We demonstrate field-induced single-ion magnetic anisotropy resulting from the multiorbital Kondo... more We demonstrate field-induced single-ion magnetic anisotropy resulting from the multiorbital Kondo effect on the diluted ytterbium alloy (Lu1-xYbx)Rh2Zn20. Single-ion anisotropic metamagnetic behavior is revealed in low-temperature regions where the local Fermi-liquid state is formed. Specific heat, low-field magnetic susceptibility, and resistivity indicate reproduction of the ground-state properties by the SU(N = 8) Kondo model with a relatively large c-f hybridization of TK = 60.9 K. Dynamical susceptibility measurements on YbRh2Zn20 support realizing the multiorbital Kondo ground state in (Lu1-xYbx)Rh2Zn20. The single-ion magnetic anisotropy becomes evident above ∼5 T, which is lower than the isotropic Kondo crossover field of 22.7 T, verifying blurred low-lying crystal field states through the multiorbital Kondo effect.
Topological defects are found ubiquitously in various kinds of matter, such as vortices in type-I... more Topological defects are found ubiquitously in various kinds of matter, such as vortices in type-II superconductors, and magnetic skyrmions in chiral ferromagnets. While knowledge on the static behavior of magnetic skyrmions is accumulating steadily, their dynamics under forced flow is still a widely open issue. Here, we report the deformation of the moving magnetic skyrmion lattice in MnSi under electric current flow observed using small-angle neutron scattering. A spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense for opposite sample edges, is observed for current densities greater than a threshold value j t ~1 MA m -2 (10 6 A m -2 ). Our result show that skyrmion lattices under current flow experience significant friction near the sample edges due to pinning, this being a critical effect that must be considered for anticipated skyrmion-based applications at the nanoscale.
Neutron polarisation correction to triple-axis data with analytical derivations
Journal of physics, Apr 1, 2023
Polarised neutron scattering is the method of choice to study magnetism in condensed matter. Pola... more Polarised neutron scattering is the method of choice to study magnetism in condensed matter. Polarised neutrons are typically very low in flux, and complex experimental configurations further reduce the count rate. Neutron polarisation corrections would therefore be needed. Here we analytically derive formulae of the corrected partial differential scattering cross-sections. The analytical method is designed for the longitudinal polarisation analysis, and the correction generally holds for time-independent polarised neutrons with a triple-axis spectrometer. We then apply the correction to recent results of our Px experiment on Y3Fe5O12. Although there is a difficulty with the experimental determination of inefficiency parameters of neutron spin polarisers and flippers, the correction appears to work properly.
High-pressure electrical resistance measurements were performed for iron-based ladder material Ba... more High-pressure electrical resistance measurements were performed for iron-based ladder material Ba 1-x Cs x Fe 2 Se 3 (x = 0.25 and 0.65) using a diamond anvil cell (DAC). Recent high-pressure study revealed that iron-based ladder material BaFe 2 S 3 exhibits an insulatormetal transition and superconductivity, and this discovery would provide important insight for understanding the mechanism of iron-based superconductors. Therefore, it is intriguing to investigate the high-pressure properties for the iron-based ladder material Ba 1-x Cs x Fe 2 Se 3 system. The parent compounds BaFe 2 Se 3 and CsFe 2 Se 3 show insulating and magnetic ordering features. For Ba 1-x Cs x Fe 2 Se 3 system, no magnetic ordering is observed for x = 0.25 and minimum charge gap was estimated for x = 0.65. The insulator-metal transitions are observed in both materials.
We performed a comprehensive study on structural, electrical, magnetic, and optical properties fo... more We performed a comprehensive study on structural, electrical, magnetic, and optical properties for iron-based ladder materials BaFe 2 (S 1-x Se x ) 3 (0 x 1), which shows pressure-induced superconductivity in the vicinity of the Mott transition at x = 0 and 1. We obtain a complete electronic phase diagram in a temperaturecomposition plane, which reveals that the magnetic ground state switches from the stripe-type to the block-type phase without any intermediate phase at x = 0.23 with increasing x. This behavior is in sharp contrast to the filling controlled system Ba 1-x Cs x Fe 2 Se 3 , in which a paramagnetic state down to the lowest temperature is realized between two magnetic ordered states. The structural transition, which is considered to be relevant to the orbital order, occurs far above the magnetic transition temperature. The magnetic and structural transition temperatures exhibit a similar composition dependence, indicating a close relationship between magnetic and orbital degrees of freedom. In addition, we found that charge dynamics are considerably influenced not only by the magnetic order but also by the structural change (orbital order) from the detailed measurements of electrical resistivity and optical conductivity spectra. We discuss the magnetism and orbital order by comparing the experimental results with the proposed theory based on the multiorbital Hubbard model. The relationship between the charge dynamics and the magnetic/orbital order is also discussed.
High-pressure Synthesis of Ba<sub>2</sub>CoO<sub>2</sub>Ag<sub>2</sub>Te<sub>2</sub> with Extended CoO<sub>2</sub> Planes
Inorganic Chemistry, May 21, 2020
Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba2CoO2Ag2Te2 (sp... more Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba2CoO2Ag2Te2 (space group: I4/mmm) with alternating stacks of CoO2 and Ag2Te2 layers, separated by Ba atoms. The CoO2 plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag2Te2 layers, causing displacement of oxide anions. Layered cobaltates with trans-CoO4X2 (X = chalcogen, halogen) coordination feature large spin-orbit coupling, which is linearly scaled by the tetrahedral factor of dCo-X/dCo-O. However, applying this relation to Ba2CoO2Ag2Te2 yields a magnetic moment of ∼4 μB, which is nearly twice the experimentally observed value of 1.87(17) μB. This result, along with a reduced Néel temperature (TN = 60 K), originates from the off-centered position of otherwise under-bonded oxide anions, which changes the crystal field splitting of Co d orbitals.
