Masafumi Sera

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (277)506.46 Total impact

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    ABSTRACT: To investigate the 4f-electronic states under the crystal electric field (CEF) in HoFe2Al10, we carried out ultrasonic and specific heat measurements on single-crystalline samples. We found elastic softening of the transverse elastic modulus C55 below 20K and two Schottky peaks at 2 and 20K in the magnetic specific heat. By theoretical analyses of the modulus and the magnetic specific heat based on the CEF theory, we reproduced the elastic softening and the two Schottky peaks, and obtained the CEF parameters.
    Preview · Article · Dec 2015 · Physics Procedia
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    ABSTRACT: We examined the chemical doping effect on the Kondo semiconductor CeFe2Al10 with a nonmagnetic ground state by means of the magnetic susceptibility, specific heat, electrical resistivity, and thermopower. The effect of Ru doping on the ground state is small. On the other hand, by a small amount of Rh doping, the magnetic susceptibility is strongly enhanced along the orthorhombic a axis, and a Curie-Weiss behavior is observed in a wide temperature range. The low-temperature specific heat is also strongly enhanced by the doping, and a metallic ground state is realized at low temperatures. These results suggest the collapse of the spin and charge gap due to the suppression of the c−f hybridization effect. From the results of a crystalline electric field analysis on the magnetic susceptibility of Ce(Fe1−xRhx)2Al10, it was revealed that the Rh-doping effect on the c−f hybridization effect is anisotropic, especially for the a axis. Similar doping effects are seen in the Rh-doped CeRu2Al10, Ir-doped CeOs2Al10, and Si-doped CeRu2Al10. From these results, we conclude that the collapse of the spin and charge gap by such an excess electron doping is one of the universal features of the Kondo semiconductor CeT2Al10 (T = Fe, Ru, and Os).
    No preview · Article · Dec 2015 · Physical Review B
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    ABSTRACT: We have measured the electrical resistivity of cerium monochalcogenices, CeS, CeSe, and CeTe, under high pressures up to 8 GPa. Pressure dependences of the antiferromagnetic ordering temperature $T_{N}$, crystal field splitting, and the $\ln T$ anomaly of the Kondo effect have been studied to cover the whole region from the magnetic ordering regime at low pressure to the Fermi liquid regime at high pressure. $T_{N}$ initially increases with increasing pressure, and starts to decrease at high pressure as expected from the Doniach's diagram. Simultaneously, the $\ln T$ behavior in the resistivity is enhanced, indicating the enhancement of the Kondo effect by pressure. It is also characteristic in CeX$_{c}$ that the crystal field splitting rapidly decreases at a common rate of $-12.2$ K/GPa. This leads to the increase in the degeneracy of the $f$ state and further enhancement of the Kondo effect. It is shown that the pressure dependent degeneracy of the $f$ state is a key factor to understand the pressure dependence of $T_{N}$, Kondo effect, magnetoresistance, and the peak structure in the temperature dependence of resistivity.
    No preview · Article · Dec 2015 · Journal of the Physical Society of Japan
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    ABSTRACT: Magnetic properties of single crystalline SmRu$_{2}$Al$_{10}$ have been investigated by electrical resistivity, magnetic susceptibility, and specific heat. We have confirmed the successive magnetic phase transitions at $T_{\text{N}}=12.3$ K and $T_{\text{M}}=5.6$ K. Resonant x-ray diffraction has also been performed to study the magnetic structures. Below $T_{\text{N}}$, the Sm$^{3+}$ moments order in an incommensurate structure with $q_1=(0, 0.759, 0)$. The magnetic moments are oriented along the orthorhombic $b$ axis, which coincides with the magnetization easy axis in the paramagnetic phase. A very weak third harmonic peak is also observed at $q_3=(0, 0.278, 0)$. The transition at $T_{\text{M}}$ is a lock-in transition to the commensurate structure described by $q_1=(0, 0.75, 0)$. A well developed third harmonic peak is observed at $q_3=(0, 0.25, 0)$. From the discussion of the magnetic structure, we propose that the long-range RKKY interaction plays an important role, in addition to the strong nearest neighbor antiferromagnetic interaction.
