arXiv subjects

Keisuke Ishigami

Publications and source records attributed to Keisuke Ishigami.

5 recordsLinked to original sources

arXiv2020

Temperature evolution of magnetic phases near the thickness-dependent metal-insulator transition in La$_{1-x}$Sr$_x$MnO$_3$ thin films observed by XMCD

Perovskite-type manganites, which are well-known for their intriguing physical properties such as colossal magnetoresistance (CMR) and half metalicity, have been considered as candidate materials for spintronics. However, their ferromagnetic (FM) properties are often suppressed in thin films when the thickness is reduced down to several monolayers (MLs). In order to investigate how the magnetic phases evolve near the paramagnetic (PM)-to-FM phase transition boundary, we have performed temperature-dependent x-ray magnetic circular dichroism (XMCD) experiments on a La$_{1-x}$Sr$_{x}$MnO$_3$ (LSMO, $x=0.4$) thin film, whose thickness (8 ML) is close to the boundary between the FM-metallic and the PM-insulating phases. By utilizing the element-selectiveness of XMCD, we have quantitatively estimated the fractions of the PM and superparamagnetic (SPM) phases as well as the FM one as a function of temperature. The results can be reasonably described based on a microscopic phase-separation model.

cond-mat.mtrl-sci↗

arXiv2018

Anisotropic Charge Distribution Induced by Spin Polarization in La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ Thin Films Studied by X-ray Magnetic Linear Dichroism

Magnetic anisotropy of epitaxially grown thin films is affected by the strain from the substrates due to a combined effect of distorted electronic structure and spin-orbit interaction (SOI). As an inverse process, one expects an anisotropy of the electronic structure induced by magnetization in the presence of SOI. We have studied the charge-density anisotropy induced by magnetization in thin films of the ferromagnetic metal La$_{1-x}$Sr$_{x}$MnO$_3$ via x-ray magnetic linear dichroism (XMLD). XMLD measurements on thin films with various thicknesses have shown that the XMLD intensity is proportional to the square of the ferromagnetic moment. Using the XMLD sum rule and cluster-model calculation, it has been shown that more Mn 3$d$ electrons are distributed in orbitals elongated along the direction parallel to the spin polarization than in orbitals elongated in the direction perpendicular to it. The cluster-model calculation has shown that the effect of tensile strain from the SrTiO$_3$ substrate on the XMLD spectra is also consistent with the observed XMLD spectral line shapes.

cond-mat.mtrl-sci↗

arXiv2018

Electronic States and Possible Origin of the Orbital-Glass State in a Nearly Metallic Spinel Cobalt Vanadate: An X-ray Magnetic Circular Dichroism Study

We have investigated the orbital states of the orbital-glassy (short-range orbital ordered) spinel vanadate Co$_{1.21}$V$_{1.79}$O$_{4}$ using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and subsequent configuration-interaction cluster-model calculation. From the sign of the XMCD spectra, it was found that the spin magnetic moment of the Co ion is aligned parallel to the applied magnetic field and that of the V ion anti-parallel to it, consistent with neutron scattering studies. It was revealed that the excess Co ions at the octahedral site take the trivalent low-spin state, and induce a random potential to the V sublattice. The orbital magnetic moment of the V ion is small although finite, suggesting that the ordered orbitals mainly consists of real-number orbitals.

cond-mat.str-el↗

arXiv2015

Room temperature local ferromagnetism and nanoscale domain growth in the ferromagnetic semiconductor GeFe

We investigate the local electronic structure and magnetic properties of the group IV based ferromagnetic semiconductor, GeFe, using soft X ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have an unusually large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin = 0.3. We find that local ferromagnetic domains, which are formed through ferromagnetic exchange interactions in the high Fe content regions of the GeFe films, exist at room temperature, well above the Curie temperature. We demonstrate the first observation of the intriguing nanoscale domain growth process in which ferromagnetic domains expand as the temperature decreases, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2014

Thickness-dependent ferromagnetic metal to paramagnetic insulator transition in La$_{0.6}$Sr$_{0.4}$MnO$_3$ thin films studied by x-ray magnetic circular dichroism

Metallic transition-metal oxides undergo a metal-to-insulator transition (MIT) as the film thickness decreases across a ritical thickness of several monolayers (MLs), but its driving mechanism remains controversial. We have studied the thickness-dependent MIT of the ferromagnetic metal La$_{0.6}$Sr$_{0.4}$MnO$_3$ by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. As the film thickness was decreased across the critical thickness of the MIT (6-8 ML), a gradual decrease of the ferromagnetic signals and a concomitant increase of paramagnetic signals were observed, while the Mn valence abruptly decreased towards Mn$^{3+}$. These observations suggest that the ferromagnetic phase gradually and most likely inhomogeneously turns into the paramagnetic phase and both phases abruptly become insulating at the critical thickness.

cond-mat.str-el↗