intensity (R b ), and the density were found to affect the B s of the films. The B s increased when the R b , Fe composition, and the density were increased; we found that a larger grain size and fewer impurities produced a higher density film. It was determined that the suppression of the impurities and the increased grain size led to the high film density and high B s .

Copper Multilayer Interconnection Using Ultravaiolet Nanoimprint Lithography with a Double-Deck Mold and Electroplating

An approach to realizing a two-step interconnection using a double-deck quartz mold based on ultraviolet nanoimprint lithography (UV-NIL) technology and Cu electroplating is discussed. The double-deck mold realizes a two-step simultaneous transcription and a two-step nanoscale interconnection. Nanoscale via and trench array patterns of Cu were formed on a silicon substrate using UV-NIL in combination with electroplating. Imprinted via and trench patterns were simultaneously formed on a photocurable resin by the double-deck mold with a two-step structure. Using this mold, 73 nm metal via and 190 nm metal trench patterns were successfully fabricated by electroplating.