Y. K. Takahashi

Preparation and magnetic properties of highly coercive FePt films

The magnetization processes of highly ordered FePt(001) films with large perpendicular magnetic anisotropy have been studied. The film morphology was controlled from isolated particles to continuous film by varying the nominal thickness (tN) of the FePt film sputter deposited directly on a MgO(001) substrate at an elevated temperature. A drastic change in the coercivity by one order of magnitude has been found at the critical thickness (tN=45 nm) where the film morphology changes from a particulate to a continuous state. A huge coercivity exceeding 40 kOe has been achieved in the film with tN=10 nm, which comprises single domain particles with an average lateral size of approximately 50 nm.

Coercivity exceeding 100 kOe in epitaxially grown FePt sputtered films

Microstructure and magnetization processes of highly ordered FePt(001) films with large perpendicular magnetic anisotropy have been studied. The film morphology was controlled from assemblies of single-domain nanoparticles to those of multidomain islands by varying the nominal thickness (tN) of the FePt films sputter-deposited on a heated MgO(001) substrate. The change in the magnetization process from magnetization rotation to domain wall displacement is clearly demonstrated by the initial magnetization curves. Huge coercivities as high as 70 and 105kOe have been achieved in the film with single-domain particles at room temperature and 4.5K, respectively.

All-optical control of ferromagnetic thin films and nanostructures

All-optical magnetic state switching Magneto-optical memory storage media, such as hard drives, use magnetic fields to change the magnetization of memory bits, but the process is slow. Light can often reveal information about the magnetization state of a sample, such as its field direction. Lambert et al. show that under the right circumstances, light can also switch the magnetization state of a thin ferromagnetic film. Using light pulses instead of magnetic fields led to ultrafast data memory and data storage. Science, this issue p. 1337 The all-optical control of magnetization in thin ferromagnetic films is demonstrated. The interplay of light and magnetism allowed light to be used as a probe of magnetic materials. Now the focus has shifted to use polarized light to alter or manipulate magnetism. Here, we demonstrate optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits.

Size dependence of ordering in FePt nanoparticles

We have investigated the size effect of A1→L10 ordering of FePt nanoparticles in FePt–Al2O3 granular and FePt/SiO2 particulate films by transmission electron microscopy (TEM). The TEM results have shown convincingly that ordering does not progress when the particle size has a diameter of less than 4 nm. Calculation of the order parameter profile from the surface to the volume of the FePt nanoparticles based on diffuse-interface theory justified the experimentally observed size dependence of the ordering. The transition length from disorder to order depends on the interfacial energy, hence the critical particle size of ordering should vary depending on the type of matrix and substrate.

Size effect on the ordering of FePt granular films

We have investigated the A1→L10 ordering process of FePt granular films with different particle sizes. Although FePt particles larger than ∼7 nm ordered by annealing at 600 °C, the ordering did not proceed when the particle size was smaller than ∼4 nm in diameter. This suggests that there is a size dependence on the ordering of FePt particles. Calculation of the Helmholtz free energy using the Lennard-Jones potential suggested that Tc decreases below the annealing temperature when the particle size is decreased to a few nanometers suggesting that experimental observation is physically feasible.

Sm(Co,Cu)5∕Fe exchange spring multilayer films with high energy product

Sm(Co,Cu)/sub 5//Fe multilayer films are prepared by sequentially depositing Sm-Co, Cu, and Fe layers on Cr underlayer. Films are heat-treated at different temperatures and their magnetic properties are measured using a superconducting quantum interference device (SQUID) magnetometer. Preliminary examination of the film structure is made by X-ray diffraction (XRD) and the detailed microstructure was investigated by transmission electron microscopy (TEM).

Structure, magnetic property, and spin polarization of Co2FeAlxSi1−x Heusler alloys

We report the spin polarization of Co2FeAlxSi1−x (x=0.0, 0.3, 0.5, 0.7) bulk alloys measured by the point contact Andreev reflection method. All the Co2FeAlxSi1−x alloys had an L21 structure along with A2- and B2-type disorder. Several off-stoichiometric alloys (CoxFeyAl0.5Si0.5) were prepared to understand the effect of the compositional deviation from the stoichiometry on the spin polarization. By substituting Al for Si, the spin polarization changed from 0.57±0.01 for x=0.0 to a maximum value of 0.60±0.01 for x=0.5. The off-stoichiometric alloys had spin polarizations of 0.57−0.60±0.01. Ab initio calculations were performed to interpret the effect of Al addition as well as the effect of disorder on the magnetic properties and on the electronic structure.

Bulk and interfacial scatterings in current-perpendicular-to-plane giant magnetoresistance with Co2Fe(Al0.5Si0.5) Heusler alloy layers and Ag spacer

We report the transport properties of a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device with Co2Fe(Al0.5Si0.5) (CFAS) Heusler alloy ferromagnetic layers and a Ag spacer layer. The CPP-GMR devices showed relatively high ΔRA values and MR ratios up to 17 m Ω μm2 and 80% at 14 K, and 8 m Ω μm2 and 34% at 290 K. The spin diffusion length ∼3 nm and the bulk spin asymmetry ∼0.77 for the CFAS alloy at 14 K were estimated by the Valet–Fert model, indicating a large contribution of the interfacial scattering.

Current-perpendicular-to-plane giant magnetoresistance in spin-valve structures using epitaxial Co2FeAl0.5Si0.5/Ag/Co2FeAl0.5Si0.5 trilayers

A current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve using epitaxial layers of Co2FeAl0.5Si0.5 (CFAS) Heusler alloy as ferromagnetic electrodes is reported. A multilayer stack of Cr/Ag/CFAS/Ag/CFAS/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO (001) single crystal substrate. Epitaxial growth of the Cr, Ag, and CFAS layers in the (001) orientation up to the top CFAS layer was confirmed. Large MR ratios of 6.9% at room temperature and 14% at 6 K were observed for the CPP-GMR device. High spin polarization of epitaxial CFAS is the possible reason for high MR ratios.

Current-perpendicular-to-plane magnetoresistance in epitaxial Co2MnSi∕Cr∕Co2MnSi trilayers

Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) of the multilayer thin film using a full-Heusler Co2MnSi (CMS) phase as ferromagnetic electrodes has been investigated. A multilayer of Cr buffer (10nm)∕CMS (50nm)∕Cr spacer (3nm)∕CMS (10nm)∕Cr cap (3nm) was grown on a MgO(100) substrate. The 50nm thick CMS layer which was deposited on the Cr buffer at 573K was epitaxially grown and had an L21 structure. The resistance change-area product (ΔRA) at room temperature was 19mΩμm2, which is one order of magnitude larger than those in previously reported trilayer systems, resulting in the MR ratio of 2.4%. A possible origin of the enhanced ΔRA is considered to be the large spin polarization in a high-quality L21 CMS film.