    Preview · Article · Nov 2015 · Physical Review B
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    ABSTRACT: A Kondo semiconductor CeRu$_2$Al$_{10}$ with an orthorhombic crystal structure shows an unusual antiferromagnetic ordering at rather high temperature $T_0$ of 27.3 K, which is lower than the Kondo temperature $T_{\rm K}\sim$ 60 K. In optical conductivity [$\sigma(\omega)$] spectra that directly reflect electronic structure, the $c$-$f$ hybridization gap between the conduction and $4f$ states is observed at around 40 meV along the three principal axes. However, an additional peak at around 20 meV appears only along the $b$ axis. With increasing $x$ to 0.05 in Ce(Ru$_{1-x}$Rh$_x$)$_2$Al$_{10}$, the $T_0$ decreases slightly from 27.3 K to 24 K, but the direction of the magnetic moment changes from the $c$ axis to the $a$ axis. Thereby, the $c$-$f$ hybridization gap in the $\sigma(\omega)$ spectra is strongly suppressed, but the intensity of the 20-meV peak remains as strong as for $x=0$. These results suggest that the change of the magnetic moment direction originates from the decreasing of the $c$-$f$ hybridization intensity. The magnetic ordering temperature $T_0$ is not directly related to the $c$-$f$ hybridization but is related to the charge excitation at 20 meV observed along the $b$ axis.
    Full-text · Article · Jun 2015
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    ABSTRACT: We have studied the Pr- and La-doping effects on the magnetic anisotropy in the antiferro-magnetic (AFM) phase of ${\mathrm{CeRu}}_{2}{\mathrm{Al}}_{10}$. The crystalline electric field (CEF) splitting in ${\mathrm{PrRu}}_{2}{\mathrm{Al}}_{10}$ was found to be as large as $\ensuremath{\sim}800$ K with a singlet ground state. In ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$, the CEF level scheme of the Pr ion is not changed with $x$. The AFM moment (${m}_{\mathrm{AF}}$) is rotated from $c$ to $b$ axis in both systems at ${x}_{c}^{\mathrm{sr}}\ensuremath{\sim}0.03$ and $\ensuremath{\sim}0.07$ for Ln=Pr and La, respectively. As the ionic radius of La and Pr is larger and smaller than that of Ce, respectively, these results indicate that the chemical pressure effect is not associated with the rotation of ${m}_{\mathrm{AF}}$, but is caused by the suppression of the $c\ensuremath{-}f$ hybridization originating from the decrease of $4f$ electrons of Ce ions by Ce-site substitution. Since a small amount of Pr or La doping changes easily the magnetization easy axis of all the moments on Ce sites, the origin of the magnetic anisotropy is not the local single ion effect but the bandlike effect through the anisotropic $c\ensuremath{-}f$ hybridization. The magnetic phase diagrams of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Ln}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$ indicate that above ${x}_{c}^{\mathrm{sr}}$, the AFM order with ${m}_{\mathrm{AF}}\ensuremath{\parallel}b$ continues to exist up to ${x}_{c}$, which is $\ensuremath{\sim}0.4$ and $\ensuremath{\sim}0.6$ in Ln=Pr and Ln=La, respectively. This indicates that even in the sample with an AFM transition temperature (${T}_{0}$) near ${x}_{c}$, the anisotropic $c\ensuremath{-}f$ hybridization dominates the AFM order. A large positive transverse magnetoresistance is seen below ${T}_{0}$, but a very small one above ${T}_{0}$. Together with the results of Hall resistivity and the observation of Shubnikov\char21{}de Haas oscillation, we propose that there exist large Fermi surfaces above ${T}_{0}$ and small ones below ${T}_{0}$. A gap is opened by the AFM order on almost the area of the large Fermi surface, and small Fermi surfaces are constructed below ${T}_{0}$, although we do not know the mechanism, which might be specific to the AFM order in Kondo semiconductors. The largest suppression of the magnetic scattering below ${T}_{0}$ is observed for the current $I\ensuremath{\parallel}a$ and the smallest one for $I\ensuremath{\parallel}b$. This anisotropy may be associated with the anisotropic $c\ensuremath{-}f$ hybridization, which may contribute to the anisotropic magnetic scattering of the conduction electron below ${T}_{0}$.
