K. Takanashi

Low-temperature fabrication of L10 ordered FePt alloy by alternate monatomic layer deposition

L10 ordered FePt alloy films with large magnetic anisotropy have been successfully prepared by alternating Fe(001) and Pt(001) monatomic layers on MgO (001) substrates at low temperatures below 230u200a°C. In addition to the fundamental (002) peak, (001) and (003) superlattice peaks have clearly been observed in the x-ray diffraction patterns for all the samples, indicating the formation of L10 ordered structure. The magnetization measurements show that all the samples are perpendicularly magnetized. Large uniaxial magnetic anisotropy (Ku=3.0×107u2009erg/cc) and high chemical ordering (long-range order parameter S=0.7±0.1) have been obtained even at the substrate temperature Ts=200u200a°C. The magnetization curves show good magnetic squareness (Mr/Ms∼0.9) for the samples grown at Ts⩾160u200a°C.

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=45u2009nm) 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=10u2009nm, which comprises single domain particles with an average lateral size of approximately 50 nm.

Giant spin Hall effect in perpendicularly spin-polarized FePt/Au devices

Conversion of charge current into pure spin current and vice versa in non-magnetic semiconductors or metals, which are called the direct and inverse spin Hall effects (SHEs), provide a new functionality of materials for future spin-electronic architectures. Thus, the realization of a large SHE in a device with a simple and practical geometry is a crucial issue for its applications. Here, we present a multi-terminal device with a Au Hall cross and an FePt perpendicular spin injector to detect giant direct and inverse SHEs at room temperature. Perpendicularly magnetized FePt injects or detects perpendicularly polarized spin current without magnetic field, enabling the unambiguous identification of SHEs. The unprecedentedly large spin Hall resistance of up to 2.9 mOmega is attributed to the large spin Hall angle in Au through the skew scattering mechanism and the highly efficient spin injection due to the well-matched spin resistances of the chosen materials.

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.

Large Interface Spin-Asymmetry and Magnetoresistance in Fully Epitaxial Co2MnSi/Ag/Co2MnSi Current-Perpendicular-to-Plane Magnetoresistive Devices

Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) characteristics were investigated in a Co2MnSi (CMS)/Ag/CMS fully epitaxial device and compared to those in a CMS/Cr/CMS device systematically. Reflection high-energy electron diffraction and transmission electron microscopy images showed the two samples had no remarkable differences and little interdiffusion. The large spin-asymmetry of electron scattering was found at the CMS/Ag inteface compared with that at the CMS/Cr interface. Finally, the largest magneto-resistance (MR) ratio of 28.8% was observed at room temperature in the CMS/Ag/CMS CPP-GMR device.

Artificial fabrication of an L10‐type ordered FeAu alloy by alternate monatomic deposition

An FeAu ordered alloy has been fabricated artificially by depositing alternately monatomic layers of Fe and Au. Neither intermediate phase nor intermetallic compound is known for the Fe–Au system in the equilibrium state. Nevertheless, the x‐ray diffraction for prepared FeAu films shows definite superlattice lines corresponding to the L10 ordered structure, and it is metastable at room temperature. The obtained FeAu ordered alloy is ferromagnetic, and it has the Fe moment of 2.5±0.3 μB, a considerably high Curie temperature over room temperature, and a large uniaxial magnetic anisotropy perpendicular to Fe and Au atomic planes.

L10 ordering of off-stoichiometric FePt (001) thin films at reduced temperature

We have investigated the effect of extensive compositional change on the structure and magnetic properties of FePt sputtered films deposited at a substrate temperature of 300u200a°C. The Fe concentration in the FePt films was varied from 19 to 68 at.u200a%. In the x-ray diffraction patterns, (001) and (003) superlattice peaks were observed at a Pt-rich composition range, indicating the formation of a L10 ordered structure. A L10 ordered Fe38Pt62 film exhibited perpendicular magnetization with a large uniaxial magnetic anisotropy of Ku=1.8×107 erg/cm3.

Formation of octahedral FePt nanoparticles by alternate deposition of FePt and MgO

Two-dimensional assemblies of octahedral-shaped FePt nanoparticles embedded in a MgO matrix were formed by alternate deposition of FePt and MgO layers on MgO (001) single-crystal substrates with a thermal cycling process. Highly ordered L10 structure with the [001] orientation perpendicular to the plane and large coercivity of more than 60kOe were obtained. Cross-sectional transmission electron microscopy observation revealed that the equilibrium crystal shape of L10 ordered FePt particles was octahedral structure due to the anisotropy of the surface energy. The {111} plane shows the minimum surface energy in the L10 structure, which is similar to the case of face-centered-cubic structure.

Oscillatory Perpendicular Magnetic Anisotropy and Lattice Plane Spacing in Fe/Au Superlattices

Perpendicular magnetic anisotropy has been investigated for Fe(xu2009ML)/Au(xu2009ML) superlattices (ML: monatomic layer thickness) with x ranging from 1 to 4. The x-ray diffraction analyses indicate the formation of coherent layered structures both for x=integers and nonintegers. However, the perpendicular magnetic anisotropy for x=nonintegers shows different behavior from that for x=integers, and consequently, it oscillates as a function of x with a period of 1 ML. Furthermore, the out-of-plane lattice spacing also oscillates in the same manner.

A novel hysteresis loop and indirect exchange coupling in Co/Pt/Gd/Pt multilayer films

We have investigated the magnetization behavior of Co/Pt/Gd/Pt multilayers in order to study the indirect exchange coupling between Co and Gd through Pt. A novel hysteresis loop has been observed for the samples with Pt layer thicknesses larger than 10 A. This loop is considered to be due to the combined effect of the antiferromagnetic coupling between Co and Gd layers and the large magnetic anisotropy acting on the Co layer magnetization. The magnetization data suggest that the coupling strength oscillates with the Pt layer thickness.