J. Echigoya

Magnetization curves of plastically deformed Fe metals and alloys

The hysteresis loop of plastically deformed Fe metals and alloys was measured for single crystals, polycrystalline iron, and A533B steel samples, and the dislocation density and structure of these samples were observed by electron microscopy. The relation between structure-sensitive properties and applied stress σ was analyzed in connection with grain boundaries and dislocations. The coercive force Hc increases with σ and is represented by one curve in all the samples. The magnetic susceptibility χc above the coercive field can be described by a simple relation to the magnetic field H. χc=c/H3 in a limited region of H. The parameter c depends only on lattice defects such as dislocations and the grain size and has a simple relation to them, but is independent of the kinds of samples or of the process of plastic deformation. The susceptibility χc is explained in comparison with the susceptibility χr in the range of approach to saturation. The magnetic properties c and Hc are discussed considering the disloc...

Directional solidification and interface structure of BaTiO3-CoFe2O4 eutectic

The directional solidification and the interface structure of BaTiO3-CoFe2O4 eutectic grown by the floating zone melting method were investigated. The microstructure of the eutectic were found to consist grains of lamellar or fibrous morphology. Two types of crystal structures of BaTiO3 were observed for different growth conditions. The orientation relationships between BaTiO3 and CoFe2O4 (the spinel-type crystal structure) are a) hcp BaTiO3: (111) CoFe2O4 // (001) BaTiO3 and (110) CoFe2O4 // (110) BaTiO3, and b) tetra/cubic BaTiO3: (001) CoFe2O4 // (001) BaTiO3 and (100) CoFe2O4 // (100) BaTiO3. The interface has misfit dislocations due to the accommodation of lattice mismatch.

Effect of antiferromagnetic grain size on exchange-coupling field of Cr70Al30/Fe19Ni81 bilayers

The exchange-coupling mechanism of Cr70Al30/Fe19Ni81 bilayer films was investigated in terms of the crystallographic orientation, the interface roughness, and the grain size. The exchange-coupling field Hex appears when the Cr70Al30 layer is thick enough to accommodate an antiferromagnetic domain wall. The value of Hex was found to increase with decreasing the grain size in association with the [110] orientation. This tendency can be explained by considering the exchange-coupling field evaluated from the random field approximation.

Detection of martensite transformation in high temperature compressively deformed austenitic stainless steel by magnetic NDE technique

The present work demonstrates that a magnetic non-destructive evaluation technique can be useful for detecting the presence and extent of ferromagnetic α′ martensitic phase in high temperature deformed 304 austenitic stainless steel.A good correlation between the martensitic transformation and magnetic parameters; saturation magnetization, coercive force and magnetic susceptibility have been obtained. Saturation magnetization was increased depending on the volume percentage of α′ martensite transformation. The volume percentage of α′ martensite was found to be dependent on the temperature and level of plastic strain. At temperatures below 623 K, martensitic transformation was detected after deformation of 10 to 40% plastic strain. A massive increase in α′ martensite phase was observed in the specimen deformed at RT to 40% plastic strain.Compressive deformation at RT formed thermodynamically more stable long and broad shape of martensite. But as the temperature of deformation increases lath shape gradually converted into needle shape.Coercive force was exclusively connected with size, shape and spatial distribution of martensite. For specimens deformed at 523 K coercive force were much higher than those of the specimens deformed at RT. Coercive force decreased remarkably at temperatures above 623 K. Formation of ferromagnetic α′ martensite in a paramagnetic matrix was also accompanied by an increase in magnetic susceptibility. Low magnetic susceptibility at temperatures above 623 K was due to disappearance of martensitic phase.

The Néel temperature of Mn100−xRhx alloys in ordered and disordered states

The concentration dependence of the room-temperature lattice constant and the Neel temperature of Mn100−xRhx (15⩽x⩽30) disordered and ordered alloys was investigated in order to clarify the difference in the antiferromagnetic properties. The room-temperature lattice constant increases with increasing x and the difference between the disordered and ordered alloys was hardly observed. The Neel temperature increases with increasing x in both the disordered and ordered states. This behavior is not explained by the variation of the strength of the exchange interaction as a function of the Mn–Mn distance but by the number of 3d electrons in the Mn site. The Neel temperature in the ordered state is 130–160 K higher than that in the disordered state, indicating that the exchange interaction is enhanced by ordering.