    No preview · Article · Jun 2015 · Physical Review B
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    ABSTRACT: We have investigated the anomalous ordered phase of CeTe under high pressure, which has been suggested to be an antiferroquadrupole ordered phase. An anisotropic magnetic phase diagram has been obtained from magnetization and specific heat measurements for the three main field directions along [100], [110], and [111]. We discuss the magnetic phase diagram using a two-sublattice mean-field calculation including antiferromagnetic and antiferroquadrupolar interactions. The anomalous ordered phase can be interpreted as an antiferromagnetic ordered phase, which is strongly affected by the antiferroquadrupolar interaction through the off-diagonal matrix element between the Gamma(7) crystal-field ground state and the Gamma(8) excited state.
    No preview · Article · Apr 2015 · Journal of the Physical Society of Japan
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    ABSTRACT: Numerical investigations on muon sites in Ce-based Kondo semiconductors, Ce(Ru,Rh)2Al10 were carried out by using the Density Functional Theory. From the view point of simple electrostatic potential calculations, we found all the previously reported muon sites, suggested by di↵erent groups (Kambe S et al. 2010 J. Phys. Soc. Jpn. 79 053708 and Khalyavin D D et al., 2010 Phys. Rev. B 82 100405(R)), can be possibly chosen as muon stopping sites. We also investigated the changes in the potential of the Rh-doped case. We discovered that the electronic potential around the nearest Ru atom to the substituted Rh atom is a↵ected and the potential becomes asymmetric around the nearest Ru ion. Although big changes in hyperfine fields at muon sites have been reported (Guo H et al. 2013 Phys. Rev. B 88 115206), the muon positions estimated from the potential calculations do not change much.
    Full-text · Conference Paper · Dec 2014
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    ABSTRACT: We have performed Ru-NQR measurements on CeRu2Al10 exhibiting novel phase transition at an abnormally high temperature T-0 = 27.3K and on NdRu2Al10 with a magnetic transition temperature T-m = 2.4K as a reference RKKY system. The splitting of the NQR line due to internal fields below T-0 shows a mean-field-like monotonic increase, indicating no change in the magnetic structure below T-0. The internal field strength is one order larger than those at Al sites in CeRu2Al10, and is comparable to that at the Ru site in NdRu2Al10 despite the sevenfold smaller magnitude of the 4 f moment, being indicative of an enhanced conduction electron polarization at the Ru site. One of the causes of the high T-0 might be the enhanced exchange coupling through the Ce-Ru-Ce path mediated by the enhanced conduction electron polarization. Being similar to that in the Al site, the nuclear spin-lattice relaxation rate 1/T-1 shows a gap-like decrease below T-0 without enhancement owing to a critical slowing down at T-0, in contrast to the mean field 2nd-order transition.
    No preview · Article · Oct 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: The effect of electron doping by the substitution of Rh for Ru on unconventional magnetic order in CeRu2Al10 was investigated via neutron powder diffraction. In Ce(Ru1-xRhx)(2)Al-10 with x = 0.05, 0.12, and 0.2, reorientation of the ordered moment from the c-axis as in pure CeRu2Al10 to the a-axis takes place in all samples, while the ordering vector q = (0, 1, 0) remains unchanged within this concentration range. The moment reorientation is accompanied by an increase in its size by a factor of similar to 2.4, from mu = 0.43 mu(B) at x = 0 to mu = 1.06, 1.04, and 1.02 mu(B) for x = 0.05, 0.12, and 0.2, respectively. The continuous decrease in the Neel temperature T-0(T-N), despite an abrupt increase in mu, underlines the strong anisotropy in the exchange interaction in CeRu2Al10 and the fact that this anisotropy is easily suppressed by electron doping.
