Yasukazu Murakami

Low temperature crystal structure of Ni–Mn–Ga alloys

Transmission electron microscope (TEM) observation at room temperature of the low temperature phase of Ni2.14Mn0.86Ga shows stress-induced 5 periodicity modulation along a pseudo-[111] cubic zone axis. On cooling to 173 K, the periodicity changes to 7. In stress-free areas, a [001] zone axis with new lattice parameters for the tetragonal phase was observed. The new lattice parameters are smaller by a factor of √2 than the expected lattice parameters. A new model for the tetragonal compound Ni2MnGa and the non-stoichiometric phases was developed with the help of TEM data, X-ray powder diffraction measurements, and a computational simulation based on the X-ray powder diffraction patterns.

Magnetic domain structures in Co–Ni–Al shape memory alloys studied by Lorentz microscopy and electron holography

Abstract Magnetic domain structures in recently developed Co–Ni–Al ferromagnetic shape memory alloys were examined by Lorentz microscopy and electron holography, and relations of the martensite variants (crystallographic domains) and the magnetic domains were discussed. Direct observations of the magnetic domain walls by Lorentz microscopy and the magnetic lines of force by electron holography revealed that each martensite variant was divided into fine magnetic domains under a low magnetic field, e.g. about 0.2 mT. Although an applied magnetic field of about 0.4 T made each variant a large single magnetic domain, a similar configuration of multiple magnetic domains to the previous one appeared when the applied field was removed. In situ Lorentz microscopy studies have demonstrated that magnetic domain structures are sensitive to the crystal structure and/or microstructure in Co–Ni–Al alloys, i.e. a magnetic domain structure favorable to the parent phase is not inherited to the parent phase, but a distinct domain structure is observed in the martensitic phase.

Ferromagnetic domain nucleation and growth in colossal magnetoresistive manganite

Colossal magnetoresistance is a dramatic decrease in resistivity caused by applied magnetic fields, and has been the focus of much research because of its potential for magnetic data storage using materials such as manganites. Although extensive microscopy and theoretical studies have shown that colossal magnetoresistance involves competing insulating and ferromagnetic conductive phases, the mechanism underlying the effect remains unclear. Here, by directly observing magnetic domain walls and flux distributions using cryogenic Lorentz microscopy and electron holography, we demonstrate that an applied magnetic field assists nucleation and growth of an ordered ferromagnetic phase. These results provide new insights into the evolution dynamics of complex domain structures at the nanoscale, and help to explain anomalous phase separation phenomena that are relevant for applications. Our approach can also be used to determine magnetic parameters of nanoscale regions, such as magnetocrystalline anisotropy and exchange stiffness, without bulk magnetization results or neutron scattering data.

Magnetization distribution in the mixed-phase state of hole-doped manganites.

The effect of ‘colossal magnetoresistance’ (CMR) in hole-doped manganites—an abnormal decrease of resistivity when a magnetic field is applied—has attracted significant interest from researchers in the past decade. But the underlying mechanism for the CMR phenomenon is not yet fully understood. It has become clear that a phase-separated state, where magnetic and non-magnetic phases coexist, is important, but the detailed magnetic microstructure of this mixed-phase state is so far unclear. Here we use electron microscopy to study the magnetic microstructure and development of ferromagnetic domains in the mixed-phase state of La1-xSrxMnO3 (x = 0.54, 0.56). Our measurements show that, in the absence of a magnetic field, the magnetic flux is closed within ferromagnetic regions, indicating a negligible magnetic interaction between separated ferromagnetic domains. However, we also find that the domains start to combine with only very small changes in temperature. We propose that the delicate nature of the magnetic microstructure in the mixed-phase state of hole-doped manganites is responsible for the CMR effect, in which significant conduction paths form between the ferromagnetic domains upon application of a magnetic field.

Electron holography of Nd-Fe-B nanocomposite magnets

Abstract Magnetization distribution in Nd–Fe–B nanocomposite magnets was investigated by electron holography, using a new pole piece apparatus dedicated to observations of nanocrystalline ferromagnetic materials. The exchange coupling between the magnetically soft and hard grains of 20–30 nm was experimentally verified by this microscopic study with improved resolution.

