Toshiki Kingetsu

Perpendicular magnetic anisotropy and structures of epitaxial Co/Ag and Co/Au metallic superlattices

The magnetic anisotropy and structures of Co/Ag and Co/Au metallic superlattices (SLs) grown by molecular‐beam epitaxy were studied. In the Co/Ag SLs, the total magnetic anisotropy energy Keff was larger for the flatter Co/Ag interfaces and for the smaller strains of the Ag buffer layers. Particularly at 5 K, the perpendicular magnetic anisotropy was observed in the Co/Ag SLs with the nominal Co layer thickness t being 1 monolayer, and was strongly temperature dependent. The dependence of Keff⋅t on t was linear both at 5 and 300 K. The difference of the magnetic anisotropy in the Co/Ag and the Co/Au SLs was discussed in terms of structural characteristics. The origin of the perpendicular magnetic anisotropy was suggested to be the Neel‐type surface anisotropy in the Co/Ag SLs and to be mainly the magnetoelastic anisotropy in the Co/Au SLs.

Growth of short-period epitaxial superlattices for X-ray multilayer mirrors

Soft-X-ray multilayer mirrors with amorphous structures, which are currently widely used, have almost reached their inherent maximum performance, when used with short periods for high or normal incident angle reflection. Therefore, a breakthrough that would enable further progress in the performance, such as that of reflectance, of X-ray short-period multilayer mirrors has been sought to date. To solve this issue, atomic-layer epitaxy-grown semiconductor superlattices have recently been applied to soft-X-ray short-period multilayer mirrors. Moreover, conventional molecular beam epitaxy of metallic superlattices, which was difficult to apply to X-ray mirrors in the past, has been investigated, in order to enable its application to soft-X-ray mirrors by considering the combinations of various chemical elements, taking crystal structures into account. As a result, short-period Al-based metallic superlattices have been grown successfully very recently. This paper presents a review for the current status and outlook of the development of soft-X-ray short-period multilayer mirrors, focusing on the growth behavior, structures and X-ray reflectance of these promising short-period superlattices that have potential to provide soft-X-ray mirrors with high performance.

Field-ion microscope observation of ordering in the near-surface of Ni4Mo alloy using in situ annealing

Abstract Field-ion microscope observations were made on Ni 4 Mo alloy to investigate surface effects on ordering during in situ annealing after quenching from high temperature α phase region. Surface ordering occurred preferentially at {200} FCC and/or {111} FCC facets (at 880–835°C). Surface layers with low degree of order ranging from ten to a few tens angstroms in thickness were formed (at 760–720°C). The degree of order in relatively large ordered domains decreased near the surface (at 825–810°C). In the interior of the specimens at a range of depths 100–300 A from the surface, ordering behavior was essentially the same as that for bulk specimens (at 825–720°C). Homogeneously nucleated ordered domains coexisted with relatively large heterogeneously nucleated ones in the interior of the specimen (at 670°C). Certain relatively low index (FCC) planes faceted (at 880-720°C). Faceting originating from β structure (ordered structure) occurred on the surface (at 670°C). In addition, in situ formed deformation twin was observed (at 810–825°C).

Molecular‐beam‐epitaxial growth and magnetic properties of (111)Pt/Co/Ag, Pt/Co, and Ag/Co/Pt superlattices

The structural and the magnetic properties of (111)Pt/Co/Ag, Pt/Co, and Ag/Co/Pt superlattices (SLs) were studied. The samples were grown at 373 K on Ag buffer layers preformed on 7×7‐(111)Si substrates by molecular‐beam epitaxy. Reflection high‐energy electron diffraction indicated that in‐plane lattice constants of Co, Pt, and Ag layers changed continuously during layer growth in Pt/Co/Ag SLs with thin Co layers and Pt/Co SLs. The effective magnetic anisotropy energy Keff of a Pt/Co/Ag SL was similar to Keff, an average of Keff values of a Pt/Co and an Ag/Co SL with the same Co layer thicknesses. This suggests that the origins of perpendicular magnetic anisotropy in the Pt/Co system are different from those in the Au/Co system investigated earlier, in which the difference between the Au/Co and Au/Co/Ag SLs is small in Keff value.

Epitaxial growth of binary and ternary metallic strained superlattices and their magnetic properties

Abstract Since the structure at/near the interface of superlattices influences physical properties such as magnetic property, it is important to investigate details of the structure. The interface structure is characterized by the factors like atomic species, strain, mixing and roughness. The reflection high-energy electron diffraction (RHEED) system installed in our molecular-beam epitaxy (MBE) system enables us to observe, continuously, the change of the surface in-plane lattice constant, which is affected by atomic species, strain and/or mixing, on a realtime basis. Ternary superlattices consisting of three elements can clarify the effect of stacking sequence by comparison between the two types of superlattices with the reverse deposition sequences, since the effect caused by the combination of the same atomic species is cancelled out and the effect caused by the different stacking sequences remains. In the present paper, we review growth behaviors of binary and ternary metallic strained superlattices, especially magnetic ones, investigated mainly by our group, and summarize the discussion on their magnetic properties, mainly on the magnetic anisotropy, in terms of their structural characteristics. First, we introduce our RHEED system that works efficiently under a magnetic field arising from evaporation sources for low vapor-pressure materials. Then, MBE-grown binary strained superlattices, Co/Au, Co/Pt and Cu/Au, are discussed, with comparing to incoherent superlattices of Co/Ag and Cu/Ag having nearly the same lattice mismatch of constituents. Next, we review ternary strained superlattices with immiscible constituents with reverse deposition order, Au/Co/Ag and Ag/Co/Au superlattices, and Au/Co/Cu and Cu/Co/Au superlattices, in relation to the growth behaviors of binary superlattices. Finally, ternary strained superlattices containing both miscible and immiscible constituents, Pt/Co/Ag and Ag/Co/Pt superlattices, and Au/Ni/Ag and Ag/Ni/Au superlattices, are reviewed.