    No preview · Article · Oct 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: We investigated the Ce- and Ru-site substitution effects on the spin gap in CeRu2Al10 by measuring the magnetic and thermal properties of Ce(1-y)Ln(y)Ru(2)Al(10) (Ln = La, Pr, Y) and Ce(Ru1-xTx)(2)Al-10 (T = Re, Rh) system, respectively. The magnetic susceptibility shows that the itinerant and localized nature is enhanced by Re and Rh doping, respectively. In the latter, the orientation of the magnetic moment in the antiferromagnetic ordered phase suddenly changes from the c to a axis at x = 0.03. The orientation of the magnetic moment to the a axis is consistent with the large anisotropy of the magnetic susceptibility in the paramagnetic region. The specific heat shows that the exponential temperature dependence, e(-Delta/kBT), and the electronic specific coefficient gamma is not changed in the Ce-site substituted systems at least up to y = 0.1 but in the Ru-site substituted systems, the e(-Delta/kBT) dependence disappears, and the gamma value increases rapidly with x. These indicate that although the spin gap is robust against the Ce-site substitution by Ln ion at least up to y = 0.1, the spin gap is rapidly collapsed by the Ru-site substitution and in place, the conduction electron with heavy effective mass appears at the Fermi level. The spin gap which is formed under the subtle balance between the localized and itinerant nature is easily collapsed by a small amount of Re or Rh doping.
    No preview · Article · Oct 2014 · Physical Review B
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    ABSTRACT: We have performed a neutron diffraction experiment on Pr-doped Ce0.5Pr0.1La0.4B6, in which an antiferromagnetic octupole order with q=(1/2,1/2,1/2) could be anticipated by analogy with Ce0.7La0.3B6. Contrary to this natural expectation, we detected an unambiguous magnetic peak at q =(1/4,1/4,1/2), which is the same q-vector frequently realized in the magnetic ordered phases of RB6 (R = rare earth) compounds. No signi ficant signal was observed at q=(1/2,1/2,1/2) at zero magnetic field. This result shows that the normal antiferromagnetic dipole moment is also one of the competing multipole order parameters in the CexLa1-xB6 system. The relevant order parameters are close in energy and can be tuned by a weak perturbation.
    No preview · Article · Sep 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: The compound NdFe$_2$Al$_{10}$ ($Cmcm$, space group #63) has been studied by both powder and single-crystal neutron diffraction. Below T$_N$ = 3.9 K, the Nd$^{3+}$ magnetic moments order in a commensurate magnetic structure, whose unit cell consists of four orthorhombic unit cells stacked along the $b$ direction. It can be described either as double-$k$ [$\mathbf{k}_1$ = (0, 1/4, 0), $\mathbf{k}_3$ = (0, 3/4, 0)] on the original base-centered orthorhombic lattice or, equivalently, as single-$k$ (wave vector $\mathbf{k}_1$ alone) on the primitive orthorhombic lattice obtained by considering corners and centers of (0 0 1) faces as inequivalent. The intensity refinements point to a structure consisting of (0 1 0) ferromagnetic planes stacked along the $b$ direction, in which the moments are collinear and oriented along the $a$ axis (easy direction according to bulk magnetization measurements). The alternating sequence providing the best refinement turns out to be that which yields the lowest exchange energy if one assumes antiferromagnetic near-neighbor exchange interactions with $J_1 \gg J_2, J_3$. Information is also presented regarding the temperature and magnetic field dependence of the magnetic structure.
    Full-text · Article · Aug 2014 · Physical Review B
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    ABSTRACT: We examined the magnetic and transport properties of the Kondo semiconductor CeFe2Al10. A large increase in the electrical resistivity along the a- and b-axes (rho(a), rho(b)) is observed below 20 K, but only a small increase in rho(c). A clear shoulder is seen at T similar to 20 K in the thermoelectric power along the b-axis. The upturn of rho(a) and rho(b) below 20 K is quite easily suppressed by pressure as well as by a magnetic field along the a-axis. From these results, we conclude that the c-f hybridization gap below 20 K is characterized by a charge gap along the a- and b-axes, and a spin gap along the a-axis, while a small charge gap effect is observed along the c-axis.