The correlation between magnetic and dielectric properties has been investigated for the single c... more The correlation between magnetic and dielectric properties has been investigated for the single crystal of chiral triangular-lattice helimagnet MnSb 2 O 6 . We found that the spin-spiral plane in the ground state has a considerable tilting from the (110) plane, and that the sign of spin-spiral tilting angle is coupled to the clockwise/counter-clockwise manner of spin rotation and accordingly to the sign of magnetically-induced electric polarization. This leads to unique magnetoelectric responses such as magnetic-field-induced selection of a single ferroelectric domain as well as reversal of electric polarization just by a slight tilting of magnetic field direction, where the chiral nature of the crystal structure plays a crucial role through the coupling of the chirality between the crystal and magnetic structures. Our results demonstrate that the crystallographic chirality can be an abundant source of novel magnetoelectric functions with coupled internal degrees of freedom.
Using small-angle neutron scattering (SANS), we investigate a deformation of the magnetic skyrmio... more Using small-angle neutron scattering (SANS), we investigate a deformation of the magnetic skyrmion lattice in the bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j t ~ 10 6 A/m 2 . We show this peak broadening to originate from a spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense observed for opposite sample edges aligned with the direction of current flow. The peak broadening and the corresponding skyrmion lattice rotations remain finite even after switching off the electric current. These results indicate that skyrmion lattices under current flow experience significant friction near the sample edges, and plastic deformation due to pinning effects.
We report positive muon spin relaxation measurements on the triangular lattice magnetic system Fe... more We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa2S4. A magnetic transition not previously detected by specific heat and magnetic susceptibility measurements is found in zero field at T * ≃ 30 K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a conventional magnetic phase transition. Since persistent spin dynamics is observed down to 0.1 K, the ground state cannot be of the canonical spin-glass type, which could be suggested from hysteresis effects in the bulk susceptibility below T f ≃ 16 K. These results are compared to those found for the isomorph NiGa2S4. It is argued that the fate of the transition, which has been interpreted in terms of the Z2 topological transition in this latter system, is probably different in FeGa2S4.
Bulletin of the American Physical Society, Mar 17, 2010
order in newly discovered orthorhombic CaFe 4 As 3 was examined through neutron diffraction on po... more order in newly discovered orthorhombic CaFe 4 As 3 was examined through neutron diffraction on powder and single crystalline samples. The structure can be described as interleaving FeAs strips extending along the b axis. Longitudinally polarized magnetic order with an incommensurate propagation vector 0.37b * < q 0 < 0.39b * develops below a 2nd order phase transition at T N = 89.6 K with critical exponent β = 0.365(6). At T 2 = 25.6 K there is a 1st order phase transition below which q 0 = 0.375(2)b * appears locked to a commensurate value and a transverse component develops. As opposed to the 122 systems, no structural modifications were found at the magnetic transitions. Ab initio LDA+U and dynamic mean field theory calculations indicate that Fermi surface nesting may drive the magnetic instabilities.
Nuclear Instruments and Methods in Physics Research, Oct 1, 2020
We report on a wide angle neutron spin analyzer for neutron energies up to 40 meV (= 1.4 Å) that ... more We report on a wide angle neutron spin analyzer for neutron energies up to 40 meV (= 1.4 Å) that was recently realized in collaboration with Tohoku University and KEK, Japan (Ohoyama et al., 2014; Yokoo et al., 2014). Curved polarizing Fe/Si supermirrors with m = 5.5/1.5 (Schanzer et al., 2016) are arranged radially around the sample covering an angular range of 40 • if all channels are equipped with supermirrors. The mirrors are saturated by a vertical permanent magnetic field ≃ 40 mT generated by NdFeB magnets. It was optimized using finite element analysis. The polarization and transmission of the spin filter have been determined at the beamline BOA at the Swiss spallation neutron source (SINQ) to be = (0.975 ± 0.01) and = = (0.55 ± 0.017) for spin-up neutrons at = 3 Å (= 9 meV), respectively. The transmission is excellent considering that approximately one third of the neutrons hit the front faces of the polarizing blades. The experimental results are in good agreement with Monte Carlo simulations using the software package McStas.
Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified t... more Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is found for complete, anisotropic phase diagrams as well as magnetic properties.
Muon spin rotation (µSR) experiments reveal unconventional spin freezing and dynamics in the two-... more Muon spin rotation (µSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time 10 −6 s at 2 K) sets in below T f = 8.5 ± 0.5 K with a mean-fieldlike temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above T f is strong evidence for 2D critical spin fluctuations. Slow Ni spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields 10 mT, in marked contrast with the field-independent specific heat. The µSR and bulk susceptibility data indicate a well-defined 2D phase transition at T f , below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid.
High-pressure electrical resistance measurements were performed for iron-based ladder material Ba... more High-pressure electrical resistance measurements were performed for iron-based ladder material Ba 1-x Cs x Fe 2 Se 3 (x = 0.25 and 0.65) using a diamond anvil cell (DAC). Recent high-pressure study revealed that iron-based ladder material BaFe 2 S 3 exhibits an insulatormetal transition and superconductivity, and this discovery would provide important insight for understanding the mechanism of iron-based superconductors. Therefore, it is intriguing to investigate the high-pressure properties for the iron-based ladder material Ba 1-x Cs x Fe 2 Se 3 system. The parent compounds BaFe 2 Se 3 and CsFe 2 Se 3 show insulating and magnetic ordering features. For Ba 1-x Cs x Fe 2 Se 3 system, no magnetic ordering is observed for x = 0.25 and minimum charge gap was estimated for x = 0.65. The insulator-metal transitions are observed in both materials.
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Papers by Yusuke Nambu