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Oxygen Isotope Effect on the Spin State Transition in (PrSm)CaCoO
Oxygen isotope substitution is performed in the perovskite cobalt oxide
(PrSm)CaCoO which shows a sharp spin
state transition from the intermediate spin (IS) state to the low spin (LS)
state at a certain temperature. The transition temperature of the spin state
up-shifts with the substitution of by O from the resistivity
and magnetic susceptibility measurements. The up-shift value is 6.8 K and an
oxygen isotope exponent () is about -0.8. The large oxygen isotope
effect indicates strong electron-phonon coupling in this material. The
substitution of O by O leads to a decrease in the frequency of
phonon and an increase in the effective mass of electron (), so that
the bandwidth W is decreased and the energy difference between the different
spin states is increased. This is the reason why the is shifted to high
temperature with oxygen isotopic exchange.Comment: 4 pages, 3 figure
Large magnetothermal conductivity of HoMnO_3 single crystals and its relation to the magnetic-field induced transitions of magnetic structure
We study the low-temperature heat transport of HoMnO_3 single crystals to
probe the magnetic structures and their transitions induced by magnetic field.
It is found that the low-T thermal conductivity (\kappa) shows very strong
magnetic-field dependence, with the strongest suppression of nearly 90% and the
biggest increase of 20 times of \kappa compared to its zero-field value. In
particular, some ``dip"-like features show up in \kappa(H) isotherms for field
along both the ab plane and the c axis. These behaviors are found to shed new
light on the complex H-T phase diagram and the field-induced re-orientations of
Mn^{3+} and Ho^{3+} spin structures. The results also demonstrate a significant
spin-phonon coupling in this multiferroic compound.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
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