    No preview · Article · Aug 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: We carried out a high-resolution x-ray diffraction experiment on ${\mathrm{Ce}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{B}}_{6}$ that exhibits staggered order of $4f$ octupole moments below ${T}_{O}=1.4$ K. Theoretically, ferroquadrupole moments that accompany the antiferro-octupole order and a resultant rhombohedral deformation of the cubic lattice were predicted. In contrast, experimentally, no direct evidence of the rhombohedral lattice has been obtained. We observe the splitting of Bragg peaks below ${T}_{O}$ and find that the unit cell is a rhombohedron being elongated along the [111] axis. The response of rhombohedral domains to magnetic fields also well agrees with theoretical calculations. A particular outcome of this experiment is that the magnitude of the induced quadrupole moments is precisely evaluated from the obtained shear strain. Using this result, the magnitude of the hidden octupole moments is also discussed.
    No preview · Article · Jul 2014 · Physical Review B

  • No preview · Conference Paper · Jun 2014

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  • No preview · Conference Paper · Jun 2014
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    ABSTRACT: Kondo semiconductors CeT2Al10 (T = Ru and Os) with the orthorhombic YbFe2Al10-type structure have attracted attentions due to the extremely high Néel temperatures. We have investigated the magnetic properties of NdFe2Al10 with the same structure as CeT2Al10 to obtain information of magnetic interactions in these compounds. From the results of magnetization measurements, it was confirmed that the long range antiferromagnetic order occurs at TN = 3.77(1) K in NdFe2Al10. We have measured 57Fe Mössbauer spectra of NdFe2Al10 in the temperature range from 2.8 to 300 K. All observed Mössbauer spectra can be fitted using one Fe site. The temperature dependence of the refined magnetic hyperfine field below TN cannot be explained by using the Brillouin function with J = 9/2. Furthermore, the Mössbauer spectra in the temperature from TN to 4.2 K indicate magnetic hyperfine fields at the Fe site in the paramagnetic state. These results reveal that there are magnetic fluctuations in the paramagnetic state of NdFe2Al10.
    No preview · Conference Paper · Jun 2014
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    ABSTRACT: The Γ5u-type antiferro-octupole (AFO) ordered state of Ce0.7La0.3B6, emerging below 1.5 K, has been studied by resonant x-ray diffraction in magnetic fields. We studied in detail what kind of multipole moments are induced by the field in the AFO phase. In a mean-field model for the Γ5u-AFO order within the Γ8 quartet crystal-field ground state, the Γ3g-type antiferroquadrupole (AFQ) is expected to be induced most strongly. However, contrary to this expectation, the main induced moment was the Γ5g-type AFQ, which is the order parameter of the field induced AFQ phase above 1 T.
    No preview · Conference Paper · Jun 2014

Publication Stats

3k Citations
506.46 Total Impact Points

Institutions

  • 1999-2015
    • Hiroshima University
      • • Department of Quantum Matter
      • • Graduate School of Advanced Sciences of Matter
      Hirosima, Hiroshima, Japan
  • 2014
    • Zhejiang University
      • State Key Lab of Silicon Materials
      Hang-hsien, Zhejiang Sheng, China
  • 2001-2012
    • Kochi University
      • • Graduate School of Integrated Arts and Sciences
      • • Faculty of Science
      Kôti, Kochi, Japan
  • 1998-2001
    • Aoyama Gakuin University
      • Department of Physics and Mathematics
      Edo, Tōkyō, Japan
  • 1983-2000
    • Tohoku University
      • • Institute for Materials Research
      • • Department of Physics
      Sendai-shi, Miyagi, Japan
  • 1996
    • Nippon Telegraph and Telephone
      Edo, Tōkyō, Japan
  • 1990-1994
    • Nagoya University
      • Department of Material Science
      Nagoya, Aichi, Japan