arXiv subjects

T. Arima

Publications and source records attributed to T. Arima.

60 records · Page 4Linked to original sources

arXiv2005

Change in lattice modulation upon gigantic magnetoelectric transition in GdMnO3 and TbMnO3

Single-crystal synchrotron x-ray diffraction measurements in strong magnetic fields have been performed for magnetoelectric compounds GdMnO3 and TbMnO3. It has been found that the P || a ferroelectric phase induced by the application of a magnetic field at low temperatures is characterized by commensurate lattice modulation along the orthorhombic b axis with q = 1/2 and q = 1/4. The lattice modulation is ascribed to antiferromagnetic spin alignment with a modulation vector of (0 1/4 1). The change of the spin structure is directly correlated with the magnetic-field-induced electric phase transition, because any commensurate spin modulation with (0 1/4 1) should break glide planes normal to the a axis of the distorted perovskite with the Pbnm space group.

cond-mat.str-el↗

arXiv2005

New s-process path and its implication to 187Re-187Os nucleo-cosmochronometer

We study a new s-process path through an isomer of $^{186}$Re to improve a $^{187}$Re-$^{187}$Os nucleo-cosmochronometer. The nucleus $^{187}$Re is produced by this new path of $^{185}$Re(n,$γ$)$^{186}$Re$^m$(n,$γ$)$^{187}$Re. We measure a ratio of neutron capture cross-sections for the $^{185}$Re(n,$γ$)$^{186}$Re$^m$ and $^{185}$Re(n,$γ$)$^{186}$Re$^{gs}$ reactions at thermal neutron energy because the ratio with the experimental uncertainty has not been reported. Using an activation method with reactor neutrons, we obtain the ratio of $R_{th}$ = 0.54 $\pm$ 0.11%. From this ratio we estimate the ratio of Maxwellian averaged cross sections in a typical s-process environment at $kT$ = 30 keV with a help of the temperature dependence given in a statistical-model calculation because the energy dependence of the isomer/ground ratio is smaller than the absolute neutron capture cross-section. The ratio at $kT$=30 keV is estimated to be $R_{st}$ = 1.3 $\pm$ 0.8%. We calculate the s-process contribution from the new path in a steady-flow model. The additional abundance of $^{187}$Re through this path is estimated to be $N_{s}$ = 0.56 $\pm$ 0.35% relative to the abundance of $^{186}$Os. This additional increase of $^{187}$Re does not make any remarkable change in the $^{187}$Re-$^{187}$Os chronometer for an age estimate of a primitive meteorite, which has recently been found to be affected strongly by a single supernova r-process episode.

astro-ph↗

arXiv2004

Ferroelectric properties controlled by magnetic fields in DyMn2O5

Cross-correlation between magnetic and dielectric properties has been attracting renewed interest because of the fundamental as well as technological importance of controlling the electric (magnetic) polarization by an external magnetic (electric) field. Here, we report the novel phenomenon that an external magnetic field induces and/or modifies ferroelectric states in a magnetic material, DyMn2O5. Measurements of temperature dependence of hysteretic polarization curves, pyroelectric current, specific heat, optical second harmonic generation, and x-ray superlattice peaks have revealed successive phase transitions between 43 K and 4 K, accompanying three ferroelectric phases. The zero-field lowest-temperature phase (<8 K) induced by the Dy-moment ordering is a reentrant paraelectric state, but is turned to a ferroelectric state with increasing the magnetic field. The phenomenon is closely related to the metamagnetic transitions of the Dy f-moment, indicating that all the ferroelectric phases of this material are strongly tied to the antiferromagnetic Mn spin structure affected by the f-d exchange interaction. The electric phase diagram for DyMn2O5 is presented in the plane of temperature and magnetic field.

cond-mat.str-el↗

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↗

arXiv2001

Observation of the antiferroquadrupolar order in DyB2C2 by resonant x-ray scattering

We have investigated the antiferroquadrupolar(AFQ) order in DyB$_2$C$_2$ by resonant x-ray scattering. X-rays with energies near the $L_3$ absorption edge of Dy were employed. Superlattice peaks that correspond to three kinds of propagation vectors of (1 0 0), (1 0 1/2) and (0 0 1/2) were investigated in detail with polarization analyses. The experimental results are analyzed using a formalism on resonant x-ray scattering and a model of the AFQ order. The magnetic and quadrupolar scatterings are explained by the model satisfactorily. A detailed investigation on the critical behavior of the AFQ ordering is also reported. The critical exponent $β$ is deduced to be 0.35, not far from the three-dimensional Heisenberg system. We have also succeeded in detecting the diffuse scattering above $T_{Q}$.

cond-mat.str-el↗

arXiv2001

A Resonant X-ray Scattering Study of Octahedral Tilt Ordering in LaMnO$_3$ and Pr$_{1-x}$Ca$_x$MnO$_3$

We report an x-ray scattering study of octahedral tilt ordering in the manganite series Pr$_{1-x}$Ca$_x$MnO$_3$ with x=0.4 and 0.25 and in LaMnO$_3$. The sensitivity to tilt ordering is achieved by tuning the incident x-ray energy to the L$_I$, L$_{II}$ and L$_{III}$ absorption edges of Pr and La, respectively. The resulting energy-dependent profiles are characterized by a dipole-resonant peak and higher energy fine structure. The polarization dependence is predominantly $σ$-to-$π$ and the azimuthal dependence follows a sin-squared behavior. These results are similar to those obtained in recent x-ray scattering studies of orbital ordering carried out in these same materials at the Mn K edge. They lead to a description of the cross-section in terms of Templeton scattering in which the tilt ordering breaks the symmetry at the rare earth site. The most interesting result of the present work is our observation that octahedral tilt ordering persists above the orbital ordering transition temperatures in all three samples. Indeed, we identify separate structural transitions which may be associated with the onset of orbital and tilt ordering, respectively, and characterize the loss of tilt ordering versus temperature in LaMnO$_3$.

cond-mat.str-el↗