X-ray magnetic circular dichroism (XMCD) is a powerful method to study the magnetic properties of magnetic thin films. However it provides depth-averaged information. Recently, we have developed a simple but effective depth-resolved technique in XMCD by using an imaging type microchannel plate detector. The performance of this method is demonstrated by applying to the depth-resolved analyses of Ni/Cu(100) and the effect of CO adsorption on Ni/Cu(100), Fe/Cu(100) and Fe/Ni/Cu(100) magnetic thin films. For Ni/Cu(100), magnetic orbital moments of the surface and internal layers are extracted to give deep insight for the spin reorientation transitions. For 7 ML Fe/Cu(100), the surface two layers are ferromagnetically coupled, while the inner layers are non-magnetic at 200 K and are in the spin density wave (SDW) state at 130 K. For Fe/Ni/Cu(100), magnetically live surface layers and some thickness-dependent magnetic coupling between the Fe surface and Ni film have been observed.
Spin-polarized scanning electron microscopy is one of the magnetic domain observation methods. It has excellent capabilities, including high spatial resolution, high surface sensitivity, and detection of magnetization direction. Taking advantage of these capabilities, we studied the magnetism of various materials. In this paper, I select three topics from our research works, and show the results. The first is a ferromagnetism appearing at the bulk-paramagnetic material surface. The sample is FeO formed by oxidation of Fe(110). We found that the FeO surface is reconstructed (111), and ferromagnetism is localized at this surface with magnetization antiparallel to that of the underlying Fe(110). The second is an exchange coupling between ferro-and antiferromagnetic materials. Using Fe/NiO(001) we found 90o coupling between ferromagnetic Fe spin and antiferromagnetic NiO spin, which is proposed by the theory. The third is the magnetic structure of a layered manganite of La1.4Sr1.6Mn2O7. We found that it was spatially separated into two phases: a major phase of layered antiferromagnetism and a minor phase of ferromagnetism. The spin direction of the antiferromagnetic phase area is laid down form perpendicular to parallel to the layer plane with the increase of temperature.
Recent first-principles studies on antiferromagnetic oxide surfaces of MnO(001) and NiO(001) are reviewed. By structural optimization for the surface relaxation, small surface contraction and rampling relaxation where the surface transition-metal ions are displaced outwards are obtained. Surface electronic structures of the both oxides are still insulating and the spin magnetic moment at the transition-metal site is almost unchanged from its bulk value. The most interesting feature is the existence of finite spin magnetic moment at the surface oxygen site despite of no moment at oxygen in bulk by symmetry. The surface spin density distribution may be possibly observed by using exchange force microscopy technique recently proposed.
Competition between surface reconstruction and magnetic ordering plays an important role to determine geometries and electronic properties of semiconductor surfaces. Thus, it is expected that magnetic ordering is enhanced under certain conditions in which the surface reconstruction is suppressed. Here, we find that the magnetic ordering of the dangling-bond (DB) networks on the hydrogenated Si(111) surfaces is realized by controlling the DB network topology (or the deposition of H atoms) which suppresses flexibility of the surface reconstruction. The density functional calculations reveal that a triangular DB unit exhibits a high spin state and that two different types of hexagonal networks consisting of the DB unit exhibit ferrimagnetic ordering. The magnetic ordering on DB nanostructures on Si surfaces may be promising elements for the nanoscale Si based spintronic devices in future.
Spin-polarized metastable atom deexcitation spectroscopy (SPMDS) demonstrates the ultimate surface sensitivity for measuring electron spin polarizations. We firstly explain the principles and the technical aspects to detect surface electron spins with spin-polarized metastable He atom beams. We then apply SPMDS to the studies on the spin polarization of Na- and oxygen-adsorbed Fe(100) surfaces, the spin polarization induced in copper phthalocyanine (CuPc) molecules, and the magnetism of perpendicularly magnetized Fe ultrathin films on Cu(100) substrates.
We have tried to clarify the magnetic property of a c(2×2) CuMn two-dimensional ordered surface alloy, which has been probed by Mn L23 X-ray magnetic circular dichroism (XMCD) spectra using a helicity modulation technique for the incident circularly polarized synchrotron radiation at BL 25 SU of SPring-8. In this work, a local magnetic susceptibility has been firstly applied to this surface alloy system, which has been deduced from the temperature dependence of the XMCD spectra. It is found that the local magnetic susceptibility above 40 K obeys the Curie-Weiss law. The evaluated positive value of the Weiss temperature Θ = 20 K clearly suggests the ferromagnetic interaction between the Mn atoms, although the expected Curie temperature is much reduced compared to the predicted value.
Spontaneous growth of whiskers from electroplated zinc is known to occur at room temperature. Since whiskers may occasionally cause short-circuiting, prevention of the whisker growth is a technologically important problem in the electronics industry. Recently, a new problem that zinc whiskers from zinc plated props at raised access floor are transported into a computer through the cooling fan and cause shut down of the computer, is occurred. High concentrations of cyanide in zinc baths contribute to the role of the brightener and lead to declined internal stress levels in the plating, which significantly affects the inhibition of whisker growth.