Growth‐temperature dependence of Co layer strain and magnetic anisotropy in epitaxial Co/Au superlattices

We studied the variation of the in‐plane lattice constant of Co layers as a function of layer thickness in Co/Au superlattices. The samples were grown at various temperatures by molecular‐beam epitaxy and were investigated using reflection high‐energy electron diffraction. It was found that the strains of Co layers are larger in a sample grown at a higher temperature. It might be thought that larger strains possibly introduce larger magnetoelastic anisotropy, contributing to the perpendicular magnetic anisotropy of the superlattices. However, magnetization measurements show that the magnetic anisotropy energy is smaller in the sample grown at a higher temperature. Possible causes of this discrepancy are discussed.

Field-Ion Microscope Studies of Ni4W Type Ordering in the Near-Surface of Ni-16.6 at%W Alloy

Field-ion microscope observations were made on Ni-16.6 at.%W alloy quenched from the high temperature α phase region to investigate Ni4W type ordering in the near-surface during in-situ annealing. Somewhat below the peritectoid temperature, disc-shaped Ni4W domains of the particular variants were formed preferentially at some of {420}FCC planes on the surface regions with their disc faces being {110}BCT superlattice planes (at 950°C for ca. 5 min). The relaxation time of occurrence of an ordered domain at one of the {420}FCC nucleation sites and the rate of its thickness-increase were evaluated to be 520±260 s and 0.10±0.08 A/s, respectively. Surface layers of a few to fifteen angstrom in thickness with lower degree of order were formed upon annealing for a few minutes at 920–860°C. The degree of order in the near-surface also decreased upon annealing partially ordered tips at the same temperatures.

In situ observation of epitaxial growth of [Au/Co/Cu] and [Cu/Co/Au] superlattices and their magnetic interface anisotropies

We studied crystal growth of [Au/Co/Cu](111) and [Cu/Co/Au](111) superlattices during molecular-beam epitaxy and their magnetic anisotropies, and discussed the relationships between the interface structures and the perpendicular magnetic anisotropies. To study the structure at or near the interface of the superlattices, we continuously observed the change of surface in-plane lattice constant during growth using reflection high-energy electron diffraction (RHEED) on a real-time basis. From the RHEED observations, we deduce that gradually decreasing strain in the thickness direction exists in the Co layers at the Co/Au interfaces in the [Cu/Co/Au] superlattices, in which the Co layers are grown on the Au layers, and that coherency strain due to the Cu underlayers and strain due to the Au overlayers coexist in the Co layers in the [Au/Co/Cu] superlattices. From the magnetic measurements and detailed considerations, we conclude that both the magnetocrystalline interface anisotropy (or the Neel-type magnetic s...

Influence of base pressure on structure and magnetoresistance in sputter-deposited Co/Cu metallic multilayers

Abstract Structures and magnetoresistance (MR) of sputtered Co/Cu multilayers were studied to clarify the influence of the base pressure during film synthesis by preparing the samples under ultrahigh vacuum (UHV) and high vacuum (HV) conditions, with Ar pressures kept constant. X-ray diffraction (XRD) scans revealed that the films were composed of 〈111〉- and 〈100〉-oriented textures in the both cases, and the fraction of the 〈100〉 columns was much larger in the HV samples. Secondary ion mass spectroscopy analyses revealed higher oxygen intensities in the HV samples especially in the early stages of film synthesis. The XRD indicated that periodic structures were formed in all samples, and suggested that oxygen atoms were localized at CoCu interfaces and grain boundaries in the HV samples. Transmission electron microscopy observations disclosed that there was no significant difference in CoCu interface roughness between UHV and HV samples. MR measurements showed that the MR ratio was larger in the UHV samples. The difference in the MR ratio was mainly due to the difference in saturation field resistivity and in orientation of textures, but about 7% difference remained even after subtracting these factors. This was presumably ascribed to the more ferromagnetic character of the HV samples, possibly brought about by the impurity oxygen.

Large Magnetic Interface Anisotropy in Molecular Beam Epitaxy Grown Pt/Co and Pt/Co/Ag Superlattices

The magnetic anisotropy of (111)Pt/Co and Pt/Co/Ag superlattices (SLs) grown on 7×7 -(111)Si by molecular beam epitaxy was studied. The interface anisotropy K s of Pt/Co SLs is found to be 1.29 mJ/m2, which is much larger than the value reported previously. The effective anisotropy energy K ef f of Pt/Co/Ag SLs is similar to the average of K ef f values of Pt/Co and Ag/Co SLs with the same Co layer thickness. This suggests that the origins of perpendicular anisotropy in the Pt/Co system are different from those in the Au/Co system investigated earlier, in which the difference between the K ef f values of Au/Co and Au/Co/Ag SLs is small.