arXiv subjects

T. Kimura

Publications and source records attributed to T. Kimura.

At least 73 records · Page 4Linked to original sources

arXiv2005

Switching magnetization of nano-scale ferromagnetic particle using non-local spin injection

We have performed non-local spin injection into a nano-scale ferromagnetic particle configured in a lateral spin valve structure to switch its magnetization only by spin current. The non-local spin injection aligns the magnetization of the particle parallel to the magnetization of the spin injector. The responsible spin current for switching is estimated from the experiment to be about 200 $μ$A, which is reasonable compared with the values obtained for conventional pillar structures. Interestingly the switching always occurs from anti-parallel to parallel in the particle/injector magnetic configurations, whereas no opposite switching is observed. Possible reasons for this discrepancy are discussed.

cond-mat.mtrl-sci↗

arXiv2005

Quantitative analysis of Sr2RuO4 ARPES spectra: Many-body interactions in a model Fermi liquid

ARPES spectra hold a wealth of information about the many-body interactions in a correlated material. However, the quantitative analysis of ARPES spectra to extract the various coupling parameters in a consistent manner is extremely challenging, even for a model Fermi liquid system. We propose a fitting procedure which allows quantitative access to the intrinsic lineshape, deconvolved of energy and momentum resolution effects, of the correlated 2-dimensional material Sr2RuO4. For the first time in correlated 2-dimensional materials, we find an ARPES linewidth that is narrower than its binding energy, a key property of quasiparticles within Fermi liquid theory. We also find that when the electron-electron scattering component is separated from the electron-phonon and impurity scattering terms it decreases with a functional form compatible with Fermi liquid theory as the Fermi energy is approached. In combination with the previously determined Fermi surface, these results give the first complete picture of a Fermi liquid system via ARPES. Furthermore, we show that the magnitude of the extracted imaginary part of the self-energy is in remarkable agreement with DC transport measurements.

cond-mat.str-el↗

arXiv2005

Estimation of spin-diffusion length from the magnitude of spin current absorption

We demonstrate the method to calculate the spatial distributions of the spin current and accumulation in the multi-terminal ferromagnetic/nonmagnetic hybrid structure using an approximate electro-transmission line. The analyses based on the obtained equation yield the results in good agreement with the experimental ones. This implies that the method allows us to determine the spin diffusion length of additionally connected electrically floating wire from the reduction of the spin signal.

cond-mat.mtrl-sci↗

arXiv2005

Magnetically driven ferroelectric order in Ni$_3$V$_2$O$_8$

We show that for Ni$_3$V$_2$O$_8$ long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition. The temperature and magnetic field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry of this system by constraining the data to be consistent with Landau theory for continuous phase transitions. This phenomenological theory explains our observation the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

cond-mat.mtrl-sci↗

arXiv2004

Tunable Thermal Expansion Behavior in the Intermetallic YbGaGe

We investigate the effects of carbon and boron doping on the thermal expansion in the hexagonal (P63/mmc) intermetallic YbGaGe. X-ray powder diffraction was used to measure the lattice constants on pure and doped (C or B at nominal levels of 0.5 %) samples from T~10 K to T~300 K. Also measured were resistivity, specific-heat, and magnetic susceptibility. While the pure YbGaGe samples exhibit positive thermal volume expansion, (V300K-V10K)/V300K = 0.94%, the volume expansion in the lightly C and B-doped samples, contract and tend towards zero volume expansion. Such a strong response with such light doping suggests that the underlying mechanism for the reported zero volume expansion is substitutional disorder, and not the previously proposed valence fluctuations.

cond-mat.mtrl-sci↗

arXiv2004

Current distribution inside Py/Cu lateral spin-valve device

We have investigated experimentally the non-local voltage signal (NLVS) in the lateral permalloy (Py)/Cu/Py spin valve devices with different width of Cu stripes. We found that NLVS strongly depends on the distribution of the spin-polarized current inside Cu strip in the vicinity of the Py-detector. To explain these data we have developed a diffusion model describing spatial (3D) distribution of the spin-polarized current in the device. The results of our calculations show that NLVS is decreased by factor of 10 due to spin flip-scattering occurring at Py/Cu interface. The interface resistivity on Py/Cu interface is also present, but its contribution to reduction of NLVS is minor. We also found that most of the spin-polarized current is injected within the region 30 nm from Py-injector/Cu interface. In the area at Py-detector/Cu interface, the spin-polarized current is found to flow mainly close on the injector side, with 1/e exponential decay in the magnitude within the distance 80 nm.

