Naomi Kawamura

Direct Observation of Ferromagnetic Spin Polarization in Gold Nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.

Colossal negative thermal expansion in BiNiO3 induced by intermetallic charge transfer

The unusual property of negative thermal expansion is of fundamental interest and may be used to fabricate composites with zero or other controlled thermal expansion values. Here we report that colossal negative thermal expansion (defined as linear expansion <−10−4 K−1 over a temperature range ~100 K) is accessible in perovskite oxides showing charge-transfer transitions. BiNiO3 shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer that is shifted to ambient pressure through lanthanum substitution for Bi. Changing proportions of coexisting low- and high-temperature phases leads to smooth volume shrinkage on heating. The crystallographic linear expansion coefficient for Bi0.95La0.05NiO3 is −137×10−6 K−1 and a value of −82×10−6 K−1 is observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet. Colossal negative thermal expansion materials operating at ambient conditions may also be accessible through metal-insulator transitions driven by other phenomena such as ferroelectric orders.

Chemically Induced Permanent Magnetism in Au, Ag, and Cu Nanoparticles: Localization of the Magnetism by Element Selective Techniques

We report a direct observation of the intrinsic magnetization behavior of Au in thiol-capped gold nanoparticles with permanent magnetism at room temperature. Two element specific techniques have been used for this purpose: X-ray magnetic circular dichroism on the L edges of the Au and 197Au Mössbauer spectroscopy. Besides, we show that silver and copper nanoparticles synthesized by the same chemical procedure also present room-temperature permanent magnetism. The observed permanent magnetism at room temperature in Ag and Cu dodecanethiol-capped nanoparticles proves that the physical mechanisms associated to this magnetization process can be extended to more elements, opening the way to new and still not-discovered applications and to new possibilities to research basic questions of magnetism.

HELICITY-MODULATION TECHNIQUE USING DIFFRACTIVE PHASE RETARDER FOR MEASUREMENTS OF X-RAY MAGNETIC CIRCULAR DICHROISM

A diamond X-ray phase retarder has been combined with a phase-sensitive detection method to improve the accuracy of X-ray magnetic circular dichroism (XMCD) measurements. Fluctuation of the absorption coefficient of a magnetized sample was induced by photon-helicity switching at 40 Hz by flipping the phase retarder, and directly detected through a lock-in amplifier referring to the helicity-modulation frequency. This new technique for XMCD measurements was applied to the Fe K-edge in pure Fe foil. XMCD spectra of higher signal-to-noise ratio were obtained in shorter measuring time.

CaFeO2: a new type of layered structure with iron in a distorted square planar coordination.

CaFeO(2), a material exhibiting an unprecedented layered structure containing 3d(6) iron in a high-spin distorted square-planar coordination, is reported. The new phase, obtained through a low-temperature reduction procedure using calcium hydride, has been characterized through powder neutron diffraction, synchrotron X-ray diffraction, Mossbauer spectroscopy, XAS experiments as well as first-principles DFT calculations. The XAS spectra near the Fe-K edge for the whole solid solution (Sr(1-x)Ca(x))FeO(2) supports that iron is in a square-planar coordination for 0 </= x </= 0.8 but clearly suggests a change of coordination for x = 1. The new structure contains infinite FeO(2) layers in which the FeO(4) units unprecedentedly distort from square-planar toward tetrahedra and rotate along the c-axis, in marked contrast to the well-studied and accepted concept that octahedral rotation in perovskite oxides occurs but the octahedral shape is kept almost regular. The new phase exhibits high-spin configuration and G-type antiferromagnetic ordering as in SrFeO(2). However, the distortion of the FeO(2) layers leads to only a slight decrease of the Neel temperature with respect to SrFeO(2). First-principles DFT calculations provide a clear rationalization of the structural and physical observations for CaFeO(2) and highlight how the nature of the cation influences the structural details of the AFeO(2) family of compounds (A = Ca, Sr, Ba). On the basis of these calculations the driving force for the distortion of the FeO(2) layers in CaFeO(2) is discussed.

Single-crystal epitaxial thin films of SrFeO2 with FeO2 “infinite layers”

Single-crystal thin films of SrFeO2, which is an oxygen-deficient perovskite with “infinite layers” of Fe2+O2, were prepared by using CaH2 for low-temperature reduction of epitaxial SrFeO2.5 single-crystal films deposited on KTaO3 substrates. This reduction process, removing oxygen ions from the perovskite structure framework and causing rearrangements of oxygen ions, topotactically transforms the brownmillerite SrFeO2.5 to the c-axis oriented SrFeO2.

Glitch-free X-ray absorption spectrum under high pressure obtained using nano-polycrystalline diamond anvils

Nano-polycrystalline diamond (NPD) [Irifune et al. (2003), Nature (London), 421, 599] has been used to obtain a glitch-free X-ray absorption spectrum under high pressure. In the case of conventional single-crystal diamond (SCD) anvils, glitches owing to Bragg diffraction from the anvils are superimposed on X-ray absorption spectra. The glitch has long been a serious problem for high-pressure research activities using X-ray spectroscopy because of the difficulties of its complete removal. It is demonstrated that NPD is one of the best candidate materials to overcome this problem. Here a glitch-free absorption spectrum using the NPD anvils over a wide energy range is shown. The advantage and capability of NPD anvils is discussed by a comparison of the glitch map with that of SCD anvils.

Magnetic dichroism in angle-resolved hard x-ray photoemission from buried layers

This work reports the measurement of magnetic dichroism in angular-resolved photoemission from in-plane magnetized buried thin films. The high bulk sensitivity of hard x-ray photoelectron spectroscopy (HAXPES) in combination with circularly polarized radiation enables the investigation of the magnetic properties of buried layers. HAXPES experiments with an excitation energy of 8 keV were performed on exchange-biased magnetic layers covered by thin oxide films. Two types of structures were investigated with the IrMn exchange-biasing layer either above or below the ferromagnetic layer: one with a CoFe layer on top and another with a Co2FeAl layer buried beneath the IrMn layer. A pronounced magnetic dichroism is found in the Co and Fe 2p states of both materials. The localization of the magnetic moments at the Fe site conditioning the peculiar characteristics of the Co2FeAl Heusler compound, predicted to be a half-metallic ferromagnet, is revealed from the magnetic dichroism detected in the Fe 2p states.

Reversible changes of epitaxial thin films from perovskite LaNiO3 to infinite-layer structure LaNiO2

An infinite-layer-structure epitaxial thin film of LaNiO2 was prepared by low-temperature reduction with CaH2 from a LaNiO3 epitaxial thin film grown on a SrTiO3(100) substrate. The oxygen content changed reversibly from the perovskite LaNiO3 to the infinite-layer LaNiO2 without losing the structural framework and the topotactic relationship. Consequently, the oxidation state of Ni ions in the film changed from trivalent to divalent to monovalent.

Polarization tunability and analysis for observing magnetic effects on BL39XU at SPring-8

Polarization tunability and analysis of X-rays is one of the most advancing features of third-generation synchrotron radiation sources. In order to apply such developments to the observation of magnetic effects, a diffractometer for X-ray magnetic absorption and scattering experiments was constructed on BL39XU at SPring-8. The efficiency of the apparatus is clearly demonstrated by several observations of the magnetic effects. In particular, a diamond phase plate plays an essential role in regulating both circular and linear polarization states.