Magnetic domain structure in a ferromagnetic shape memory alloy Ni51Fe22Ga27 studied by electron holography and Lorentz microscopy

Behaviors of magnetic domains with cooling in a Ni51Fe22Ga27 ferromagnetic shape memory alloy were examined by electron holography and Lorentz microscopy. A peculiar meshy pattern was observed in the Lorentz microscope image of the parent phase, being concurrent with the anomaly in the thermomagnetization curve. The meshy pattern was found to stem from the heavily bent lines of magnetic flux. The dramatic change in the magnetic domains is presumably due to some intrinsic magnetic instability that is pronounced by cooling, rather than a phenomenon triggered by the lattice modulation as the precursor effect of martensitic transformations or formation of the intermediate phase as observed in other systems.

Growth of SiGe bulk crystal with uniform composition by directly controlling the growth temperature at the crystal–melt interface using in situ monitoring system

Abstract A SiGe bulk crystal with uniform composition was successfully fabricated by clarifying and controlling the growth parameters at the crystal–melt interface. An apparatus was developed for the direct in situ observation and precise control of the interface parameters such as the temperature and the position. The dynamical change of the growth rate of a SiGe bulk crystal in a temperature gradient can be known by utilizing the apparatus. The growing crystal was continuously pulled down at the pulling rate balanced to the growth rate to keep the interface temperature constant, which resulted in the excellent uniformity of the grown crystal. Our technique opened the possibility to incorporate multicomponent semiconductor substrates to the semiconductor heterostructure technology.

Submicron-scale spatial feature of ultrafast photoinduced magnetization reversal in TbFeCo thin film

Submicron-scale spatial feature of magnetization reversal dynamics induced by femtosecond optical pulse irradiation in a small external magnetic field was investigated by time-resolved magneto-optical Kerr microscopy on TbFeCo thin film. The magnetization reversal time near the magnetic domain boundary is dominated by an effective magnetic field generated from the peripheral domain by dipole-dipole interaction. The magnetization reversal is accelerated as high as 4.5 times (from 3.4 ns to 750 ps) when reducing the reversed domain size from 1.5 to 0.4 μm due to concentration of dipole-dipole interaction.

Precursor effects of martensitic transformations in Ti-based alloys studied by electron microscopy with energy filtering.

Precursor effects of martensitic transformations in two well‐known shape memory alloys, Ti50Ni48Fe2 and Ti50Pd34Fe16, were studied extensively by energy‐filtered electron microscopy, including in‐situ observations, and high‐resolution electron microscopy. Energy‐filtered dark‐field images, where weak diffuse scattering was utilized, clearly showed the microstructure in the premartensitic state of Ti50Ni48Fe2. Tiny domains observable in this state were close to spherical rather than thin and slender, and the temperature dependence of the domain‐like structure was clarified by the in‐situ observations. It was found that Ti50Pd34Fe16 exhibited a domain‐like structure similar to that of Ti50Ni48Fe2, which was attributed to a transverse lattice displacement. High‐resolution images of Ti50Pd34Fe16 showed that a domain was coupled with other ones with different orientations of distortion, so as to reduce the total strain due to their formations. Furthermore, effects of including a fundamental reflection, in addition to the diffuse scattering, on dark‐field images were discussed based on the observations and the image processing.

Electron holography of magnetic materials

Electron holography, which visualizes magnetic and/or electric fields in materials on the nanometre scale, is a powerful tool for the study of fundamental issues in physics as well as the characterization of advanced materials. This paper presents an overview of recent electron holography studies on advanced magnetic materials, which include hard magnetic materials (both nucleation-type and pinning-type magnets), soft magnetic materials (both classical alloys and recently developed nanostructured materials), magnetic recording materials (Co–CoO tape and other related topics) and magnetic functional materials (ferromagnetic shape memory alloys and colossal magnetoresistive manganites).