cond-mat.other↗

arXiv2004

3-dimensional distribution of spin-polarized current: application to (Cu/Co) pillar structures

We present a formalism determining spin-polarized current and electrochemical potential inside arbitrary electric circuit within diffusive regime for parallel/antiparallel magnetic states. When arbitrary nano-structure is expressed by 3-dimensional (3D) electric circuit, we can determine 3D spin-polarized current and electrochemical potential distributions inside it. We apply this technique to (Cu/Co) pillar structures, where pillar is terminated either by infinitely large Cu layer, or by Cu wire with identical cross-sectional area as pillar itself. We found that infinitely large Cu layers work as a strong spin-scatterers, increasing magnitude of spin-polarized current inside the pillar twice and reducing spin accumulation nearly to zero. As most experimentally studied pillar structures are terminated by such a infinitely large layers, we propose modification of standard Valet-Fert formalism to simply include influence of such infinitely large layers.

cond-mat.other↗

arXiv2004

Technique for bulk Fermiology by photoemission applied to layered ruthenates

We report the Fermi surfaces of the superconductor Sr2RuO4 and the non-superconductor Sr1.8Ca0.2RuO4 probed by bulk-sensitive high-energy angle-resolved photoemission. It is found that there is one square-shaped hole-like, one square-shaped electron-like and one circle-shaped electron-like Fermi surface in both compounds. These results provide direct evidence for nesting instability giving rise to magnetic fluctuations. Our study clarifies that the electron correlation effects are changed with composition depending on the individual band.

cond-mat.str-el↗

arXiv2004

Neutron diffraction study of TbMnO3: Magnetic structure revisited

Magnetic ordering in TbMnO3 was revisited by a neutron diffraction study. In addition to the previously reported A-type and G-type modulated structures with a propagation vector (0,q_Mn,0), we found C-type and F-type orderings of the Mn moments. All the components appear below T_N^Mn = 46 K, and the G, C,and F-type components are enhanced below T_lock = 28 K, where q_Mn ordering locks at its low temperature value. The locking of the propagation vector also yields the squaring up of the spin arrangement. The magnetic moments of Tb ions show a quasi long-range ordering below T_N^Tb = 7 K. It drastically promotes the development of the G, C, and F-type components while suppressing the A-type components.

cond-mat.str-el↗

arXiv2003

Field-Induced Thermal Metal-to-Insulator Transition in Underdoped LSCO

The transport of heat and charge in cuprates was measured in single crystals of La_{2-x}Sr_xCuO_{4+δ} (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T goes to 0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.

cond-mat.supr-con↗

arXiv2003

Distorted Perovskite with e_g^1 Configuration as a Frustrated Spin System

The evolution of spin- and orbital-ordered states has been systematically investigated for a series of insulating perovskites $R$MnO$_3$ ($R$=La,Pr,Nd,...). $R$MnO$_3$ with a large GdFeO$_3$-type distortion is regarded as a frustrated spin system having ferromagnetic nearest-neighbor and antiferromagnetic (AF) next-nearest-neighbor (NNN) interactions within a MnO$_2$ plane. The staggered orbital order associated with the GdFeO$_3$-type distortion induces the anisotropic NNN interaction, and yields the unique sinusoidal and $E$-type ("up-up-down-down") AF ordered states in the distorted perovskites with $e_g^1$ configuration.

cond-mat.str-el↗

arXiv2003

Thermal Conductivity across the Phase Diagram of Cuprates: Low-Energy Quasiparticles and Doping Dependence of the Superconducting Gap

Heat transport in the cuprate superconductors YBa$_2$Cu$_3$O$_{y}$ and La$_{2-x}$Sr$_x$CuO$_4$ was measured at low temperatures as a function of doping. A residual linear term kappa_{0}/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from kappa_{0}/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.

