Yuri Nukaga

Structure and Magnetic Properties of Co Epitaxial Thin Films Grown on MgO Single-Crystal Substrates

Co epitaxial thin films were prepared on single-crystal MgO substrates of (100), (110), and (111) planes heated at 300degC by ultra high vacuum molecular beam epitaxy. The film crystal structure varies depending on the substrate orientation. On MgO(100) substrate, a Co(112macr0) epitaxial bi-crystalline film with an hcp structure is obtained. A Co(110) single-crystal film with an fcc structure is formed on MgO(110) substrate, whereas a Co(0001) single-crystal film with an hcp structure is grown on MgO(111) substrate. Co film growth on MgO substrates follows the Volmer-Weber mode. The in-plane magnetization properties reflect the magnetocrystalline anisotropy of bulk hcp-Co or fcc-Co crystal. The Gilberts damping constants of these 40-nm-thick epitaxial Co films are determined.

Preparation of Co(0001)hcp and (111)fcc Films on Single-Crystal Oxide Substrates

Co thin film were prepared on oxide single-crystal substrates of Al2O3(0001)D51, MgO(111)B1, and SrTiO3(111)E21 by ultra high vacuum molecular beam epitaxy. Effects of substrate material and substrate temperate on the film growth and the crystallographic properties were investigated. Co epitaxial thin films of hcp(0001) and/or fcc(111) orientations are obtained on all the substrate. With increasing the substrate temperature, the volume ratio of hcp to fcc increases for the Co films grown on Al2O3 and MgO substrates, whereas the ratio decrease for the Co film grown on SrTiO3 substrate. The in-plane lattice strain is larger than the out-of-plane strain due to accommodation of lattice mismatch between the film and the substrate

Effects of substrate temperature and Cu underlayer thickness on the formation of SmCo5(0001) epitaxial thin films

SmCo{sub 5}(0001) epitaxial thin films were prepared on Cu(111) underlayers heteroepitaxially grown on Al{sub 2}O{sub 3}(0001) single-crystal substrates by molecular beam epitaxy. The effects of substrate temperature and Cu underlayer thickness on the crystallographic properties of SmCo{sub 5}(0001) epitaxial films were investigated. The Cu atoms of underlayer diffuse into the SmCo{sub 5} film and substitute the Co sites in SmCo{sub 5} structure forming an alloy compound of Sm(Co,Cu){sub 5}. The ordered phase formation is enhanced with increasing the substrate temperature and with increasing the Cu underlayer thickness. The Cu atom diffusion into the SmCo{sub 5} film is assisting the formation of Sm(Co,Cu){sub 5} ordered phase.

Effects of Co/Sm Composition on the Ordered Phase Formation in Sm-Co Thin Films Grown on Cu(111) Single-Crystal Underlayers

Sm-Co thin films were prepared on Cu(111) single-crystal underlayers by changing the Sm composition from 12 to 35 at.% and the effects of Co/Sm composition on the ordered phase formation were investigated. The epitaxial SmCo5 thin films were obtained for all samples. The SmCo5 epitaxial layer grown on Cu underlayer has two different types of domains whose orientations are rotated around film normal by 30 degrees to each other. Cu atoms substitute a part of the Co sites in the SmCo5 structure, and an alloy compound of Sm(Co,Cu) 5 phase was formed for all samples. The Sm(Co,Cu)5 phase was more Cu enriched with increasing the Sm composition. The Cu atom diffusion into Sm-Co layer plays an important role in assisting the formation of Sm(Co,Cu)5 ordered phase.

Microstructure of Co(112̄0) epitaxial thin films, grown on MgO(100) single-crystal substrates

Co(110) epitaxial thin films with hcp structure were prepared on MgO(100) single-crystal substrates heated at 300 °C by ultra high vacuum molecular beam epitaxy. The microstructure is investigated by employing X-ray diffraction and high-resolution transmission electron microscopy. The film consists of two types of domains whose c-axes are rotated around the film normal by 90° each other. Stacking faults are observed for the film along the Co[0001] direction. An atomically sharp boundary is recognized between the film and the substrate, where some misfit dislocations are introduced in the film at the Co/MgO interface. Dislocations are also observed in the film up to 15 nm thickness from the interface. Presence of such stacking faults and misfit dislocations seem to relieve the strain caused by the lattice mismatch between the film and the substrate. X-ray diffraction analysis indicates that the out-of-plane and the in-plane lattice spacings of the film are in agreement within 0.5% and 0.1%, respectively, with those of the bulk hcp–Co crystal, suggesting the strain in the film is very small.