In situ growth of Co nanofibers in In2O3–SnO2 matrix during sputtering deposition

Co-sputtering onto (001) cubic zirconia from a target of indium tin oxide (ITO) partially covered by cobalt (Co) was carried out at substrate temperatures of 470–770K in order to investigate the growth of Co nanofibers. During film growth, Co forms fibers in the growth direction in the single-crystalline ITO matrix. The cross section of the Co fibers, the size of which depends on the substrate temperature, was a rectangle with an edge 1–5nm in length. The edge length of the Co fiber increased with the increase of the substrate temperature. The present method is attractive for application to produce magnetic recording media.

Effect of cold rolling and subsequent annealing on hot pressed Ni/Al laminates

Nickel/Aluminum laminates have been prepared by hot pressing of stacked alternate layers of very thin foils of nickel and aluminium. It is found that ductile nickel layers exist in hot pressed Ni/Al laminates which are annealed up to 600 °C. However, at these temperatures Al-rich intermetallic compounds are also formed with a very thin irregular layer of pure aluminium and few voids. As the annealing temperature is increased, nickel layer is consumed and Ni-rich intermetallic compounds are formed. Cold rolling up to 8% reduction in area resulted in the cracking of the regions having intermetallic compounds with cracks perpendicular to the rolling direction. The tensile strength values are high for the laminates which contain ductile Ni layers and reach a value of 320 MPa for the specimens which are hot pressed at 420 °C. However, the laminates which are annealed at 800 °C show brittle fracture and lower tensile strength of 174 MPa. The fractographic examination indicated that Ni/Al laminates which are hot pressed at 420 °C undergo plastic deformation before breaking. On the other hand, Ni/Al laminates which are annealed at 800 °C with Ni present in the form of intermetallic compounds only, exhibit brittle intergranular fracture.

Growth of diamond films on SiC, WC and cubic BN substrates

The growth morphology of diamond films grown on single crystals of SiC and on sintered WC and cubic BN (CBN) substrates by hot filament assisted chemical vapour deposition was examined using transmission electron microscopy and scanning electron microscopy. Diamond was found to have the form of particles on the substrates of SiC and WC in the initial stage of film growth. Both an amorphous layer and a directly bonded area were seen at the interface. Several orientation relationships, different from the cube/cube relation, were observed in these systems. On the other hand, in the case of diamond films on CBN substrates, the growth morphology of diamond was affected by the surface condition of the substrates. When CBN substrates were polished with a diamond paste before deposition, diamond grew in the form of particles. The growth morphology was changed by ion sputtering of the surface of the substrate from particle growth to uniform film growth. These results are discussed on the basis of lattice mismatch at the interface.

Magnetic properties of ion irradiated epitaxial Fe films

Magnetic properties of a heavy-ion irradiated single crystalline iron film were investigated. A high quality Fe (001) film with a thickness of 250 nm was fabricated on MgO (001) using the molecular beam epitaxy technique. The film was irradiated by 3.2 MeV Ni ions at room temperature using a tandem accelerator. Formation of dislocation loops with nanometer size was observed by TEM observation, and that of sub-nanometer size vacancy clusters was confirmed indirectly from a resistivity increase. However, M-H hysteresis curves and magnetic domain structure did not change significantly. These results indicate the formation of irradiation defects of pure iron in nanometer scale range has little influence on the magnetization process of the iron.

Effect of buffer layer on antiferromagnetic grain size and exchange-coupling field of Cr70Al30/Fe19Ni81 bilayers

MgO/Cr70Al30/Fe19Ni81 and ZnO/Cr70Al30/Fe19Ni81 layered films with different buffer layer thicknesses were prepared on (100)Si substrates in order to investigate the relationship among the exchange-coupling field, the grain size, and the lattice constant of the antiferromagnetic layer. The exchange-coupling field was found to consist of the intragrain exchange coupling and the intergrain coupling fields. The former field is inversely proportional to the product of the lattice constant and the grain size. The latter field is constant and changes its sign in association with the increase of the buffer layer thickness.