cond-mat.supr-con↗

arXiv2002

Resonant photoemission spectroscopy study of insulator-to-metal transition in Cr- and Ru-doped Nd_{1/2}A_{1/2}Mn_{1-y} O_{3} (A=Ca, Sr)

Electronic structures of very dilute Cr- or Ru-doped Nd_{1/2}A_{1/2}MnO_{3} (NAMO; A=Ca, Sr) manganites have been investigated using the Mn and Cr 2p -> 3d resonant photoemission spectroscopy (PES). All the Cr- and Ru-doped NAMO systems exhibit the clear metallic Fermi edges in the Mn e_g spectra near E_F, consistent with their metallic ground states. The Cr 3d states with t^3_{2g} configuration are at ~ 1.3 eV below E_F, and the Cr e_{g} states do not participate in the formation of the band near E_F. Cr- and Ru-induced ferromagnetism and insulator-to-metal transitions can be understood with their measured electronic structures.

cond-mat.str-el↗

arXiv2002

Magnetic Anisotropy in La_0.8Sr_0.2MnO_3: Electron Spin Resonance

We report on Ferromagnetic-Resonance experiments in a single crystal of La_0.8Sr_0.2MnO_3 in the temperature range from 4 to 300 K. The observed anisotropy of the resonance line changes on crossing the transition from the orthorhombic O-phase to the rhombohedral R-phase at T~100 K and indicates a reorientation of the spins at about 130 K.

cond-mat.str-el↗

arXiv2002

Coherent X-ray Scattering from Manganite Charge and Orbital Domains

We report coherent x-ray scattering studies of charge and orbital domains in manganite systems. The experiments were carried out on LaMnO_3 and Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed to be constant over a timescale of at least minutes, which is indicative of static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both charge and orbital speckle patterns were observed. The observation of the latter rules out the presence of fast orbital fluctuations, while long time series data-- on the order of several minutes-- were suggestive of slow dynamic behavior. In contrast, the charge order speckle patterns were static.

cond-mat.str-el↗

arXiv2001

Intrinsic Tunneling in Cuprates and Manganites

The most anisotropic high temperature superconductors like Bi2Sr2CaCu2O8, as well as the recently discovered layered manganite La1.4Sr1.6Mn2O7 are layered metallic systems where the interlayer current transport occurs via sequential tunneling of charge carriers. As a consequence, in Bi2Sr2CaCu2O8 adjacent CuO2 double layers form an intrinsic Josephson tunnel junction while in in La1.4Sr1.6Mn2O7 tunneling of spin polarized charge carriers between adjacent MnO2 layers leads to an intrinsic spin valve effect. We present and discuss interlayer transport experiments for both systems. To perform the experiments small sized mesa structures were patterned on top of single crystals of the above materials defining stacks of a small number of intrinsic Josephson junctions and intrinsic spin valves, respectively.

cond-mat.supr-con↗

arXiv2001

Fermi Surface Nesting and Nanoscale Fluctuating Charge/Orbital Ordering in Colossal Magnetoresistive Oxides

We used high resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered Colossal Magnetoresistive (CMR) oxide La1.2Sr1.8Mn2O7. With these parameters the calculated in-plane conductivity is nearly one order of magnitude larger than the measured DC conductivity. This discrepancy can be accounted for by including the pseudogap which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and many other properties are the parallel straight Fermi surface sections which are highly susceptible to nesting instabilities. These nesting instabilities produce nanoscale fluctuating charge/orbital modulations which cooperate with Jahn-Teller distortions and compete with the electron itinerancy favored by double exchange.

cond-mat.str-el↗

arXiv2001

Surface electronic structure of Sr2RuO4

We have addressed the possibility of surface ferromagnetism in Sr2RuO4 by investigating its surface electronic states by angle-resolved photoemission spectroscopy (ARPES). By cleaving samples under different conditions and using various photon energies, we have isolated the surface from the bulk states. A comparison with band structure calculations indicates that the ARPES data are most readily explained by a nonmagnetic surface reconstruction.

cond-mat.supr-con↗