Preparation and structure characterization of SmCo5(0001) epitaxial thin films grown on Cu(111) underlayers

SmCo5(0001) epitaxial films were prepared on Cu(111) single-crystal underlayers formed on Al2O3(0001) substrates at 500 °C. The nucleation and growth mechanism of (0001)-oriented SmCo5 crystal on Cu(111) underlayer is investigated and a method to control the nucleation is proposed. The SmCo5 epitaxial thin film formed directly on Cu underlayer consists of two types of domains whose orientations are rotated around the film normal by 30° each other. By introducing a thin Co seed layer on the Cu underlayer, a SmCo5(0001) single-crystal thin film is successfully obtained. Nucleation of SmCo5 crystal on Cu underlayer seems controllable by varying the interaction between the Cu underlayer and the SmCo5 layer.

Epitaxial growth of fcc-CoxNi100−x thin films on MgO(110) single-crystal substrates

Co x Ni 100 − x ( x = 100 , 80, 20, 0 at. %) epitaxialthin films were prepared on MgO(110) single-crystal substrates heated at 300 ° C by ultrahigh vacuum molecular beam epitaxy. The growth mechanism is discussed based on lattice strain and crystallographic defects. CoNi(110) single-crystalfilms with a fcc structure are obtained for all compositions. Co x Ni 100 − x film growth follows the Volmer–Weber mode. X-ray diffraction analysis indicates that the out-of-plane and the in-plane lattice spacings of the Co x Ni 100 − x films are in agreement within ±0.5% with the values of the respective bulk Co x Ni 100 − x crystals, suggesting that the strain in the film is very small. High-resolution cross-sectional transmission microscopy shows that an atomically sharp boundary is formed between a Co ( 110 ) fcc film and a MgO(110) substrate, where periodical misfit dislocations are preferentially introduced in the film at the Co/MgO interface. The presence of such periodical misfit dislocations relieves the strain caused by the lattice mismatch between the film and the substrate.

Formation of SmFe5(0001) ordered alloy thin films on Cu(111) single-crystal underlayers

SmFe5(0001) single-crystal thin films are prepared by molecular beam epitaxy employing Cu(111) single-crystal underlayers on MgO(111) substrates. The Cu atoms diffuse into the Sm-Fe layer and substitute the Fe sites in SmFe5 structure forming an alloy compound of Sm(Fe,Cu)5. The Sm(Fe,Cu)5 film is more Cu enriched with increasing the substrate temperature. The Cu underlayer plays an important role in assisting the formation of the ordered phase.

Preparation and characterization of Co epitaxial thin films on Al2O3(0001) single-crystal substrates

Co epitaxial thin films were prepared on Al2O3(0001) single-crystal substrates in a substrate temperature range between 50 and 500 °C by ultra high vacuum molecular beam epitaxy. Effects of substrate temperature on the structure and the magnetic properties of the films were investigated. The films grown at temperatures lower than 150 °C consist of fcc- Co(111) crystal. With increasing the substrate temperature, hcp-Co(0001) crystal coexists with the fcc crystal and the volume ratio of hcp to fcc crystal increases. The films prepared at temperatures higher than 250 °C consist primarily of hcp crystal. The film growth seems to follow island-growth mode. The films consisting primarily of hcp crystal show perpendicular magnetic anisotropy. The domain structure and the magnetization properties are influenced by the magnetocrystalline anisotropy and the shape anisotropy caused by the film surface roughness.

Preparation of SmNi5 and Sm(Ni,T)5[T=Co,Fe] ordered alloy thin films on Cu(111) underlayers

SmNi 5 and Sm ( Ni , T ) 5 [ T = Co , Fe ] epitaxialthin films with the c -axis perpendicular to the substrate surface are successfully prepared on Cu underlayers heteroepitaxially grown on MgO(111) single-crystal substrates. The nucleation behavior of SmNi 5 and Sm ( Ni , T ) 5 crystals on Cu(111) underlayers were investigated. The SmNi 5 and the Sm ( Ni , Co ) 5 films consist of two types of domains whose orientations are rotated around the film normal by 30° each other, whereas the Sm ( Ni , Fe ) 5 film is a single crystal. The nucleation and the domain volume ratio seem to be controllable by adjusting the kind and the composition of transition metal elements.